This article will take an in-depth look at hydraulic valves and their advantages.
The article will bring more detail on topics such as:
This chapter will explore hydraulic valves, including their functions and the various techniques they employ for controlling flow.
A hydraulic valve is a mechanical component used to control the movement of hydraulic fluid within a hydraulic system. These systems often operate under high pressure, generally starting at 200 bar and commonly reaching up to 700 bar or more. Therefore, the valves must be made from materials robust enough to handle these pressures. The control methods for these valves are diverse, including physical, mechanical, electrical, hydraulic, and pneumatic systems.
The various methods of flow control used by hydraulic valves are:
Throttling flow control involves adjusting the size of the fluid's pathway to regulate the flow rate. By altering the cross-sectional area of the valve, as shown in the image above, the flow rate can be adjusted. This concept is well explained by Bernoulli’s principle.
In the tapered tube illustrated below, changing the pipe's diameter from d1 to d2 will result in an increase in the fluid's velocity (V1 < V2), regardless of whether the pipe is inclined. This increase in velocity also raises the flow rate, as previously demonstrated. Thus, any mechanism that adjusts the cross-sectional area of the valve will effectively alter the flow rate.
Pressure-compensated flow control valves are engineered to keep the volumetric flow rate constant, regardless of fluctuations in pressure across the valve.
Pressure-compensated flow control valves include a variable orifice and a pressure compensation mechanism. As depicted, the fluid follows a designated path. It enters through an inlet whose size is adjusted by the pressure compensator, which in this case is a compensator spool. This spool is spring-loaded, and the spring's force, combined with the hydraulic load and incoming fluid pressure, positions it to adjust the inlet's size. This adjustment ensures that the volumetric flow rate remains constant, even if there are pressure variations in the system.
Temperature-compensated flow control valves are an adaptation of pressure-compensated flow control valves. They address the issue where rising operational temperatures can affect the accuracy of orifice tolerances. To accommodate these temperature fluctuations, temperature compensators are integrated into the valve design.
This is the most basic method of fluid flow control. It consists of a drilled hole in what acts as a passage on an otherwise blocked fluid passage. When employed for flow control, it is usually put in series with the hydraulic pump.
A widely used adjustable flow control valve is the priority valve. This type of valve directs flow to a specific outlet based on the system’s needs. For example, if the system pressure falls below a preset level, the priority valve will restrict flow to non-essential outlets and ensure that only the critical outlet receives the necessary flow. This is managed by a spring-loaded mechanism that adjusts according to the pressure in the system.
Hydraulic valves serve three primary functions, which categorize them into distinct classes:
This section will outline each of these three classes of hydraulic valves and provide examples of the specific valve types within each category.
In hydraulic systems, these valves are employed to regulate or control the flow rate of hydraulic fluid. They typically feature a mechanism that allows for flow rate adjustment, such as an adjustable opening or port. By modifying the size of this opening, the flow area can be changed, which in turn impacts the flow rate.
A typical example would be controlling the speed of extending or retracting a hydraulic cylinder. It can sometimes be on hydraulic motors or any other hydraulic actuator. The speed of operation is directly related to the flow rate of the hydraulic fluid.
Lowering the flow rate will decrease the speed of operation, while increasing the flow rate will enhance it. A higher flow rate results in more force being applied to the piston, leading to faster extension or retraction of the cylinder.
Flow control valves differ based on the method they use to modify flow rate. Flow rate itself is a broad term encompassing various types. For instance, volumetric flow rate, measured in mm³/sec, is often used to calculate the linear speeds of hydraulic piston rods.
Another type is weight flow rate, typically measured in lb/sec, which is used for calculating power in imperial units. Mass flow rate, measured in kg/sec, is employed to determine forces related to inertia during acceleration or deceleration.
Flow control valves are designed to regulate the volume of fluid passing through them per unit of time. As such, they are versatile and can manage different types of flow rates, varying primarily in the mechanism used to control the flow. For example:
Each flow control valve operates based on a particular mechanism designed to implement a specific principle for regulating flow rate. The principle refers to the scientific concept that governs how the flow is adjusted, while the mechanism is the actual tool or method used to achieve this regulation.
