Hydraulic valves direct the flow of a liquid or gas through a hydraulic assembly. The direction of the hydraulic fluid is determined by the spool, which is the knob on the exterior of the valve. A hydraulic system cannot function without the use of valves. Every hydraulic assembly requires a valves, be it one valve or multiple valves. Their uses can be as simple as one relief valve to protect the pump and actuator, or their uses can be complex, with a variety of different types of valves used for one function. The size of the required valves is determined by the maximum flow of the hydraulic fluid through the valve and the maximum system pressure.
Different valves have different functions. Check valves let fluid flow in one direction and block flow in the opposite direction. Directional control valves are used to pass on the pressure medium (the flow) in a particular direction. Pressure control valves switch on or off at a certain pressure. Flow control valves regulate the flow of hydraulic fluid by adjusting the size of the bores (orifices). Needle valves use a tapered pin to gradually open a small space for precise control of flow.
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Hydraulic valves direct the flow of a liquid or gas through a hydraulic assembly. The direction of the hydraulic fluid is determined by the spool, which is the knob on the exterior of the valve. A hydraulic system cannot function without the use of valves. Every hydraulic assembly requires a valves, be it one valve or multiple valves. Their uses can be as simple as one relief valve to protect the pump and actuator, or their uses can be complex, with a variety of different types of valves used for one function. The size of the required valves is determined by the maximum flow of the hydraulic fluid through the valve and the maximum system pressure.Different valves have different functions. Check valves let fluid flow in one direction and block flow in the opposite direction. Directional control valves are used to pass on the pressure medium (the flow) in a particular direction. Pressure control valves switch on or off at a certain pressure. Flow control valves regulate the flow of hydraulic fluid by adjusting the size of the bores (orifices). Needle valves use a tapered pin to gradually open a small space for precise control of flow.
Hydraulic solenoid valves are popular in industrial and commercial applications. These electrically controlled valves are used in hydraulic systems to open, close, or change the direction of the flow of fluids, ensuring that the system runs at peak efficiency.
The valves are used in a wide range of applications, including:
Process control in manufacturing plants
Turbine systems
Water supply systems
Wastewater treatment plants
Fuel supply systems
Hydraulic machinery in the aerospace industry
Hydraulic pumps, motors, and brakes in the automotive industry
Heavy machinery in construction
Choosing the right solenoid valve for an application is essential to ensure that the system performs as intended and that it can handle any unexpected challenges along the way. In this article, we’ll look at criteria for selecting the best hydraulic solenoid valve for your needs.
The suitable number of ports and positions in a solenoid valve is based on the application’s overall requirements. The more ports you have, the greater your selection flexibility. This is because solenoid valves come in manifolds that can support multiple positions with one valve or a single position per valve (lift type). If you need less than five positions and three ports, there’s no reason to select an electromagnetic solution over pneumatic.
On the other hand, if you have more than five positions and three ports, or a mix of single-position valves with manifolds that support multiple positions in one valve (lift type), then an electromagnetic solution is required. It is important to remember that not all solenoid valves are available for lift types, while most pneumatic flow control valves are lift types, which means you will have less selection flexibility.
Additionally, if the valves need to handle high pressures or flows, then an electromagnetic solution is necessary. Pneumatic valves cannot be used for those applications due to their inherent limitations on higher pressures and flows. Understanding the number of ports and positions you need is the first step in selecting an appropriate solenoid valve type for your hydraulic application.
The flow requirements for a hydraulic application play a significant role in determining the size of the solenoid valve. The flow rate must be considered in conjunction with the pressure drop across the valve, especially in applications like commercial hydroponics, where a constant flow requirement is needed. Generally speaking, the higher the solenoid coil current, the greater the energy consumption and heat generation. If you’re using a DC power supply that’s not regulated by a PWM controller, this can result in heating the coils and eventually damaging them.
