ANGULAR MISALIGNMENT: A measure of the angle between the centerlines of driving and driven shafts, where those centerlines would intersect approximately halfway between the shaft ends. Coupling catalogs will show the maximum angular misalignment tolerable in each coupling. A coupling should not be operated with both angular and parallel misalignment at their maximum values.
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AXIAL: A projection or movement along the line of the axis of rotation. Example: Sliding the hub in either direction may change the position of a coupling hub, on its shaft. Thus affecting its axial position on the shaft.
AXIAL DISPLACEMENT: One type of misalignment that must be handled by the coupling. It is the change in axial position of the shaft and part of the coupling in a direction parallel to the axial centerline. Can be caused by thermal growth or a floating rotor. Some couplings limit this displacement and are called limited end float couplings.
AXIAL FORCES: The driver or driven equipment can generate axial forces (thrust) in which case the coupling will pass those forces to the next available bearing with thrust capability. Because of the inherent construction of some couplings, forces may be generated in the axial direction when operating at high speeds or under misalignment. Such forces can place additional loads on the support bearings.
AXIAL FREEDOM: This characteristic allows for variation in coupling position on the shaft at time of installation.
BACKLASH: The amount of free movement between two rotating, mating parts. If one half of a coupling is held rigid and the other half can be rotated a slight amount (with very little force), you have some amount of backlash. The freedom of movement, or looseness, is the backlash and may be expressed in degrees. Backlash is not the same as torsional stiffness.
BORE: The central hole that becomes the mounting surface for the coupling on the shaft. Close tolerances are required. Bores/shafts are not always round, although that is the most common shape. Other bore types can include hex, square, d-shaped, tapered, and spline. A spline bore is one with a series of parallel keyways formed internally in the hub and matching corresponding grooves cut in the shaft. Spline bores and shafts most commonly conform to Society of Automotive Engineers (SAE) standards.
DAMPING: Some couplings greatly reduce the amount of vibration transmitted between driver and driven shafts because of the damping capacity of an elastomer in the coupling. It is a hysteresis effect that will generate heat. The coupling must dissipate this heat or risk losing its strength by melting down. The stiffness of the elastomer affects the rate at which vibration is damped. All-metal couplings, for the most part have poor damping capacity.
DISTANCE BETWEEN SHAFTS: The distance between the faces (or ends) of driving and driven shafts, usually expressed as the “BE” (between ends) dimension or “BSE” (between shaft ends) dimension.
FACTORS OF SAFETY: The coupling designer applies these factors to compensate for unknown elements of the product design. The factors can compensate for temperature, material variations, fatigue strength, dimensional variations, tolerances, and potential stress risers to name a few.
FAIL-SAFE: A fail-safe coupling is one that will continue to operate for a period of time after the torque-transmitting element has failed. This is characteristic of couplings in which some portion of both halves operate in the same plane, allowing direct contact between those portions. An example of this is the jaw coupling, in which driving jaw faces push the driven jaw faces through an elastomer in compression between them; if the elastomer breaks away, the driving faces simply advance to push the driven faces directly.
FINITE LIFE VS. INFINITE LIFE IN COUPLINGS:
All couplings fall into one of these two categories:
1.). Finite-life couplings are those that wear in normal operation, because of using sliding or rubbing parts to transmit torque and compensate for misalignment. This group includes jaw, gear, grid, sleeve (shear), nylon sleeve gear, chain, offset and pin & bush types. These types usually have lower purchase costs than infinite-life couplings. They won’t last as long, but their life span may be sufficient for the life expectancy of the application. Periodic maintenance is required.
2). Infinite-life couplings (a name given to “non-wear” couplings) transmit torque and compensate for misalignment by the distorting of flexing elements. The distortion results in fatigue stresses rather than wear, and the couplings are designed and rated to operate within the fatigue capabilities of the coupling material. “Infinite life” couplings do not necessarily last forever. This group includes tire, disc, diaphragm, some donut types, wrapped-spring, flex-link, and most motion-control types. “Infinite life” couplings remain infinite only as long as the load, including those caused by misalignment, is kept within the coupling’s design capabilities. An overload will fail an infinite-life coupling (but may only reduce the life of a finite-life coupling). Infinite-life designs are most often used on maintenance-free systems where maximum torque requirements – including transient, cyclic and start-up torque – are known.
HORSEPOWER: The unit of power used in the U.S. engineering system. It is the time rate of doing work. For power transmission it is the torque applied and rotational distance per unit of time. Applied torque causes a shaft and its connected components to rotate at a certain RPM (revolutions per minute). Horsepower (HP) is converted to torque as follows:
T = the torque in inch-pounds Where T = BHP x /RPM BHP = the motor or other horsepower RPM = the operating speed in revolutions per minute = a constant used for inch-pounds; use for foot-pounds, and for Newton-meters The metric system uses kilowatts (kW) for driver ratings. Converting kW to torque: Where T = BHP x /RPM T = the torque in inch pounds kW = the motor or other kilowatts RPM = the operating speed in revolutions per minute = a constant used when torque is in inch-pounds. Use for foot-pounds, and for Newton-metersKEYWAY: A rectangular opening formed by matching rectangular slots cut axially (lengthwise) along both the coupling bore and shaft. A square or rectangular metal key is then inserted into the opening to lock the coupling and shaft in position. Torque is transmitted from shaft to coupling through the keyway and key.
LENGTH THROUGH BORE: The effective length of the bore in the hub, or that portion of the length that is useable and may be attached to the shaft.
OUTSIDE DIAMETER: The largest effective diameter of the coupling.
OVERALL LENGTH: The largest effective length of the complete coupling assembly.
PARALLEL MISALIGNMENT: A measure of the offset distance between the centerlines of driving and
driven shafts. Coupling catalogs will show the maximum parallel misalignment tolerable in each coupling. A coupling should not be operated with both parallel and angular misalignment at their maximum values.
RADIAL: Any projection outward from the center of a shaft or cylindrically shaped object, or any motion along that line. The centerline of the projection or motion normally passes through the axial centerline of the object.
REACTIONARY LOADS: When two shafts are offset (parallel misalignment), the coupling’s radial stiffness will cause a broadside force to be exerted on the shafts. This is called a “reactionary load”, as it causes the shafts to bend slightly in reaction to the broadside force. It may also be called a “restoring moment”, as a force produced by the coupling in an effort to restore, or correct, the parallel misalignment.
RESTORING MOMENT: see REACTIONARY LOADS
SERVICE FACTORS: Multipliers that are assigned to common applications to compensate for their typical load characteristics. These are used for the purpose of guiding coupling size selection to a torque rating that will allow for unforeseen demands those characteristics might make on the coupling. Such characteristics can include peak torque, start-up torque, transients or cyclic torque, or any other empirical factor.
Among couplings that have no wear parts (see Finite/Infinite life), service factors are intended to prevent premature failure due to overload damage. Among couplings that use wear parts to transmit torque, service factors are intended to prevent premature failure of those parts due to accelerated wear or degradation.
Caution: Resist the temptation to specify in excess of the published service factors. An oversized coupling will not perform better or last longer, but will be unnecessarily expensive and force the system to waste energy. Always base coupling size and service factor on the actual torque requirements at the point of installation within the drive system.
SET SCREW: A headless screw, with hexagon shaped socket, used over a keyway to keep the key stock in place and prevent the coupling from moving axially along the shaft. It can also be used for torque transmission on low torque applications
STIFFNESS
STATIC TORSIONAL STIFFNESS: A resistance to twisting action (rotational displacement) between driving and driven halves of the coupling. (The opposite – low resistance to twist – is termed “torsional softness”) Stiffness is expressed in lb.-inch/radian and measures the amount of angular displacement about the coupling’s axis of rotation at its static torque rating. Even seemingly stiff all-metal couplings can have some degree of torsional twist.
TORSIONAL SOFTNESS: Torsional soft or hard is determined by dividing the dynamic torsional stiffness by the nominal coupling torque rating. Values greater than 30 are hard (very stiff). Values between 10 and 30 are torsionally flexible. Values less than 10 are considered very soft.
DYNAMIC TORSIONAL STIFFNESS: It is the relationship of the torque to the torsional angle under the load of actual operation. The dynamic stiffness will be greater than the static. The dynamic torsional stiffness can be linear, a constant value, or non-linear, an increasing value.
TOLERANCES: The amount of variation permitted on dimensions or surfaces of machined parts. It is equal to the difference between maximum and minimum limits of any specified dimensions
TORQUE: In rotary motion it is the force multiplied by the radius, to the axis of rotation, at which the force is applied. Force (F) multiplied by radius (r) = F * r = Torque. In English units (F) is in pounds and (r) is in inches, expressed as in.-lbs. In metrics (F) is in Newtons and (r) is in meters, expressed as Newton-meters (Nm).
TORSIONAL VIBRATION: The periodic variation in torque of a rotating system. Some causes of torsional variation are the geometry of the rotating parts of internal combustion engines, cyclic and irregular torque demands of the driven equipment, and variations in the output of certain types of electric motors at startup.
The donut shaped elastomeric coupling consists of a rubber donut fastened with cap screws to hubs. The hubs provide the shaft connection, the elastomer mounts in between the hubs to transmit the torque and allow misalignment. Metal inserts (either aluminum or steel) are bonded into the elastomer and provide a durable material through which the fasteners attach to the hubs. The elastomer donut is precompressed between the fasteners to make certain that the torque is always transferred in a compression mode. The elastomer is stronger in compression than in tension. By preloading the donut, any tensile forces merely relieves the compression and does not put the unit into a tensile load-carrying situation. Donuts can have a square, rectangular, octagonal or other cross-section design. They do not have to be round.
Donut couplings have one hub that is smaller than the other to fit inside the donut. It is called the cylindrical hub. The donut is fastened to the inner or cylindrical hub by radial fasteners. The other hub is a flanged hub to which the donut is attached by axial fasteners. The elastomer uses metal inserts that transfer torque by friction between the metal inserts and the metal hubs then through the elastomer to the next set of fasteners attached to the other hub. The torque path alternates from one leg of the donut to the next. The fasteners are tightened so make a high friction joint to avoid loading the bolts in shear. Donut couplings that use the cylinder and flange hub system have bore limits on the cylindrical hubs compared to other couplings of similar torque capabilities.
The donut style elastomeric coupling is primarily used on torsional damping and tuning systems associated with Diesel drivers. In such a case a flywheel plate replaces the flanged hub. The flywheel plate is drilled to match various SAE designated or DIN designated flywheel dimensions. The coupling is configured to dissipate heat that is generated by hysteresis. It is also rated for a maximum torque, a nominal torque, and a vibratory torque. Each of the values are different, the maximum torque is limited to a specific number of cycles.
Donut type couplings can handle a load in either direction as the load shifts to alternate legs still in compression. The donut can accommodate alternating loads and cyclic loads without backlash. There is windup in elastomeric couplings. These couplings when constructed of rubber exhibit a quality of hystersis. That quality enables the coupling to dampen the vibration energy that passes through the coupling.
Elastomers for Donut Type couplings
The base elastomer is a natural rubber with binders. It is suitable to about 190°F temperature before it loses strength. When the temperature increases the coupling must be derated. The formulations of this elastomer are identified by the shore hardness. Each successively harder rubber carries more torque, but is torsionally less resilient. The variations allow the application engineer to tune the system for critical speed as well as torsional vibration damping. Hystersis, a characteristic exhibited by rubber with binders, allows the elastomeric material to adsorb dynamic energy. The energy is in turn is lost in heat generation.
If the material is able to radiate or otherwise conduct the heat to a sink, damping will occur without damage to the coupling elastomer. If the heat builds up in the elastomeric element it will fail or melt down.
Alternate elastomers include Hytrel® and Zytel®. Each is considerably more stiff than rubber. The change in material may require the coupling design to change to accommodate the fastening of the elastomer to the metal hub. The increase in stiffness changes the unit from torsionally soft to torsionally stiff, and as a result the tuned critical moves from a value below operating speed to one above operating speed. The change in materials will mean an increase in normal torque capability. Refer to the chapter on torsional applications for more information on critical speeds and damping requirements.
This type of coupling is similar to the jaw coupling in that torque is transferred from one hub to the other by compressing captured rubber blocks. In this case the hubs consist of an external claw hub matched to an internal pocket hub that contains the elastomer. There are several varieties of these couplings.
Variations include the shape of the elastomer blocks and the type of elastomer. The ideal shape for the elastomer is a cylinder, loaded radially. Alternatives use rounded-off rectangular shapes. The coupling is used for both shaft-to-shaft connections as well as shaft-to-flywheel connections. A popular application for this coupling is the diesel driven generator. Another common application is the synchronous motor driven compressor. Both of these applications are very high horsepower units. An example of a lower horsepower application is the electric motor driven reciprocating compressor.
This style of torsional coupling is manufactured with OD’s of 3 inches to several feet. Obviously the large size carries a very high torque that is associated with the large generator sets and ship propulsion. The hubs can be a casting of iron or bar stock or a forging of steel. Torsional stiffness ratio for these units is in the medium range, which is consistent with the application requirements. A very soft unit would either not have the torque capability or would have to be dimensionally too big to get the torque capability.
Bonded Elastomer in Shear Couplings
This coupling was developed in for diesel flywheel applications. There are two basic types and both use an elastomer element in-shear. These couplings have a very low torsional stiffness ratio, in the range of 1.5 to 12. The normal torque capability of these couplings range from 900 inch-pounds to more than 1,000,000 inch-pounds depending on the size and type. They are used with the largest of diesel engines and small ones when extremely low torsional stiffness is needed. The couplings can be configured with elements in series to reduce the torsional stiffness even more, or in parallel to increase the torque capabilities. As with other rubber couplings there are several elastomer variations that are identified by the shore hardness. The higher the “shore” number ithe stiffer the torsionaly coupling is.
The first type of bonded elastomer in-shear coupling is a rubber disk bonded to an inner (or driven) metal ring. The inner ring can be combined with various hub types for fastening to a shaft. The types include tapered OD split hubs, bolted straight bore cylindrical hubs, and special hubs for connecting to U-Joint shafting. The outer diameter of the rubber disk is an external toothed form that slides into a circular metal ring with internal teeth. The circular metal ring is cast aluminum to keep the weight, and therefore the inertia, low. The outer ring OD is configured to bolt to a diesel engine flywheel.
The rubber disk has a designed shape to ensure that equal stress occurs over most of its section, thus providing a large torsional angle and avoiding high stress in these areas. Loading at the inner ring and outer teeth is reduced below normally accepted levels by the design of those two areas. The load is carried in shear from its periphery to its center. This style has a non-linear torsional stiffness. The element could have some backlash in the tooth form at the periphery, although normally it is a tight fit. The tooth area becomes a wear point when the coupling is misaligned. Coupling life therefore, is dependent on the wear as well as the torsional loading cycles.
The second type of bonder elastomer in-shear is a four-sided closed ring of elastomer with a special cross sectional shape. The OD, the ID, and one side are flat and perpendicular to each other, the fourth side is tapered from OD to ID in a conical shape. The torque load is carried from one side to the other via shear forces. Both the OD and the thickness from side to side determine torque capability. The elastomer is bonded to metal plates on each side. The plate on the flat side is configured to attach to a flywheel adapter or a shaft hub disk at the OD. The plate on the other side matches the conical shape and is configured to bolt to coupling hubs or half couplings at the ID. The ID may include plain bearings to carry minor radial and axial loads.
There is a wide variety of secondary couplings that are bolted to the side opposite the flywheel. They include gear coupling halves, link couplings, and disc plates. The secondary couplings provide misalignment capabilities not available from the primary torsional coupling. Cardan shaft adapters and clutches have also been attached to the coupling.
The rubber element again has a designed shape to provide for equal stresses across the element. The element has a linear torsional stiffness. There is no backlash in this style of element, however there is torsional windup. This coupling is a non-wear configuration and coupling life is dependent on the torsional damping and maximum load cycles.
Torsionally Stiff Couplings (Flywheel)
While torsional softness can be a benefit for elastomeric couplings, there are some applications that require stiff elastomers. Most of the elastomeric coupling types have an alternative stiff elastomeric material. Jaw coupling, donut shaped compression loaded, and unclamped donut in shear are sometimes supplied with Hytrel® or other stiff elastomers such as Zytel® or urethane. The stiff elastomer is used for greater torque capability without going to a larger size. Stiff elastomers have less resilience and may restrict the angular misalignment capability to much lower values.
Many times the switch to a torsionally stiffer elastomer is to tune the torsional system to a higher natural frequency. This is done on some diesel driven systems with light inertia loading. One example is a diesel driven hydraulic pump for off-highway equipment. Torsionally stiff couplings for these applications are a significant coupling need. The couplings are designed for attachment from a flywheel to a driven shaft. The couplings can be of the compression type as described under the “Donut Shaped Elastomeric Coupling” section or can be a stiff elastomeric disc loaded in shear.
