What Are Hydraulic Pressure Relief Valves And How To Test

Modular hydraulic pressure relief valve and hydraulic cartridge pressure relief valve are available in almost all hydraulic systems, so it is important to have a full and deep understanding of the characteristics of the relief valve used.

If you want to learn more, please visit our website Huade Hydraulic.

Hydraulic Relief Valve Function and Application
Hydraulic relief valve consists of valve housing A, valve poppet B and spring C and other basic components, at least there are one inlet (P) port and one outlet (T) port. The main role of hydraulic pressure relief valve is essentially pressure limiting: to limited the working pressure is by pressure relief.
The load determines the pressure, the hydraulic relief valve only limits the pressure of the hydraulic system, but the pressure relief valve itself cannot generate pressure.

Basic Common Features Of Various Types Of Hydraulic Pressure Relief Valves:
1) When the inlet pressure reaches the pressure presetting point, the pressure relief valve will open and relief the pressure then.
2) The spring chamber is generally connected to the outlet port and the back pressure is equal to the outlet pressure. Therefore, in addition to the type of unloading outside, the pressure at outlet port is directly increased by 1:1 proportion. If you want the opening pressure to be completely independent of the outlet pressure, you should consider using a sequence valve.

Basic Functions Of Hydraulic Relief Valves:
A. As a safety valve used to limit the maximum pressure of hydraulic system, it plays the following role:
1. Avoid the high pressure damages to the hydraulic system, components and piping
2. Avoid hydraulic power source such as electric motors, diesel engines, or gasoline engines stops while operation due to heavy loads
3. Avoid excessive force/torque generated by the hydraulic cylinder or hydraulic motor, which may damage connected or pushed components; Avoid hydraulic cylinder or hydraulic motor with large inertia load, due to excessive inertial force during acceleration braking or torque, resulting in damage.
   In these operating conditions, the pressure relief valve is normally closed.

B. As pressure control valve, hydraulic pressurepressure relief valves maintain the pressure in whole hydraulic system or partial system within a specific level or range, for example:
1. Relief flow for the inlet and outlet throttling speed control circuit, which can be referred to as the constant pressure valve at this time
2. Generate the back pressure on the return oil to improve motion stability, which is often referred to as back pressure valve.
3. Unloading the pressure in double pump circuit or accumulator circuit, which is often referred to as pressure unloading valve.
In these conditions, the hydraulic pressure relief valve is normally open.

There are many types of hydraulic pressure relief valves, which can be classified from many different angles.
Classification according to valve poppet structure:
1) Ball type: The ball type relief valve is simple in structure and low in cost, but it is only suitable for small flow.
2) Poppet type: The poppet hydraulic pressurerelief valve can pass through a large flow, with less leakage, quick response and longer life, it is most widely used.
3) Spool type: The spool type relief valve can flow through a larger volume but with a smaller pressure range regulation.

Classification According To The Type Of Action
1) Direct acting pressure relief : The direct-acting type has fast response and small overshoot and is suitable as a safety valve to reduce the impact, but the pressure regulation deviation is large, that is, the control pressure fluctuates greatly with the flow rate.
2) Pilot operated pressure relief: The pressure regulation deviation of pilot type is small, control pressure accuracy is high, and it is used in occasions where pressure control is more accurate needed, but the response is slower
3) Soft pressure relief type : Soft pressure relief type will be able to relief the pressure before the pressure reaches to presetting value and surges in the inlet port.
The pressure relief valve in the constant rate pump with the open central spool directional control valve, or the (constant pressure) variable pump with the closed neutral directional control valve circuit generally requires a low leakage, fast response, anti-pollution, vibration reduction, can be considered using direct-acting type.
In the hydraulic constant rate pump throttle circuit, continuous flow supply and high-precision pressure control are generally required. When small amounts of internal leakage have little effect, pilot type pressure relief valves may be considered.

