Content

• The basic function of actuators.
• Types of actuators.
• Hydraulic cylinder types and their applications.
• Types of hydraulic motors and their applications.

The basic function of the actuator

The actuator is a converter or drives element in a hydraulic system. It is the component that converts hydraulic energy into mechanical energy. In any hydraulic system, our ultimate intention is to generate force and displacement at the user end. Hence, we use actuators in the hydraulic system. Let’s see different types of actuators.

Hydraulic actuator Characteristics

The characteristic of the hydraulic actuator or cylinder is to transmit the large force. But this has the limitation that the force can transmit only in its effective length and linear motion. Depending on its construction type, the hydraulic cylinders can transmit forces either in one or two directions.

People also read “Formula for converting cc/rev to LPM.“

Types of Actuator

1. Linear Motion Actuator or Cylinder
2. Rotary Motion Actuator or Motor

Linear Motion Actuator or Cylinder

A hydraulic cylinder or linear actuator is the component that converts hydraulic energy into linear motion (mechanical energy). A hydraulic cylinder or linear actuator works on the displacement principle. They convert the flow directed into the cylinder chamber into linear motion.

Hydraulic Actuator types and their application

1. Single Acting Hydraulic Cylinder
2. Double Acting Hydraulic Cylinder
3. Special Design Cylinder

Depending on the direction, a hydraulic cylinder or linear actuator can transmit force in one direction or two directions. If the force can transmit only in one direction then it is a Single-acting cylinder. Similarly, if the force can transmit in both the directions (i.e. to and fro motion) then it is a Double-acting cylinder.

The function of the Single Acting Actuator or Cylinder

In a single-acting cylinder, the hydraulic pressure built up only on one side of the piston or piston rod and converted into mechanical force. With pressure, the piston rod can thus extend or retract only in one direction. That is why it is referred to as a single-acting cylinder.

Types of single acting cylinder or actuator

1. Plunger cylinder: The plunger cylinder is sometimes called the trunk piston cylinder. Here the piston rod is used as a piston. For example, hydraulic jack. In a hydraulic jack pressurized fluid pushes the plunger out. But the piston retracts again under its curb weight after the pump is switched off.
2. Spring return hydraulic cylinder: The cylinder with return spring, the spring force pushes the piston back after the pump is switched off. although, Most small cylinders use a spring return mechanism. This type of single-acting cylinder can be used in a horizontal position where the piston cannot move from its weight to its initial position after the pump is stopped. The rod side spring presses the piston back when the pressure drops. Spring returns can be either push-type or pull-type depending on the cylinder design.
3. Single-acting cylinder without spring: The function of a single-acting cylinder without a spring is similar to a plunger cylinder. After the pump is stopped the piston returns to its original position due to its own weight or external forces.

Double Acting Actuator cylinder

In contrast to single-acting cylinders, double-acting cylinders have two effective piston surfaces (piston side and Piston-rod side). Therefore, they can extend as well as retract using hydraulic pressure.

Type of double acting cylinder

1. Differential double-acting cylinder
2. Synchronizing double-acting cylinder.

Differential double acting cylinder

In a differential double-acting cylinder, the two effective areas of the piston are of different sizes. Due to this difference in the piston area, there is also a difference in the force transmission and speed of the cylinder. In the above figure, when the piston extends, it can transfer larger force with lower speed than retraction. Hence, it is a differential double-acting cylinder. It is a very common and mostly used cylinder.

Synchronizing double-acting cylinder.

In a synchronizing double acting cylinder, there is the piston rod on both sides as shown above. The effective areas of the piston are identical on both sides and hence, the same speed and force transmission when the same working pressure applied during extension and retraction. Thus, for constant flow, piston stroke speed is identical in both directions.