The stiff elastomeric disk, loaded in shear, has a torque capability up to 21,240 inch pounds. The torsional stiffness ratio is above 100. Normally this is a high volume molded disk to make an economical coupling for small diesel production engines.
Shear loaded disks are molded of Zytel® or nylon with strengthening fibers. The disk is designed with boltholes on the periphery to match a flywheel-drilling pattern. The ID is designed to mate with a coupling hub in a sliding fit. The coupling hub can have typical gear coupling teeth with crowning or can have four to six crowned dogs. The crowning accommodates a limited amount of angular misalignment while transferring torque from the element to the hub. Hubs are made from steel bar stock or from powdered metal. The hub bore is usually a spline to match a standard hydraulic pump shaft, but could be a straight bore with key.
The torsionally stiff coupling for flywheel applications is designed for blind assembly. Having the shaft hub slide into the flywheel attached elastomer does this. This coupling type is often supplied with pump mounting plates and flywheel enclosures.
Gear-couplings are the king of coupling types. They can do things that many other couplings cannot do, can only do with difficulty or with expensive modifications and de-rating. Gear couplings are more power intensive, offer more modifications, and a wider size, torque bore range than any other type, and can perform at extremely high speeds. Gear couplings have axial slide capability, low speed high torque capability, shifter capability and spindle capability not found in other couplings. They are easily modified for shear pin service, floating shaft type, vertical type, insulated type, limited end float, and can have a brake drum or disc added. While those latter items may be available on other couplings, it is usually easier and less costly to modify the gear coupling. With all these advantages the gear coupling is used on twice as many applications versus the nearest competitor type.Gear couplings can also perform at extremely high rates of speed. As implied by the name, gear couplings use the meshing of gear teeth to transmit the torque and to provide for misalignment. External gear teeth are cut on the circumference of the hub. Both toothed hubs fit inside the ends of a tubular sleeve that has matching gear teeth cut around its interior circumference, with each tooth extending axially the full length of the sleeve. Hub and sleeve teeth mesh, so torque transfers from the driving hub’s teeth to the sleeve teeth and back to the driven hub’s teeth.
Gear couplings achieve their misalignment capability through backlash in the teeth, crowning on the tooth surfaces, and major diameter fit. Backlash is the looseness-of-fit that results from gear teeth being narrower than the gaps between the teeth. In addition to contributing to the misalignment capabilities, the backlash provides space for the lubricant. The loose fit provides misalignment capability by allowing the sleeve to shift off-axis without binding against the hub teeth. Some gear couplings have more backlash than others. Those with the least (roughly one-half of the backlash present in those with the most) are known as “minimum backlash” couplings. Some users prefer this type, most prefer normal backlash. Crowning, or curving the surface of the hub teeth, further enhances this capability. The crowning can include tip crowns, flank crowns, and chamfers on the sharp edges. This also helps improve tooth life by broadening the contact area along the “pitch line” (where the teeth mate and transfer torque), thereby reducing the pressure of torque forces. In addition, it prevents the sharp squared edges of the tooth from digging in and locking the coupling. Vari-Crown, which varies the curvature radius along the tooth flank, maintains greater contact area between teeth during misalignment compared with standard crowning, and reduces those stresses that cause wear. Note that crowning applies to hub teeth only; sleeve teeth are straight except for a chamfer on the minor diameter edge.
While the hub and sleeve teeth are cut to fit loosely side to side, they are cut to fit closely where the tip diameter of the hub teeth meet the root diameter of the gaps between the sleeve teeth. That is called a major diameter fit. When the coupling is not rotating, those two surfaces rest upon each other if it is a horizontal installation. Minor diameter fits (where the tips of the sleeve teeth meet the root diameter of the hub teeth) are purposely avoided, because a close fit here would preclude suitable misalignment capability and torque transmission capability.
It was noted earlier that gear couplings are power intensive. That means more torque transmitted per pound of coupling weight and per cubic inch of space consumed than other couplings. In many cases the gear coupling has more torque capability than the shaft can transmit. The resulting relatively small size of the gear coupling allows the addition of attachments without having the coupling grow to impracticable proportions. It also allows the OEM designer more latitude to locate the coupling in small, out-of-the-way places with confidence that it will be reliable. Gear couplings eventually wear, but rarely to a catastrophic failure. They can be sized to make sure that wear life is consistent with the rest of the machine design.
Coupling Configuration
Sleeve Alternatives
Gear coupling sleeves can be a single piece, termed a “continuous sleeve”, or can be split laterally (radially) into two half sleeves, one on each hub. The split version is termed a “flanged sleeve”, because each half has a flanged end, drilled for bolt holes, which allows them to be bolted together.
Because the continuous sleeve needs neither flanges nor bolts, it provides the advantage of making the coupling lighter and smaller in diameter than comparably rated flange types. With that comes lower inertia values, which helps lighten motor load during start-up. Bolt stress, which can be a weak point in some applications, is eliminated. The absence of bolts is an advantage in high-speed applications, because bolts add potential points of unbalance and bolted connections can be another point of non-concentricity.
When two halves of a flanged sleeve are bolted together, the bolting becomes an important part of the power transmission path. Best designs have the power transmitted across the face by friction, in which case the bolts simply provide enough clamping force to provide face friction. Other designs could allow the bolts to carry the load in shear, but those are in the minority. Both cases require a proper analysis of the multiple loads on the bolts. In addition the bolt bodies may provide the centering action to pilot the two halves of the coupling.
Bolts can either be exposed, or shrouded for safety reasons. However, with the advent of OSHA coupling-guard requirements, shrouding becomes unnecessary. The two types also have different windage loss and that affects high speed applications. Windage losses cause a heat generation inside the coupling guard. Note that flange bolts are specially made for their purpose, and should never be replaced with common hardware-store bolts. Flanged sleeve gear couplings built to American Gear Manufacturers Association (AGMA) dimensional standards will mate half-for-half with all other gear couplings made to those same standards.
While AGMA standards are U.S. based, many European manufacturers build to match the dimensions. However, matching dimensions include the interface only, such as outside flange diameter, number of boltholes, bolt hole size, bolt circle, and flange thickness. Although length-through-bore of the hub is often identical as well, torque and bore capability are likely to be different and should be compared carefully.
Flex Planes and Misalignment Capability
Planes of flexibility (“flex planes”) are those pivot points along the shaft-to-shaft connection where rigid components engage but can move independently of each other The standard gear coupling (two toothed hubs engaging opposite ends of the same rigid sleeve) has two flex planes, one at each hub-to-sleeve gear mesh. When both flex planes work together in series, flexing in the same direction, they give the gear coupling an angular misalignment capability of up to 1½° at each flex plane.
This standard configuration is called “full flex” or “double engagement” coupling.
The full-flex gear coupling, with two flex planes in series flexing in opposite directions, allows for parallel (radial) misalignment of 0.055 to 0.165 inches in standard models with short sleeves. The longer the sleeve (i.e. the greater the axial distance from one flex plane to the other), the greater the parallel misalignment. The greatest parallel capability results from floating shaft, spacer and spindle versions, described later, which greatly lengthen the distance between flex planes.
Gear couplings can be configured with only one flex plane, for applications where parallel misalignment capability is unwanted. In flanged type couplings, this is accomplished by using a single-piece flanged hub with no teeth, as the rigid half, bolted to a flexible half that uses a standard flanged sleeve with teeth and a standard hub with crowned teeth. These are called “flex-rigid”, “single-engagement” or sometimes “half couplings”. In continuous-sleeve couplings, a flex-rigid configuration is accomplished by mating the sleeve at the rigid end with a hub having straight teeth that fits into the sleeve like a spline shaft into a spline hub. While the full flex design is the most popular in gear couplings, flex rigid designs are often useful in systems with three bearings or floating shafts. Sometimes one flex-rigid coupling is used in series with another flex-rigid coupling at a distance to allow much more parallel misalignment.
While gear couplings will normally provide from ½° to 1½° of angular misalignment per flex half, they can be designed for up to 6° with reduced load capability and with accommodating grease seals.
Axial Displacement
Gear couplings naturally accommodate axial (in-out) shaft movement better than other competing designs, because their hub teeth easily slide along their sleeve teeth with no effect on coupling operation or torque load capability. Axial movement often results from thermal expansion/contraction of the shaft, as in hot applications, or a rotor seeking its magnetic centers (floating rotor). Thrust bearings can limit or prevent shaft movement at the coupling end, but if positioned at the far end of the machine, they can force the shaft movement back toward the coupling. The amount of axial displacement the gear coupling can handle depends primarily on the length of the sleeve, and specials are available for long sliding application.
The Gear Coupling Tooth
The gear coupling tooth has evolved over many years. The first gear couplings had straight teeth, and depended purely on backlash to achieve misalignment. Later improvements included tooth crowning that increased misalignment capability and coupling life. The original tooth form followed the spur gear form with modification. Various pressure angles were used that walked the line between life and strength. The 40° pressure angle tooth was chosen for strength. It proved to have problems with wear life and with reactionary loading on the machinery. Eventually the 20° pressure angle tooth became the standard, and it still is the standard. Some 25° teeth are used to achieve added strength for special designs. The additional strength of today’s materials alleviates the need for 40° teeth and still provides low sliding friction.
The gear coupling tooth, like the spline tooth, is not a full height tooth. Where the spline is 50% height, the gear coupling tooth is about 80%. Gear coupling teeth do not need full height because the torque load is carried at the pitch line of the tooth and many teeth are in contact with each other in the hub and the sleeve to carry the load. The number of teeth in contact is a function of the true form of the teeth. If all teeth in the hub and sleeve are identical the maximum number will be in contact. As the teeth wear into place the more teeth come into contact. Therefore initial tooth wear makes the coupling stronger, but can increase the friction loading too.
The strength of the gear tooth is the subject of many questions in determining the amount of load to be carried. The tooth is the strongest of all the elements of a gear coupling. The tooth strength is calculated as a bending moment at the root of the tooth, the shear strength at the pitch line, and the Hertzian loading at the contact surface. All of these forces act concurrently.
The most likely failure mode of a gear coupling tooth is that which comes from wear rather than any other factor. As the teeth wear, they move from being the strongest element to being the weakest element.
Severe misalignment that causes a lock up of the teeth will also result in premature failure. Most other loading on the coupling will not result in failed teeth.
Lubricant must always be available in the tooth mesh. The lack of lubricant will, of course, cause the coupling to fail almost instantly. The gear coupling is fitted together so as to prevent the lubricant from leaking. Most gear couplings are lubricated with grease. The sleeve to hub interface at the boundaries will need elastomer O-rings, gaskets, or labyrinths to prevent grease leakage. (Note that O-ring material might limit the coupling’s ambient temperature capability.) When oil lubrication is used, it is usually a continuous flow through the tooth mesh, but can be a batch lube in some applications. Oil lubrication is a special case.
Misalignment may allow grease to leak out the seal surface, or some modifications may need a wiper seal rather than an O-ring. One type of flange coupling uses a high misalignment seal with more flex than the regular seal. The seals can be held in place by several means. The Oring is the simplest; it fits into a groove in the sleeve.
The continuous sleeve coupling seal is held in place by a spiral ring. The seal has stiffeners molded into the inside face. It is a U or C shape that stays closed under load. It also provides the movement limit for the coupling and is actually rated to withstand an axial force.
Sometimes the seal holder is bolted to the coupling sleeve. This is always the case on couplings larger than size 9. It makes the assembly of the coupling to the shaft easier, and makes replacement of seals easier. The couplings with bolt on seal carriers are designated heavy duty (HD). Flange series couplings size 7 through 9 can be either the “HD” version or the plain version.
Remember that the coupling grease is not ordinary grease but is specially formulated so the oils do not separate from the soaps. The result is that the lubricant is contained within the needed space and sludge is not allowed to accumulate. Oil and soaps separate in ordinary lubricants because of centrifugal forces on the heavier particles. Use only coupling grease for best results.
Variations to Gear Couplings
1. Fill the Space between Shafts
Couplings often must fill a space between shafts as one of their primary attributes. It would seem a simple enough task, but not all couplings offer flexibility doing that job. This is another reason why the gear coupling is very popular.
2. “BSE” Dimension
The distance between shaft ends (BSE) will vary with different machine systems to accommodate design standards, product line alternatives,different motor frames and maintenance needs. The “BSE” dimension is important for all couplings. Gear couplings have the advantage of allowing a variable “BSE”. That variation can be achieved by machining the hub face or can be achieved by reversing one or both of the hubs. An infinite number of possibilities can be obtained from catalog minimum to catalog maximum. Note this gap (BSE) does not always affect the distance between flex planes unless the hubs are reversed. A combination of facing and reversing is possible too. All couplings have a certain “BSE” dimension variability, but few are able to tolerate as great a variance as gear couplings can.
3. Spacer Couplings
Spacer couplings consist of two flexible hub and sleeve assemblies i.e. a half coupling on both the driving and driven shafts. These are connected by a tubular center section of various lengths that can easily be removed to allow space for removal of the hub or other components on one side of the system without disturbing the hub or component mountings on the other side. The tubular center section can have flanged ends for bolting to hub flanged sleeves, or toothed ends that mate with hubs using continuous sleeves. Spacers are built to the standards of the rotating machinery builders. Pumps have several standard spacers such as 3½ inches, 7 inches and others. Compressors could have a different set of standard spacers. Spacers can serve to separate the flex planes and can be part of the torsional tuning of a coupling.
They have practical limits on length in regard to cost, weight and critical speed. The flanged hollow tube is machined to varying tolerances depending on speed and balance. As the tube gets longer, deflection of the unsupported center section forces the cylinder walls to be made thicker. As the walls get thicker the cost grows more and so does the weight. The weight then reduces the critical speed. That is a cross combination of events that eventually makes the spacer a poor choice. When the spacer becomes impractical, the next step is to use a floating shaft coupling to achieve the necessary spacing.
4. Floating Shafts
Floating shaft couplings consist of flex rigid couplings on both driving and driven shafts connected by a piece of solid shafting between the couplings. Usually the coupling hubs on the equipment ends are rigid while the two center hubs connected to the floating shaft are flexible.
While these two can be used to provide service spacing, the primary reason for a long floating shaft is to allow for greater radial misalignment between shafts. The secondary reason is to reach a long distance between the driver and the rotating equipment. Weight and critical speed are important considerations for floating shafts. They are found on bridge cranes and steel rolling mills.
The couplings and center shaft are designed as a unit to suit their specific application. The parameters include the usual torque and bore, but must include length and speed because, as in any spacer, critical speed and deflection are interrelated. These issues may require a larger diameter center shaft to reduce deflection. In that case the rigid hubs are on the floating shaft, taking advantage of the rigid hub’s greater bore capability, to accommodate the oversized center shaft.
Otherwise, the center shaft may need to be necked down (reduced) to fit a flex coupling hub. The rigid hub could also be placed on the outside to fit a shaft that is made larger than is necessary to carry torque, as would be the case with bending problems. The center shaft would be smaller to carry torque only and thus fit the flex hub. When the flex hub is on the center shaft it is called a marine style coupling. When the flex hub is on the equipment shaft it is called a reduced moment style. The floating shaft designer must always balance the effects of weight (which causes deflection) and diameter (which determines torque capacity and resists deflection but increases weight and cost).
5. Limited End Float Couplings
Gear couplings can be modified to allow shaft growth in the axial direction or to limit movement in the axial direction. Limiting the movement calls for a plate and possibly a button to be inserted between the coupling halves. As the shaft tries to move in the axial direction, it is stopped after moving a predetermined distance.
These are called limited end float couplings. They are necessary with sleeve bearing motors, a design commonly found in larger sizes of 200 horsepower or more. The same plates and
buttons are used on vertical couplings as explained below.
6. Sliders
In addition to thermal growth, gear couplings can be arranged to slide great distances. Extra long sleeves enable the hub to slide 10 inches or more, either at rest or while in operation, to serve applications where equipment must be temporarily removed from the system and the coupling is the most suitable point of movement. Refiners, Jordan machines, and roll winders found in paper mills utilize this sliding capability. The Jordan coupling is a special variation that can move its hub relative to its shaft with a clamping mechanism.
Two dimensions are important when considering the slider coupling. One is the minimum BSE and the other is the total amount of slide. Those are in addition to the usual gear coupling requirements. If a Jordan is involved the amount of clamping movement is necessary to know.
7. Spindle Couplings
Spindle couplings are special floating shaft gear couplings that are used in rolling mills. They are designed for high torque, shock loads, and high angular misalignment. They have replaceable wear parts and customized accessories. Spindle couplings also have some slide capability to adjust to the installation or operational requirements of rolling mills. The spindle coupling uses the continuous sleeve principal to reduce the overall outer diameter.