Classification According to the location of the application in the loop
1) Main hydraulic pressure relief valve, installed in the same direction of directional valve, the side of the hydraulic pump
2) Secondary hydraulic pressurerelief valve mounted after the directional valve, the side of driving device

Classification according to function

1) Normal type
2) Pressure-safe type. After opening, it will not be closed until the inlet pressure drops to zero. Not suitable for circuits that require load holding.
3) With reverse check valve type. The general relief valve does not pass in the reverse direction.
4)  Bi-directional type. In the hydraulic motor drive circuit, the pressure relief valve, or a two-way relief valve, should generally be installed on both sides to prevent excessive pressure on one side due to external loads during neutral conditions.
5) External control type. The opening pressure can be changed by an additional control type such as a hydraulic control, air control, a solenoid switch, or an electrical proportional control.
* There is also a so-called thermal protection relief valve. Actually, it is a small direct flow relief valve that is used as a safety valve. It starts to relief the pressure when the temperature rises causing thermal expansion of the closed liquid to protect the components (mainly hydraulic cylinders) from high pressure damage.
From the point of oil connection, ordinary types are two-way. The three-port and four-port ports are generally available for external control.

Differential Pressure Flow Characteristics Test For hydraulic pressureRelief Valve
(1) Test circuit
See the figure according to 1S003: and GB/105-

1. Hydraulic power source. It is required that the flow rate be smoothly adjustable throughout the test range, which is difficult to achieve with a single variable pump or a fixed pump with a speed control valve. Especially in the initial stage, because the required flow rate is very small, it is often below 0,1 L/min. Using multiple speed control valves in parallel, or bypass throttle valve can take in to consideration.
2. The pressure relief valve in the system is used for safety protection only. The pressure set value must be higher than the test range, but it does not exceed the allowable pressure of the valve under test.
4.Thermometer
5. Pressure sensor. 5a measures the inlet pressure. 5b measures the outlet pressure. If the outlet pipe is very short, thick, the pressure loss is negligibly small even can be omitted. Or only by a low pressure gauge is used for monitoring.
6. Tested valve
7. Flow sensor. Gear flow sensors may be considered here because they have a much larger measurement range than turbine flow sensors.
8. X-Y recorder, or digital oscilloscope, or computer data acquisition record display system.

Test Process Of Differential Pressure Flow Characteristics
Preparation stage:
Connect the X-Y recorder with flow qv7 as the X axis, differential pressure P5a-P5b or pressure P5a as the Y axis. Allow the oil temperature to reach the predetermined value.

Test procedure:
1. Turn on the hydraulic pressure source
2. The tested valve 6 is adjusted to the minimum value of the given pressure adjustment range.
3 start recording. Slowly increase the output flow of the hydraulic source from zero to the maximum test flow, then slowly decrease it to zero and stop recording. 4. The tested valve 6 is adjusted to the maximum value of the given pressure adjustment range and the steps are repeated step 3.
5. Set a few more values between the maximum and minimum values of the pressure adjustment range and repeat step 3.
*Keep the oil temperature relatively constant throughout the test. The test curve thus obtained is the valve&#;s characteristics under the operating conditions of the valve, which is typical test curve.

Transient Response Characteristics Test Of hydraulic pressurePressure Relief Valves
Test circuit: According to ISO03: and GB/T8m5-:

1. Hydraulic power source. It is best not to use a variable pump to avoid the dynamic response characteristics of the variable mechanism affecting the test results.
2. The system pressure relief valve is for safety protection only. The pressure presetting value must be significantly higher than the test range. Pressure relief should not occur during the test, otherwise it will reduce the system pressure rise gradient.
3. Bypass valve. Where 3a is used for pressure loading, it must be a quick-response valve, otherwise it will not be able to obtain a sufficient gradient of system pressure rise. 3b is used to control the pressure in the pilot chamber of an externally controlled relief valve. This part of the circuit must be modified accordingly with the type of valve being used.
4. Thermometer.
5. Pressure sensor. Response characteristics better than Hz
6. The valve being tested.
7. Flow meter. If the flow is known, it can be removed or moved to the return line of the tested valve in order to reduce the volume of the pipe.
8. Throttle valve. Set initial pressure.
9. Zero flow display. It is used to monitor whether the flow bypasses the test. It can be a measuring cup or an ordinary container.
10. Quick recorder. If digital, sampling time is shorter than 0.2ms.