Special Design Cylinder

1. Telescopic cylinder
2. Tandem cylinder
3. High-speed cylinder

Hydraulic Rotary Actuator or Hydraulic Motor

The function of a hydraulic motor or rotary actuator is to converts the hydraulic energy into rotary motion (mechanical energy). The working principle is the same as a hydraulic cylinder (i.e. Displacement principle). But the difference is that the hydraulic motor can give continue rotation regardless of stroke length.

Types of Hydraulic motor

1. Fix displacement motor
2. variable displacement motor

The hydraulic motor comes in very different designs. The selection of a motor depends on the purpose of the motor. Before selecting a hydraulic motor, the manufacturer’s datasheet must always be observed. A hydraulic motor can be a fixed displacement motor or a variable displacement motor. For example, external gear motor, internal gear motor, gerotor, vane motor are fixed displacement type. Whereas radial piston motor or axial piston motor can be fixed or variable both.

Difference Between Hydraulic Cylinder and Motor

Related Article

• Pressure reducing valve and pressure relief valve
• Hydraulic accumulator working principle
• Hydraulic Tank Design, Task & Type

Reference: Wikipedia, and author own experience.

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1. What is the pump’s basic purpose in a hydraulic system?

2. What are the different types of hydraulic displacement pump?

3. What is the most important design and how hydraulic displacement pump works?

What is the pump’s basic purpose in a hydraulic system?

To understand pump’s basic purpose, let’s see an example.

Here you can see an external gear pump. There are two gears, one is the driver and the other one is the driven gear. Also, there are two ports, one is the low-pressure port (suction) and the other is the high-pressure port (delivery).

The external gear pump is the simplest and easy to understand. There are many different pumps, but all of them operate on the same principle. They convert mechanical energy, rotational motion for example, into hydraulic energy, in other words into flow and pressure.

Simply, the purpose of the hydraulic displacement pump is to draw the liquid and creates a flow. But how the pump do that?

As you can see the gear wheels gather the liquid between two adjacent teeth and transport it. It flows into the area behind the gear wheels. The gear wheels closed up against each other so that the liquid cannot be pushed back. Hence, due to restriction in flow, the pump creates pressure.

It is important that the cross section of the pump’s suction side must larger than delivery side. Thus the danger of cavitation is reduced.

What are the different types of hydraulic displacement pump?

The types of hydraulic displacement pump are:

1. Positive Displacement pump
2. Non-positive displacement pump (or dynamic pump)

Other related articles

• Hydraulic system Designing Tips for Engineers
• formula for converting cc/rev to LPM.
• Difference between pressure relief valve and pressure reducing valve

References:

• Wikipedia

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For example, consider a system that only requires one pump to deliver oil to a double-acting cylinder. The designer may select a variable displacement piston pump over a fixed displacement gear pump. Because a variable displacement pump will de-stroke when no flow is required and therefore reduce power consumption and improve performance. Here we have listed many other factors that need consideration for designing a hydraulic system.

What are the factors for designing a hydraulic system?

Let us design a simple double-acting cylinder operation. For this simple hydraulic system, we need to answer the following questions.

1. What is the duty cycle of the actuator?
2. How to calculate maximum working pressure in the system?
3. What is the maximum flow required by the system?
4. How to calculate the required pump size (cc/rev)?
5. What should be the maximum rpm of the pump?
6. What elevation will the system be operating at?
7. ISO 4406:2017 cleanliness level?

1. What is the duty cycle of the actuator?

Answer: Defining the duty cycle of the actuator is very important for a hydraulic system. The duty cycle is the number of strokes per hour. If the duty cycle is very less (4-6 strokes per hour) than the cleanliness level (i.e. ISO 4406:1999) can not be achieved with a variable displacement pump. Because when there is no cylinder actuation, the pump will de-stroke and stop the oil flow. The only oil flow is from the pump’s casing. Moreover, any pump manufacturer does not recommend any restriction in case drain including filter.

So, the duty cycle of an actuator is important for selecting pump type and cleanliness of the hydraulic system.

