Practical Valve Use in Hydraulics: Flow Control & Safety Guide

Hydraulic valves are quintessential regulators. They direct, control, and modulate the flow of hydraulic fluid, often determining the very performance of the system. Imagine orchestrating a symphony where every instrument’s volume, pace, and sequence need precise control. In the hydraulic realm, valves are the conductors, ensuring that every note, or in this case, fluid movement, is perfectly timed and executed.

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Without valves, a hydraulic system would be like a city without traffic signals—chaotic, uncontrollable, and prone to mishaps. They manage the direction of fluid flow and regulate the pressure and volume, ensuring that each part of the system receives the exact amount of hydraulic fluid it requires. This precision is critical, especially when hydraulic systems are used in applications where even a minor error can lead to significant consequences, be it in heavy machinery, aerospace, or medical devices.

Furthermore, safety is paramount in hydraulic systems. Valves play a crucial role in preventing potential hazards such as over-pressurization, which can lead to equipment failure or even catastrophic explosions. By acting as checkpoints and relief mechanisms, they ensure the system operates within safe parameters, safeguarding both the equipment and its operators.

In essence, while pumps, motors, and cylinders might get most of the limelight in discussions about hydraulics, it’s the valves that quietly ensure everything runs smoothly, efficiently, and safely. As we delve deeper into the intricacies of hydraulic valves in the sections that follow, it becomes clear just how instrumental they are in the grand hydraulic symphony.

Basics of Hydraulic Valve Operation

To truly appreciate the value and intricacy of hydraulic valves, one must first understand their basic operation. Much like how the human heart has valves that regulate blood flow, hydraulic valves control the flow of fluid, ensuring it reaches where it’s needed when it’s needed, and in the required amount.

How Hydraulic Valves Function

At its core, a hydraulic valve is designed to do one or a combination of three primary tasks: allow fluid flow, prevent fluid flow, or partially allow fluid flow. This simple but vital operation is what makes hydraulic systems flexible, responsive, and safe.

Opening, Closing, and Modulating Flow

• Opening (or On Position): In this state, the valve permits the flow of hydraulic fluid through it. This is often achieved when an internal component, such as a spool or poppet, moves to align with fluid channels, creating an unobstructed path.
• Closing (or Off Position): In this mode, the valve prevents any fluid from passing through. The internal components of the valve block or obstruct the fluid channels, effectively sealing off the flow.
• Modulating (or Throttling): Perhaps the most nuanced of the three, modulating involves partially opening or closing the valve to regulate the volume or speed of the fluid flow. It’s akin to adjusting a faucet to get the desired water flow. In hydraulic systems, this function is crucial for tasks that require varying power or speed, ensuring precision and efficiency.

By mastering these three basic operations, hydraulic valves can effectively manage complex tasks, from lifting heavy loads with precision to ensuring smooth operations in intricate machinery. The true genius, however, lies in the design variations and additional features that cater to specific needs, as we’ll explore in the subsequent sections.

Responding to Pressure Changes

Hydraulic valves are not just passive conduits for fluid flow; they’re sensitive to the dynamic nature of hydraulic systems. Pressure changes are commonplace in these systems, be it due to load variations, fluid compressibility, or system operations like starting and stopping. Valves play a pivotal role in responding to these changes to maintain system stability and safety.

• Pressure Sensing: Many hydraulic valves come equipped with mechanisms that can detect pressure changes. This can be through spring-loaded components, diaphragms, or even electronic sensors. When the sensed pressure deviates from a set threshold, the valve takes corrective action.
• Relief and Regulation: Upon detecting an overpressure situation, relief valves, a subtype of hydraulic valves, open to divert excess fluid, often back to the reservoir. This prevents potential damage to the hydraulic components and potential safety hazards. On the flip side, when system pressure drops below a certain level, valves can work in tandem with pumps to increase fluid flow and restore the required pressure.
• Compensation: Some advanced valves can adjust their position dynamically to compensate for pressure changes, ensuring a steady flow rate or consistent system pressure. This compensation is crucial in applications where precision is paramount, like in CNC machinery or flight control systems.

Understanding the sensitivity of valves to pressure changes underlines their importance in safeguarding hydraulic systems against failures and ensuring optimal performance.