Ball valves operate using a ball with a hole through its center. When the hole aligns with the input and output ports, it allows hydraulic fluid to pass through. Ball valves come in various configurations based on the number of ports they connect, including two-way, three-way, and four-way designs.
A 2-way ball valve controls flow between a single input and output. When the ball is turned perpendicular to the flow path, it completely blocks the passage. In contrast, a 3-way ball valve can connect any two ports as needed, offering more flexible flow routing options.
Ball valves function as "switches" to either fully open or close the flow. While they can be used for throttling by adjusting the ball to a partially open position, they are generally not recommended for precise throttling applications.
The needle valve is designed for precise flow rate control in low-pressure scenarios. It is particularly useful for applications requiring accurate flow regulation, such as in pressure-compensated flow control systems.
Needle valves feature a plunger that fits into a tapered orifice to control the flow. By adjusting the position of the plunger, you can regulate or completely stop the flow.
(A) represents the handle attached to the plunger, also known as the stem (F). Turning this handle moves the plunger up and down along the threads (C), with the Locknut (B) preventing it from unscrewing completely. When the plunger is lowered, its tapered end or stem (I) seals against the valve seat, closing off the (H) orifice completely. The inlet port is labeled (G), the Bonnet is (D), and the valve housing is (E).
In some configurations, the needle pin valve can be operated by electric or pneumatic actuators, which allow for remote control, particularly in closed-loop systems with feedback mechanisms.
The butterfly valve is a widely used method for controlling fluid flow. It features a disc that rotates to either open or close the flow path. This disc can be adjusted manually or operated via an electric motor connected to the valve's stem.
Butterfly valves are a very affordable means of flow control. They are also lightweight and the disk material comes in vast materials to cater to different hydraulic fluid properties. They can be used to shutout flow as well as to throttle flow.
When choosing a flow control valve, consider the following scenarios and types of valves.
| Scenario | Valve Type |
|---|---|
| Consistent pressure and constant load on the cylinder or hydraulic motor. | Fixed Flow Control Valve (Orifice) Variable flow control |
| Varying Load on the cylinder or hydraulic motor and varying pressure in the system | Pressure compensated valve |
| Varying load on the cylinder or hydraulic motor, varying pressure in the system, and varying temperature in the system | Pressure and temperature compensated |
Pressure-control valves are designed to manage the fluid pressure within a hydraulic system. They achieve this by ensuring that the system pressure stays below a predetermined threshold.
Several types of pressure control valves are available for hydraulic systems, including the following:
These valves maintain the system pressure below a predefined threshold. They can regulate pressure either upstream or downstream of the valve and provide protection to equipment by preventing pressure surges or spikes.
These are pressure-activated valves that are typically in a normally closed state and open when the fluid pressure reaches a specific level.
Counterbalance valves allow fluid to flow freely into the actuator but restrict reverse flow until a predetermined pressure is achieved.
Unloading valves stop or divert the pump’s flow back to the tank, particularly when the machine is not in use.
The details of these valves and their variants will be discussed further in the following section.
In many fluid power systems, maintaining a specific pressure range is crucial to avoid damage to the equipment and ensure safe operation. Relief valves play a critical role in protecting both the machinery and operators by releasing excess pressure.
The pressure at which a relief valve begins to allow fluid to pass is known as the cracking pressure. The difference between this pressure and the system's current pressure is referred to as the pressure differential or pressure override.
Below are the different types of relief valves:
A direct acting relief valve features a poppet ball that is directly subjected to the system pressure on one side. The opposite side is connected to a spring that applies pressure against the system force. In a normally closed state, the force from the spring exceeds that of the system pressure, keeping the valve closed.
The spring in a direct acting relief valve can be adjusted to change its length, which in turn adjusts the cracking pressure. When the system pressure exceeds the spring force, it moves the ball away from the seat, allowing excess fluid to escape until the pressure returns to an acceptable level.