Depending on your application’s flow requirements, choose a solenoid valve with an appropriately sized orifice (nozzle). The nozzle size affects the pressure drop across the orifices at varying pressures and flows. In some cases, flow requirements may be too low to warrant using a solenoid valve at all. When choosing the appropriate orifice size for your application, consider how it will affect energy consumption and operational costs as well as meeting performance expectations.
Flow requirements vary greatly depending on the application, so consult a hydraulic solenoid valve expert for assistance with flow calculations and to determine the appropriate orifice size. When choosing a pressure drop, you should consider the trade-off between increased energy consumption and heat generation on one hand, and achieving adequate performance based on the application’s parameters, including process-fluid viscosity, on the other hand.
Determine the solenoid valve’s spool action based on whether your hydraulic application requires the spool to stay in place when de-energized or return to the center. For spool action valves, the center position is typically not a rest state. That means it takes more energy to move the spool back into the resting/intermediate position after being in an energized or de-energized state. In short, if you need your solenoid valve to stay in a rest state when de-energized, you’ll need to take this into account.
The type of solenoid valve you choose matters. For example, spool action valves typically have faster response rates than paddle-style valves. But that doesn’t mean they are better suited for every hydraulic application. It’s crucial to consider all the features, benefits, and requirements to determine which spool action solenoid valves are right for your application. The flow direction and speed required for optimal performance determines which spool action valve is best.
The solenoid valve’s materials must be compatible with the properties of the application’s flowing media. For example, the valve body must be strong enough to handle high pressure and temperature. At the same time, it shouldn’t corrode due to interaction with corrosive chemicals such as chlorine or sulfuric acid in the flowing media. Different types of materials are used for the valve body, which is why it’s important to understand your material options.
The most common materials used for solenoid valves are:
Cast iron (body)
Aluminum (bodies and internals)
Carbon steel (internals only in some cases). Valves made with these bodies work best when the media is corrosive.
Stainless steel (bodies, internals, and trim). Valves made with these bodies have a longer life span because they’re resistant to corrosion.
If the flowing media contains sulfuric acid or chlorine chemicals, consider valves made of stainless steel. Brass is a more expensive option than cast iron or aluminum, but it has excellent properties for solenoid applications in high-temperature areas. If the flowing media contains oxygen, nitrogen, or carbon dioxide gases, select valves made of aluminum or brass because the solenoid coils have a high chance to oxidize.
The solenoid valves you choose should be able to withstand the maximum pressure required for your hydraulic applications. If the valves cannot handle high pressure, they can fail or become a safety hazard. Pressure is a serious consideration, and it should be covered in your engineering specs. This is especially important when dealing with hydraulic applications for lifting or other heavy-duty purposes. The valves should also have a metal-to-metal seat, which can prevent leaks under pressure.
For low-flow and nonshock applications, standard elastomer seats work well. Select a valve that is rated for the highest pressure you expect to see. The valves should be able to handle the maximum anticipated flow levels without excessive leakage. If the system has multiple stages, each stage needs its own solenoid valve and coil assembly to maintain equal pressures on both sides of the piston inside each cylinder. This is especially important for multicylinder hydraulic systems, which are typically used in heavy machinery.
Currently, it’s not possible to use a single coil assembly and solenoid valve to control the movement of multiple pistons inside individual cylinders because each piston has its own pressure zone based on fluid volume. You must purchase one coil assembly and solenoid valve for each piston. Keep in mind that the maximum operating pressure in hydraulic applications can vary widely. Be sure you select a solenoid valve that can handle the maximum pressure you expect in your system while maintaining sufficient flow capacity for the safe operation of all necessary components within each cylinder.
The final selection criteria concern ensuring that your hydraulic solenoid valves and their materials can withstand the minimum and maximum temperature requirements of the applications. Temperature is essential in determining solenoid valve capacity; it affects the fluid’s flow and viscosity. If the temperature is too low, you might experience a slow response time, or fluid may not flow at all. At higher temperatures, your solenoid valve could be damaged by the heat generated from its coil, and the higher temperature could affect the fluid’s viscosity.