Testing Process:
Preparation:
1. Connect the pressure sensor output P5 to the quick recorder.
2. Start the hydraulic power source. Allow the oil temperature to reach the predetermined value.
3. The tested valve 6 under is adjusted to a certain opening pressure.
4. Bypass valve 3a open. Using the throttle valve 8 to set the initial pressure, it should approach but not exceed the opening pressure of the valve 6 under test.
Test Procedure:
1. At the beginning of the test, the flow from the hydraulic power source 1, is directly drawn through the bypass valve 3a and the inlet pressure P5 of the valve under test is lower than the opening pressure and is closed by the tested valve 6 .

2. Quickly close the bypass valve 3a, P5 quickly rises, open the tested valve 6. After a while, P5 has stabilized. By recording the transient changes in pressure P5, the transient behavior of the tested valve will be able to be seen.

3. If the pressure in the pilot chamber of the tested valve is externally controlled, the bypass valve 3b can be quickly connected to make the measured 6 unloaded. P5 quickly decreased, which can measure the transient characteristics of the tested valve when unloading the pressure

In this section, you will learn about the various sub-categories of valves. This will give you an in-depth knowledge on what has been mentioned previously in chapters 3 and 4.

So, chapter 5 will focus mainly on the sub-categories of:

• Pressure control hydraulic valves
• Flow control hydraulic valves
• Directional control hydraulic valves

The main areas of focus will be the key applications and features of each sub-category. Moreover, it will also include either a video or image of every hydraulic valve.

With this, you&#;ll be able to choose the right valve for a specific application. The section will cover over 20 different types of hydraulic valves.

Now, let&#;s start with the:

5.1 Pressure Control Hydraulic Valve

As you can see, these are broad functions. With this information alone, it is nearly impossible to choose the right pressure control valve for a hydraulic system.

For sure, this can be a nightmare if you&#;re new to hydraulic systems. Therefore, in this section, you will learn about these 7 different types of pressure control valves:

• 5.1.1 Hydraulic relief valve
• 5.1.2 Pilot operated hydraulic relief valve
• 5.1.3 Spool type hydraulic relief valve
• 5.1.4 Hydraulic pressure reducing valve
• 5.1.5 Hydraulic sequence valve
• 5.1.6 Hydraulic load control valve
• 5.1.7 Hydraulic counterbalance valve

Remember, these are pressure control valves. They come in unique designs suitable for specific applications.

Let&#;s review each of these pressure control valves:

5.1.1 Hydraulic Relief Valve

Hydraulic relief valves play an integral role in hydraulic systems. They are the most commonly used pressure control valves.

In hydraulic systems, you can use these valves mainly for two major roles:

• Protect hydraulic circuits from overload
• Limit force or toque from rotary motors or linear actuators

These functions may vary depending on the design of a hydraulic system. However, this does not imply that the relief valves will have a completely unique design.

The internal design of all these valves is basically the same. Broadly, hydraulic valves have two sections.

These include a piston that is normally retained by springs and a pilot-valve or cover that controls the piston&#;s movement.

Normally, you can adjust the control range using screws.

This is an image of hydraulic relief valve. Image source: Hydraulic Valve.

To ensure that these relief valves remain effective in hydraulic systems, it is advisable to adopt an appropriate cleaning process. That is, you should reduce the operating pressure since this will help flush out any possible sludge deposits.

This implies that, operating hydraulic relief valves at reduced pressure removes sludge deposits.

In most cases, the complexity of relief valves in a hydraulic circuit will depend on the specific functions you intend to achieve. Still, the working principle will still remain the same.

Here is what you should expect:

You can connect a relief valve in a hydraulic system such that one port is in line with pressure line while the other port is connected to a reservoir. The ball will remain in position and this will be due to a force exerted on it by the spring.