8. Insulating Couplings
Gear couplings can also be equipped to block galvanic (electrical) currents, which can cause pitting and corrosion at the close running fits of gear teeth and other mechanical components. One half of the coupling is electrically insulated from the other half by adding insulating plates and bushings. It is not necessarily a high voltage insulator as found in wiring systems.
Modification for Special Needs
Vertical Couplings
Both continuous sleeve and flanged sleeve gear couplings can operate in the vertical position with the addition of a vertical kit, which is a limited end float plate or plate-and-button that supports the loose weights above the coupling. The button is rounded to allow the load to transfer under misalignment. Therefore, load is transferred to the lower shaft and ultimately supported by a thrust bearing in that equipment. Since a gear coupling is normally a shrink or interference fit, the upper hub is fixed to the shaft as is the lower one. In a vertical-floating shaft coupling, where both outer hubs are rigid and inner hubs are flexible, the entire center rotor is loose weight that needs to be supported by a plate in the upper coupling and a plate and button in the lower coupling.
A special vertical coupling is the rigid adjustable pump coupling. This coupling is designed for use with vertical circulating pumps that need clearance adjustments in the impeller. As indicated in the coupling name, it is a rigid coupling with no teeth in either half, with no provision for misalignment. The entire rotor weight is hung from the motor or driver bearings. Special designs of hanging load gear couplings can provide misalignment capability.
Other Gear Coupling Special Configurations
Gear couplings can be configured to do special jobs. Possibilities include the shear pin, cutout shifter, and brake coupling. Shear pin couplings disconnect when subjected to predetermined torque overloads thus protecting other equipment. Torque overloads could come from stalls or cyclic overloads.
Cutout couplings allow the driving/driven halves to be disengaged without disassembling the coupling. They use a special sleeve in which the teeth are interrupted at one end by a flat-bottomed annular groove. When the sleeve is shifted axially to align the groove with the teeth of one hub that hub spins freely, disengaged from the torque transmission path. A cutout pin (set screw) holds the sleeve in engaged or disengaged positions. They can be used on a dual drive machine to isolate the unused driver, or for a turning gear that rotates heavy equipment when it is off line, and helps prevent a permanent set in the shaft. Automatic cutout is available for temporary disconnect “on the fly” to allow adjustment of relative position between driving/driven halves.
Brake Drums and Brake Discs
Gear couplings are easily modified for the attachment of a brake, which saves system space by eliminating a separate brake. In other situations putting the brake at the coupling prevents the high cyclic torque from reaching low torque shafts. Brake wheel couplings are often attached near the gearbox shaft since high gear inertia is in the box. The brake drum or disk is a piece of metal, machined to standard brake sizes and clamped between the coupling’s bolted flanged sleeves, requiring longer bolts. The coupling manufacturer does not include the brake and actuator.
Whenever a brake is installed in the system, it flags the need to check the stopping torque requirements. Stopping torque, like starting torque, depends on the amount of time that is available to stop or start. See the section on torque for a torque formula.
Moderate and High Speed Applications
As noted earlier, gear couplings are capable of very high speeds and high torque together. The limits have always been the need for lubrication of the mating gear surfaces and the need for balance. While high speeds increase the wear rate and can be the cause of high stresses within the coupling, the bigger issue is balance. Couplings operating at high RPM or high rim speed will cause vibration problems if they are not in balance.
Balance
A full discussion of balance will be found in another section of this handbook so only a few issues that relate specifically to gear couplings will be referenced here. Balance concerns itself with how the weight of the rotating mass or inertia is positioned or displaced relative to the center of rotation. If that weight is perfectly distributed around the center of rotation, the coupling is in balance. Since nothing is perfect in couplings, there is always a potential unbalance.
Coupling balance is achieved through design, manufacturing and remedial balancing machines. Off center bores, out of round circumferences, non-parallel sides, or even loose fits lead to mass displacement. In castings some of the potential unbalance could come from voids or air space internal to the casting. When a coupling consists of an assembly, the component design and the assembly process can result in an unbalance condition.
If the hub OD is not perfectly concentric with the hub bore the center of mass and center of rotation will be different. This means the gear teeth must be carefully cut with a pitch diameter concentric with the bore. That is controlled by the arbor or mandrel used on the hobbing machine and the concentricity of the pilot bore. The hub face must be perpendicular to the bore or to the hub OD. If it is not it becomes a trapezoid. Trapezoidal hubs have poor weight distribution and therefore unbalance.
Sleeves must likewise be concentric with the hub bore at the pitch diameter, the OD and at the pilot fits if any exist. Flanged sleeves must have a concentric bolt circle as well as a proper hole size and location. Flange-to-flange alignment before bolting will have a big effect on the balance of the assembled coupling.
Once the equipment is designed and the tolerances are established, it is possible to calculate the mass displacement of each component. The mass displacement of each component is added algebraically by a method that is called the square root of the sum of the squares. The total mass displacement can then be called the potential unbalance of the coupling. That total unbalance of the coupling could then be compared to recognized standards to see if it is acceptable. Refer to AGMA standard -C90 for more on this subject.
Component & Assembly Balancing
It is unlikely that calculation of the mass displacement would be sufficient to satisfy a high-speed specification. That leads to the next process. Each component or piece of the coupling could be subjected to a balancing procedure on a balancing machine. Single-plane or two-plane balancing is also a consideration. If the coupling’s width-to-diameter ratio is 1:1, or greater in diameter, single-plane balancing is sufficient. If width (axial dimension) is greater, two-plane balancing is needed. (See chapter on balancing for more information.) Machine balancing results in adding or subtracting weight from the piece to counter the unbalanced weight and lessen the unbalance. The remaining unbalance of the part while on the balance machine is called the residual unbalance. The coupling can be assembled after component balancing and left at that potential unbalance. The total unbalance of the assembly at that point would depend on the distribution of the individual high points within the assembly. The worst case would be to end up with all heavy points in one quadrant.
For further reduction of unbalance, the coupling could be assembled and returned to the balance machine, again with corrective adding or subtracting of weight. The result would be called an assembly balanced coupling. With these, all individual pieces are match marked before the coupling is disassembled so they can be reassembled exactly the same way on the users equipment.
A gear coupling is not easily assembly balanced. First the coupling must be assembled with tight fits between the hub teeth and sleeve teeth so that loose parts will not fool the balancing machine. After the coupling is balanced, the teeth are relieved so the coupling can be installed in a system with possible misalignment.
The final balance, after the coupling is removed from the machine, will be affected by the concentricity run-out and bearing surfaces of the mandrels, arbors and mounting devices of the balance machine. The coupling, unlike machinery rotors, is not balanced on its own shaft. A half coupling might be balanced on the equipment rotor, but then the two half couplings from the two different rotors must be joined together. Why should one worry so much about balance? The balance is critical on high-speed applications to prevent destructive vibration. Different applications have different definitions of high or low speed, but generally for couplings, anything greater than RPM is high speed.
High Speed Gear Couplings
Most high-speed gear couplings are spacer types, that would acknowledge the need for maintenance on the connected equipment. Two important attributes of high-speed couplings are lightweight and low inertia. If the coupling is to be accelerated from zero to 10,000 or 15,000 rpm the torque required to reach those speeds quickly is substantial if inertia is allowed to be too high. High-speed machines are sensitive to overhung weights too. Everything is built for speed, which means small, light and precise.
We mentioned that high-speed couplings are precision made to tight tolerances. They are also made with ground bores, body fitted bolts and reamed holes in the flanges. Since the couplings are highly stressed the materials are magnetic particle inspected to make sure of the integrity of the piece. The material may be standard steel, but it often has papers to prove its strength and chemical composition. Hubs are attached to the shaft by hydraulic fits on a taper in the really high-speed units. That eliminates keys and keyways that could affect the balance. Other methods might include an integral flange on the rotor that bolts up to a marine style spacer coupling.
Sometimes the need for maintainability or rigidity forces the coupling to be a marine type of spacer coupling. Marine style refers to the tooth location not the application. In a marine type unit the gear teeth are on the spacer section not the hub section. This increases the overhung moment so a trade off is being made.
Materials for High Speed Units
While balance is most important to high speed gear couplings, it must also be noted that high speed has the potential for high wear of the teeth. For that reason extreme high-speed units utilize hardened teeth to extend the coupling life. However, this requires material that will be compatible with induction hardening, carbonization, or nitride hardening. The hardened tooth must retain its strength to carry the torque. Iron carbides and carbon or other nitrides provide the surface hardness. While AISI carbon steel is the most popular for gear couplings, AISI high alloy steel is used on high-speed units. Coupling materials and hardening will be discussed more thoroughly a bit later in this chapter.
Lubrication of High Speed Units
High-speed couplings are lubricated with oil rather than grease. The oil, which is circulated through filters and coolers, is sprayed into the sleeve on one side of the teeth and drained from the sleeve on the other side of the teeth. Circulating oil has the advantage of constant renewal, but even with the circulation it is necessary to prevent sludge build-up in the coupling. Sludge will prevent oil from reaching the necessary surfaces that need lubrication. Anti-sludge features in a coupling prevent the build up by putting drains and dams in the passages.
Grease-lubricated high-speed couplings are limited in their application possibilities. Even though grease labeled as “coupling type”, will resist separation of soaps and oils, it is not enough for the true high-speed application. Another problem with grease is temperature build up. Oil that is circulating is also cooled. Grease that is static would heat up from the rubbing friction at the high speeds.
Mounting the Gear Coupling in a Shaft System
Metric Versus English Units
The metric and English systems of size and tolerance were developed without a desire to interchange with each other. Simple conversions are not satisfactory because different bore dimensions are used, along with different tolerances and different formulas defining tight and loose fits. Metric bores are defined in ISO standards while English bores are defined in AGMA and ANSI standards. Those standards are also summarized in coupling manufacturers’ catalogs.
Hub to Shaft Interface
There are several methods to fasten the hub to the shaft. In all cases the objective is to have a joint that facilitates the transfer of torque from shaft to hub, is easy to install or remove, and does not make the alignment more difficult.
Clearance or Loose Fits
Loose fits are easiest to manufacture and to install. But, loose fits are not the first choice for gear couplings, except low torque applications or some nylon sleeve applications. The loose fit does not provide sufficient restraint for the forces found in gear couplings, so interference fits are used. Loose or clearance fit hubs use a keyway and a loose fit key to transmit torque, with a setscrew to hold the hub tight to the shaft and key to prevent wobble and fretting wear. The key and setscrew also help if some cyclic loading is present. Since that is the only means of transferring the torque, the length through bore for clearance fits is longer than that of other fits. The preferred length is 1.25 to 1.5 times the diameter of the bore. Keyways on clearance fit bores are a square cross section. Key sizes are matched to shaft sizes to ensure sufficient surface is available for the torque transfer. The key also has a loose fit within the keyway.
Interference (or Shrink) Fits
The interference or shrink fit is the hub mounting choice in the majority of gear couplings. It utilizes a hub bore diameter that is slightly smaller than the shaft diameter under all tolerance combinations. There are many combinations to the amount of interference, but a popular number is . inches per inch of shaft diameter.
The interference fit installation is accomplished by heating the hub to the point where it expands enough to fit over the shaft. Heating can be done in ovens, oil baths or by induction. The induction method is popular as a hub removal method too. A temperature of 300° F to 350° F is sufficient to do the job. Excess heat may change the metallurgical properties of the hub, and excess shrink or interference may split the hub.
The interference fit hub has a straight bore with a keyway so the friction between shaft and hub and the key are not used to transmit torque. The key is the main means of torque transfer, and may be either a loose or interference fit. Again a square key is used, and most times a radius is included in the keyway and on the key to reduce stress concentrations.
Reduced keys, known as shallow, half height or rectangular keys can be used to allow greater shaft diameters within the hub limits. All are wider than they are tall. Metric keys are of the reduced or rectangular key variety. When using reduced keys, torque capability must be carefully assessed. On large couplings and shafts two half-height keys are sometimes used to strengthen torque transmission. Interference fit hubs use a 1 to 1 ratio between the hub contact length and the shaft diameter. That ratio may vary in applications prone to high cyclic loads or sudden peaks in the torque from transitory conditions.
Tapers and Mill Motor Bores
Two types of taper bores are also common on gear couplings. One type is the tapered and keyed mill motor bore. This hub fits a standard mill motor shaft that has a like taper. As the hub slides up the shaft it forms a tight fit with the shaft. A shaft end nut is used to hold it in place. This method achieves good torque transfer, with a tight fit. It is an easy assembly or disassembly feature. Tapered shafts of this type can be used with machinery other than mill motors.
Another type of taper bore is the shallow taper hydraulic type. In this type there is no key. The hub is expanded by hydraulic pressure and pushed up the shaft to a predetermined point. When the pressure is removed, the hub shrinks to the shaft. The shaft can have a nut or plate attached to the end for retention of the hub. Removal also is accomplished by hydraulic pressure. The hubs have oil grooves machined in the bore to facilitate the application of oil pressure. Taper bore shaft hub combinations require a very complete match between the hub and shaft. The contact area of the hub bore to a gage acting as a shaft is measured in the manufacturing of the hubs to make sure a proper fit will be obtained when the hub is mounted on the shaft. Standards have been established to use as a guide for percentage of contact.
Shrink fit and hydraulic fit hubs are the choice for the heavy torque applications. One of the weak points in the power transmission train is the interface between hub and shaft. It is also the place where cyclic loads and peak loads can cause slippage or fretting damage. The tightness of the fit contributes to a more secure connection for torque transmission.
Sleeve to Sleeve Interface
Interchangeability
Gear couplings from size 1 to size 9 will match up half for half with other flange type gear couplings made to AGMA standard dimensions. However, while the dimensional standard ensures compatibility of the face to face match between sleeve flanges, it does not assure matching torque capability or bore. This should always be checked. When a labyrinth seal coupling is matched to an O-ring-sealed coupling, the bore capability and torque may both be different despite the fact that their flanges match and bolt together.
Bolts and Torque
Flange bolting is important to coupling reliability, as bolting can be a potential weak point. Most designs use a friction basis for transferring the load across the face to face match of the two coupling halves. Bolts are designed for tension loading, and primarily serve the purpose of clamping the two flanges together to enable face friction to transfer torque. In fact, the maximum outer diameter of the flange on flanged sleeve couplings is partially determined by the needs of space for bolts and surface for friction. Although friction is the main means of torque transfer, if the coupling is overloaded to the point of overcoming friction, it becomes a shear load on the bolts before becoming a coupling failure. Since the bolts are loaded by several types of forces one must be sure the bolt threads are not in the shear plane between the flanges.
Other specifications could allow body fit bolts to carry the load in shear, although from an engineering standpoint the concept of carrying load on bolts in shear is not favored. The body fit bolt has a tight fit to the bolt holes that keep the two halves concentric. To carry that method to the extreme one would drill and ream the boltholes at assembly and then match mark the two coupling halves.
Bolting will also affect and be affected by balance requirements. Balanced couplings may require weigh-balanced bolts. In addition, bolting can provide a means of piloting the two half couplings. To use the bolts as a pilot, the boltholes must be drilled to a close tolerance or line reamed at assembly.
Remember that the continuous sleeve coupling is not affected by any of the issues associated with bolting. The continuous sleeve coupling provides a bolt-free method of transferring torque through a continuous cylinder of metal with the additional advantage of a smaller outside diameter.
Alignment
Although alignment is covered in another section of this handbook, the gear coupling has some special alignment considerations that should be noted here. As mentioned in the bolting section, it is necessary for the two halves of flanged type to have some sort of piloting for best alignment practice. That can be achieved by piloted bolts or better achieved by pilot rings or rabbet fits. The alignment needs depend on the connected machinery and the speed of operation. High-speed operation always needs close alignment. Always refer to the machinery specification’s first, not the coupling specifications, when setting the alignment parameters. Since continuous sleeve couplings do not have bolts, alignment is done hub face to hub face.
Indexing Couplings
Once in a while there is a call for an “indexing” coupling. That type of coupling aligns two shafts in a rotational circular position that is the same each time. To accomplish that, the hub keyway is cut to be in line with a tooth or a space. The second hub is cut the same way. If it is a continuous-sleeve coupling, the continuous sleeve might be marked to identify the same tooth or space on both ends of the sleeve. The procedure on flanged sleeve couplings is more complex. In addition to the keyway meeting the hub tooth or space, a bolthole on the flange also must be lined up with a tooth or space. The mating flange must be drilled the same way so that when it is assembled the unit will be aligned or indexed. Of course, to make this work, the shaft keyway must also be aligned with a significant part of the machinery. Indexing is done to a specified tolerance on the location of that alignment.
Additional indexing is accomplished with floating shaft couplings when the coupling on each end of the unit has a different number of teeth. The indexing can then have a number of set points equal to the product of the two numbers of teeth.