Temperature is critical if you’re using hydraulic solenoid valves for steam applications. Many materials used in their construction cannot withstand temperatures above 200°F (93°C). If your application requires high operating pressure but doesn’t allow for a temperature adjustment, it may be necessary to use a solenoid valve with the required temperature rating. If your application may require working at high temperatures, select a hydraulic solenoid valve that can withstand these conditions without breaking down or experiencing reduced performance.
When selecting hydraulic solenoid valves, it’s important to have a thorough understanding of the factors that affect their performance. It’s also valuable to research users’ experiences with different brands and models. This can be an indicator of how well suited a valve is to particular applications. Depending on your application, solenoid valves can be a great choice for controlling flow in a hydraulic system and should work well with most other components.
MIL-H- is a fire-resistant hydrogenated polyalphaolefinbased fluid developed in the s to overcome the flammability characteristics of MIL-H-. MIL-H- is significantly more flame resistant than MIL-H-, but a disadvantage is the high viscosity at low temperature. It is generally limited to –40 °F. However, it can be used in the same system and with the same seals, gaskets, and hoses as MIL-H-. MIL-H- is the rust-inhibited version of MIL-H-. Small aircraft predominantly use MIL-H-, but some have switched to MIL-H- if they can accommodate the high viscosity at low temperature.
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These fluids are used in most commercial transport category aircraft and are extremely fire-resistant. However, they are not fireproof and under certain conditions, they burn. The earliest generation of these fluids was developed after World War II as a result of the growing number of aircraft hydraulic brake fires that drew the collective concern of the commercial aviation industry. Progressive development of these fluids occurred as a result of performance requirements of newer aircraft designs. The airframe manufacturers dubbed these new generations of hydraulic fluid as types based on their performance.
Today, types IV and V fluids are used. Two distinct classes of type IV fluids exist based on their density: class I fluids are low density and class II fluids are standard density. The class I fluids provide weight savings advantages versus class II. In addition to the type IV fluids that are currently in use, type V fluids are being developed in response to industry demands for a more thermally stable fluid at higher operating temperatures. Type V fluids will be more resistant to hydrolytic and oxidative degradation at high temperature than the type IV fluids.
Due to the difference in composition, petroleum-based and phosphate ester-based fluids will not mix; neither are the seals for any one fluid usable with or tolerant of any of the other fluids. Should an aircraft hydraulic system be serviced with the wrong type fluid, immediately drain and flush the system and maintain the seals according to the manufacturer’s specifications.
Experience has shown that trouble in a hydraulic system is inevitable whenever the liquid is allowed to become contaminated. The nature of the trouble, whether a simple malfunction or the complete destruction of a component, depends to some extent on the type of contaminant. Two general contaminants are:
Abrasives, including such particles as core sand, weld spatter, machining chips, and rust.
Nonabrasives, including those resulting from oil oxidation and soft particles worn or shredded from seals and other organic components.
Whenever it is suspected that a hydraulic system has become contaminated or the system has been operated at temperatures in excess of the specified maximum, a check of the system should be made. The filters in most hydraulic systems are designed to remove most foreign particles that are visible to the naked eye. Hydraulic liquid that appears clean to the naked eye may be contaminated to the point that it is unfit for use. Thus, visual inspection of the hydraulic liquid does not determine the total amount of contamination in the system. Large particles of impurities in the hydraulic system are indications that one or more components are being subjected to excessive wear. Isolating the defective component requires a systematic process of elimination. Fluid returned to the reservoir may contain impurities from any part of the system. To determine which component is defective, liquid samples should be taken from the reservoir and various other locations in the system. Samples should be taken in accordance with the applicable manufacturer’s instructions for a particular hydraulic system. Some hydraulic systems are equipped with permanently installed bleed valves for taking liquid samples, whereas on other systems, lines must be disconnected to provide a place to take a sample.
