HYDRAULICS VALVES

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1) Energy conversion

Diagram of a simple hydraulic pump-motor system

1 Reservoir, 2 hydraulic pump, 3 pressure gauge, 4 hydraulic motor (suitable for left and right hand rotation)

In fact every hydraulic system can be reduced to a simple pump-motor system as shown in the diagram

The hydraulic pump is driven bij an eclectic motor or a combustion engine. The hydraulic pump (2) sucks oil from the reservoir (1) and pumps the oil through the pipelines and hoses to the hydraulic motor (4). The hydraulic motor for example drives a winch. So the hydraulic pump converts mechanical energy into hydraulic energy (pressure and flow) and the hydraulic motor converts the hydraulic energy into mechanical energy again! From the exhaust side of the hydraulic motor the oil flows back to the reservoir. In the return line the pressure is almost zero! The pressure needed to drive the hydraulic motor can be read on the pressure gauge (3), and is determined by the resistance in the system. The most important resistance is the load to be driven by the hydraulic motor (4)! Lines and hoses also have a certain influence on the level of pressure.

The speed of the hydraulic motor is determined by its dimension (displacement) and by the flow that is pump into it.

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2) The axial piston pump

The axial piston pump with rotating swashplate.

In hydraulic systems with a working pressure above aprox. 250 bar the most used pump type is the piston pump. The pistons move parallel to the axis of the drive shaft. The swash plate is driven by the shaft and the angle of the swash plate determines the stroke of the piston. The valves are necessary to direct the flow in the right direction. This type of pump can be driven in both directions but cannot be used as a hydro motor.

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3) The vane pump

On many industrial installations with a maximum pressure of about 200 bar, vane pumps are applied. The advantage of vane pumps is the pulse free delivery and low level of noise. The shaft of the rotor with the radial mounted vanes is driven by an engine or motor. The stator ring is circular in form and is held in an eccentric position. The amount of eccentricity determines the displacement of the pump. When the amount of eccentricity is decreased to zero, the displacement of the pump becomes 0 cm3: from that moment on the pump doesn't deliver any oil.

Suction and delivery: The chambers between the vanes rotate with the rotor. At the suction side the chamber volume increases and the chamber is filled with oil from the suction line. At the pressure side the chamber volume decreases and the oil is forced into the pressure line.The pressure at the pressure side is determined by the resistance in the system. The most important resistance is the load on the hydraulic cylinder or hydraulic motor. In order to prevent cavitations, the pressure at the suction side of the pump should not exceed 0.1 to 0.2 bar (10 to 20 kPa) below atmospheric pressure (minimum absolute pressure: 0.8 bar or 80 kPa).

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4) The gearmotor

For simple systems with a relatively low level of pressure (about 140 to 180 bar or 14 to 18 MPa) the gear motor is the most used type of hydraulic motors. The gearmotor is a very simple, reliable, relatively cheap and less dirt sensitive hydraulic motor. In the animation you can see that the direction of rotation is determined by the direction of the oil flow. The pressure at the pressure side is determined by the load (torque) on the shaft of the hydraulic motor.

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5) Draining a hydraulic pump or motor

Draining a hydraulic pump or motor.

In hydraulic pumps and motors there always leaks oil from the pressure side into the housing. If this oil is not removed, a pressure will be built up inside the housing with the result that the shaft seal is pushed out of the housing! Therefore the prescribed maximum housing pressure (often 2 bar or 0.2 MPa) should not be exceeded. To prevent this problem hydraulic pumps and motors generally are equipped with a drain port. This drain port should be connected directly to the reservoir and the pump/motor should be mounted in that position that the drain port is at the upper side. This to assure that the housing is always kept filled with oil for greasing and cooling purposes. If the drain line has insufficient capacity, the pressure will increase and the shaft seal shall, as you can see in the animation, be pushed out of the housing as well.

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The pressure relief valve

Drawing and simulation of a direct operating pressure relief valve: left: valve closed; middle: symbol of a direct operating pressure relief valve according to ISO 1219; right: simulation of an operating pressure relief valve

Description:

The pressure relief valve is mounted at the pressure side of the hydraulic pump. It's task is to limit the pressure in the system on an acceptable value. In fact a pressure relief valve has the same construction as a spring operated check valve. When the system gets overloaded the pressure relief valve will open and the pump flow will be leaded directly into the hydraulic reservoir. The pressure in the system remains on the value determined by the spring on the pressure relief valve! In the pressure relief valve the pressure (=energy) will be converted into heat. For that reason longtime operation of the pressure relief valve should be avoided.

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The pilot operated pressure relief valve

The pilot operated pressure relief valve is applied in systems with a considerable amount of flow. Its task is to limit the pressure in the system on an acceptable value.