Selecting Gear Couplings
Gear coupling selection parameters include two very important items and many more secondary items. The most important items are the bore and torque capabilities, in that order. Bore refers to the nominal shaft size where the coupling will be used. Torque in this case refers to the normal operating torque that the coupling must transmit. The secondary items can include a whole host of things like speed, misalignment, weight, spacer length, inertia, etc.
Bore and Torque: First Pass Selection
The gear coupling size in most cases will be determined by the nominal shaft size. The nominal shaft size is a mixed number of units and fractions that represent a specific diameter of shafting. The actual shaft is the decimal equivalent of that number plus .000 minus . or .001 inches. Nominal sizes are not just any number, but are chosen from a list of preferred numbers. Preferred numbers can also be metric in origin. This is part of our discussion is limited to inch numbers. That nominal number would also be the coupling bore with the actual size as a function of the class of fit.
Gear couplings typically use interference fits, so the coupling bore usually is smaller than the shaft size. The amount of interference varies by the designer’s requirements, but a value of . inches per inch of bore diameter is often used. For details on shaft size for interference or clearance fits refer to AGMA -A86. That is an inch series document; if metric is of interest, refer to “Preferred Metric Limits and Fits” ANSI B4.2 reaffirmed .
If the nominal shaft size is equal to or less than the published coupling bore capability, the gear coupling is usually okay for the service. “If it fits it is okay” is the gear coupling motto. For example smooth running, RPM, machinery without high starting torque or stopping requirements can use bore size to select the coupling.
The second step in gear coupling selection is to check the torque requirement of the application vs. the torque rating of the coupling. Normal operating torque is used unless a peak or cyclic torque is known. If the application calls for peak torque or cyclic torque, more care must be taken. The application description is also important to see if further investigation is needed. At this point, the nominal torque requirements of the system times an application factor that could be used to select the coupling.
The normal or continuous operating torque of the system is that torque value that is required for design point operation on a continuous basis. Coupling ratings are sometimes listed as HP per 100 RPM, but torque and horsepower can be derived from one another if the speed in RPM is also known.
Service Factors
Service factors (sometimes called Application Factors) are applied to the normal torque to account for variations that are typical of specific applications. They are based on a combination of empirical data and experience, and provide a quick reference to guide selection of a coupling for torque, and perhaps life, without going into the details of the application. Service Factor tables usually are provided in coupling catalogs, and will be different for different types of couplings. Another source of Service Factors (application factors) is AGMA standard 922-A96. Factors of Safety and Service Factors should not be confused with each other or interchanged. The former is for design work and the latter is for applications work.
Stretching the Bore
This subject is included to highlight the fact that it is not recommended. Never exceed the bore associated with the coupling size and the key type. Square keys have a maximum bore, rectangular keys have another and metric has its own. Do not mix them. When extra shrink is requested, or an over bore is requested for low torque applications, engineering should review the application. The gear coupling is the most power intensive coupling as it is designed, but the shaft to hub connection can be the weak point of the coupling. Stretching the limits can result in machinery failure as well as coupling failure.
Other Considerations
Bigger Than Size 7
There are several magic numbers when it comes to gear couplings. One is the size cut-off between big and small. That number is arbitrarily set at 7, but could be 9. The AGMA dimensional interchange goes to size 9 for gear couplings, but once the size rises to 7 and above, the number of applications become very limited. A size 7 gear coupling has a bore capability of nine or more inches (depends on key size too) and a torque of one million inch pounds. That torque corresponds to 16,000 horsepower at 1,000 RPM. Not many applications go that far and when they do the situation is special or low speed. Generally, big gear couplings are used on very low RPM and very high torque applications such as those found in the steel and aluminum rolling mills, crushers, rubber processors or mine concentrators.
For an idea of how big the gear couplings can be made, the catalogs will show gear couplings up to size 30. Loosely, the number equates to half the pitch diameter for flanged sleeve couplings. That means the coupling overall diameter will exceed sixty inches. Continuous sleeve coupling numbers are roughly equal to the maximum bore.
When the coupling size reaches the double-digit numbers, the torque rating is nebulous. Couplings are often re-rated based on improved materials, heat treating, and hardening. In reality the user and designer are trading wear life for torque rating. The torque rating can be used as a peak load or cyclic high and not always as the normal operating torque.
Not many modifications are made to these large coupling sizes. At this size, added functions are too expensive to build into the coupling and may be available as a separate device. Torque limiters fall in the latter category as they replace shear pins. The weight of the coupling and the other pieces of the rotating system also may preclude the desire for modifications. We should point out that large coupling bores are not always the ordinary bore and keyway because they may have special shapes and non-standard dimensions.
Speed
Catalog ratings are often accompanied by speed limits in RPM. It is possible to increase the RPM limit by balancing the coupling to minimize vibration. Balancing combined with special manufacturing tolerances can increase the speed even more. However, a perfectly balanced coupling will eventually have a speed limit set by stress, friction between the teeth, and lubricant breakdown.
Misalignment
All couplings have a misalignment limit. The standard gear coupling is capable of 1½° angular misalignment per mesh. Specially designed gear couplings can push that limit to 6° or more, per mesh. However, high misalignment limits can reduce the torque capability of the coupling.
Misalignment accelerates tooth wear, because it causes the hub and sleeve to rub harder against each other. Sometimes high misalignment capability is sought for and limited to non-operational conditions, such as moving a shaft aside for maintenance.
Modifications used to achieve high misalignment capability in gear couplings include increases in backlash (tooth gap), additional crowning, 25° or more tooth pressure angles, hardened wear surfaces, modified grease seals, increased clearance between sleeve and hub (makes the teeth look taller), and a torque de-rating. High misalignment couplings may also have modifications to make coupling maintenance easier or less expensive such as replaceable wear surfaces.
Materials of Construction
Gear couplings are typically made of two common steels, AISI carbon steel, and AISI -alloy steel. Alloy steel means elements other than carbon have been added to give additional properties to the steel.
Standard gear couplings use AISI steel. It can be bar stock or forging depending on the size and the component. Couplings needing higher strength or hardness for greater wear resistance are made from AISI which also can be bar stock or forging.
Gear couplings can be specified in 303 SS, but that is expensive and usually done only when required for the food processing or the pulp and paper industry.
Steel can be treated in many ways to improve hardness and strength. Hardness is the key to improving wear resistance for longer life under increased friction from high speed or misalignment, because gear couplings typically wear out under load rather than break. Strength provides resistance to the impact and cyclic loads.
The terms heat treatment, hardening, annealing, quenching and tempering are used in conjunction with the materials. Each of these terms represents a process that conditions the steel. Heat-treating is the general description that includes variations of all the others. Heat-treating does not have to mean hardening of the steel although it is usually taken in that context. Hardening of steel can mean in-depth hardened or surface hardened, which is also called casehardening. Hardness is measured in Brinnell units or Rockwell units, abbreviated as Bhn or Rc. The Rockwell Rc method of measurement is more popular on hardened surfaces of gear couplings while Bhn is used for overall hardness of a batch of steel.
For AISI steel, expected properties of strength for gear couplings would require a range of 190-260 Bhn. For AISI the range would extend up to 300 Bhn in the higher strength versions of the steel. The basic process in simple terms is that the steel is heated to a critical temperature held for a period of time and then rapidly cooled. After the rapid cooling the steel has a very hard structure that may need further tempering or annealing to trade hardness for strength. Rapid cooling is called quenching. Tempering or annealing is heating to a temperature and then cooling at a predetermined rate that is slower than a quench. The intent of these processes is to obtain a strong hard material that is ductile and tough.
For wear resistance we want to increase the surface hardness to 50 Rc or better. That requires an additional process known as hardening, case hardening, or nitriding. The process is to load the surface with iron carbides by exposure to carbon and heat or carbon nitrides and other nitrides by exposure to nitrogen and heat. The heat is provided by a heat treating furnace and the other elements are provided by the atmosphere in the case of nitriding or by packing the piece in carbon in the case of carburizing. The base steel has to be suitable for the process. In the case of nitriding the end product retains the original dimensions, but in the case of carburizing the end product grows and needs to be ground if the original dimensions are to be held. There are many methods, beyond these mentioned, which can harden steel surfaces. It is a complex subject. The process of hardening the surface of gear coupling teeth can extend the useful life of gear couplings.
Gear Coupling Applications
Reduced Moment, Three Bearing and Four Bearing Systems
The weight of the coupling and any reactionary forces all act at the center of the flex plane and cause a bending moment on the equipment shaft. When the coupling is placed close to a support bearing, the close support reduces that bending moment arm and the coupling can be called a “reduced moment” coupling. Reduced moments mean smaller loads and less wear on the equipment bearings. Placing the flex point close to a bearing also helps keep the system stable. Increasing the distance between flex point and bearing invites vibration, or wobble. For the most part, a three-bearing system has one bearing in the driven equipment and two bearings in the driver. The one-bearing side of the equipment is given a rigid half coupling without a flex plane. The two bearing side of the equipment, which is more stable, is given a flexible half coupling. With only one flex plane, this type of system can only have angular misalignment. Three bearing systems are commonly found in motor generator sets, and a long-shaft situation such as bridge crane traction drives.
The more common system is the four-bearing system with two bearings each in the driving and driven equipment. The system is more expensive and usually needs two flex planes because two bearings on each shaft make shaft locations rigid, usually in parallel misalignment.
Standard Couplings vs. Spacers
The simplest application for a coupling is a pump, compressor or centrifuge or the input side of a gearbox. These usually involve an electric motor drive mounted on the same base plate as the driven equipment. The coupling connects the two shafts and the most complicated issue is usually the BSE dimension. As the gear coupling has some range in BSE, the equipment designer can use a common size base plate for many different models of his equipment. The torque requirement of this type of rotating equipment is usually a smooth curve from zero to full speed and does not have any cyclic content. The coupling can be selected by torque and bore with a minimum service factor.
When the designer wants to make his equipment easier and cheaper to maintain, a spacer is installed between the two flex halves of the coupling. When the designer needs to span a long gap between driving and driven equipment (as when reaching up to a big-diameter roll, removing a large piece of equipment from an on-line position, or extending through a wall or bulkhead) a floating shaft is needed. This arrangement is often used with pinion stands, where the output is a double shaft that drives a meshing pair of rolls or mixers that are part of a large machine, such as a rolling mill.
Separating the Driver & Driven
Rotating equipment such as fans, pumps and compressors can have two separate drivers on the same piece of equipment. The drivers might be an electric motor for start-up and a steam turbine for running. That occurs on co-generation applications where steam is available and the operator wants to conserve electricity or use the electricity for other purposes.
Sometimes the equipment has an electric motor for normal purposes and some other device like an internal combustion engine for emergency operation. Other times the equipment sits idle but the driver runs. While these sound like applications for clutches, they also can be places where cut-out gear couplings might be the wiser choice. The gear coupling in many cases is less expensive and takes less space in the system than a clutch.
Save the Equipment from Torque
Rotating equipment shafts are often oversized because they are designed to limit deflection, which can lead to oversize couplings. Motors are sized as the next larger standard unit compared to the application requirements. Those issues plus a service factor can result in a drive system that has torque capability well in excess of the driven equipment needs. In such systems, torque spikes or overloads are easily passed to components that are not designed to withstand them and may be severely damaged. To prevent that, a torque limit device is installed in the drive train. The gear coupling, which probably is needed in the system for other reasons anyway, can provide the same protection at much lower cost than many devices sold as torque limiters, with the simple addition of a shear pin.
All flexible couplings fall into one of two categories, “finite life” or “infinite life.”
Finite-life couplings are those that wear in normal operation, because of using sliding or rubbing parts to transmit torque and compensate for misalignment. This group includes jaw, gear, grid, sleeve (shear), Nylon sleeve gear, chain, offset and pin & bushing types. All usually have lower purchase costs than infinite-life couplings. They won’t last as long, but their life span may be sufficient for the life expectancy of the application. Periodic maintenance is required.
Infinite-life is something of a misnomer, as these couplings do not necessarily last forever. It simply means the couplings do not wear in normal operation and by design the acceptable loads do not exceed the fatigue life of the parts. This is because they transmit torque and compensate for misalignment through distortion within their flexing elements rather than by the sliding or rubbing movement of loosely fitted parts. This group includes tire, disc, diaphragm, some donut types, wrapped-spring, flex-link, and most motion-control types. (Sleeve types are excluded here because their torque and misalignment capabilities are served by the flexing of their elastomeric element, the interface between the element and its hubs is a loose gear-like fit that wears.)
Distortions of the flexible element results in fatigue stresses rather than wear. Infinite life in couplings remains infinite only as long as the operating load stresses, considering misalignment is kept within the fatigue capabilities of the coupling’s material. Elastomeric couplings do not have the same fatigue capabilities as metal couplings and they also experience reduced load capability as time passes. For that reason, the shelf life of the elastomer must be factored into the couplings design rating.
An overload that will fail an infinite-life coupling might only reduce the life of a comparably rated finite-life coupling. Accordingly infinite-life designs are most often used on maintenance-free systems where maximum torque requirements (including transient, cyclic and start-up torque) are known.
Single Flex, 3 Bearing Systems and Two Flex Plane 4 Bearing Systems
We have previously mentioned that some couplings, the metallic flex type in particular can be built as single flex element or two flex element couplings. Single flex element couplings were noted to be limited to angular misalignment and possibly axial displacement whereas the two flex element units were needed to achieve the additional parallel (radial) misalignment capabilities. Couplings of the single flex element type could be expected to have a lower cost. Elastomeric couplings may provide parallel misalignment in a single element through distortion of the element.
There are applications that require two elements and therefore two flex planes, and other applications that either allow or require the single element. The coupling installed in a four bearing system will be of the two flex plane type or of a type that allows radial misalignment. The four bearing system consists of two pieces of rotating equipment, each having a set of two bearings. Each set of bearings will hold the associated shaft in a straight line when the equipment is installed on its foundation. Alignment of the two pieces of equipment will make the shafts close to coaxial and coplanar. However, since some misalignment will occur, a flexible coupling is needed. Unless one piece of equipment can swivel about a point, parallel misalignment will eventually show up in the system and a coupling with that capability will be required.
The coupling installed in a three bearing system will be of the single flex type. The single bearing is a self aligning type which provides the swivel possibilities. (It does not have to be a three dimensional swivel.) The system only needs an angular misalignment capability as associated with a single flex coupling. There are two types of single bearing systems. Note that both types have a radial load that is carried across the coupling. Most elastomeric couplings will not be able to carry that radial load and should not be used in the system unless checked for radial capabilities. Two flex plane couplings will be unstable in these systems and cause vibrations or wobble.
The first type of a single bearing system is one that places the load between the coupling and the outboard single bearing. This is typical of a three bearing generator coupled to a driver. The load can be heavy. Usually the flex half of the coupling is mounted near to the driver bearing to reduce the overhung moment.
The second type of single bearing system is the overhung pulley application. The pulley has one side open to allow for easy changing of the belts. The bending moment caused by the pulley load has to be passed through the coupling. The bearing between the coupling and the pulley is a pivot point and a load carrying position.
The floating shaft or floating tube coupling is a special case of using two single flex couplings in a four bearing system. The connection between the two single flex couplings is a long unsupported shaft or tube. The length of the shaft or tube is limited by critical speed, the diameter is a function of the torque. Tubes are used to lighten the weight and improve the critical speed. The flex halves can be on the center shaft, a marine type, or next to the equipment bearing, a reduced moment type. Mixed coupling designs with one side being a reduced moment and the other a marine style is acceptable. Vertical floating shafts are available. Floating shafts that use full flex couplings on both ends of the floating shaft are unstable when operating and should be avoided. Elastomeric floating shaft couplings are possible but must be reviewed and approved by the coupling manufacturer. Floating shafts are used when the connected equipment has offset shafts and space is available for the long shaft. They are found in paper and steel mills.
Some systems have a mix of floating shafts and semi-floating shafts. Usually single flex couplings are needed to provide stability in the rotating system. The flex half should be mounted nearest to a bearing for best results.
Torque – Limited and Bore – Limited Couplings
Some coupling designs are limited by the torque capability of the flexing element. They are called “torque-limited” couplings. Other couplings are limited by the hub bore size because the flex element is capable of transmitting all the torque that shaft size will normally deliver. These are termed “bore-limited”.
The elastomeric coupling is considered a torque limited coupling device because the flexing element uns out of torque carrying capacity before the connecting shafts reach their full torque potential. This is because elastomers have much lower tensile and compressive strength than the metals otherwise used in flexing elements. Consequently, elastomeric couplings must become larger in diameter to achieve higher torque-carrying capabilities. The hub naturally follows the elastomer in becoming large, giving the hub a bore capability that is unnecessarily large in relation to the torque capability of the coupling. Enterprising designers use that extra bore capability to fit tapered bushings and other easy-assembly devices into the hubs. (For more discussion on these devices see the chapter on mounting the coupling hubs to the shafts.) All this means that elastomeric couplings must always be checked for both torque and bore capability.