Routine sampling—each system should be sampled at least once a year, or every 3,000 flight hours, or whenever the airframe manufacturer suggests.
Unscheduled maintenance—when malfunctions may have a fluid related cause, samples should be taken.
Suspicion of contamination—if contamination is suspected, fluids should be drained and replaced, with samples taken before and after the maintenance procedure.
Pressurize and operate hydraulic system for 10–15 minutes. During this period, operate various flight controls to activate valves and thoroughly mix hydraulic fluid.
Shut down and depressurize the system.
Before taking samples, always be sure to wear the proper personal protective equipment that should include, at the minimum, safety glasses and gloves.
Wipe off sampling port or tube with a lint-free cloth. Do not use shop towels or paper products that could produce lint. Generally speaking, the human eye can see particles down to about 40 microns in size. Since we are concerned with particles down to 5 microns in size, it is easy to contaminate a sample without ever knowing it.
Place a waste container under the reservoir drain valve and open valve so that a steady, but not forceful, stream is running.
Allow approximately 1 pint (250 ml) of fluid to drain. This purges any settled particles from the sampling port.
Insert a precleaned sample bottle under the fluid stream and fill, leaving an air space at the top. Withdraw the bottle and cap immediately.
Close drain valve.
Fill out sample identification label supplied in sample kit, making sure to include customer name, aircraft type, aircraft tail number, hydraulic system sampled, and date sampled. Indicate on the sample label under remarks if this is a routine sample or if it is being taken due to a suspected problem.
Service system reservoirs to replace the fluid that was removed.
Submit samples for analysis to laboratory.
Filters provide adequate control of the contamination problem during all normal hydraulic system operations. Control of the size and amount of contamination entering the system from any other source is the responsibility of the people who service and maintain the equipment. Therefore, precautions should be taken to minimize contamination during maintenance, repair, and service operations. If the system becomes contaminated, the filter element should be removed and cleaned or replaced. As an aid in controlling contamination, the following maintenance and servicing procedures should be followed at all times:
Maintain all tools and the work area (workbenches and test equipment) in a clean, dirt-free condition.
A suitable container should always be provided to receive the hydraulic liquid that is spilled during component removal or disassembly procedures.
Before disconnecting hydraulic lines or fittings, clean the affected area with dry cleaning solvent.
All hydraulic lines and fittings should be capped or plugged immediately after disconnecting.
Before assembly of any hydraulic components, wash all parts in an approved dry cleaning solvent.
After cleaning the parts in the dry cleaning solution, dry the parts thoroughly and lubricate them with the recommended preservative or hydraulic liquid before assembly. Use only clean, lint-free cloths to wipe or dry the component parts.
All seals and gaskets should be replaced during the reassembly procedure. Use only those seals and gaskets recommended by the manufacturer.
All parts should be connected with care to avoid stripping metal slivers from threaded areas. All fittings and lines should be installed and torqued in accordance with applicable technical instructions.
All hydraulic servicing equipment should be kept clean and in good operating condition.
Contamination, both particulate and chemical, is detrimental to the performance and life of components in the aircraft hydraulic system. Contamination enters the system through normal wear of components by ingestion through external seals during servicing, or maintenance, when the system is opened to replace/repair components, etc. To control the particulate contamination in the system, filters are installed in the pressure line, in the return line, and in the pump case drain line of each system. The filter rating is given in microns as an indication of the smallest particle size that is filtered out. The replacement interval of these filters is established by the manufacturer and is included in the maintenance manual. In the absence of specific replacement instructions, a recommended service life of the filter elements is:
Pressure filters—3,000 hours
Return Filters—1,500 hours
Case drain filters—600 hours
This document provides installation and maintenance instructions for an Electromatic Relief Valve Type VX. It describes safety precautions for working with pressurized systems and components of the valve system. Instructions are provided for storage, installation, setup, operation, testing, disassembly, repair, and maintenance of the valve and associated controller. Detailed diagrams are included to illustrate the valve components and assembly/disassembly procedures.
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