You can change this by adjusting the hydraulic relief valve&#;s screw.

As the hydraulic system operates, the hydraulic relief valve will remain closed as long as the pressure remains below the minimum threshold to overcome the force exerted by the spring.

Whenever the pressure at the inlet exceeds the force exerted by the spring, the ball moves from its position. This opens the valve and the hydraulic fluid flows through the pressure line to the reservoir.

Normally, this diversion prevents increase in pressure in the hydraulic system. As the pressure at the inlets reset back to the initial value, the valve will close again.

You should note these key aspects here:

1. Cracking pressure: This is the pressure at which the hydraulic relief valve begins to function.
2. Full-flow pressure: It is the pressure at which the valve allows maximum fluid to pass through.

In short, before you choose this valve, it is important to understand the hydraulic fluid circuit structure and its function.

5.1.2 Pilot Operated Hydraulic Relief Valve

Under normal circumstances, you may have to choose between a direct acting valve and pilot-operated valves. But, what brings the difference?

A pilot operated valve mainly regulates the pressure. On the other hand, a direct-acting valve will prevent over pressure.

The way these two valves operate, may depend on their design. For now, let&#;s focus on pilot operated hydraulic valves.

Below is a video showing how a pilot operated relief valve works:

https://youtu.be/q5NV2gBsryk

The pilot operated hydraulic valve designs are such that they may regulate pressure based on two distinct factors:

• Use internal pressure setting to control pressure
• Depend on a remote circuit to regulate pressure

In most cases, they are a perfect choice in situations where multiple settings are required to control hydraulic fluid.

Normally, you can use a pilot operated hydraulic relief valve to unload the pump at low pressure. This practice is common in periods between work cycles.

5.1.3 Spool Type Hydraulic Relief Valve

These hydraulic relief valves have a spool located in their housing. Normally, there must be an actuation force to control the spool.

Spool type hydraulic relief valves have unique features such as:

• High switching power
• Low actuating force
• Robust control function

The working principle here is not different from that of the directional spool valves. Below is an example of spool type hydraulic relief valve:

This an example of a water hydraulic relief valve with a spool. Image source: The American Society of Mechanical Engineers.

In most cases, the spool type valves unload flow after the hydraulic system reaches a required pressure setting. The design of the spool type hydraulic valves guarantees a fast responding and reliable performance.

Whenever you are using a spool type hydraulic valve, it is important to take care of any possible leakages. You will find that operating a spool type hydraulic relief valve at high pressure may result in leakage losses.

The leakages are common around the spool. They depend on the following key aspects:

• Gap between the spool and the hydraulic relief valve housing
• Hydraulic system pressure
• Viscosity of the hydraulic fluid

In short, a good understanding of a hydraulic system will help you to choose the right spool type hydraulic relief valve.

5.1.4 Hydraulic Pressure Reducing Valve

A hydraulic pressure reducing valve is yet another important accessory in hydraulic systems. It makes redesigning the existing hydraulic systems cost competitive.

In hydraulic circuits, these valves limit pressure in brand circuits. At times, the average pressure in the main hydraulic system can be 750 psi, and the maximum pressure requirement in one of the branch-circuits is 250 psi.

This is a hydraulic pressure reducing valve. Image source: Valve Hydraulic.

In such situations, you will need a hydraulic pressure reducing valve in the system and it must be set to 250 psi. Like in other hydraulic valves discussed earlier, the valve will remain open as long as the pressure is not sufficient to overcome the force of the spring.

Otherwise, the valve will close partially or fully, depending on the amount of pressure.

5.1.5 Hydraulic Sequence Valve

These valves play an integral role in a sequence of operations in hydraulic circuits. That is, one action cannot take place before the other.

Definitely, all these actions must be related. Take for example, the landing gears of an aircraft.

The process is such that, the landing gear only begins to extend after the landing gear doors are fully open. Furthermore, the door only closes after the landing gears have fully retracted.