Metallic element couplings tend to keep a close relationship between hub, bore and torque capability. One notable exception is the gear coupling which is truly bore-limited because it can transmit more torque than its maximum shaft size will normally deliver.
Some composite materials offer strength capabilities somewhere between elastomers and metals. These materials sometimes offer weight-to-strength advantages that can be important.
Coupling selection always needs consideration of torque, speed, misalignment, connecting shaft sizes, and appropriate service factors. However, in old installations needing coupling replacement, the real torque values might be unknown or uncertain. In such situations the gear coupling could be selected purely on shaft diameter and speed, with limited risk.
Selecting elastomeric couplings purely on shaft diameter and speed is very risky. In some cases, however, that risk can be an advantage. When overloaded, the elastomeric coupling will fail before the rotating equipment shaft fails, provided the overall design is correct, thus sacrificing the less expensive coupling to protect the more expensive rotating equipment. When using this strategy, the overall design considerations should include the wear life of the coupling and the damping energy to be absorbed by the coupling.
The fact that the coupling can be the weakest element does not necessarily mean that the coupling will provide a fusible link. To have the coupling serve that function, it is necessary to pick the type of coupling with that feature. These types are termed non-failsafe. In failsafe design, coupling failure does not automatically disconnect the two rotating shafts, but will require the coupling to be maintained as soon as possible. If the coupling is a wearable device, as are most elastomeric couplings, both load and misalignment are factors in total life. The end of usable life of the coupling might not be the result of equipment problems when wear is a consideration.
Coupling torque requirements can be defined many ways, and specifiers need to decide which definition to use. We will first review the various definitions, then discuss how they are used in coupling selection.
System Torque
Normal Operating Torque – The steady state torque required by the system when operating at normal design conditions. This is usually the level at which the equipment designer certifies the equipment performance.
Starting Torque – The torque needed when the system starts its operation. This torque can be greater or less than the normal operating torque.
Peak Torque – The maximum torque required by the system. This torque is normally a one time event or limited to a specified number of occasions. In torsional vibration coupling systems it is the maximum vibratory response torque that could pass through the coupling.
Cyclic Torque – It is any torque requirement of the system that varies with time. It can be of a smooth, periodic variation like a sine wave or could be an erratic variation. It does not go through zero to a negative value, but can be equal to zero. In torsional vibrating systems it is the vibratory torque that occurs at the operating speed.
Reversing Torque – This is a cyclic torque that passes through zero and becomes negative or “reverses” to the opposite direction.
Transient Torque – A transient torque is of short duration, not necessarily expected, not happening on a regular basis but occurring when a system is upset. It may or may not be equal to or greater than peak torque.
Normal Braking Torque – It is the torque used to decelerate or reduce the speed of the equipment when the brakes are applied in a normal manner. The torque is time dependent, and moves through the system.
Emergency Braking Torque – In this case the brakes are applied to stop the equipment in a very short time. The torque will exceed the normal braking torque by the inverse ratio of the time required to stop in each case.
Stall or Lockup Torque – This is the torque that passes through the system when the system stalls or otherwise come to a stop because of some activity within the driven system.
Shutdown Torque – The torque required to bring the equipment from operating conditions to a shut down condition. This can be the normal braking torque or could be a result of friction or load in a system that is coasting to a stop.
Torque to Accelerate or to Decelerate – The torque required to increase or to decrease the equipment operating speed. In the case of acceleration, the available torque for acceleration is the difference between the driver capability and the system requirements at the current speed. Decelerating torque comes from braking devices or from frictional drag or other energy drains within the system that cannot be overcome by the driver. A formula for calculating this torque is found at the end of this section.
Driver Horsepower (Torque)
Nameplate Rated Horsepower (Torque) – The torque value is derived from the driver capability shown on the nameplate as a horsepower and a speed. It is based on specific inputs to the driver such as voltage, and amps or kVA if it is an electric motor. A formula to convert horsepower and speed to torque is found at the end of this section.
Service Factor Rated Horsepower (Torque) – Some drivers have additional capabilities beyond the name plate rating. The nameplate capabilities are multiplied by the service factor. The service factor is also shown on the nameplate.
Start-Up Torque – The driver torque capability at start-up available to accelerate the driven equipment to operating speed. Some drivers have a fixed percentage of the rated torque available at start-up. It can be greater than 100%.
Peak Torque – This is the maximum torque available from the driver, it may not be able to operate for extended time periods at this torque.
Stall Torque – This is the system torque requirement that will cause the driver to come to a stop.
While all the torque values defined previously may exist within the system at some point in time, the torque requirements of the driven equipment are the primary consideration. The driver will not supply more torque than the driven equipment will absorb or the driver can produce. Under some conditions the maximum torque within the system may exceed the driver capability, for example when brakes are energized.
A piece of driven equipment operating at its full speed capability requires a certain amount of torque. If the driven equipment is not operating at full speed the driver will supply additional torque until equilibrium is reached for torque and speed. The driver could still have additional capability, but it will not transmit it to the driven equipment. Other than at start up, the speed variation is small and subject to driver speed limits.
Drivers have speed limits that are imposed by physics or by trip devices. The physical limits can include the effects of frequency on an electric motor, or the effects of fuel restriction on an internal combustion engine, or steam availability to a steam turbine. Trip devices can include governors and over speed switches. The speed-torque capabilities of the driver are fixed by the design of the driver and the inputs to the driver.
Coupling Selection Torque
Using the Driver Torque
The coupling can be selected based on the driver capabilities, using nameplate values or start-up torque. The capability requirements can be increased by an application service factor before choosing the coupling. This method of coupling selection usually results in a coupling that is oversized for the application, even if the service factor is 1.0. This translates into high cost and other problems. The reason oversizing results is twofold. First the driven loads may include equipment service factors that have already increased the torque value. Second, the driver is usually oversized. Drivers such as electric motors, come in standard sizes. If a piece of driven equipment requires a horsepower that is in between two standard sizes, the larger is chosen. Even when the requirements are right on the nose, the designer will usually pick a larger size out of conservatism.
When the coupling is chosen by driver horsepower one must be sure there is no gear reducer between the driver and the driven load. Gearboxes are constant horsepower devices that increase the torque or decrease the torque depending on the gear ratio input to output. Other power transmission devices may do the same. In any case those types of devices must be accounted for in the coupling selection. Couplings selected using driver torque are normally mounted with one half on the driver shaft.
Using Driven Equipment Torque and a Service Factor
The coupling can be selected by using the normal operating torque of the driven equipment, adjusted for coupling location, multiplied by a service factor. Service factors are used to account for unknowns in the driven equipment system.
Service Factors
Sometimes “Service Factors” are called “Application Factors” or “Experience Factors”. They have been empirically developed for most applications, or are known by their designer based on experience with their systems. Coupling manufacturers publish Service Factors based on their experience with their couplings on various systems. The factors are listed in coupling catalogs. Manufacturers may publish different service factors by product line. The catalog service factors will include factors for the application and the type of driver. Elastomer couplings sometimes include an environmental temperature service factor, and if intended for dampening vibratory torque, will have a frequency service factor as well.
AGMA Standard -A96 lists service factors for many different applications. Service Factors are not the same as design Factors of Safety. Service factors deal with the unpredictable nature of the application, not with unknowns in the design of the coupling.
Depending on the selection of service factor this method also could result in an oversize coupling. Oversize couplings cost more and can result in bearing overload, excess inertia, premature wear and more maintenance.
Using System Torque with Little or No Service Factor
A coupling can be selected based on the exact requirements of the system. In this case the requirements must include all the torque values to be transmitted through the coupling. That can include starting requirements, braking requirements, peaks, transients and any others listed at the beginning of this section. Check the coupling manufacturers catalog as the coupling can have various torque capabilities. It may have one rating based on normal operation with another simultaneous rating for low cycles of peak torque. It may be acceptable to compare the peak system torque with a 1.15 Service Factor, to the yield strength of the coupling, and allow that as part of the acceptable selection. The coupling manufacturer should be consulted when the coupling selection is based on peak torque, emergency torque or a high transient that comes along only once. The coupling may already have sufficient reserve to satisfy those requirements on a limited number of occurrences.
If the coupling has only one published torque value, the coupling would have to be selected to meet the maximum torque expected in that part of the system. However, some types of couplings have torque ratings based on wear life, maximum misalignment combined with torque, or conservative considerations.
If the coupling is subject to cyclic torque or reversing torque, the selection should be based on those torque values. In the case of cyclic torque, use the high value. In the case of reversing torque, it will be necessary to check the coupling’s fatigue life against the torque peaks and the acceleration/deceleration requirements associated with reversing operation.
The most economical selection will be based on the exact torque requirements of the system including peak, transients, breaking, or other expected torque values. Of course, this approach requires that all of the torque values be known with certainty. When the coupling has been sized to meet torque, it must also be checked for bore capability. Some bore-limited couplings might have the needed torque capacity but not enough bore capability to accept the shaft that will deliver it. Likewise, some torque-limited couplings might have sufficient bore capability to accept the shaft but not be able to carry all the torque that the shaft will deliver.
Using Torque Information Coupling Catalogs
Coupling manufacturers have several methods of listing the coupling capabilities in their catalogs. These capabilities vary on each manufacturer’s experience, design requirements, and testing capabilities. Torque capabilities found in the catalogs may have to be factored or reduced for misalignment, vibration frequency, temperature (elastomers), life (including elastomeric shelf life), or maximum torque. Such factors, if they are to be used, should be shown in the same catalog.
Most often the value shown in the catalog is the normal torque capability that the coupling can transmit over its design life. Some couplings have a listed maximum torque. Usually that maximum value is used when the application might involve short cycle fatigue on couplings that have infinite life. Couplings that wear over time, such as a gear coupling, may have maximum capabilities that are quite large as long as their application keeps wear low. Couplings that wear may also offer alternate materials for reduced wear and longer life or for higher torque.
Because some coupling torque capabilities are limited by wear of the flex element and others limited by on fatigue of the flex element, it is best to understand the type of coupling that is to be used in the system before selecting the size. In addition to the flex element, coupling torque capability is affected by the method of securing shaft to hub, and any other joint in the unit, bolted or otherwise. Usually it is the flex element that is the limiting factor for the catalog torque values.
Useful torque equations:
Converting horsepower to torque:
T = BHP x / RPM
Where
T = the torque in inch-pounds
BHP = the motor or other horsepower
RPM = the operating speed in revolutions per minute
= a constant used for inch-pounds, use for foot-pounds, and for Newton-meters
Converting kW to torque:
Where
T = BHP x / RPM
T = the torque in inch-pounds
kW = the motor or other kilowatts
RPM = the operating speed in revolutions per minute
= a constant used for inch-pounds, use for foot-pounds, and for Newton-meters
Determining the acceleration or deceleration torque:
T = (Wk^2 x N) / (307 x t)
T = the torque to accelerate or decelerate in foot-pounds
Wk2 = the inertia of the piece to be accelerated or decelerated in pound
feet squared
N = the absolute change of speed in RPM
t = the time for the speed change in seconds
307 = a constant that allows the speed to be in RPM, the time to be in seconds and the torque and inertia to be in pounds and feet. It is a common form of the equation.
One class of coupling applications is unique in that a secondary load is transferred through the power transmission system and the equipment connected to the system. That secondary load is torsional vibration. Torsional vibrations are associated with internal combustion engines, reciprocating (piston) type compressors, vane passing frequencies of some centrifugal pumps, grinding mill drives, kiln drives, rolling mill drives, variable speed motors, and the start up of synchronous motors. Diesel engines represent the most significant unit volume of torsional coupling applications, and will be discussed in separate detail later.
One class of coupling applications is unique in that a secondary load is transferred through the power transmission system and the equipment connected to the system. That secondary load is torsional vibration. Torsional vibrations are associated with internal combustion engines, reciprocating (piston) type compressors, vane passing frequencies of some centrifugal pumps, grinding mill drives, kiln drives, rolling mill drives, variable speed motors, and the start up of synchronous motors. Diesel engines represent the most significant unit volume of torsional coupling applications, and will be discussed in separate detail later.
Torsional vibrations cause equipment breakdowns such as wear or chatter on loose connections like spline pump shafts, or complete fatigue failure of the shaft or some other element. These harmonic torsional pulses are difficult to detect, because they do not bounce the equipment up and down, as would a lateral vibration. Nor can they be felt by touching the equipment. Usually, the result of the vibrations is known before the vibration is known. Often something else is blamed.
If the torsional vibratory frequency matches a system torsional natural frequency, the system reaches a harmonic or becomes resonant. That’s because the natural frequency is an energy balance point at which additional forces will set off uncontrolled vibration. From a technical standpoint, it is the frequency at which the kinetic energy of spinning inertia blocks is equal to the potential energy of the torsional spring connecting the inertia blocks.
In such systems, inertia blocks can be impellers, pistons, mill rolls, motor rotors or any other device that is mounted on the shaft, which all rotate together as a single wheel. The torsional spring is a combination of the shaft and coupling’s flexible element, plus other potentially flexing components such as a spacer or floating shaft.
When the wheel and spring rotate as parts of the same system, the inertia of the spinning wheel is balanced against the windup of the spring. Any additional forward pulsing force on the wheel will cause the spring to windup more and that will in turn react with a reverse force to the wheel. Between pulses, when that additional force is removed the spring unwinds, and the wheel surges forward. When the pulsing force returns, the spring winds up again, reapplying the reverse force to the wheel, etc. That pulsing force which is being applied at some time cycle or frequency at or related to the operating RPM, is the torsional vibration. If the timing is right, the winding and unwinding of the spring and the energy changes in the wheel resonate back and forth. The point where the timing is right is the system’s natural frequency.
Determining the Natural Frequency
All rotating systems have a torsional natural frequency. It is a function of the driven inertia, driver inertia and the torsional stiffness of the shaft, spacer and/or coupling connecting the two. There is a natural frequency for each combination of inertia and spring. Aside from the kinds of torsionally sensitive systems discussed in this section, most systems have torsional natural frequencies so high as to be inconsequential. By itself, the natural frequency is harmless and does not generate torsional vibration, but is simply a sensitive spot along the systems RPM curve. It is a “forcing frequency”, i.e. it is not self-initiating or self-sustaining, rather it must be triggered by a vibratory force pulsing at that frequency.
Many systems can be reduced to a two-mass system. For a two-mass system, the frequency can be determined mathematically from the following equation.
CPM= (60/2π) SqRt (Ctdyn x (JA + JL) / (JA x JL))
CPM is the frequency in “cycles per minute”.
JA is the polar inertia of the driver
JL is the polar inertia of the driven
Ctdyn is the dynamic torsional stiffness of the coupling.
60/2π is a constant.
Reducing a system to a two-mass system is done by lumping inertias connected by torsionally stiff shaft elements. For example the lumped polar moment of inertia of the driver JA and the lumped polar moment of inertia of the driven equipment JL are determined by adding all the individual inertias that are connected by stiff shafts. When a gear reducer or increaser is involved, the downstream inertia must be factored by the square of the gear ratio (speed). It is an inverse function.
A coupling is between the driver and the driven. The coupling stiffness Ctdyn is obtained from the coupling manufacturer. It is called the dynamic torsional stiffness, which is higher than the static torsional stiffness.
Inertia is marked by the symbol “J”, the units in the English system are inch-pounds second squared. It is related to WR2 by “g” the acceleration due to gravity. For a method of calculating the inertia value and the stiffness of connecting pieces refer to AGMA Standard .
In a multi-mass system that includes more than two inertias connected by torsionally soft shafts, couplings, or sections, the natural frequency can be determined using the Holzer method. Refer to a textbook for an example of the Holzer analysis.
As long as the torsional natural frequency is more than 40% above or 30% below (.7 Nc to 1.4 Nc, where Nc is the critical numerical value) the system’s operating frequency or idling frequencies (RPM) or associated torsional vibration frequencies (CPM) no resonance problems should occur. If it is in between those values there is a good chance the system vibratory response will cause damage to one or more components. If it is close to any of those frequencies, resonance is likely to occur.
Campbell diagrams are graphic plots of operating speeds and pulse frequency. They are used to identify the potential trouble spots where operating or idle RPM is equal to a torsional pulse frequency in CPM, (cycles per minute).
If it is decided to operate the system normally at an RPM above the torsional critical speed, (natural frequency) then the driver must have enough torque available to accelerate the load quickly through the critical speed zone (RPM). Comparing the speed torque capabilities of the driver and the load will determine the system’s ability to accelerate through the critical zone quickly enough.