Pilot operated relief valves are ideal for applications requiring the relief of large flows with a small pressure differential or minimal pressure override.
The pilot-operated relief valve operates in two distinct stages. Initially, in the pilot stage, a smaller relief valve (illustrated as a rod with a piston above) is activated. This action then triggers the main relief valve (depicted with a spring above).
The main relief valve typically remains closed when the system's pressure is below the force exerted by the spring of the main valve. It is important to note that the main relief valve is subjected to pressure on both ends, with the front end having a smaller surface area in contact with the fluid compared to the back end.
This difference in surface areas means that an increase in the system fluid pressure will exert a greater force on the smaller surface area (since pressure is inversely proportional to area). Consequently, this enables the main relief valve to open and direct excess fluid to the tank, thereby managing and reducing any surges in pressure.
Pressure reducing valves are employed in hydraulic systems to control and maintain a secondary lower pressure. Typically, these valves are normally open and function as two-way valves. They close off the flow when the downstream pressure exceeds the desired level, ensuring the system operates within the specified pressure limits.
There are two main types of pressure reducing valves: pilot-operated and direct-acting valves.
This type of pressure reducing valve manages the pressure in the secondary circuit (the outlet circuit) independently of the pressure in the main circuit. Unlike a pressure relief valve, which reacts to pressure changes in the main circuit, the pressure reducing valve monitors the pressure in the downstream circuit, or at the outlet.
As depicted in the diagram, when the pressure in the secondary circuit rises, it exerts a hydraulic force on area A of the valve, partially closing it. The force of the spring counteracts this hydraulic force, allowing only enough oil to flow through the valve to maintain the desired pressure in the secondary circuit. The spring tension can be adjusted to set the desired pressure level.
Once the outlet pressure reaches the valve's preset level, the valve will close. However, a small amount of oil typically escapes from the low-pressure side of the valve. This leakage often flows through an orifice in the spool, passes through the spring chamber, and is then directed to the reservoir.
In cases where the valve closes completely, any leakage around the spool might lead to increased pressure in the secondary circuit. To prevent this, a bleed passage to the reservoir ensures the valve remains slightly open, preventing the downstream pressure from exceeding the valve setting. The excess leakage is directed back to the tank through the drain passage.
Constant-pressure-reducing valves maintain a steady pre-set pressure in the secondary circuit, as long as the main circuit pressure is higher than the secondary circuit pressure.
These valves adjust the secondary circuit pressure by using an adjustable spring that opposes the pressure in the secondary circuit. If the secondary circuit pressure drops, the spring force opens the valve wider to restore and maintain the desired pressure level.
The valve functions by balancing the main circuit pressure against the combined forces of the secondary circuit pressure and the spring. Since the areas on either side of the poppet are equal, the spring provides a consistent reduction in pressure.
The spool functions as a pilot-controlled pressure-reducing valve, designed to maintain balanced hydraulic pressures from downstream at both ends. It remains open due to a gentle spring. When the reduced pressure meets the setting of the pilot relief valve's spring, the valve allows hydraulic fluid to flow into the tank. This results in a decrease in pressure across the spool, which, in turn, causes the spool to move towards a closed position, counteracting the spring’s light force.
The pilot valve is responsible for discharging just enough fluid to adjust the main valve spool or poppet, ensuring that the flow through the main valve matches the needs of the reduced pressure circuit. During periods when the low-pressure circuit no longer requires flow, the main valve will close. Subsequently, any high-pressure fluid that leaks into the reduced-pressure section is redirected back to the reservoir via the pilot-controlled relief valve.
In hydraulic systems with multiple actuators, it's typical to operate components like hydraulic cylinders in a specific sequence. This is sometimes managed by selecting cylinder sizes based on the load they need to handle.
Space constraints and force requirements often dictate the appropriate cylinder size. In these situations, sequence valves can be employed to ensure that cylinders are activated in the desired sequence.