2. How to calculate maximum working pressure in the system?

Answer: The maximum pressure depends on the two factors. First is the maximum force that the cylinder will handle and the second is cylinder bore size.

Once the required force is known along with the cylinder bore, the designer can calculate hydraulic pressure in the cylinder using the calculation shown below.

P = F/A

 P = Pressure (N/m2)
 F = Force(N)
 A = Area (m2)

Once working pressure at the cylinder is determined the designer will need to determine which type of hydraulic connection will be used. He must ensure that the working pressure of the connection meets or exceeds the pressure required at the cylinder.

3. What is the maximum flow required by the system?

To calculate the maximum flow required for the system, the designer will need to know the required extend and retract times for the cylinder. And also the diameter of bore, rod, and length of stroke.

4. What size of the pump should use?

The selection of the pump size depends on the flow required by the hydraulic system. A pump can be measure in cc/rev (cubic centimeter per revolution). Bigger the size of the pump means it can displace more liquid per revolution and hence more flow. And smaller the pump, lesser the flow.

5. What should be the maximum rpm of the pump?

Normally an electric motor or engine drives the pump. To achieve the desired rpm of the pump designer either use VVVF drive or gearbox. RPM of the pump is also a factor of flow calculation. Higher the rpm means higher the flow. Thus flow, size of the pump, and rpm of the pump are related to each other.

So, a hydraulic engineer or designer can select a smaller pump if the rpm of a pump can increase. Similarly, if the rpm of a pump is not in his control than, he has to increase the pump size to fulfill the flow demand in the hydraulic system.

6. What elevation will the system be operating at?

Once maximum pressure, flow, rpm, and pump size calculation has done, it is very important for a pump in a hydraulic system to meet the minimum inlet condition. For this, maximum elevation is specified by the designer at where the hydraulic system can work efficiently. For example:

Take a PVG-048 pump. At the speed of 2100 rpm, the minimum inlet pressure should be 6.2 psi. The atmospheric pressure at sea level is 14.7 psi which is higher than the recommended inlet pressure. But if the hydraulic system has to work on 25000 feet above the sea level ( atmospheric pressure will be only 5.45 psi), then there is a problem. It may damage the pump due to cavitation.

7. ISO 4406:2017 cleanliness level?

we have already discussed every factor as per the designing aspects. But the designer may have to consider all of the design if it does not meet the required cleanliness level.

The required ISO 4406:2017 (previously ISO 4406:1999) cleanliness level will depend on what type of components are using in the hydraulic system.  For instance, gear pumps are more tolerant to contamination than piston pumps.  Similarly on/off valves are more tolerant of contamination than proportional valves. The cleanliness recommendation for different components are showing below:

Conclusion

“How to design a hydraulic system” is a long and tricky process. Although here we tried to design a simple double-acting actuator circuit and discussed some basic considerations. If you are designing a complex hydraulic circuit with more than two operations, you have to consider many more factors that will play key roles in the circuit. Such as cooling of fluid, fluid viscosity, types of fluid used, tank size, etc.

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The formula for converting “cc/rev to LPM” is quite simple. But before knowing just one formula, the basic concept must be understood. cc/rev (centimeter cube per revolution) is a general term in a hydraulic system for the displacement capacity of a pump.¹ While LPM (liter per minute) is the flow rate of hydraulic fluid.

For converting cc/rev to LPM we need pump speed or in other words, the number of revolution of the pump in one minute.

cc/rev to LPM (centimeter cube per revolution to Liter per minute)

The diagram shows a fixed displacement pump having 90 cc/rev displacement capacity. Q indicates the flow rate of the pump and P indicates the power of the pump.

Now, suppose the pump is rotating with the speed of 2222 RPM (The pump can be powered by an engine, gearbox, or electric motor).

Flow = (Displacement x RPM)/1000

Flow is in Liter per minute (LPM), Displacement is cc per revolution while RPM is rotation per minute.