Types of Hydraulic Valves and Their Applications

The world of hydraulic valves is vast and varied. Each valve type is tailored for specific functions and applications. This section delves into some of the most common types, starting with directional control valves.

Directional Control Valves

As the name suggests, directional control valves (DCVs) primarily govern the path that hydraulic fluid takes within a system. By directing fluid flow, these valves play an essential role in determining the movement and operation of hydraulic machinery.

• Design: At their core, DCVs consist of a spool housed within a cylinder. The position of the spool, which can be shifted manually, electronically, or pneumatically, determines the fluid path. Ports on the valve connect to the pump, actuator, and reservoir, allowing the fluid to be channeled as needed.
• Operation: The fundamental operation of DCVs is to connect or disconnect fluid pathways. For instance, in a simple DCV, pushing the spool might connect the pump to a hydraulic cylinder, extending it. Conversely, pulling the spool might connect the cylinder to the reservoir, retracting it.
• Applications: DCVs are ubiquitous in hydraulic systems. From construction equipment like excavators and backhoes to industrial machines and even some automotive systems, the ability to direct fluid flow is a fundamental requirement.

The depth and breadth of DCVs extend to various subtypes, each with unique features catering to specific needs. Exploring each one would reveal the ingenuity and engineering marvel behind these seemingly simple components.

Design and Mechanism

Directional control valves (DCVs) are elegantly engineered to perform their function efficiently. The foundational design typically includes:

• Spool and Sleeve: At the heart of most DCVs is the spool-and-sleeve mechanism. The spool is a rod with lands (thicker sections) and grooves (thinner sections) that align with ports in the valve body or sleeve. As the spool shifts position, different ports get opened or closed, directing the flow of hydraulic fluid.
• Ports: DCVs have multiple ports – primarily the pressure port (P) connected to the pump, the tank port (T) leading to the reservoir, and service ports (A and B) connecting to actuators. The specific arrangement and connectivity of these ports change as the spool moves, thus determining fluid direction.
• Actuation Methods: The method used to shift the spool position varies. Manual levers, solenoids, hydraulic pilots, and pneumatic mechanisms are some of the common actuation methods. The choice depends on the application and the level of control precision required.
• Center Configurations: Many DCVs have a neutral or center position, wherein no work is performed. The configuration in this position can vary, leading to terms like “open center,” “closed center,” or “tandem center.” These configurations determine how the fluid flows (or doesn’t) in the valve’s neutral position.

Applications and Use Cases

The versatility of DCVs makes them invaluable in a multitude of hydraulic systems:

• Mobile Equipment: In construction machinery like excavators, backhoes, and cranes, DCVs direct fluid to specific actuators, determining whether a shovel digs into the ground, an arm extends, or a crane hoists a load.
• Industrial Machinery: Manufacturing equipment, from injection molding machines to hydraulic presses, relies on DCVs to sequence operations, ensuring tasks like molding, pressing, or stamping occur in the right order and at the right time.
• Automotive Systems: Some vehicles, particularly those with advanced suspension systems or convertible tops, utilize DCVs to channel fluid effectively and achieve desired outcomes.
• Safety Systems: In applications where safety is paramount, like in certain braking systems, DCVs ensure that fluid is channeled correctly, preventing accidents and malfunctions.

Given their pivotal role, a deep understanding of DCVs, their design, and their operational mechanisms is crucial for anyone working in the realm of hydraulics.

Pressure Control Valves

Pressure control valves (PCVs) are instrumental in maintaining the stability of hydraulic systems. Tasked with regulating and modulating pressure levels, they ensure the system operates within safe and efficient parameters. By safeguarding against over-pressurization or pressure drops, they play a pivotal role in optimizing performance and preventing potential hazards. Different designs cater to varied functions, making them versatile components in hydraulic applications. Let’s explore their design intricacies and specific use cases.