Here is a video illustrating how hydraulic sequence valves work in fork lifts:

https://www.youtube.com/watch?v=Hn8GKKVM-DQ

So, what makes hydraulic sequencing valves different from the other types of hydraulic relief valves?

Ideally, the working principle of sequence valves is similar to that of the relief valves. However, in the case of hydraulic sequence valves, after reaching the set pressure, it diverts the fluid to the next actuator to perform other tasks within the system.

You can learn more about hydraulic sequencing circuits by watching this video:

https://www.youtube.com/watch?v=nyrDHoAVihs

The sequence valves come in a wide range of shapes and designs. For instance, when it comes to hydraulic systems, you can choose either a mechanical or pressure control sequence valve.

It is important that you understand the order in which every operation should take place.

5.1.6 Hydraulic Load Control Valve

The load control valves play an integral role in the hydraulic systems. They come in a wide range of shapes and designs.

Hydraulic load control valves are mainly designed to prevent the load from running ahead of the hydraulic pump. That is, hydraulic load control valves provide a positive load holding.

Basically, they are special types of hydraulic pressure valves that prevent loads on either motors or cylinders from dropping in an uncontrollable manner.

They are common in both positioning, and moving systems. Some of the most common designs may include screw-in cartridge or inline housing valves.

This is a hydraulic load control system. Image source: Hydraulics and Pneumatics.

Depending on the intended application, you may go for the following types of hydraulic load control valves:

• Pilot vented or assisted load control valves
• Manifold mounted motor control valves
• Counter balance valves
• Load control valve cartridge

In short, there are many series depending on the specific hydraulic system requirements. At times, hydraulic valve manufacturers may decide to combine different functionalities into one valve.

That is, you may find a valve with three different functions such as: Pipe-rapture safety protection, safety valve and load control.

So, before you purchase a hydraulic load control valves, know all possible functionalities and specifications.

5.1.7 Hydraulic Counterbalance Valve

In hydraulic systems, you&#;ll find counter balance valves in applications that work with either suspended or overriding loads. The hydraulic counterbalance valves create a back pressure to prevent any possibility of losing control over the load.

They are designed such that they allow free flow in one direction and prevent flow of hydraulic fluid in another direction until a certain system pressure is reached.

These are two different designs of counterbalance valve.

The hydraulic counterbalance valves are also called the load holding valves. They are designed such that they can control the motion of the load in the following ways:

• In case of tube or hose failure, they prevent load from dropping
• Prevent the load from drift whenever there is a leakage in the spool of directional control valves.

The hydraulic counterbalance circuits come in a wide range of designs. This depends on the complexity of the hydraulic circuit and the specific tasks it should handle.

At this juncture, I am sure you can clearly distinguish the various pressure control valves available. As you may have realized, some of these valves may perform the same function in hydraulic system when modified accordingly.

Apart from these, another set of valves that play an important role in hydraulic systems are the flow control hydraulic valves. You will learn more about them in the next section of this hydraulic valve eBook.

5.2 Flow Control Hydraulic Valve

The flow control hydraulic valves regulate flow rate of hydraulic systems. As a result, you can perform the following key functions easily:

• Control the speed of an actuator
• Regulate rate of energy transfer in the hydraulic system

Ideally, a hydraulic flow control valve acts as a &#;fluid switch&#;.

These flow control hydraulic valves come in a wide range of designs and shapes, for various applications.

Some of the key applications may include material processing, conveyor systems, manufacturing, transport industries, etc.

Normally, a high quality flow control hydraulic valve is regarded as one that guarantee consistent flow irrespective of the load. That is, they can effectively limit the amount of fluid flow in hydraulic circuits.

Furthermore, a high quality flow control valve should reduce the effect temperature. This will also depend on the nature of the hydraulic valve design.

For more information, please visit Hydraulic pressure valve.

Furthermore, a high quality flow control valve should reduce the effect temperature. This will also depend on the nature of the hydraulic valve design.