Sequence valves are generally closed 2-way valves that manage the order of operations within a circuit. Although they resemble direct-acting relief valves, their spring chambers are usually drained to the reservoir externally, as opposed to internally through an outlet port, as seen with relief valves.
A sequence valve is designed to allow pressurized fluid to move to a secondary path only after the initial priority path or task has been completed. In its normally closed position, the valve lets hydraulic fluid flow freely to the primary circuit, where it performs its intended function until reaching the preset pressure level of the valve.
Once the primary function is achieved, the pressure in the primary circuit increases and is detected by the pressure-sensing mechanism. This pressure acts on the spool, overcoming the spring force. As the spring compresses, the valve spool shifts, redirecting hydraulic fluid to the secondary circuit.
Normally closed counterbalance valves are used to maintain a specific pressure in a part of a hydraulic circuit, typically to counteract weights or external forces, such as in a platen or hydraulic press, and prevent uncontrolled free fall. The main port of the valve connects to the rod end of the hydraulic cylinder, while the secondary port is connected to the directional control valve. The valve is set to a pressure slightly higher than needed to prevent the load from falling freely.
The counterbalance valve blocks the flow of hydraulic fluid from its inlet port to the outlet port until the pressure at the inlet port exceeds the spring force. When pressurized fluid enters the hydraulic cylinder cap, the cylinder extends, raising the pressure at the rod end. This increased pressure shifts the main spool in the counterbalance valve, creating a path for fluid to flow through the secondary port to the directional control valve and ultimately to the reservoir. As the load is lifted, the check valve opens, allowing the cylinder to retract freely.
When there is a need to boost cylinder force and relieve backpressure at the bottom of the stroke, the counterbalance valve can be controlled remotely. Typically, counterbalance valves are internally drained. During cylinder extension, the valve opens its secondary port to allow fluid to flow to the reservoir. When the cylinder retracts, the check valve ensures that the spool remains unaffected.
These valves are commonly used to offload pumps. They regulate the pump's output flow—typically from a standalone pump in a multi-pump system—directly to the reservoir at low pressure once the system reaches its pressure set point.
The valve remains closed due to the force of the spring. When an external pilot signal exerts enough force on the opposite end of the valve spool to overcome the spring's force, the spool shifts, redirecting the pump output to the reservoir at a reduced pressure.
Being spring-loaded, the unloading valve naturally stays closed. It opens only when the system pressure exceeds the force of the adjustable spring.
Unloading valves can also be equipped with a pilot control feature for the main valve. This setup includes a port through the main valve plunger, allowing system pressure to act on both sides of the plunger.
The valve remains closed due to the combination of a light spring and the system pressure acting on the larger surface area at the spring end of the plunger.
A built-in check valve maintains system pressure. When the system pressure drops to the set point, the pilot valve closes. The flow from the pump through the port in the main valve spool then causes the valve to close.
In a piloted unloading valve, a piston is subjected to pump pressure on both ends.
Directional control valves rank third in our classification of main hydraulic valves. These valves manage the flow of fluid by directing it into one or multiple paths, whether the fluid is coming from several sources or just one. Inside these valves, a spool controls which paths are allowed to receive or release flow. The position of the spool dictates which paths are active. Directional control valves can be operated either electronically or manually.
Directional control valves serve three key functions:
These functions often work in combination.
There are several types of directional control valves, but the main ones are:
The most common type of directional control valve is the 2-way valve. This valve either permits or blocks fluid flow. An example of a 2-way valve is a water tap, which manually controls the flow of water by either allowing it or stopping it.
A single-acting hydraulic cylinder requires both supply and exhaust at its port to function, which necessitates a 3-way valve. This valve allows fluid flow to the actuator in one position and drains the fluid from it in another. Some 3-way valves also include a third position that blocks flow in all ports.
For a double-acting actuator, a 4-way valve is required. This valve independently pressurizes and exhausts two ports. A 3-position, 4-way valve can either stop the actuator or allow it to float, making it commonly used in hydraulic systems for handling applications.