Q = (90 x 2222)/1000 = 200 l/min (approx).

Read More, for Basic of Hydraulic.

How to determine the displacement of a pump?

To determine the pump displacement you need the RPM and fluid flow rate (in LPM) of the pump.

Displacement (in cc) = (Flow x 1000)/RPM

If flow = 200 LPM & RPM = 2222 then,
Displacement of pump = (200 x 1000)/2222 = 90 cc/rev.

Definition

1. Displacement of the pump: The capacity of the pump to deliver or displace the liquid from one place to another in one complete rotation or stroke.

Abbreviation

• LPM: Liter Per Minute.
• RPM: Rotation Per Minute.
• CC: Centimeter Cube.
• CC/rev: Centimeter Cube per Revolution.
• l/min: Liter per Minute.
• kW: Kilo Watt.

Click to know some important formulas.

People also read

• Hydraulic accumulator working principle
• Pressure reducing valve and pressure relief valve

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Pressure reduction valve is a control valve that lowers the inlet pressure of the fluid and gives the desired pressure at the output. The pressure reduction valve maintains a constant pressure in a part of the system that operates at a pressure lower than the normal system pressure. Often we use a variable pressure reduction valve to set it to any desired downstream pressure within its design range. Once the valve is set, it will maintain reduced pressure regardless of changes in supply pressure and system load variations.

Types of Pressure Reduction Valve

There are numerous designs and types of pressure reducing valves. But most common are:

• The spring-loaded pressure reducing valve
• The pilot-operated pressure reducing valve

Spring Loaded Pressure Reducing Valves

Spring-loaded pressure reducing valve structure and function

The spring-loaded pressure-reducing valve is most commonly used in hydraulic and pneumatic systems. It is generally referred to as a pressure regulator. The valve is normally open all the time. The adjusting spring pressure acts against a diaphragm to open the valve.

When the pressure at the outlet is less than the spring pressure, the spring pressure overcomes the outlet pressure and pulls the valve stem downward. Thus the valve will remain open.

When the outlet pressure is greater than the spring pressure, outlet pressure overcomes the spring pressure and pushes diaphragm and stem upward. Thus the valve will be close.

We can adjust the outlet pressure of the valve by adjusting spring pressure on the diaphragm. Normally, when we turn the adjusting screw clockwise, we are increasing the downstream pressure.

Pilot-Operated Pressure Reducing Valve

The simple diagram above illustrates a pilot operated pressure reducing valve. This valve is similar to the spring-loaded valve in addition to the pilot-controlled valve. High pressure will come from the pump to the inlet port. Normally due to the open condition, the oil will flow to the outlet port. Actually, the pressure at the outlet depends on the spring pressure of the reducing valve.

When pressure builds up at the outlet it will act on the bottom of the main spool. Simultaneously, a pilot pressure acts on the ball against a spring-2 and spring chamber through an orifice. This pressure keeps the main spool open until the pressure at the outlet port is less than the pressure at Spring-2. If the outlet pressure is above the set pressure of spring 2, the ball will move forward and the pilot line will connect with the tank. This will empty the spring chamber and the main spool will move upward and close the inlet port. Thus, a pilot operated pressure reducing valve works.

Pressure reducing valve Symbols

References

• ScienceDirect

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The industrial hydraulic symbol or just the hydraulic symbol is the first step towards an understanding of the hydraulic system. Without a basic understanding of symbols, one cannot efficiently troubleshoot the hydraulic system completely. A hydraulic system consists of cylinders, motors, valves, and pumps connected via hydraulic hoses and pipes. Due to the complexity of showing these components in a diagram we use symbols instead of absolute components.

Industrial Hydraulic Symbol

Most hydraulic industries follow standard DIN ISO 1219 for the design of hydraulic circuits, however, one can find differences in individual drawings with the company.