By the end of this section, you should be able to choose an appropriate flow control valve for your application.

So, let&#;s start with:

5.2.1 Hydraulic Needle Valve

Hydraulic needle valves are valves with a needle-shaped plunger or long tapered point on one end of their stem. These are unique valves that are common in hydraulic systems with relatively low flow rates.

Due to their unique design and how they control fluid flow, you can use them as throttle valves. Ideally, these valves are suitable when controlling fluid flow in delicate components in a hydraulic system.

Below, is a video showing how hydraulic needle valves work:

https://www.youtube.com/watch?v=CzBB1CEAAS4

As you can see in the video, a tapered pin gradually opens and closes the conduit. This allows for fine and smooth flow of the hydraulic fluid.

Clearly, by turning the screw, you can comfortably regulate the fluid flow using a hydraulic needle valve.

But, where exactly can you use a hydraulic needle valve?

Needle valves are suitable for the following key applications:

• Control flow into delicate gauges; such components that may get damaged by a sudden surge in fluid pressure.
• Ending working cycle; a hydraulic system can slowly be stopped by adjusting the needle valve
• Common in metering applications
• Bleed valves in heating applications

Although a number of hydraulic valve manufacturers are trying to adopt other mechanisms of opening and closing these valves, most of them are manually operated.

Apart from the hydraulic needle valve, another essential valve is the:

5.2.2 Hydraulic Flow Divider

A hydraulic flow divider is a key component of a flow divider cartridge. They are available in different designs and configurations.

Depending on the nature of the application, you may use this hydraulic valve as a hydraulic flow divider or hydraulic combiner. Generally, you can represent hydraulic valve divider as:

This is a simple circuit of hydraulic divider valve circuit. Image source: Hydraulic & Pneumatics

Depending on the nature of the flow control requirements, you may opt for divider valves with varying pressures and flow rates. Furthermore, the complexity and dynamisms in the hydraulic circuits have led to the design of more efficient valves.

Examples of hydraulic applications where these valves play a critical role include: Flow partitioning, cylinder synchronization, transmission differentials and motor synchronization. Normally, advanced hydraulic flow divider valves have interlocking spools that help in pressure compensation.

Such hydraulic flow divider valves have a complex circuit diagram. Here is a good example:

Spool-type hydraulic flow divider valves

As you can see from the images above, the hydraulic fluid that passes through the valves splits into two. Moreover, from the circuit diagram above, you can easily tell the direction of hydraulic fluid flow.

In most cases, a spool type hydraulic flow divider valve does not allow for reverse flow. It is for this reason that spool type hydraulic divider valves used to synchronize cylinders features a check valve.

The check valves pass the reverse flow.

Apart from the hydraulic flow divider/combiner valve, another essential valve is the pressure compensated flow control valve.

5.2.3 Pressure Compensated Flow Control Valve

Are you operating a hydraulic system that requires constant flow with varying pressures and loads? Well, the modern pressure compensated flow control valves feature unmatched features such as:

• Almost no change flow with pressure variation
• Durable with reverse pressure function
• Tamperproof options

This makes pressure compensated flow control valves suitable for hydraulic power packs/units, accumulator bleed-off and motor/cylinder speed control.

Below is a diagram showing a pressure compensated flow control valve:

This is an image of pressure compensated flow control valves. Image source: Valve Hydraulic

So far, you&#;ve learned about flow control valves; but what makes pressure compensation an important aspect here?

The pressure compensated flow control valves feature a setting that allows for safe variations in pressure in the hydraulic system. This hydraulic valve eliminates undesired change in actuator speed with any change in load.

In short, the pressure compensated flow control valves are suitable in applications where hydraulic actuators should maintain a constant speed irrespective of any change in load or pressure.

5.3 Directional Control Hydraulic Valve

Directional control hydraulic valves are versatile components of hydraulic systems. They are uniquely designed to: Change direction, stop and allow hydraulic fluid flow.

Depending on the type of hydraulic circuits, they are available in simple designs such as the 2-way valve to those with complex structure such as the 4-way control valves in the double-acting actuators.