Design and Mechanism

Pressure control valves (PCVs) serve as guardians of hydraulic systems, ensuring that pressure levels remain within desired limits. Their inherent design revolves around the following key components and principles:

• Pressure Sensing Element: Integral to PCVs is an element, typically a spring-loaded piston or diaphragm, which senses pressure. When the hydraulic fluid pressure reaches a certain set limit, this element responds by either opening or closing the valve.
• Set Pressure: The desired or set pressure is predetermined, often adjustable, and is the point at which the valve starts to act. This is generally achieved by adjusting the spring tension.
• Relief and Reducing Functions: Some PCVs are designed as relief valves, which open to relieve excess pressure, directing the fluid back to the tank. On the other hand, reducing valves maintain a particular downstream pressure, irrespective of variations in upstream pressure.
• Pilot Operation: Certain advanced PCVs operate on a pilot principle. Here, a small pilot valve senses the pressure and controls the main valve. This allows for more precise control and responsive action.
• Direct and Indirect Operation: Direct-operated valves respond immediately to changes in pressure. In contrast, indirect or pilot-operated valves use pressure from the system itself to control the valve’s opening or closing.

Applications and Use Cases

The necessity to manage and control pressure efficiently makes PCVs indispensable in a variety of settings:

• Industrial Hydraulics: In factories, where hydraulic systems power everything from lifts to presses, PCVs ensure equipment operates safely, preventing over-pressurization, which could damage machinery or pose safety risks.
• Mobile Hydraulics: In heavy-duty vehicles such as bulldozers, loaders, or tractors, PCVs play a role in maintaining consistent operations, safeguarding systems from pressure spikes and drops.
• Aerospace: Aircraft hydraulic systems, responsible for functions like landing gear extension and wing flap control, rely on PCVs to ensure smooth, safe operations, especially during critical phases of flight.
• Renewable Energy: In emerging sectors like hydroelectric power generation, PCVs manage water flow and pressure, optimizing turbine performance.
• Safety Systems: Elevator systems, hydraulic brakes, and safety interlocks often incorporate PCVs to guarantee fail-safe operations, ensuring user safety.

With their pivotal role in managing pressure across a broad spectrum of hydraulic applications, understanding the nuances of PCVs is essential for professionals in the field.

Flow Control Valves

Flow control valves serve as the arbiters of fluid velocity within a hydraulic system, ensuring that fluid moves at the desired rate to achieve optimal system performance. By adjusting or modulating the rate of fluid flow, they influence the speed of actuators and the efficiency of the entire hydraulic process. This adaptability allows systems to tackle a wide variety of tasks with precision and consistency. Flow control valves come in diverse designs, each tailored to specific applications. Dive into the mechanics of their design and the myriad ways they’re implemented in real-world scenarios.

Design and Mechanism

Flow control valves are generally characterized by their means of adjusting fluid flow. Their design often includes a movable component, like a spool or a needle, which can be positioned to increase or decrease the flow passage size. The positioning might be manually set or automatically adjusted based on system feedback. Some valves offer a simple on/off function, while others can finely modulate flow rates. Additionally, they might be combined with check valves to allow free flow in one direction and regulated flow in the opposite direction.

Applications and Use Cases

Flow control valves find their place in a myriad of hydraulic applications. For instance:

• Speed Regulation of Hydraulic Cylinders: By controlling the rate of fluid entering or leaving a cylinder, the speed of the piston can be precisely regulated.
• Hydraulic Motors Speed Control: Similar to cylinders, controlling fluid flow can dictate the speed of hydraulic motors.
• Temperature Regulation: By modulating flow rates, these valves can assist in managing system temperature, as fluid speed can influence heat generation and dissipation.
• Sequential Operations: In complex hydraulic systems where tasks need to be performed in a specific order, flow control valves can be timed to direct fluid at precise moments, ensuring orderly operation.

Their adaptability and precision make flow control valves indispensable in both simple and complex hydraulic systems.

Proportional and Servo Valves

In the realm of precision control within hydraulic systems, proportional and servo valves reign supreme. These valves offer dynamic control capabilities, adjusting fluid flow in proportion to an input signal. This allows for a high degree of accuracy in controlling motion, speed, and force within hydraulic applications. While they share similarities, there are distinct differences in their operation and design that lend them to specific applications. Let’s delve into the intricate workings of these valves and the scenarios where they shine.

Design and Mechanism

Proportional Valves:

These valves modulate flow or pressure in direct proportion to an electrical command signal, typically provided by a potentiometer or a PLC. A solenoid inside the valve receives this signal and adjusts the valve spool position accordingly. The positioning is analog in nature; the more the signal, the more the valve opens, and vice versa.