The symbols in a hydraulic schematic give the following information about the system:

1. Type of Prime mover (Engine or Electric Motor)
2. General information about pump (CC/rev & RPM)
3. The number of connections and joints for the representation of the hydraulic flow.
4. Different types of valve and their functions.
5. The number of switching positions of the valves.
6. Actuation and return method of the valves.

Although we get all necessary information from schematic and symbols, it can define the following details:

1. The exact location of the components.
2. Dimensions of the component.
3. Manufacturer of the components.

Now, by keeping the intention of a hydraulic schematic in mind, one can understand the basic symbol for the hydraulic system very easily.

Basic hydraulic symbols

	Pressure line or Return line.
	Quick release coupling.
	Shuttle Valve.
	Pressure Gauge.
	Multiport Swivel.
	Manifold
	Line Junction.
	Connection for measuring port.

Symbols for actuation type

	Actuation by pneumatic (compressed air).
	Actuation by spring force.
	Actuation by Hydraulic pressure.
	Actuation by the plunger.
	Actuation by the roller movement.
	Actuation by solenoid (electrically)
	Actuation by electro-hydraulically.
	Actuation by foot paddle.
	Actuation by a lever.

Symbols for filters and coolers

	Filter
	Cooler

Symbols for Valves

	Pressure Relief Valve (non-adjustable)
	Pressure relief Valve (adjustable)
	Check Valve
	Pilot Operated Check Valve.
	Pressure reducing Valve
	3 Port Pressure Reducing Valve

Symbols for Pump and Motor

	Mono direction fix displacement pump
	Mono direction variable displacement pump
	Mono direction fix displacement motor
	Mono direction variable displacement motor
	Bi-directional fix displacement pump
	Bi-direction variable displacement pump
	Bi-direction fix displacement motor
	Bi-direction variable displacement motor
	Electric Motor

Symbols for Cylinders

	Single Acting Cylinder
	Double Acting Cylinder
	Telescopic Cylinder

FAQ

(Que.1) What is the hydraulic symbol for concrete pumps?

Answer: The symbol will be the same for all pumps. Concrete pumps are designed differently because it has to handle dense liquid or semi-solid materials, but the symbol will be similar. See the Symbol

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Hydraulics is the science of flowing and stationary fluid (liquid). It can be understand as the generation of force and motion by hydraulic fluid.

The word hydraulic comes from two Greek words “Hydor” which means water and “Aulos” which means pipe. Hydraulics definition may be as simple as it defines here but it has a very wide application.

Historical Development

Human already started the use of hydraulic in ancient time in the form of the water wheel at around 350 BCE. According to the author and historian Helaine Selin, there is evidence that ancient people from the Persian Empire were using the water wheel for millstones. The water wheels still are in use with today’s advanced technologies. Moreover, in 6 BCE human started irrigation system that played an important role for civilisation like Mesopotamia.

Development in hydraulics in the modern era

Blaise Pascal a French physician studied about the fluid hydrostatic nature and laid the foundation of industrial hydraulics in 1653. By using Pascal’s law British engineer Joseph Bramah invented hydraulic press in the year of 1795.

Later, William George Armstrong an English industrialist and engineer invented hydraulic accumulator in 1851. When hydraulic pressure was not available he used weighted accumulator for pressurised flow. Although he first used his accumulator in his hydraulic cranes, later it found in many applications.

In 1905 two American engineers Reynold Janney and Harvey D Williams first used mineral oil in the hydrostatic transmission in the axial piston design. In this way, the use of oil in the hydraulic system begins.

After oil hydraulic invented, The world of hydraulic grows very rapidly. We begin to use hydraulic in agriculture, construction like excavators, cranes etc, Industrial uses, in automobile, in aerospace and many more.

Now, because hydraulic are using every field, in order to classify the possibilities of uses we can divide it into two categories.

1. Industrial Hydraulics.
2. Mobile Hydraulics.

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In this article, you will learn about hydraulics flow control valve, basic jobs, the principle of working, construction and some type of hydraulic flow control valve.