A directional hydraulic control valve

The directional hydraulic valves may have sophisticated control mechanisms that enhance performance and functionality. These may include either manually or solenoid controlled directional control valves.

Ideally, with the advancement in technology, you can adapt directional control valves to nearly any application. For instance, you may opt for poppet type, sliding spool or screw-in cartridge valves; just to mention a few.

The unique designs of these valves aim to reduce leakage in the hydraulic system while preventing load drop.

Below are different types of directional hydraulic valves you need to know:

5.3.1 Hydraulic Solenoid On/Off Valve

Hydraulic solenoid On/Off valve is an electromechanically controlled valve. They are controlled by an electric current that passes through the solenoid.

Solenoid valves are used in many applications such as hydraulic manifolds to achieve the desired performance.

These are a series of solenoid valves. Image source: Wikipedia

In certain hydraulic fluid applications, you may use a solenoid operated On/Off hydraulic valves. However, you need to deal with certain problems such as hysteresis and saturation.

The hydraulic solenoid On/Off valves are available in a wide range of shapes and designs where the solenoid may be activated by a pressure switch, electrical On/Off device or level probe.

For solenoid pilot valves, there is an option for manual operation.

5.3.2 Hydraulic Spool Valve

As you have learnt in the previous sections, there are many types of hydraulic valves. They come in a wide range of shapes and designs.

A good example is a hydraulic spool valve. In these valves, the position of the spool determines the direction where the oil will flow.

Some examples of hydraulic spool valves include:

• ON/OFF directional spool valves
• Proportional spool valve
• Valve combinations

The directional spool valves, are popular for low actuating force, a range of control functions and high switching power. Normally, the spool within the valve housing either connects or separates depending on the available actuating force.

This is an example of a directional spool valve. Image source: Valve University

For the scope of this section, we&#;ll discuss the following three types of spool valves:

• 3W2P hydraulic spool valve
• 4W2P hydraulic spool valve
• 4W3P hydraulic spool valve

So, let&#;s start with:

5.3.2.1 3W2P Hydraulic Spool Valves

A 3 way spool type solenoid valve is common in most hydraulic applications. Normally, you&#;ll find valves with a wide range of flow rates and pressures.

All these depend on specific designs of the 3 way valve. Can you remember how a 3 way 2 position directional control valve works?

I had discussed this earlier in this hydraulic valve eBook.

This is a complete diagram showing the working principle of 3W2P hydraulic valve. Image source: MathWorks

The only difference here is the hydraulic valve spool. You&#;ll learn more about the hydraulic valve parts in chapter 6 of this hydraulic valve eBook.

Under normal circumstances, the 3W 2P hydraulic spool valve designs ensure predictable and reliable performance even in minimal hysteresis or low pressure drop. This makes these 3 way 2 position valves a perfect choice for unloading or switching applications that do not require low poppet leakage.

Closely related to this is the:

5.3.2.1 4W2P Hydraulic Spool Valves

A 4 way 2 position hydraulic spool valve equipped with a solenoid offers a perfect and effective fluid flow control mechanism. They are suitable for both either actuation or reversing applications.

Depending on their structural design, you can use these valves for a range of flow rates and pressures.

This is a schematic drawing of a 4 way 2 position hydraulic valve.

The working principle of the 4W2P hydraulic spool valve is quite similar to those I had discussed earlier in this eBook.

5.3.2.2 4W3P Hydraulic Spool Valves

In case you have forgotten how a 4 way 3 position hydraulic valve works, here is a practical video of the same:

https://youtu.be/XiC6PXtDnTU

The 4W3P hydraulic spool valves are common in a number of complex hydraulic systems such as reverse flow control, actuator applications, pilot circuits and bi-directional motors, just to mention a few.

The working principle and specification is not different from the two hydraulic spool valves I have discussed above.

Now, at this juncture, all that remains is your ability to understand the basic parts of hydraulic valves and their respective functions. You will learn about this in chapter 5.