Servo Valves:

Servo valves, on the other hand, offer even higher precision and are typically used in closed-loop control systems. They use a feedback mechanism to ensure the desired output is achieved. The valve continuously corrects its position based on feedback, ensuring that any disturbances or changes in load are automatically adjusted. The heart of the servo valve is its torque motor and flapper nozzle mechanism, which finely tunes the valve’s spool position.

Applications and Use Cases

Proportional Valves:

• Industrial Automation: For applications where varying speed or force is necessary but without the stringent demands of ultra-fine control.
• Load Sensing Systems: Proportional valves can adjust pump flow based on load requirements, improving energy efficiency.
• Machine Tooling: Offering variable control over tool speed and direction based on user input.

Servo Valves:

• Aerospace: Employed in flight control systems, where minute adjustments can drastically affect aircraft behavior.
• Robotic Systems: Providing high-precision motion control for robotic arms, ensuring accuracy in tasks like welding or assembly.
• Simulation and Testing Equipment: Delivering precise motion feedback in simulators, be it for pilot training or automotive crash tests.

Both types of valves, while distinct in their mechanisms, offer a level of control that’s unparalleled in hydraulic systems, ensuring efficiency, accuracy, and adaptability in a wide array of applications.

Valve Selection: Ensuring System Efficiency and Safety

The correct valve selection is paramount not only for the efficient functioning of a hydraulic system but also for ensuring its safety. As with other hydraulic components, there’s no one-size-fits-all when it comes to valves. The selection is influenced by a myriad of factors, each interplaying to dictate which valve is the best fit for a given application. Let’s explore the critical considerations one must account for during this selection process.

Factors to Consider When Choosing Valves

The intricacies of hydraulic systems necessitate a thorough understanding of various operational demands before deciding on a specific valve. Below are the key factors that guide this selection:

System Requirements and Valve Specifications

• Flow Rate: The chosen valve should be capable of handling the system’s maximum flow rate without inducing excessive pressure drops, which can affect performance and efficiency.
• Operating Pressure: Every valve comes with a maximum allowable operating pressure. This specification must be aligned with the system’s pressure levels to avoid premature valve failure or potential hazards.
• Temperature Range: Hydraulic systems might operate in varying temperature conditions. The selected valve should be resistant to temperature-induced wear or changes in performance.
• Fluid Compatibility: The valve’s internal materials must be compatible with the hydraulic fluid in use. Incompatibilities can lead to accelerated wear, corrosion, or even valve failure.
• Response Time: A valve with a quick response time is crucial for applications demanding rapid changes in flow or pressure.
• Mounting and Size: The physical dimensions and mounting options of the valve must fit within the constraints of the hydraulic setup.
• Operational Modes: Whether the system requires manual, electric, or pneumatic valve operation, the valve type should match the desired mode of operation.

By aligning the specific needs of the hydraulic system with the appropriate valve specifications, one can ensure not only optimal performance but also enhanced safety and system longevity.

Material and Design Considerations

The materials used in constructing a valve and its design intricacies play a pivotal role in its performance and longevity. Here are the key aspects to consider:

• Material Compatibility: The valve’s material should be resistant to the hydraulic fluid used and any potential contaminants. Materials like stainless steel, brass, and certain plastics are commonly employed, but their appropriateness varies depending on the fluid.
• Corrosion Resistance: In environments where the valve is exposed to corrosive elements, opting for corrosion-resistant materials or coatings is vital.
• Structural Integrity: The design should ensure that the valve can withstand the mechanical stresses it will be subjected to during operation, such as high pressures or rapid flow changes.
• Sealing Elements: Valve designs often incorporate seals to prevent leaks. The material and design of these seals should ensure minimal leakage and long-term durability, especially under fluctuating pressures.
• Temperature Tolerance: The materials used should maintain their mechanical properties over the system’s expected temperature range, ensuring consistent performance.

By weighing these material and design factors, one can select a valve that not only performs efficiently but also stands the test of time.