Definition of a flow control valve

Definition: A hydraulic flow control valve is a hydraulic device which controls the flow rate by giving a definite amount of passage to the flow of hydraulic fluid. Basically, it controls the speed of actuators or complete operation in the hydraulic system. A hydraulics flow control valve has two main jobs.

1. Speed regulation
2. The equalisation of pressure fluctuation.

A hydraulics flow control valve is also called a flow restrictor valve because it restricts the flow and allows the fluid in the desired flow rate. For example,

In this hydraulic circuit, consider when we start the pump and position the direction control valve in position B what will happen? The cylinders piston extend rapidly. This rapid movement can cause jerking in the system and the function will not smooth. So, we need something which can control the rapid movement. Therefore, we use hydraulics flow control valve or simply a flow restrictor.

Figure 2 is the symbol for flow restrictor. Various kind of flow restrictors are available and according to that symbols also differs. Above figure represents a constant flow restrictor or flow control valve. Now, see the solution for the above-mentioned problem.

When you place a flow restrictor in the system it will reduce the cross-section of the pipe and allow less hydraulic oil flowing per unit time. Because of less quantity of hydraulic oil flowing per second to the actuator, the actuator moves slowly and you can control the speed.

Type of hydraulic flow control valve

1. onstant flow control valve
2. Adjustable flow control valve

Constant flow control valve

A constant flow control valve controls the flow rate but it will be fixed forever. We can not change the cross-sectional area and hence, we can not change the flow rate. In the above figure 3, The speed of the actuator will be fixed irrespective of load and if the load increased again operation will not smooth. So, as per changing a load the design also changes.

Adjustable flow control valve

An adjustable flow control valve is also controlling the flow rate of hydraulic fluid but we can change the cross-sectional area and hence, we can change the flow rate. If a load is increased then we can reduce the cross-sectional area and can change the speed of actuator and operation will be smoother. Figure 4 shows the symbol of a variable flow control valve.

Construction of Flow control valve

Flow control Valve can be designed in various ways but the basic designs are

1. Throttle type- It can be a constant or variable flow control.
2. Orifice type- it can only be a constant type.

In throttle type, there is an opening for flow rate limitation and it can be adjusted by a screw. But in orifice type, there is just an orifice which limits the flow rate and we can not change the size of an orifice.

Influence of pressure fluctuation on speed

With restrictor (flow control) valve either constant or variable type, minor pressure fluctuation can cause flow rate changes and speed of operation. For example,

Consider a rope winch driven by hydraulic power. The speed of winch is controlling by a variable flow restrictor. When a load is not changing or single user then speed will nearly constant but if additional users and also changing loads in the system can cause pressure fluctuations. when load increase in winch pressure will increase on the other hand addition of user (load in system) will decrease the pressure. So, pressure may change repeatedly in the system and also change the flow rate. The challenge is that the pressure in the system changes in an undefined way and not parallel. The motor will rotate with different speeds—and therefore the rope winch will too. But we do not want that. Ideally, a rope winch should always rotate at the same speed, rotational speed should remain constant even with pressure differences.

To achieve this, a pressure balance valve is added to a variable flow restrictor valve.

In this diagram, there will be some pressure drop due to flow restrictor when fluid is flowing. Let us consider the flow restrictor is preset at a particular pressure difference delta P( difference of pressure before and after restrictor valve). When the load at rope winch increase pressure will also increase after the flow restrictor and accordingly speed will effects.

Metering Throttle

See figure 8 and 9, the increased pressure above the restrictor valve called a metering throttle here, acts via the control line on the spring side of the pressure balance. The piston of the pressure balance extends toward the left and releases more flow volume. The pressure in front of the metering throttle increases, and the preset pressure difference here, delta p, is restored. The pressure fluctuation was thus balanced. And the outlet pressure of the metering throttle stays unchanged despite the pressure fluctuation. This is also called Pressure compensated flow control valve.