But before that, let&#;s have a quick overview of another critical component of the hydraulic circuit; the hydraulic check valve.

5.3.3 Hydraulic Check Valve

Hydraulic check valve is a simple and essential type of directional control valve. The design of these check valves is such that they allow hydraulic fluid to flow in one direction and stop flow in the opposite direction.

At times, they are also referred to as non-return valves. In most hydraulic fluid systems, they:

• Protect hydraulic systems from pressure surges
• Act as bypass or prefill valves
• May work as pre-tensioning valves

The available hydraulic valves are available in a range of designs and shapes. In most cases, these valves are spring-loaded with either a plate or a ball that prevents flow in the opposite direction.

One good thing about these valves is that they can isolate a hydraulic system without any leakages.

These are examples of check valves. Image source: Aeronautics Guide

These hydraulics check valves come in a wide range of shapes and designs. Also, they are made from different materials such as stainless steel, zinc plated carbon, etc.

Depending on the type of application at hand, you may choose:

• Hydraulic check valves with hardened seat
• Line rapture valve
• Shuttle valves that only allow for hydraulic fluid flow at high operating pressure

Apart from these three, there is another special type of check valve that is common in many applications. This is the pilot operated check valve.

5.3.4 Pilot-Operated Check Valves

A pilot operated check valve is a special type of valve that allows for fluid flow in two directions. Below is a standard symbol you can use to represent this valve in a hydraulic circuit.

This is a symbol of a pilot operated check valve. Image source: Hydraulic Valve

Normally, during the first phase, this valve allows fluid to flow in one direction like any other hydraulic check valve. However, in the reverse direction, a pilot pressure line will hold the valve open to allow for free fluid flow.

5.3.5 Pilot Logic Elements

Pilot logic elements are multi-purpose devices that perform a wide range of functions such as controlling pressure, direction and flow. Normally, these elements are used in cartridge valves to create a complex and functional hydraulic circuit.

Basically, these are cartridge valves you can conveniently install in hydraulic manifold blocks. With proper selection of pilot logic elements you can create a complex hydraulic circuit with optimal performance even at low pressure drop and high flow rates.

The pilot logic elements come in complex designs that may perform a range of functions such as:

• Sensing presence of load
• Bypass pressure in compensated pressure control
• Optimizing flow in areas of low pressure drop
• Designing energy efficient manifold blocks

This is a circuit diagram of a cartridge solenoid valve symbol.

There is a wide range of pilot logic elements you can incorporate in a hydraulic circuit to achieve unique performance requirements.

5.3.6 Lock Valves

Lock valves are available in a wide range of designs to lock hydraulic cylinders and sections of the circuit from possible damage and leakage. You can use lock valves whenever the directional control valve is in neutral position.

Some of the key functions of hydraulic lock valves include:

• Prevent load drop and drift
• Locks actuator and help relieve excess pressure
• Act an emergency manual release
• Prevent actuator cavitation and chatter

This is a complex circuit showing a hydraulic system with a manual lock system. Image source: Integrated Publishing

Again, hydraulic lock valves are available in a wide range of shapes and designs. You should choose one that fit your hydraulic system.

5.3.7 Hydraulic Servo Valve

A hydraulic servo valve is a device that monitors the flow of hydraulic oil in response to a specific command (electrical input signal) to control force, position, velocity or pressure in a system. You can classify these valves as closed-loop control devices and they can be single, two or three stages.

To achieve a precise control mechanism, the hydraulic servo valve uses a feedback control mechanism.

Above is an example of a two-stage servo valve. Image source: Hydraulic Valve

Normally, you can modify these systems to suit the desired application depending on the nature of the control system.

In summary, there are many types of hydraulic valves for a range of applications. Depending on the task at hand, you may choose either standard or custom hydraulic valves.

With all these in mind, now let&#;s discuss the various parts of hydraulic valve parts. This will give you an in-depth understanding of hydraulic valve design, hence, a high chance of choosing the right accessory.

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