Using the Coupling to Tune Critical Frequency
In the torsionally sensitive system, couplings take on an important extra role beyond the transfer of driving torque and the handling of misalignment. They have the ability to move the natural frequency away from those levels that will be occupied by the torsional vibratory frequency at normal operating or idling speeds. This is called “tuning” the critical frequency. It works as long as the coupling is the controlling element for the critical frequency. That is not the case when long slender shafts are in the torque path. They also can be used to damp the energy of torsional vibration to reduce its potential for damage. The coupling torsional stiffness/softness is an attribute that is important in serving these functions.
Couplings with the highest levels of torsional stiffness are not used here. Those designs primarily serve systems that must transfer motion without windup or backlash, as previously discussed under motion control. Torsionally soft systems have a normal operating speed above the torsional critical speed while torsionally stiff systems have a normal operating speed well below the torsional critical speed. Because the coupling is usually the softest torsional element in either system, the system tends to be stiff when the coupling has a relatively high torsional stiffness and soft when the coupling has relatively high torsional softness.
Stiff couplings have elastomers of the Zytel® and Hytrel® type of plastic or their flex elements are metal. Soft couplings are rubber elastomers in compression or in shear.
Changing to a torsionally stiffer coupling raises the system’s natural (critical) frequency, and reduces or eliminates the coupling’s capacity to damp vibratory energy.
Using stiffer couplings to drive the critical frequency above the operating speed is as effective on simple systems like a single hydraulic pump driven by a diesel engine as it is on sophisticated high-speed couplings that are found on turbine driven rotating equipment.
When the coupling is used in the regimen of keeping the critical frequency high, it is usually just a matter of making sure the coupling is sufficiently torsionally stiff. That could be accomplished by using a stiff spacer piece with a metallic-element coupling, or by using a very stiff elastomeric element on a flywheel coupling. The coupling manufacturer can provide the necessary information on coupling and spacer piece stiffness.
An exception would occur when a system has a long slender shaft, which usually means the lowest critical frequency would be the result of that shaft. That type of system can become complex because the coupling is no longer the element that controls the stiffness.
The torsionally stiff elastomeric coupling and the torsionally stiff metallic element coupling offer no damping between the driver and the driven equipment. That means torsional vibrations are passed into the driven system. In such stiff systems, loose parts or parts with backlash will vibrate and rattle, and may have wear problems. Typically spline shafts on hydraulic pumps and gears with backlash suffer the wear.
Changing to a torsionally softer coupling lowers the system’s natural (critical) frequency, but also may increase the coupling’s capacity to damp vibratory energy, so that function and the heat it will generate through hysteresis needs to be considered in the selection. Elastomeric couplings expected to damp torsional energy must be designed to reject the resulting heat to a heat sink. Otherwise the heat will fail the elastomer by melting from the inside out.
Elastomeric torsional couplings can be either compression type or shear types. The more common compression types are of the donut or torus configuration. However, some use elastomer blocks or elastomer cylinders. The compression block types are most often found in the high torque applications. The shear types are shaped to equalize stresses from torque and misalignment.
Torsional softness and torque capabilities are opposite coupling characteristics. A soft coupling tends to have lower torque capabilities than similar sizes of stiff couplings. The softer, lower-torque couplings generally are used on applications that require 100 HP at 2,000 RPM or less. Torque capability increases with torsional stiffness of the flex elements.
The coupling designer must balance the various attributes to achieve the desired coupling for the specific application, or to devise a coupling with broad capabilities as a standard unit that serves many applications. Dual stage torsional couplings can also be obtained. They incorporate two different elements. One is soft for low or idle speed and a stiffer one for high or operational speed.
Some torsional coupling types utilize viscous friction damping This method is found in hydraulic torque converters, which mechanically isolate the driven system from the driver, and transmit torque between them through the motion of a viscous fluid. When a system uses a torque converter, it becomes two separate torsional systems. Torsional vibrations do not pass through the torque converter, except when a lock up device is engaged to mechanically connect the two halves. Hydraulic torque converters are not included in this handbook’s discussion of flexible couplings.
Refer to the torsional coupling section and the metallic element section of this handbook for a more detailed description of the couplings used to damp torsional vibration and/or tune critical frequencies. Also refer to the bibliography for more publications on this subject.
Torsionally Sensitive Systems
Torsional vibration problems appear primarily in four types of applications briefly discussed here.
High Speed Machines
High-speed machines have torsional pulses or vibrations at high frequencies, therefore the natural (critical) frequencies must be kept even higher. A discussion about high-speed special purpose couplings and the associated equipment torsional problems can be found in many of the coupling textbooks. They are a sophisticated coupling application, which is not covered in this handbook.
Variable Frequency Drives
The VFD will produce a torsional pulse at low speeds that is larger than those generated at faster speeds. Keeping the operating speed above 10% of the maximum speed, i.e. no lower than 90% below maximum will alleviate the problem in that type of system.
Synchronous Motors
Synchronous motor start-up is a unique situation. At start-up the motor produces torque pulses at a frequency equal to two times the slip frequency. (Slip frequency is numerically equal to full synchronous speed minus operating speed.) The magnitude of the pulse is related to the torque developed by the motor. As the unit accelerates to full speed, the torque pulses drop in frequency reaching zero at full synchronous speed. The torsional vibrations or vibratory torque ceases at that point. A problem will occur if a torsional natural frequency is less than two times the AC power line frequency, as the start-up torque pulse frequency must then pass through the critical frequency. When going from startup to the running speed the driver must accelerate the load through the critical speed quickly. Acceleration through the critical frequency is a function of the torque available from the motor at starting.
High torque synchronous motors will also have high vibratory pulses that need the damping of torsionally soft couplings, but soft torsional couplings have a high vibratory response when passing through the critical speed, as well as difficulty carrying the high torque loads. The relationship between the two functions therefore must be a compromise. The coupling must be soft enough at startup to dampen some torsional vibration energy, but stiff enough to carry the high torque at running speed.
Reciprocating Internal Combustion Engine Drives
There are three main types of engines in common use. They are gasoline engines, gas engines (natural or LPG or propane or other), and diesel engines. Gasoline and natural gas engines are spark-ignited low cylinder pressure types as compared to the compression-ignited diesel engine, which requires very high cylinder pressure.
The diesel is the most efficient of the three so it is very popular for continuous duty applications in those regions of the world that have high fuel prices. Gas engines (Natural, LPG or Propane) are most popular where these gases are readily available or where air pollution is a serious problem like the inner cities.
All internal combustion engines generate a torsional vibratory pulse. The magnitude of the pulse is a function of the cylinder pressure, turbo charging, engine’s displacement, internal damping, the engine geometry, and whether it is a two or four stroke engine. The diesel drive rotating system accounts for the majority of the torsional vibration problems due to its high cylinder pressures and resulting high magnitude of torsional harmonic pulse compounded by its widespread popularity. The engine itself is designed to tolerate its internally generated forces from torsional vibration and may include some internal damping. The problems start when these harmonic vibrations pass to the driven equipment. Special attention must then be given to selecting couplings that can help reduce these problems
Diesel drives range from the simple low-horsepower single-unit hydraulic pump to a marine installation in which the diesel will drive the propeller and generators through a gear reducer. The preponderance of diesel drive systems can utilize a simple analysis to select the right coupling, however the marine system should be analyzed by an expert in that field.
While the magnitude of the torsional pulse is important, it is also necessary to know the frequency of the pulse. Like magnitude, pulse frequency is dependent on many factors. Those factors can include the number of cylinders, the configuration, such as “V” or inline, the stroke (two or four) and the firing order.
Also note that diesels typically have several torsional pulse frequencies, established at harmonic intervals. A 6-cylinder 4-stroke inline engine will have major harmonic orders of 3 and 6. Pulse frequencies are the RPM multiplied by the order. For example an engine running at RPM will have pulse frequencies of and CPM. If the natural frequency were also or CPM, the engine should not be operated at RPM. Note the frequency in CPM and speed in RPM is the same units in this case.
If any of the torsional frequencies are equal to a natural frequency the system will vibrate at resonance.
Coupling Selection for Torsional Systems
In addition to the damping possibilities, the coupling is selected with three torque values in mind. The first would be the continuous running torque. The coupling should be capable of handling this torque under all environmental conditions of the applications. The second is continuous vibratory torque. The coupling should damp this torque without a meltdown from heat generation. The third is the maximum torque pulse or peak torque. The pulse occurs as a vibratory response torque at critical speed. The coupling rating is a fatigue life in that case. The manufacturer will publish the torque value at 100,000 non-reversing cycles.
Continuous Running Torque
This is the design torque for the system. Usually it is the driver horsepower and operating speed. That value is normally in excess of the load requirements or is tied closely to the load requirements. Since the coupling also will be judged against peak or maximum transients in the system, a service factor is redundant except for high starting torque. Couplings that are oversized by using service factors can also be too stiff. Elastomer couplings may require derate factors on the coupling capabilities for temperature or frequency or speed. A derate factor is not a service factor. For more discussion on the various torque values found in an operating system see the chapter called “Applications “.
Maximum or Peak and Vibratory Torque
It is important when analyzing the torsionally sensitive coupling application to know the value for the generated vibratory torque. That value becomes the forcing torque that puts the natural frequency into critical resonant vibration. In a diesel drive system the torque pulse is a function of cylinder pressure, number of cylinders, use of turbocharging, number of strokes for firing, etc.
Lloyds Register of Shipping publishes a pamphlet that is a good source of harmonic pulse factors used to determine the harmonic vibratory torque of diesel engines. The engine manufacturers could also provide the information. The manufacturers of other drivers or driven equipment should be able to provide the similar forcing torque values for their equipment.
Once the initial pulse torque is known, it is then possible to calculate the values for the vibratory response.
If the values are plotted on a graph of torque vs. speed, the peak will occur at the critical RPM. All values to the left of peak (below critical) are higher than the initial value. Once past peak the coupling will dampen the vibratory response starting at about 1.4 times critical RPM. Actually there are several damping possibilities in a system, but the damping type coupling is the best bet. Because of the damping, the torque pulse transmitted downstream to the system is reduced from the original value.
The torque pulse transmitted down the system can trigger other forced responses or vibrations. Thus an undamped pulse, like the type transmitted through the stiff coupling system, has the potential to damage downstream components. Loose connections, such as a hydraulic pump spline shaft, are susceptible to this damage unless they are protected. It is also likely that more than one pulse is generated in the operating range. This is very true of diesel engines. All the vibrations must be accounted for if they are in or near the operating range. The Campbell diagram shows the ones in the operating range.
At continuous operating speeds the vibratory torque capability of the coupling must be greater than the vibratory response torque pulse. The damping is energy absorbed by the coupling through hystersis, which results in heat generation. The coupling must dissipate the heat to survive. Its capability to dissipate heat is reflected in the published continuous vibratory torque rating.
The peak torque generated at critical speed must not exceed the maximum torque capability of the coupling. That value is shown in the coupling capabilities as the Tkmax for 100,000 cycles or 50,000 reversing cycles.
Torsional Conclusions
Torsional systems are a special case for coupling applications. Equipment that is driven directly off the diesel flywheel or by synchronous motors should always be given an extended
system analysis for torsional vibrations.
Introduction and selection of commonly used pipes
In recent years, the biggest hot spot in building water supply and drainage is the wide application of new pipes. Traditional galvanized steel pipes and ordinary cast iron pipes for drainage have been replaced due to easy corrosion, heavy weight, and inconvenient transportation and construction. At present, the commonly used pipes for building water supply and drainage mainly include plastic pipes, metal pipes and composite pipes. Now let's briefly discuss some characteristics, advantages and disadvantages, and scope of use of commonly used pipes.
1. Plastic tube
Plastic pipe is a product made of synthetic resin and additives by melt molding.
Additives include plasticizers, stabilizers, fillers, lubricants, colorants, UV absorbers, modifiers, etc.
It has the characteristics of light weight, corrosion resistance, beautiful appearance, no bad smell, easy processing and convenient construction. There are two types of plastic pipes: thermoplastic pipes and thermosetting plastic pipes.
The main resins used in thermoplastic pipes are polyvinyl chloride resin (PVC), polyethylene resin (PE), polypropylene resin (PP), polystyrene resin (PS), acrylonitrile-butadiene-styrene resin (ABS). ), polybutene resin (PB), etc.
The main resins used in thermosetting plastics are unsaturated polyester resins, epoxy resins, phenolic resins, etc.
1.1 Performance of plastic pipes
1.1.1 Physical Properties:
The physical properties of plastic pipes affect the installation method, use, compensation measures and pipe insulation of pipes.
1). The mechanical properties of PVC-U, PP, and ABS are relatively high, and they are regarded as "rigid pipes", which are better for surface installation. On the contrary, PE, PE-X and PB are suitable for concealed application as "flexible pipes".
Want more information on Ductile Iron Flexible Coupling? Feel free to contact us.
2). The use temperature and heat resistance of plastic pipes determine that PVC-U, PE and ABS can only be used for cold water pipes, while PE-X, PP and PB can be used as hot water pipes. When the building has a hot water supply system and the hot and cold water uses a unified pipe, the heat resistance becomes the main indicator.
3). The plastic pipe has a high thermal expansion coefficient. When used as a hot water pipe, there are many thermal compensation measures such as flexible interfaces, expansion joints or various bending positions. Among them, polyolefins such as PE and PP are the most popular. If there is no sufficient
If enough attention is paid and corresponding technical measures are taken, the problem of pulling off of the expansion joint at the interface is very likely to occur.
4). Due to the low thermal conductivity, the thermal insulation performance of the plastic pipe is excellent, which can reduce the thickness of the thermal insulation layer or even eliminate the need for thermal insulation.
In addition to thermal conductivity, the comparison of thermal insulation between different plastic pipes is also related to their respective pipe wall thicknesses.
1.1.2 Pressure bearing performance:
The content involved in the pressure bearing performance is the internal pressure that the plastic pipe can withstand under certain conditions and the failure time under constant pressure. In order to determine the design parameters related to it, and to evaluate and monitor the quality of the pipe. Two tests are generally performed: hydraulic test and long-term high temperature hydraulic test.
1.1.3 Sanitary performance:
Physical and chemical health indicators. Among them, PE, PE-X, PP, PB and ABS are easy to meet the standard.
Non-toxic PVC resin and stabilizer should be used in the production of PVC-U pipes to meet the requirements of hygienic performance.
1.2 Advantages and disadvantages of plastic pipes:
The raw material composition of the plastic pipe determines the characteristics of the plastic pipe.
1.2.1 Advantages:
1). It has good chemical stability, is not affected by environmental factors and the composition of the medium in the pipeline, and has good corrosion resistance.
2). Small thermal conductivity, low thermal conductivity, thermal insulation, good energy saving effect.
3). The hydraulic performance is good, the inner wall of the pipe is smooth, the resistance coefficient is small, it is not easy to scale, and the flow area in the pipe does not occur with time
Change, the probability of pipeline blockage is small.
4). Compared with metal pipes, it has low density, light material, convenient transportation and installation, flexibility, simplicity and easy maintenance.
5). With natural bending or cold bending performance, the coil supply method can be used to reduce the number of pipe joints.
1.2.2 Disadvantages:
1). The mechanical properties are poor, the impact resistance is poor, the rigidity is poor, and the straightness is also poor, so the density of pipe clamps and hangers is high.
2). Poor flame retardancy, most plastic products are flammable and thermally decomposed when burning, releasing toxic gases and fumes.
3). The thermal expansion coefficient is large, and the expansion and contraction compensation must be emphasized.
1.3 Commonly used plastic pipes are:
●Rigid polyvinyl chloride pipe (PVC-U);
●High-density polyethylene pipe (PE-HD);
●Cross-linked polyethylene pipe (PE-X);
●Random copolymer polypropylene pipe (PP-R);
●Polybutene tube (PB);
• Acrylonitrile-butadiene-styrene copolymer (ABS).
1.4 Water supply plastic pipe:
Rigid polyvinyl chloride plastic pipes and fittings for water supply are made of food hygiene-grade polyvinyl chloride resin as the main raw material, adding non-toxic special additives, and then mixing, plasticizing, extruding or injecting. The products meet the national drinking water hygiene standards.
The water supply plastic pipe has a single structure, many kinds of materials and different properties.
The characteristics of common water supply plastic pipes are as follows:
Model Connection method Advantages Disadvantages PVC-U bonding, thread Strong corrosion resistance, easy to bond, cheap, hard texture Yes
UPVC monomer and additives bleed and are not suitable for hot water delivery.
PP-R hot melt good temperature resistance under the same pressure and medium temperature
Under the conditions, the wall of the pipe is the thickest
PEX extrusion clamping Good temperature resistance and creep resistance Only metal can be used
Piece connection; cannot be recycled and reused
HDPE extrusion clamping/thermal fusion/electric fusion Good toughness, good fatigue strength, better temperature resistance
Good, light weight, good flexibility and impact resistance Welding requires electricity, mechanical connection, connection
large
PB extrusion clamping, thermal fusion, electric fusion Good temperature resistance, good tensile and compressive strength,
Impact resistance, low creep, high flexibility There is no PB resin raw material in China, relying on imported
mouth, high price
ABS bonding, threading, electro-fusion High strength, impact resistance Poor UV resistance, long bonding curing time
In general, because of its low price, PVC pipes are the first choice for cold water water supply systems without considering the impact of water quality. When the temperature is high, polyethylene pipes or cross-linked polyethylene pipes, polypropylene pipes, polybutene pipes can be used. Tube.