Description: The pilot valve is adjusted at 150 bar. The pressure below the main valve is equal to the pressure above the main valve, for example 100 bar (determined by the load on the hydraulic motor). The spring on the main valve (about 1 to 5 bar) keeps the valve in the closed position. As long as the pressure in the system does not increase the adjusted pressure, the pump flow goes to the hydraulic motor. When the hydraulic motor is overloaded, the pressure will increase and the pilot valve will open. From that moment on the pressure above the main valve is limited on 150 bars. However, the pump flow cannot be drained by the small throttle in the by-pass canal, so the pressure below the main valve will increase with the spring pressure of about 1 to 5 bar (the pressure below the main valve will increase to 151...155 bar). Then the main valve opens and the majority of the pump flow will be drained by the main valve.

The symbol of the pilot operated pressure relief valve according to NEN3348 and ISO 1219 (complete and simplified)

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The pilot operated pressure relief valve as an unloading valve

The pilot operated pressure relief valve can also be applied as an unloading valve.

Normally the 2/2-direction control valve is activated and the opening pressure of the main valve is determined by the pilot valve. If the 2/2-direction control valve is NOT activated the pressure at the upper side of the main valve will become zero. The pressure at the bottom side of the mains valve will open the main valve: the pressure needed to do this will be about 3 bar (almost zero). From that moment on the majority of the pump flow will be drained towards the reservoir by the main valve.

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The pressure relief valve in the motor circuit

The diagram shows a hydraulic motor circuit; the direction of rotation of the motor is determined by the position of the 4/3-direction control valve. In the central position of the valve all ports are closed. After activating the left side of the valve, the hydraulic motor starts rotating in the pointed direction.

Generally in hydraulic systems the moment of inertia of the driven load is of a considerable level, so at the moment the 4/3-direction control valve is pushed in the central position the hydraulic motor starts acting as a pump, driven by the load. This will cause a tremendous increase of pressure at the right side of the hydraulic 'motor' and if there was no safety valve, the weakest component would break down or explode! In this system however the pressure relief valve will open and the oil flows back to the left side of the hydraulic motor. Because of the pressure at the right side of the motor the speed of rotation decreases to 0 rpm.The hydraulic motor has an external leakage line so there will disappear oil from the motor circuit. This may cause cavitations at the left side of the motor. In this system however the circuit is protected against cavitations by the check valves (suction valves). The diagram on this page forms a basic diagram for most motor circuits.

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The direction control valve

With a direction control valve you determine the direction of the flow and therefore the direction of operation of a hydraulic motor or cylinder. In the animation we use a so called 4/3-direction control valve; the 4/3 comes from: 4 line connections and 3 positions.

The housing, commonly made of cast iron, with 4 line connections contains a spool of steel. This spool, which is kept in the centre of the housing by two springs, can shift in the housing. In the drawn position, the middle position, the P-port is closed so the pump flow has to flow to the reservoir through the pressure relief valve. This generates a lot of heat and should be avoided if possible. The A- en B-ports are closed as well so in this case a cylinder will be hydraulically locked in its position. By shifting the spool to the left the cylinder will make its outward stroke. The oil flows from Port P to A to the cylinder and the oil from the roadside of the cylinder flows via port B to T back to the reservoir.

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The flowcontrol

In order to control the velocity of a hydraulic motor or cylinder you have to control the flow. In this example the flow to the cylinder is controlled by a simple flow control. .

The pressure behind the flow control is determined by the load on the cylinder and is in this case 80 bar. De flow control is adjusted on a flow of 8 l/min. The hydraulic pump delivers 12 l/min so a part of the pump flow, 4 l/min flows through the pressure relief valve back to the reservoir. The pressure before the flow control is determined by the pressure relief valve, in this case 120 bar. The pressure drop in the flow control (40 bar) and in the pressure relief valve (120 bar) is converted into heat. This kind of flow control is relatively cheap but has low energy efficiency.

The pressure compensated flowcontrol

Controlling the velocity of a hydraulic cylinder by controlling the flow with a pressure compensated flow control

To control the velocity of a hydraulic motor or cylinder one has to control the flow to these components. This can be done with a simple flow controlThe flow through a flow control is determined by:a) The area of the flow control: a larger area means a higher amount of flow andb) the pressure drop across the flow control: an increase of the pressure drop means an increase of flow

The flow is also determined by the construction of the flow control and by the viscosity of the fluid, but these factors are neglected.

Example: in a system with a flow control the pressure at the pump side is determined by the pressure relief valve (see also flow control). When the pressure drop across the flow control decreases as a result of an increase of the load on the cylinder the flow and velocity of the cylinder will decrease. If the velocity has to remain constant and independent of the load one has to use a pressure compensated flow control

How does it work?