This is all about Flow control valve/flow regulator valve/flow restrictor valve or throttle valve. Hope you understood. Comment below if you have any queries.

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Most of us are very familiar with the pressure relief valve and pressure reducing valve. Both valves control the pressure in the hydraulic system. Since both are pressure controlling device still both are different from each other. Therefore, we will learn in this article how they differ with each other, where it can be used, and their construction. Also, we will see some special-purpose valves.

Pressure relief valve

Consider a simple hydraulic system without a relief valve for instance.

When the pump starts and does the work by lifting the load up to its maximum limit. But when it reaches its limit what will happen? The pump is still running and tries harder to push more oil in the system and ultimately pressure will increase tremendously and damage the weaker component. Therefore we need something which will limit the pressure in the system. This component is a pressure relief valve.

Pressure relief valve: A pressure relief valve is a hydraulic component that provides safety to the hydraulic system from excessive pressure. This is normally closed but when the pressure in the hydraulic system increase beyond the set limit of a pressure relief valve, it opens the path and allows hydraulic oil to flow into the tank. In this way, it releases the excessive pressure from the system and provides protection. As soon as pressure reduces in the system, the pressure relief valve comes in the initial position and closes the open path again.

As shown above, an arrow shows the closed path for hydraulic fluid and there is also spring tension which forces the relief valve to remain closed. But when the pressure in the hydraulic system is enough or more than relief valve spring tension, a pilot line will push the arrow against the spring tension and open the path. In this way, the pressure is relieved from the system and save the components.

Click to learn Industrial Hydraulic Symbols.

Construction of pressure relief valve

A pressure relief valve can be designed in many more ways but, here it is the most common type of pressure relief valve. It consists of two passages one is inlet port and the other is outlet port. Inlet port is connected with the pressurized hydraulic system while on the other hand outlet is connected with the tank.

As in above figure 4, you can see there is a valve seat which is normally closing the inlet and seat holder is forced against a spring. At the top, there is a screw which will set the spring tension or in other word set the pressure of the relief valve. Usually, the screw is protected with a cap so, the unauthorized person can not change the pressure setting and also it protects the screw from rusting.

Pressure Reducing Valve

Pressure reduction valves are used to limit the hydraulic pressure in a system. However, instead of reducing inlet pressure, they reduce the outlet pressure. It divides a hydraulic system into different sub-systems. For Example, If a hydraulic system there are numbers of operations, and each has its own capacity. As per every hydraulic component, there may be a different pressure requirement but with only one pump this cannot be achieved but with the use of pressure reducing valve it can easily achieve.

Pressure reducing valve is normally open valve and it allows the pressurized fluid to flow but when the pressure at the output of the valve is more than the valve pressure setting, a pilot pressure pushes the arrow down and the path will close. The valve will not allow pressurized fluid to flow in the system until the pressure drops lower than the valve pressure setting.

In the above example, there are two hydraulic actuators for clamp A and B which holds a workpiece. A pressure relief valve. A pressure reducing valve. And a direction control valve and positive displacement pump. The pressure relief valve setting is 300 bar. So, the maximum system pressure is 300 bar. When the direction control valve is in the first position both the clamp cylinders extend but in clamp A the pressure will be maximum as system pressure but in clamp B due to the pressure reducing valve setting, the pressure will be 200 bar. As soon as the pressure increases in the clamp B line the pressure reducing valve will close and not allow the pressurized fluid to flow. Hence, the pressure will limit at 200 bar.

For better understanding, you may read click here this article.

Construction of pressure reducing valve

A common pressure reducing valve consists of an inlet port (C), an outlet port (D), and a pilot line (E). There is a spool against the spring tension which opens and closes the path for pressurized fluid. As the pressure increase at the outlet port (D) beyond the spring tension (i.e. Pressure setting), pilot line (E) shift the spool against the spring and make the opening narrow. Due to the pressure difference between inlet and outlet this spool continuously maintains almost constant pressure.