Cost, Lifespan, and Maintenance

When investing in hydraulic valves, it’s essential to consider the long-term implications, encompassing the initial purchase price, expected lifespan, and maintenance requirements:

• Initial Purchase Price: While cost is always a factor, it shouldn’t be the sole consideration. A cheaper valve might not always provide the desired performance or lifespan, leading to higher costs in the long run.
• Operational Lifespan: Higher-quality valves, although pricier upfront, tend to have longer operational lifespans, often making them more cost-effective over the system’s life.
• Maintenance Needs: Some valves, due to their design or material choice, might require more frequent maintenance. Considering the maintenance intervals and associated costs can provide a clearer picture of the valve’s long-term value.
• Replacement and Repair: A valve’s design might make it easier or more challenging to repair. Valves that are easily repairable or have readily available spare parts can reduce downtime and operational costs.
• Operational Costs: Some valve designs might be more energy-efficient than others, leading to savings in operational costs.

By thoroughly evaluating these cost and longevity aspects, one can make an informed decision that balances performance needs with budgetary constraints, ensuring the hydraulic system remains both efficient and cost-effective.

Challenges and Troubleshooting in Valve Operation

Hydraulic valves are the unsung heroes of fluid systems, quietly and efficiently directing, controlling, and modulating flow as needed. But like all mechanical components, they aren’t immune to issues. As systems evolve and become more sophisticated, understanding potential challenges and troubleshooting techniques becomes crucial to maintain optimal performance. Whether due to external factors, wear and tear, or operational anomalies, valves can present a range of challenges that require prompt attention.

Common Issues with Hydraulic Valves

Hydraulic valves, being a cornerstone of fluid control, are susceptible to various operational challenges. These issues, if not addressed promptly, can impede the efficiency of the entire hydraulic system. Understanding these common problems can provide insight into effective troubleshooting and preventive maintenance.

• Contamination: One of the most frequent culprits of valve malfunction is contamination. Dirt, metal shavings, and other foreign particles can obstruct the proper operation of valves, leading to sticking, leakage, or even total failure.
• Wear and Tear: Over time, repetitive use can cause wear on the valve’s internal components. This wear can result in internal leakage, erratic operation, or a reduction in the valve’s responsiveness.
• Improper Installation: If a valve is incorrectly installed or not adequately aligned, it can lead to issues like external leakage, vibration, or misoperation. It’s crucial to follow installation guidelines and ensure compatibility with the hydraulic system.
• Cavitation and Aeration: These phenomena can cause damage to the valve and lead to inefficiencies in system operation. Cavitation occurs when the fluid vaporizes due to low pressure, while aeration involves air bubbles in the fluid. Both can cause noise, reduced system efficiency, and potential valve damage.
• Electrical Failures: For electrically actuated valves, issues such as solenoid failures, wiring problems, or controller malfunctions can prevent the valve from operating correctly.
• Inadequate Maintenance: Skipping regular maintenance checks can lead to a buildup of issues that eventually cause valve malfunction. Regular inspections, cleaning, and replacements of worn-out components are vital.
• Temperature Extremes: Hydraulic valves are designed to operate within specific temperature ranges. Exposing them to temperatures outside this range can impact the fluid’s viscosity, potentially causing the valve to malfunction or wear out prematurely.
• Incorrect Specification: Using a valve that’s not specified for the system’s pressure or flow requirements can lead to inefficiencies, reduced system life, and even catastrophic failures.

Recognizing and understanding these common challenges is the first step in effective troubleshooting. By being proactive in maintenance and staying vigilant for early signs of issues, operators can ensure the longevity and efficiency of their hydraulic systems.

Best Practices for Valve Maintenance and Repair

Valve longevity and optimal performance largely depend on regular maintenance and timely repair. By ensuring that valves function at their best, one can not only improve system efficiency but also avoid potential hazards and costly downtimes. Below are some best practices that can help in maintaining hydraulic valves effectively.