1.5 Drainage plastic pipe:
The material is relatively simple: hard polyvinyl chloride.
Various structural forms: core layer foam tube, hollow wall tube, spiral tube, core layer foam spiral tube, hollow wall spiral tube.
1.5.1 Disadvantages:
1). The thermal expansion coefficient is large, which needs to be solved by setting up a telescopic compensation device.
2). The rigidity is small and the straightness is poor, so it is necessary to encrypt the pipe clamps, brackets and hangers.
3). Poor heat resistance and low softening temperature, it is necessary to limit the drainage temperature, limit the use site and control the distance from the heat source.
4). The flame retardancy is poor, and fire-stop rings and fire-proof casings should be set at the roof panels that pass through the floor, the roof, the firewall, and the wall of the pipeline well.
5). Poor mechanical shock resistance.
6.) The sound insulation is poor, the noise of the plastic pipe is greater than that of the cast iron pipe, the problem is particularly prominent when the pipe is exposed and the pipe is located close to the bedroom.
The core layer foam pipe and the hollow wall pipe are taken to improve the sound insulation effect of the pipe material to achieve noise reduction; another way to reduce noise is to change the water flow conditions, and the spiral pipe is developed based on this idea. The idea of combining the two methods resulted in a core foamed helical tube and an empty walled helical tube. Under the same conditions, the noise measurement results of drainage plastic pipes with different structures are: ordinary pipe>core layer foam pipe>empty wall pipe>spiral pipe
1.6 Commonly used plastic pipes
1.6.1 Rigid Polyvinyl Chloride Pipe (UPVC)
In the world, UPVC pipe is the most consumed variety of various plastic pipes, and it is also a new type of chemical building material that is being vigorously developed at home and abroad. The use of this kind of pipe can play a positive role in alleviating the situation of steel shortage and energy shortage in our country, and the economic benefits are remarkable. UPVC pipe has the following characteristics:
1). Good chemical corrosion, no rust;
2). It is self-extinguishing and flame retardant;
3). Good aging resistance, can be used between -15℃-60℃ for 20-50 years;
4). The inner wall is smooth and the surface tension of the inner wall makes it difficult to form scale, and the fluid transport capacity is 43.7% higher than that of cast iron pipes;
5). Light weight, easy flaring, bonding, bending, welding, installation workload is only 1/2 of the steel pipe,
Low labor intensity and short construction period;
6). Good resistance performance, volume resistance 1-3 × 105Ω. cm, breakdown voltage 23-2kv/mm;
7). Low price;
8). Save metal energy;
9). The toughness of UPVC pipe is low, the coefficient of linear expansion is large, and the operating temperature range is narrow.
Main application fields of rigid polyvinyl chloride pipe (UPVC):
1) Building water supply and drainage pipeline system;
2) Building rainwater system;
3) Pipes for building electrical wiring;
4) Air conditioning condensate water system
1.6.2 Core layer foamed pipe (PSP)
The core layer foamed pipe is a new type of pipe with a low foaming layer with a relative density of 0.7-0.9 in the middle, which is produced by a three-layer co-extrusion process. Due to the use of the I-type structure principle in material mechanics in structure, and the foam core layer with energy absorption and sound insulation effect, it has gradually become a
A plastic pipe that replaces cast iron pipes, rigid PVC solid wall pipes, etc.
Core layer foam pipe (PSP) has the following characteristics:
1). The impact strength is significantly improved: its hoop rigidity is 8 times that of ordinary UPVC pipes.
2). Wide range of use: it can be used at -30℃-100℃, and the size is stable when the temperature changes
good sex.
3). The foamed core layer can effectively block the noise transmission, which is more conducive to the drainage system of high-rise buildings.
4). Good thermal insulation, 35% lower heat transfer efficiency than non-foamed solid wall pipes.
5). The foamed core layer greatly improves the compression resistance of the inner wall.
6). Compared with solid wall pipes, it can save more than 25% of raw materials, and the larger the diameter, the more raw materials can be saved.
7). The pipe is light, easy to transport and install.
8). The service life in the bent state is more than ten years longer than that of the solid wall pipe.
The main application fields of the core layer foamed pipe (PSP);
1). Drainage system of civil buildings;
2). Industrial protection and conveying liquids;
3). Agricultural microporous irrigation, drainage and irrigation;
4). Electricity, communication cable conduit.
1.6.3 Rigid PVC muffler pipe
Rigid polyvinyl chloride muffler pipe is a new type of building drainage pipe developed by South Korea in the early s. It has obtained patents in Japan, Germany, France, Switzerland and other countries.
The inner wall of the UPVC muffler pipe is provided with six triangular convex helix lines, so that the sewage flows freely and continuously in a spiral shape along the inner wall of the pipe, and the drainage is rotated to form the best drainage conditions, so as to play a good role in energy dissipation at the bottom of the riser and reduce noise. . At the same time, the unique structure of the muffler pipe can make the air form an air column in the center of the pipe to be discharged directly, there is no need to set up a special ventilation pipe as in the past, so that the drainage and ventilation capacity of high-rise buildings can be increased by 10 times, the drainage volume is increased by 6 times, and the noise is higher than that of ordinary UPVC drainage. Pipes and cast iron pipes are 30-40Db lower. UPVC silencer with
When used together with the muffler pipe fittings, the drainage effect is good.
In addition to the characteristics of UPV, the rigid PVC muffler pipe also has the following characteristics:
1) High water-passing ability and fluid conveying ability;
2) High ventilation capacity;
3) Noise reduction and sound insulation;
4) Economical and energy saving;
5) Easy to construct and maintain;
6) Can withstand temperature and settlement deformation of buildings. No expansion joints are required.
Rigid PVC muffler pipe is mainly used in drainage pipe system, especially in high-rise building drainage pipe system.
1.6.4 Plastic bellows
The plastic corrugated pipe adopts a special "annular groove" special-shaped section in the structural design. This kind of pipe has a novel design and a reasonable structure, breaking through the "plate" traditional structure of ordinary pipes, so that the pipe has sufficient compressive and impact resistance strength. , and has good flexibility. According to the different forming methods, it can be divided into single split corrugated pipe and double wall corrugated pipe.
Features of plastic bellows:
1). It has both rigidity and flexibility, which not only has sufficient mechanical properties, but also has excellent flexibility;
2). Compared with the plate pipe, the corrugated pipe per unit length has the advantages of light weight, material saving, energy consumption reduction and price reduction.
Cheap;
3). The bellows with smooth inner wall can reduce the flow resistance of liquid in the pipe and further improve the conveying capacity;
4). Strong chemical corrosion resistance, can withstand the influence of acid and alkali in the soil;
5). The corrugated shape can strengthen the load resistance of the pipeline to the soil without increasing its flexibility, so that it can be continuously laid on uneven ground;
6). The interface is convenient, the sealing performance is good, the handling is easy, the installation is convenient, the labor intensity is reduced, and the construction period is shortened;
7). Wide temperature range, flame retardant, self-extinguishing, safe to use;
8). It has good electrical insulation performance and is an ideal material for wire sleeves.
The main application areas of plastic bellows:
1) Municipal water supply and drainage pipeline system;
2) Building electrical wiring and piping;
3) Agricultural irrigation;
4) Piping of automobile oil pipeline;
1.6.5 Chlorinated Polyvinyl Chloride Pipe (CPVC)
Chlorinated polyvinyl chloride pipe is a kind of plastic pipe with good heat resistance obtained by processing the so-called perchlorinated vinyl resin with a chlorine content of up to 66%. Chlorinated polyvinyl chloride resin is obtained by chlorination of PVC resin. With the increase of chlorine content in the resin, its density increases, its softening point, heat resistance and flame retardancy increase, its tensile strength increases, and its melt viscosity increases. Large, excellent chemical resistance, no deformation in boiling water. Features of CPVC pipe:
1). Wide temperature range: -40℃-95℃;
2). Has good strength and toughness;
3). Has good chemical corrosion resistance;
4). The flame retardant performance is self-extinguishing;
5). Low thermal conductivity, about 1/200 of steel;
6). The content of heavy ions in the medium reaches the standard of ultrapure water;
7). The hygienic performance meets the requirements of national sanitation standards;
7). The wall of the pipe is clean and smooth: it has less frictional resistance and adhesion when transporting fluid;
8). Light weight: equivalent to 1/5 of the steel pipe, 1/6 of the steel pipe;
9). Easy installation: can be connected by bonding, threading, welding, etc.;
10). Excellent anti-aging and anti-ultraviolet performance, long normal service life.
The main application areas of chlorinated polyvinyl chloride pipes:
1). Building air-conditioning system, drinking water pipeline system, underground water discharge pipeline, swimming pool and hot spring
pipeline;
2). Industrial piping system;
3). Food processing pipeline system;
4). Water supply and sewage plant piping system;
5). Agricultural irrigation.
1.6.5 High Density Polyethylene Pipe (HDPE)
High-density polyethylene pipe is valued by the pipeline industry due to its excellent chemical properties, toughness, wear resistance, low price and installation cost
, it is the second most used plastic pipe material after polyvinyl chloride. High-density polyethylene pipe (HDPE) double-wall corrugated pipe is a kind of pipe with low material consumption, high rigidity, excellent flexibility, corrugated outer wall and smooth inner wall. Compared with ordinary pipes of the same specification and strength, the double-wall pipe can save 40% of material, and has the characteristics of high impact resistance and high compression resistance. Widely used as drainage pipes, sewage pipes, underground cable pipes, agricultural irrigation and drainage pipes.
1.6.7 Random copolymer polypropylene pipe (PP-R)
Polypropylene can be divided into homopolypropylene and copolymerized polypropylene, and copolymerized polypropylene is further divided into block copolymerized polypropylene (PPC) and random copolymerized polypropylene (PP-R).
Random copolymer polypropylene, also known as tri-type polypropylene, is a copolymer in which propylene and other comonomer segments are randomly distributed on the main chain. The raw material of PP-R is polyolefin, and its molecule only has carbon and hydrogen elements, which is non-toxic and has reliable hygienic performance.
PP-R will not adversely affect the human body and the environment in the whole process of raw material production, product processing, use and disposal, and it has become a green building material with the same generation of cross-linked polyethylene pipes.
In addition to the advantages of general plastic pipes, such as light weight, good strength, corrosion resistance and long service life, PP-R pipes have the following characteristics:
1). Non-toxic hygiene: meet the requirements of national hygiene standards:
2). Heat-resistance and heat preservation: the Vicat softening point of PP-R tube is 131.3℃, and the maximum service temperature is 95℃.
The operating temperature is 70℃; the thermal conductivity is 0.21W/m, ℃, which is only 1/200 of the steel pipe, and has good thermal insulation performance;
3). Simple and reliable connection and installation: PP-R pipe has good hot melt welding performance, and the pipe is connected with the pipe fittings.
The strength of the part is greater than the strength of the pipe itself, and there is no need to consider whether the connection will leak during long-term use;
4). Good elasticity and anti-freeze cracking: The excellent elasticity of PP-R material makes pipes and fittings resistant to frost heave liquids
Expand together so that they are not burst by frost heaving liquids;
5). Good environmental performance;
6). The linear expansion coefficient is larger, 0.14-0.16mm/m. k;
7). Poor anti-ultraviolet performance: easy to age under long-term direct sunlight.
The main application areas of pipes:
1) Public and civil buildings are used to transport cold and hot water and heating systems;
2) In industrial buildings and facilities, it is used to transport daily water, oil or corrosive liquids;
3) Due to its corrosion resistance to salt water, it is used for water supply pipelines in seaside facilities;
4) Air conditioning piping system;
5) Agricultural irrigation system;
1.6.7 Polybutene Pipe (PB)
PB resin is a high molecular polymer obtained by synthesizing 1-butene, and is an isotactic polymer with a slightly lower isotacticity than polypropylene. It not only has the impact toughness of polyethylene, but also the stress cracking resistance and excellent creep resistance higher than that of polypropylene, and has the characteristics of rubber, and can withstand the stress of 90% of the gas yield strength for a long time.
In addition to the advantages of general plastic pipes, such as good hygienic performance, light weight, easy installation and long life, polybutene pipe (PB) also has the following characteristics:
1). Heat resistance: high heat distortion temperature, easy to use heat resistance, 90 ℃ hot water can be used for a long time;
2). Antifreeze: low embrittlement temperature (-30°C), freezing will not crack within -20°C;
3). Good flexibility; bending radius is only R12;
4). Good thermal insulation;
5). Good insulation performance;
6). Corrosion resistance (easy to be attacked by hot and concentrated oxidizing acid);
7). Environmental protection and economy: waste can be reused, and no harmful gas is produced when burned.
The main application areas of polybutene pipe (PB);
1). Used for various hot water pipes: residential hot water, hot spring water diversion, greenhouse hot water, road and airport snow melting
Waiting for hot water pipes;
2). Industrial pipes
3). Gas pipeline;
4. ) Used to transport abrasive and corrosive hot materials in industrial sectors such as mining, chemical and power generation.
1.6.8 ABS pipe
ABS resin is a terpolymer developed on the basis of polystyrene resin modification. ABS resin is composed of three elements of acrylonitrile-butadiene-styrene. where A represents acrylonitrile, B represents diene, and S represents styrene.
ABS pipe is a new type of corrosion-resistant pipe, it has good impact strength and surface hardness within a certain temperature range, good comprehensive performance, easy to form and machine, and the surface can also be chrome-plated. It has both the corrosion resistance of PVC pipes and the mechanical properties of metal pipes.
Features of ABS pipe;
1). Has good impact strength, which is 5-6 times that of PVC pipe; and can withstand higher working pressure
force, about 4 times that of PVC pipe;
2). Stable chemical properties: non-toxic and tasteless, resistant to acid and alkali, and also non-toxic and non-toxic when used in the food industry
taste;
3). Wide operating temperature range: the operating temperature range is -40℃-80℃;
4). Light weight and low resistance; ABS pipe is light, 0.8 times that of PVC, and the inner and outer walls of the product are smooth.
small resistance;
5). The pipeline connection is convenient and the sealing performance is good;
The main application areas of ABS pipe:
1). Purified water system;
2). Petrochemical industry piping system;
3.) Environmental protection industry.
Types and applications of plastic pipes
Types of pipes/municipal water supply/municipal drainage/building cold water/building hot water/building drainage/threading pipes/floor radiant heating/pipeline maximum diameter (mm/stiffness
rigid polyethylene pipe
（PUC-U ） ● ● ● ● ● ≤ straight pipe rigid PVC noise reduction ● ● 75, 110 straight pipe chlorinated polyvinyl chloride pipe (PVU-C ● ● ≤200 straight pipe core layer foam composite pipe (PSP ) ● ● ● 50-160 straight pipe plastic corrugated pipe ● ● ● 300 straight pipe random copolymer polypropylene pipe (PP-R) ● ● ● 110 coil/straight pipe cross-linked polyethylene pipe (PEX) ● ● ● ● 63 Coiled/Straight High Density Polyethylene Pipe (HDPE) ● ● ● 315 Coiled/Straight Polybutylene Pipe (PB) ● ● 32 Coiled/Straight ABS Pipe ● 200 Straight
2. Metal tube
2.1 Galvanized steel pipe
The replacement of galvanized steel pipe does not mean that metal pipes are replaced, nor does it mean that galvanized steel pipes are replaced in the entire building water supply field.
Steel pipes are divided into seamless steel pipes and welded steel pipes according to their manufacturing methods.
Seamless steel pipes are made of high-quality carbon steel or alloy steel, and are divided into hot-rolled and cold-rolled (drawn).
Welded steel pipes are made of rolled steel plates and welded with butt or spiral seams.
In terms of manufacturing methods, it is further divided into welded steel pipes for low-pressure fluid transportation, spiral seam electric welded steel pipes, direct coil welded steel pipes, and electric welded pipes.
Seamless steel pipes can be used for various liquid and gas pipelines, etc. Welded pipes can be used for water pipes, gas pipes, heating pipes, etc.
2.2 Cast iron pipe
Compared with steel pipes, cast iron pipes for water supply have the advantages of not easy corrosion, low cost and good durability, and are suitable for buried laying. The disadvantage is that it is brittle, heavy, and small in length. The connection method generally adopts socket connection and flexible connection. The traditional gray cast iron pipe destroys the iron base due to the "splintering" effect of the flake graphite on the iron base. In addition, the pipe wall is rough and easy to block; the cement interface needs maintenance, causing construction trouble and other reasons, which have been gradually eliminated from the market.