FAQ about pressure relief valve and pressure reducing valve

1. What exactly happens if the pressure relief valve replaced by a reducing valve?

Answer: The function of a pressure relief valve is to limit the maximum pressure in a hydraulic system. It closes normally and opens only when the system pressure exceeds the setting of the pressure relief valve, by releasing pressurized oil to the lower pressure side or tank. Whereas on the other hand, the pressure reducing valve limits the pressure for the system to less than the maximum pressure. It opens normally and closes only when the system pressure at the output of the reducing valve exceeds its set pressure. But if we replace a pressure relief valve with a pressure reducing valve, the required pressure cannot be generated. Because there is no longer the necessary restriction to generate pressure and the flow will be directed to the tank due to their constructional difference. For more understanding Read hydraulic symbols.

Hope, you like this article and learned the basic of pressure relief valve and pressure reducing valve.

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In brief, the hydraulic direction control valve is a device to direct the flow of fluid in a hydraulic or pneumatic system. It makes the hydraulic system to work repetitive.

As shown in figure 1. the pump will make the fluid to flow and extend the piston and does work. But, after complete extension, we can not retract the piston which is not intended. Therefore, we need something which makes our system repetitive. This is Direction control valve or in short DCV.

In the above figure 2. a hydraulic direction control valve (DCV) is used. In the central position, the cylinder will stay in its place and valve will be called in 0 positions or in the rest position. However, if it changes the position from 0 to b cylinder rod will extend. Similarly, when direction control valve changes its position from b to a cylinder rod will retract. Thus Hydraulic direction control valve makes the system repetitive and useful and we can get benefited to continue work.

Graphical symbol

Although the graphical symbol of the direction control valve depends on the number of port and number of position, however, figure 3 shows a general idea of the symbols. Here the number of square boxes indicates the number of positions whereas arrows indicate the direction of flow through the valve. Similarly, Closed paths show close ports. A and B are the working ports. P is the port connecting with the pump while T is the port connecting to the tank. So, this is a 4/3 direction control valve with 4 ports and 3 positions.

Here are some symbols of various hydraulic direction control valve

2/2 Direction Control Valve

3/2 Direction Control Valve

3/3 Direction Control Valve

2/2 Direction Control Valve

4/3 Direction Control Valve

5/2 Direction Control Valve

5/3 Direction Control Valve

6/3 Direction Control Valve

Controlling the hydraulic direction control valve

The hydraulic direction control valve can be controlled in various ways. However here are some mentioned.

1. Mechanically controlled.
2. Electrically controlled.
3. Pneumatically controlled.
4. Hydraulically controlled.
5. Controlled by combination of any of above.

The direction control valve either can be directl controlled or can be pilot controlled.

Illustrated figure. 4 is the example of 4/3 spring centred lever operated direction control valve.

Illustrated figure. 5 is the example of 4/3 spring centred electrically operated direction control valve.

Illustrated figure. 6 is the example of 4/3 spring centred electro-pneumatic operated direction control valve.

Illustrated figure. 7 is the example of 4/3 spring centred electro-hydraulic operated direction control valve.

Illustrated figure. 8 is the example of 4/3 spring centred electro-pneumatic operated direction control valve.

Hydraulic direction control valve types

1. Spool type
2. rotary type

Spool type

Rotary type

Spooling Position of 4/3 hydraulic direction control valve

1. Open centre: In this 4/3 DCV all the ports are connecting with each other in centre position.

2. Close Center: In this 4/3 DCV all the ports are blocked in center position.

3. Float center: In this 4/3 DCV pressure port is block but working ports are connected with the tank in cente position.

4. Tandem Center: In this 4/3 DCV, pressure port and tank port are connected in center position but working ports are blocked.

So, this is the brief description of Hydraulic direction control valve. Hope you learned something.

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