1. Regular Inspection: Schedule periodic inspections to check for signs of wear, leakage, or damage. Visual inspections can help identify external issues, while functional tests can reveal internal malfunctions.
2. Cleanliness is Key: Ensure the hydraulic fluid is free from contaminants. Use high-quality filters and change them at regular intervals. Keeping the fluid clean can prevent many valve-related issues.
3. Avoid Over-pressurization: Frequently monitor system pressure. An over-pressurized system can damage valves and reduce their lifespan. Ensure that pressure relief valves are functioning correctly to protect the system.
4. Lubricate Moving Parts: While many hydraulic valves are self-lubricating due to the hydraulic fluid, some external components may require occasional lubrication to ensure smooth operation.
5. Replace Worn Components: Parts like seals, springs, and spools can wear out over time. Instead of waiting for a complete failure, replace these components proactively based on usage and wear patterns.
6. Train Personnel: Ensure that those operating and maintaining the hydraulic system are adequately trained. Human error or oversight can often be the cause of valve malfunctions. Proper training can reduce these instances.
7. Stay Updated on Valve Specifications: As hydraulic systems evolve, so do their components. Regularly review manufacturer recommendations and guidelines to ensure that valves are being used and maintained as intended.
8. Seek Expert Advice: If a valve issue is recurrent or particularly challenging, don’t hesitate to consult with hydraulic experts or the valve manufacturer. They might offer insights or solutions that aren’t immediately apparent.

By adhering to these best practices, hydraulic system operators can ensure that their valves function efficiently and safely, prolonging the life of the entire hydraulic system.

Innovations and Future Trends in Valve Technology

As industries evolve and technological advancements become more prevalent, hydraulic valve technology is no exception to the winds of change. As we increasingly incorporate digital solutions and prioritize sustainability, the future of hydraulic valves promises remarkable innovations.

The Next Generation of Hydraulic Valves

• Smart Valves: With the integration of IoT (Internet of Things) and AI, hydraulic valves are becoming more intelligent. These “smart” valves can self-diagnose issues, predict maintenance needs, and adjust their operations in real-time for optimum performance.
• Eco-friendly Materials: As industries move towards greener practices, hydraulic valves made of environmentally friendly materials that reduce carbon footprint will gain prominence.
• Miniaturization: As equipment design focuses on compactness and efficiency, expect to see smaller valves that deliver the same or even better performance than their larger counterparts.
• Improved Energy Efficiency: Future valves will prioritize energy conservation, reducing the overall power consumption of hydraulic systems. This not only saves costs but also aligns with global energy conservation goals.
• Magnetic Technologies: Some new valves are exploring the use of magnetic technologies, allowing for smoother operations and reduced wear and tear.
• Adaptive Valves: These valves can adapt their functions based on external conditions without the need for manual recalibration, ensuring consistent performance regardless of environmental changes.
• Wireless Control: Remote operation and wireless control of valves are becoming more common, especially in large industrial setups or in hazardous environments where direct human interaction is risky.
• 3D Printing: As 3D printing technology advances, there’s potential for creating custom valve designs suited for specific applications, ensuring a perfect fit for specialized requirements.

The constant research and development in the field of hydraulics mean that valves of the future will be more reliable, efficient, and in tune with modern industrial needs. They will play a pivotal role in ensuring that hydraulic systems continue to be an indispensable part of various sectors.

Conclusion

Valves, often considered the unsung heroes of hydraulic systems, ensure flow control, safety, and operational efficiency. As we’ve explored, they come in various types, each suited for specific applications, and are crucial for the seamless operation of any hydraulic system. With the advent of new technologies and a focus on sustainability, the hydraulic valve sector is poised for significant transformations. Embracing these innovations will be crucial for industries to maintain efficiency, safety, and eco-friendliness in the years to come.

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The two main types of high-performance hydraulic valves are:

• Servo-Quality Proportional Valves
  Also called high-performace proportional valves. These are more common. Make sure the valve has a ±10V command signal (not 4-20mA).

• Servo Valves (current driven)
  Flapper-nozzle, or jet pipe. Often called just servo valves, these require a current input, such as ±50mA. To use these with an RMC, a Current-to-Voltage converter is required, such as the VC.

Servo-Quality Proportional Valves

For great performance, trouble-free set-up and machine operation (this means lower startup and maintenance costs), Delta recommends using linear valves with a zero-lapped spool and on-board electronics.

The RMC75E, RMC150E and RMC200 do support non-linear valves (with a single “knee”), but it requires extra set up. A linear valve is the easiest to use.

Linear Valve:
This means the flow vs command signal profile is linear (at least up to about 8 volts):

Terms such as “kink”, “knee”, and “progressive” indicate a non-linear valve.

Zero-lapped spool:
This means the spool is cut so that as soon as a voltage is applied to the valve, hydraulic fluid starts flowing.