Ductile cast iron, also known as spheroidal graphite cast iron, has strength and toughness comparable to that of steel, good ductility, and the same internal corrosion as gray cast iron. Ductile iron pipes produced with spheroidal graphite cast iron have good adaptability to more complex use environments
2.3 Copper and copper alloy pipes:
The most advantageous metal pipes are copper pipes. Copper pipes have been used for a long time and have many advantages. The pipes and fittings are complete, and the interface methods are various. They are mostly used in hot water pipelines. The main problem at present is the folding of copper. The output is easy to exceed the standard.
Copper pipes are mainly made of pure copper and phosphorus deoxidized copper, which are called copper pipes or red copper pipes.
Brass pipes are made of ordinary brass, lead brass and other brass.
2.4 Stainless steel tube
2.4.1 Advantages
1). Excellent corrosion resistance.
The thin and strong oxide film on the surface of stainless steel makes stainless steel have excellent corrosion resistance in all water quality and is suitable for various water quality. There is no corrosion and excessive exudate, which can keep the water pure and hygienic and prevent secondary pollution.
2). Excellent mechanical and physical properties.
Stainless steel water pipes have very high strength, can withstand vibration and shock well, and have the characteristics of no leakage, no bursting, fire resistance, shock resistance, etc., so they are very safe and reliable; at the same time, they have good thermal insulation performance, especially suitable for hot water transportation. Field corrosion test data show that the service life of stainless steel water pipes can reach 100 years, and stainless steel water pipes hardly need maintenance. Therefore, its performance-price ratio is very good, the operating life cost is low, and the economic benefits are significant. Due to the advantages of stainless steel pipes, they are widely used in food, medical, chemical, and petroleum industries. However, in the past, industrial stainless steel pipes had thick walls and were expensive to manufacture. The field of water supply has been greatly restricted, and the emergence of thin-walled stainless steel pipes has solved this problem.
2.5 Introduction of commonly used metal pipes:
2.5.1 Welded steel pipe:
1). Welded steel pipe and galvanized welded steel pipe for low pressure fluid transportation
Welded steel pipes for low-pressure fluid transportation are made of mild carbon steel and are the most commonly used small-diameter pipes in pipeline engineering. They are suitable for transporting water, gas, steam and other media. Zinc pipe (commonly known as white iron pipe) and non-galvanized pipe (commonly known as black iron pipe). The inner and outer walls coated with a zinc protective layer are about 3%-6% heavier than the non-galvanized ones. According to the different wall thickness of the pipe, it is divided into three types: thin-walled pipe, ordinary pipe and thickened pipe. Thin-walled pipes are not suitable for conveying media and can be used as casings.
2). Straight seam coiled electric welded steel pipe
The straight seam coiled electric welded steel pipe can be divided into electric welded steel pipe and straight seam welded steel coiled and welded by steel plates on site.
Tube. Several wall thicknesses can be made.
3) Spiral seam welded steel pipe:
Spiral seam welded steel pipe is divided into two types: automatic submerged arc welded steel pipe and high frequency welded steel pipe.
a). Spiral seam automatic submerged arc welded steel pipe is divided into two categories: Class A pipe and Class B pipe according to the pressure of the conveying medium. Type A pipes are generally welded with ordinary carbon steel Q235, Q235F and ordinary low-alloy structural steel 16Mn, and Type B pipes are welded with steels such as Q235, Q235F and Q195, and are used as low-pressure fluid conveying pipes. b). Spiral seam high frequency welded steel pipe Spiral seam high frequency welded steel pipe, there is no unified product standard, generally made of ordinary carbon steel Q235, Q235F and other steel.
2.5.2 Seamless steel pipe:
Seamless steel pipes are divided into hot-rolled pipes and cold-drawn (rolled) pipes according to the manufacturing method.
The maximum nominal diameter of cold drawn (rolled) pipe is 200mm, and the maximum nominal diameter of hot rolled pipe is 600mm. In pipeline engineering, when the diameter of the pipe exceeds 57mm, the hot-rolled pipe is often used, and when the pipe diameter is less than 57mm, the cold-drawn (rolled) pipe is often used.
1). General seamless steel pipe:
Generally, seamless steel pipes are referred to as seamless steel pipes. They are made of ordinary carbon steel, high-quality carbon steel, ordinary low-alloy steel and alloy structural steel. They are used to make liquid pipelines or make structures and parts.
Seamless steel pipes are supplied according to the outer diameter and wall thickness. There are various wall thicknesses under the same outer diameter, and the pressure range is large. Usually the length of steel pipe is 3-12.5m for hot-rolled pipe and 1.5-9m for cold-drawn (rolled) pipe.
2). Seamless steel pipes for low and medium pressure boilers:
Seamless steel pipes for low and medium pressure boilers are made of No. 10 and No. 20 high-quality carbon steel.
2.5.3 Cast iron pipes:
Cast iron pipes are made of pig iron.
According to the different manufacturing methods, it can be divided into: sand type centrifugal socket straight pipe, continuous cast iron straight pipe and sand type iron pipe. According to the different materials used, it can be divided into: gray iron pipe, ductile iron pipe and high silicon iron pipe.
Cast iron pipes are mostly used in plumbing projects such as water supply, drainage and gas.
1) Feed water cast iron pipe:
a). Sand type centrifugal cast iron straight pipe:
The sand type centrifugal cast iron straight pipe is made of gray cast iron, which is suitable for the transportation of pressure fluids such as water and gas. b). Continuous cast iron straight pipe:
The continuous cast iron straight pipe is the continuously cast gray cast iron pipe, which is suitable for the transportation of pressure fluids such as water and gas. c). Drainage cast iron pipe:
Ordinary drainage cast iron socket pipe and pipe fittings.
Flexible anti-seismic interface drainage cast iron straight pipe, this type of cast iron pipe is sealed with rubber rings and fastened with bolts, and has good flexibility and elasticity under internal water pressure. Can adapt to large axial displacement and lateral deflection deformation, suitable for indoor drainage pipes of high-rise buildings, especially suitable for earthquake areas.
d). Ductile iron pipe:
The advantages are outstanding:
The pipe wall is uniform, the structure is dense, the surface is smooth, and there are no defects such as trachoma and slag inclusion;
●Excellent sound insulation effect;
●Flexible interface, easy to install and save construction cost; It can be slightly disturbed and has excellent shock resistance; ●High strength, impact resistance, corrosion resistance, long service life and good fire resistance;
Ductile iron pipes can also be lined with various protective layers according to the use environment and the nature of the conveying medium to meet different use requirements. It has been widely used in construction, municipal, petrochemical and other industries.
2.5.4 Aluminum and aluminum alloy pipes
Aluminum and aluminum alloys refer to industrial pure aluminum containing 98% aluminum and aluminum alloys with copper, magnesium, manganese, zinc, chromium and other alloying elements as the main body. Aluminum and aluminum alloy pipes are drawn or extruded from industrial pure aluminum or aluminum alloys.
Aluminum has good acid corrosion resistance.
3. Composite pipe
Most of the composite pipes are composed of working layer (requires water corrosion resistance), support layer, and protective layer (requires corrosion resistance). The composite pipe is generally made of metal as the supporting material, and the lining is mainly epoxy resin and cement. It is characterized by light weight, smooth inner wall, low resistance and good corrosion resistance;
There are also high-strength soft metals as support, instead of metal pipes on both sides, such as aluminum-plastic composite pipes, which feature that the inner wall of the pipe will not corrode and scale to ensure water quality;
There are also metal pipes on the inside and non-metal pipes on the outside, such as plastic-clad copper pipes, which use the poor thermal conductivity of plastics for thermal insulation and protection.
According to the metal material can be divided into:
(1).Steel-plastic composite pipe
(2).Stainless steel-plastic composite pipe, plastic coated stainless steel pipe
(3).Plastic clad copper tube
(4). Aluminum-plastic composite pipe, cross-linked aluminum-plastic composite pipe
(5) Plastic-lined aluminum alloy pipe
The connection of the composite pipe should adopt the cold working method, and the hot working method is easy to cause the expansion, deformation and even melting of the lining plastic. Generally, there are thread, ferrule, clamp and other connection methods.
3.1 Brief introduction of several commonly used composite pipes:
3.1.1 Steel-plastic composite pipe
Steel-plastic composite pipe is a kind of metal and plastic composite pipe, which is rolled into a steel pipe with galvanized steel strip and welded by argon arc, and has a plastic layer inside and outside.
The steel-plastic composite pipe overcomes the defects of easy rust, pollution, bulky, short service life, low strength, large expansion and easy deformation of the steel pipe, and has the common advantages of the steel pipe and the plastic pipe, such as oxygen barrier. good sex, high rigidity
High strength and high strength, buried pipes are easy to detect, etc.
advantage:
1). Excellent physical properties:
Compared with plastic pipes, it has higher strength, rigidity and impact resistance; it has low expansion coefficient and creep resistance similar to steel pipes. The buried pipe can withstand the external pressure much higher than that of the all-plastic pipe; excellent corrosion resistance, can be installed without any anti-corrosion treatment, saving engineering costs; small expansion coefficient, linear expansion coefficient is only 12 × 10-5/℃ .
2). Small damping coefficient
The pipe wall is smooth, the fluid resistance is small, and there is no scaling, and the head loss is 30% lower than that of the metal pipe under the same pipe diameter and pressure conditions.
3). Stable pressure bearing performance
The complete steel pipe layer is the main pressure-bearing layer of the pipe body, and the pressure-bearing capacity of the pipe is not affected by the performance change of the plastic layer.
4). Self-traceability
Magnetic metal detectors can be used for tracking, and there is no need to bury tracking or protection marks, which can avoid excavation damage and provide great convenience for emergency repair and maintenance.
5). Windability
Can be bent, underground installation can effectively withstand sudden shock loads caused by settlement, slip, vehicles, etc. Improved operational reliability. The fixed-length (12m) single steel-plastic composite pipe can be bent 25° in one direction, saving the consumption of small-angle steering elbows.
6). Wide temperature range
7). Low thermal conductivity
8). Light weight, easy to transport and install; long service life and low comprehensive cost.
3.1.2 Aluminum-plastic composite pipe
Generally, the middle layer is aluminum tube, and the inner and outer layers are polyethylene, which are compounded by hot-melt co-extrusion. The general working pressure is 1.0MPa, and the burst pressure is 7.0MPa.
advantage:
(1). Non-toxic and tasteless, fully comply with hygienic standards,
(2). It has the characteristics of high temperature resistance and low temperature resistance.
(3). Oxygen barrier, completely eliminate penetration.
(4). Absorb the rebound energy of the tube when bending, so that the tube can be arbitrarily formed; it can be bent without rebound, and a single tube can be
Up to several hundred meters in length, providing convenient conditions for construction and installation.
(5). With antistatic properties, it can be used to transport gas and oil.
(6). At room temperature, the inner and outer polyethylene layers are insoluble in any known solvent, and can resist the corrosion of various acids, alkalis and salt solutions.
(7) The buried position of the pipe can be detected with a metal detector,
(8). The thermal conductivity is 0.45W/m*k, which is far less than the thermal conductivity of metal pipes.
(9) The inner surface is smooth and the head loss is small.
(10). The cutting is convenient and the construction period is shortened.
Four, pipe fittings
Pipe fittings are also called pipe fittings, connectors, joint parts, etc. The pipes in various piping systems are connected with different joints and fittings to form a pipeline.
4.1 Malleable cast iron (Mal Steel) pipe fittings:
Made of malleable iron, threaded to the pipe. The working pressure is within 0.1Mpa. Its appearance is characterized by thick edges at the ends. to increase the connection strength.
Malleable iron pipe fittings are mainly used for extension, branching and turning of pipes. Malleable iron pipe fittings mainly have the following varieties:
4.1.1 Direct:
Used to connect pipes with the same diameter on the same line, there are two kinds of through wire and non-through wire;
The reducer is directly called the size head, which is used to connect two pipes with different diameters on the same line;
Different diameter eccentric head, the center lines of the big and small ends do not overlap, and it is used to connect two horizontal pipes with different diameters located on the upper and lower sides of the same horizontal line;
4.1.2 Elbow:
It is used at right-angle bends of pipes to connect two equal-diameter pipes that are perpendicular to each other;
Reducing elbow, connecting two pipes of unequal diameter that are perpendicular to each other;
4.1.3 45° bend;
4.1.4 Tee:
The two ends in the straight line have the same diameter and the end that is perpendicular to it is a small pipe diameter, which is used for the connection of small diameter branch pipes; the 45° inclined tee is also called a Y-shaped branch pipe. less resistance;
4.1.5 Four-way:
It is used for the vertical cross-connection of the pipe; the reducing cross is used when connecting two branch pipes with smaller pipe diameters vertically on the pipeline;
4.1.6 Tonic for heart:
Also known as inner and outer wire, inner and outer wire, the inner wire is small, the outer wire is large, the outer wire is connected with other pipe fittings, and the inner wire is directly connected to the pipe, which is used for the reducing connection of the pipe;
4.1.7 External screw short connector:
Used to connect two abutting pipe fittings; often replaced by short pipe joints spun by lathes, very short joints are called butt wires.
4.1.8 Thread plug:
Also known as a plug, the plug is an external thread used to block the orifice of the pipe fitting.
4.1.9 Union:
It consists of two male and female joints that can be buckled with each other, and a female set connecting the male and female mouths.
Use rubber pads or asbestos paper pads to prevent water leakage. The union is used in the pipeline where the same diameter pipeline needs to be connected, so as to facilitate disassembly and maintenance.
4.1.10 Lock nut:
Also known as root mother affix, root hoop, a short pipe section with a short thread at one end and a long thread at the other end (with no tip at the root) and a root mother, plus another inner wall with a straight wire directly forms a long thread. Silk, which acts as a live joint, is used as a detachable live connection.
4.1.11 Flange:
The left and right two pieces form a pair, and the function is the same as the live joint. The specification is more than d50mm in diameter. Since the two ends of the large-sized valve are mostly flanged interfaces, the flanges are also mostly used for the connection between the pipeline and the valve.
4.2 Steel and malleable iron pipe fittings:
When the steel pipe is connected by thread, if the working pressure is high (but within 1.6Mpa), steel pipe fittings can be used. Steel pipe fittings are made of carbon steel, commonly known as wrought iron pipe fittings. It has good weldability and can be used where welding is required.
There are mainly the following varieties:
4.2.1 Press the reducer
4.2.2 Welding elbows
4.2.3 Welded reducers (concentric, eccentric)
4.2.3 Welding three-energy, four-way
4.3 Cast iron pipe fittings for water supply:
Cast from grey cast iron.
There are two types of cast iron pipe fittings for water supply: socket type and flange type. Their uses are similar to malleable cast pipes. They mainly have the following types:
Cross tube, T-tube (including three-column cross tube, three-coil cross tube, four-column cross tube, four-coil cross tube, double-column T-tube, double-coil T-tube, three-column T-tube, three-coil T-tube, double-column single tube 90° elbow, 45° elbow; fork pipe; B-shaped pipe; fire hydrant pipe; casing, etc.
4.4 Drainage cast iron pipe fittings:
Mainly include socket and socket bending pipe, 90° elbow, 45°Y-shaped tee, 90°T-shaped tee, 90°TY-shaped tee, inclined tee, positive tee, TY-shaped reducing tee, reducing four Tee, pipe clamp, T-shaped bottle mouth tee, 45° elbow, Y-shaped reducing tee, socket and socket sweeping mouth, S-shaped water trap, P-shaped storage trap, floor drain and other varieties.
4.5 Plastic pipe fittings:
The specifications and functions are similar to the above-mentioned fittings, but the materials of manufacture are different.
4.6 Standard of pipe fittings:
4.6.1 Elbow:
The current international standard for elbows is the American national standard ANSIB16.9 and 16.28.
The standard outer diameter range is 1/2"~ 80". Generally, those within 24" are made of seamless steel pipes, and those from 26" to 80" are stamped with steel plates and then welded. The elbow is based on the radius of curvature. It can be divided into long-radius elbow and short-radius elbow. Long-radius elbow refers to its radius of curvature equal to 1.5 times the outer diameter of the pipe, that is, R = 1.5D. Short-radius elbow refers to its radius of curvature equal to the pipe Outer diameter, that is, R = D. The elbow is divided into 45 ° elbow, 90 ° elbow and 180 ° elbow. The maximum wall thickness can reach 60mm, and the minimum is
1.24mm.
4.6.2 Tee:
The outer diameter is in the range of 2.5"-60", from 26"-60" is welded tee, and the wall thickness is 28-60mm.
4.6.3 Size header:
Conventionally, we first talk about the big head specifications, and then talk about the small head specifications.
DN100-80;DN80-50
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