Check valves are 2-port valves, with one port for fluid entry and the other for fluid exit. They come in various materials, including polymers, metal, and rubber. Common designs include swing or flap types, where a metal disc pivots on a hinge to prevent reverse flow. Larger check valves typically use this swing or flap design. Additionally, there are spring and ball check valves, which feature a ball that rests in a specially shaped seat.
Typically, a check valve remains closed due to the spring force. When fluid pressure exceeds the cracking pressure, the ball or disc moves away from its seat, allowing the valve to open and permit flow.
Duckbill check valves use a rubber diaphragm that keeps the valve normally closed unless positive pressure is applied. Unlike metal swing or flap check valves, rubber duckbill check valves are highly reliable. They do not experience issues like seizing, rusting, or binding. Additionally, rubber check valves are less prone to mechanical wear compared to their metallic counterparts.
A key factor to consider in check valves is the cracking pressure, which refers to the minimum pressure on the upstream side needed for the valve to begin functioning. Typically, check valves are designed to manage fluid flow in a single direction and can be calibrated for a particular cracking pressure.
This solenoid-operated directional valve is utilized in hydraulic systems to control the opening, closing, or redirection of fluid flow. It operates with a solenoid, which is essentially a coil of wire carrying electrical current wound around a ferromagnetic core. The valve includes multiple chambers known as ports. The solenoid actuates the spool inside the valve, enabling the opening or closing of these ports. The spool, a cylindrical component similar to a piston, either obstructs or permits fluid flow through the ports depending on its position.
The valve features solenoids labeled X and Y in the diagram above, positioned on opposite sides for actuation. It includes a cylindrical spool marked as Z. This spool has "lands," which are sections with a larger diameter, and "grooves," which have a smaller diameter. The lands are designed to obstruct the flow, whereas the grooves facilitate the passage of fluid through the valve.
Cartridge valves regulate the pressure, direction, and flow of hydraulic fluid internally. These inline valves operate parallel to the fluid flow and are ideal for applications requiring high flow rates and effective leakage control.
Cartridge valves are often considered bodiless because they are installed directly into a cavity without an integral housing of their own. A single cartridge can serve multiple valve functions, including relief, sequence, pilot-operated, flow control, or counterbalance. These valves are lightweight, easy to install, cost-effective, resistant to leaks, and straightforward to repair.
Cartridge valves come in two primary types: slip-in and screw-mounted.
Often referred to as a panel-type cartridge valve, this valve requires additional pilot control to operate. It is pressed against the cover plate of the manifold and remains securely in place there.
In this setup, hydraulic threaded cartridge valves are secured into the manifold block using threaded connections. Each valve is designed to perform a specific function, such as relief, flow control, or directional control.
Hydraulic valves offer several benefits, including:
The table below outlines the advantages of cartridge valves.
| Feature | Benefit |
|---|---|
| Increased power density | Smaller size |
| Several functions from a single mounting position | Lower system cost |
| Fast acting | Improved system response |
| Mounted within a manifold | Lesser chance of oil leakage |
| Soft switching | Fewer system pressure spikes |
| Higher permissible working pressures | Cost effective in the control in high flow systems |
| Low pressure drop | Reduced energy consumption |
| Large flow range | More cost effective control |
| Not sensitive to contamination | Long Service Lifespan |
| Very long holding time | Perfect for safety circuits |
| Not sensitive to water based fluids | Greater stability across all operating conditions |
| Not Sensitive to high pressure drops | Can be utilized in high flow systems and hazardous environments |
What function you want to control: this is what you would want the valve to achieve for your system. Either Controlling pressure, flow rate or changing direction.
How you want to control the function: would you like it to be electrically controlled, or automatically with the mechanical systems in the hydraulics or manually.
Hydraulic fluid type: this will mean you have to choose material of the valve that works well with your chosen hydraulic oil.
Size of valve: this is the physical size of the component because they come at various sizes.
Pressure rating: this is the maximum pressure that the valve will perform in.
Ports and connection type: this is the number of interface ports you have, number of inputs and number of outputs.
Working temperature: this is the extreme end of the working temperature for the hydraulic system.
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