On-board electronics:
This means you don’t need to wory about a separate card, and typically, with a good valve, you don’t need to make any adjustments on the electronics.

Important Notice
If you enjoy headaches, choose a valve with an overlapped spool (we call that deadband), and a non-linear flow versus command signal profile. Notice, however, that headaches can result in costs savings. Especially for OEM applications, where you just need to go through the difficulty once, and then pump out volume, it can be well worth it to get a valve that is barely good enough to do the job. On single applications, the time savings in setup usually justify a good valve.

Recommended Servo-Quality Proportional Valves
Here are some valves that Delta knows have been used successfully. Many valves offer +/-10V or 4-20mA input. Make sure to choose +/-10V.

Atos
DLHZO-TEB and DLKZOR-TEB: single-stage, sizes NG6 and 10. The first L in the part numbers indicates a sleeve, which gives more precise control and faster response.
DPZO-LEB: two-stage, sizes 10-35
The EB indicates the Basic Integral digital driver, which is all that is necessary when using the RMC to control the valve.
F180.pdf (1.54 MB)

Bosch Rexroth
4WRREH: Size NG6. Very fast response. Make sure to choose the ±10V command option.
4WRPEH: Sizes NG6 and 10. The NG6 is not as fast as the 4WRREH, but still good. Make sure to choose the ±10V command option.
4WRLE: Sizes 10-35. Make sure to get the V spool with the L (linear) flow characteristic.

Continental
VED03MX: Size D03. Very fast response.
This valve is quite linear, but not perfectly so. Setting the Velocity Feed Forward for a large command voltage typically results in the best tuning.
The VED03MK is not as high performance as the VED0M3X, but will also work quite well.

Domin
The Domin valves S4 Pro, S6 Pro, and S10 Pro all perform very well for position and force control. They have fast step response. The S10 controlled force exceptionally well.

Eaton
Axis Pro valves are very high response and work well for position control, but we have had several reports that they are not suitable for pressure or force control. D03 and D05. Choose level 1 in order to work with the RMC. Make sure to choose the ±10V command option.
K(B)SDG4V-3: http://www.eaton.com/ecm/groups/public/@pub/@eaton/@hyd/documents/content/pll_.pdf

Moog
D661: Very fast response and high flow.
D681-D685: Very fast response and high flow.
D633/634: http://www.moog.com/literature/ICD/Moog-Valves-D633_D634-Catalog-en.pdf
D636/637: http://www.moog.com/literature/ICD/Moog-ServoValves-D636_D637-Catalog-en.pdf Less expensive valve with pretty good performance.
79-series: Crazy high response for 200 gpm (750 lpm), and very expensive.

Parker
Very fast response valves with great control.
D1FP: Size NG6. Make sure to choose the ±10V command option.
D3FP: Size NG10. Make sure to choose the ±10V command option.
These valves are quite linear, but not perfectly so. Setting the Velocity Feed Forward for a large command voltage typically results in the best tuning.
The D1FH is not as high performance as the D1FP, but will also work quite well.

Santest DA03S
As of this writing, Santest offers only this one valve, a D05 size, with or without on-board electronics. It is possibly the fastest valve we have tested, and provides exceptional control. The fastest response requires a 48 VDC power supply, and it also supports 24 VDC. The testers say this valve may be the easiest tuning valve they have tried. It tuned a cylinder in pressure control unusually easily. Pressure control is where the differences between valves becomes more pronounced.

Star Hydraulics
Star primarily offers high quality servo valves.
200: Delta has used this valve. It is very small in physical size, 7 lpm, performs well and needs a current output. The VC or RMC200 CA4 module can provide current for this valve.
Make sure to choose a linear zero-lapped spool. If you have a very fast application, choose the high response version.
Most of the sero valves take a current input. However, some of the servo valves have on-board electronics. In this case, choose the ±10 volt option.

Yuken
The LSVG-01-EH and The LSVG-03-EH are crazy high response and linear with on-board electronics. D03 and D05, but maxes out at 60 lpm. Make sure to choose the ±10V command option. Yuken also makes a similar high response D05 valve, the SE, with more flow, 130 L/min. In addition, there are two-stage valves with very high flow (up to lpm) and very fast 0-100% step response (8-13 msec).