The pressure at the outlet of the pressure compensated flow control is determined by the load on the cylinder. The load is 50 bar and increases to 90 bar when the mouse cursor is put on the picture. The pump pressure, determined by the pressure relief valve is 120 bar.The pressure compensated flow control is adjusted on 10 l/min. The pump delivers 12 l/min: this means that the a flow of 4 l/min flows through the pressure control valve back to the reservoir. The pressure compensated flow control in fact has two parts: a flow control valve (the needle valve) and a pressure reducing valve or pressure compensator. The desired flow is adjusted with the needle

valve. The pressure compensator with spring loaded plunger at the left side measures the pressure at the inlet of the needle valve (p2). At the right side of the plunger the pressure of the load (p3) and of the spring are pushing the plunger to the left. The pressure of the spring is 8 bar. The plunger finds it's balance when:p2 = p3 + pspring ==> p2 - p3 = pspring and because of the fact that pspring= constant (8 bar) the pressure compensator keeps the pressure drop across the needle valve on a constant value of 8 bar. This means that the flow through the needle valve remains constant! When the load increases the pressure p3 increases and the plunger is out of balance and pushed to the left. Then the pressure p2 will increase as well and the plunger finds it's balance again. The pressure drop across the needle valve is still 8 bar so the flow remains 10 l/min and therefore the velocity of the cylinder remains constant and independent of the load!!

The pilot operated checkvalve

Picture of a pilot operated check valve; at the right: application of a pilot operated check valve on the cylinder of the outrigger legs of a crane

A pilot operated check valve is used to keep a part of the system free from internal leakage for example a hydraulic cylinder or motor. A very good example is the application of the pilot operated check valve on the cylinder of the outrigger legs of a crane. The cylinder is connected to port B of the check valve. When oil is supplied to port A the oil can flow freely towards port B and to the cylinder. When the leg has to be retracted oil is supplied to the roadside of the cylinder. The pressure at the roadside of the cylinder is used as pilot pressure on port Z for opening the check valve. Now the oil can flow back from port B to port A. The pressure at port Z needed to open the check valve against the cilinderpressure behind the main valve is about 1/3 to 1/10 (called the opening ratio) of the cilinderpressure.

The counterbalance valve

Cross section of a counterbalance valve

In fact a counterbalance valve is an improved pilot operated check valve. An important and major difference between these two valves is:

- The opening pressure of a pilot operated check valve depends on the pressure (applied by the load) behind the valve;

- The opening pressure of a counterbalance valve depends on the spring pressure behind the valve.The dynamic performance of a balance valve is many times better than the dynamic performance of a pilot operated check valve

The balance valve is applied as a 'brake valve' on relatively small crane systems in order to get a positive control on a hydraulic cylinder or motor with a negative load.

Functioning (see diagram):

When the left side of the 4/3-direction control valve is activated the cylinder will make its 'OUT-stroke'. The oil flows through the check valve which is integrated in the housing of the balance valve. In order to lower the cylinder, the right side of the 4/3-direction control valve has to be activated. From that moment on pressure is built up at the rod side of the cylinder. This pressure opens the balance valve and the oil at the bottom side of the cylinder flows through the balance valve and the direction control valve back to the reservoir.

As the load helps lowering the cylinder, the cylinder might go down faster than the oil is applied to the rod side of the cylinder (the cylinder isn't under control at that moment). However, the pressure at the

rod side of the cylinder and therefore the pilot pressure on the balance valve will decrease and the spring moves the balance valve to the direction 'close' as long as it finds a new 'balance'.

When the direction control valve is suddenly put in the middle position while lowering the loaded cylinder, the counterbalance valve closes immediately. This will cause an increase of pressure at the bottom side of the cylinder. However, the counterbalance valve will open at the adjusted pressure and thus protects the cylinder against overpressure!

The accumulator

Accumulators are used:

when the system needs a considerable flow during a short period; when the system or a part of the system has to be kept under pressure;

to accumulate peak pressure or pressure vibrations;

as a cushioning element.

In hydraulic systems the following types of accumulators are used:

the piston accumulator; animation (to supply oil; reliable; relatively slow accumulator as a result of friction between piston and cylinder)

the bladder accumulator (to supply oil; 'fast' accumulator)

the diaphragm accumulator (cushioning element; pressure compensator)

This example explains the functioning of the piston accumulator (animation) ; the functioning of the other types is similar to this one. At one side of the piston the accumulator is filled with nitrogen gas. The pressure of the gas at the gas side of the accumulator has to have a certain pressure, in this case 80 bar (8 MPa). This pressure, prescripted by the manufacturer of the system, has to be checked when there is no oil at the other side of the piston.

At the moment that the accumulator is filled with oil, the pressure at the oil side increases to the level of the gas pressure immediately. In the animation you can see this happen. For an appropriate functioning of the system, the gas pressure has to have the right value. The manufacturer prescribes how often the pressure has to be checked.

Watch out: accumulators accumulate hydraulic energy and therefore can be very dangerous, especially when you are not familiar with the system and accumulators!!

When repairing or modifying a hydraulic system be sure that the accumulator is drained and shut off as instructed by the manufacturer!

The cylinder with end position cushioning

When a cylinder reaches the end of the stroke the piston and rod are decelerated to standstill. The kinetic energy resulting from this must be absorbed by the end stop, the cylinder head or cylinder cap. Its capacity to absorb this energy depends on the elastic limit of the material. If the kinetic energy exceeds this limit the cylinder needs an external or internal end position cushioning. In this example we use an internal end position cushioning. When the piston with the cushioning bush travels into the

bore in the cylinder cap the fluid must exhaust from the piston chamber by means of the adjustable throttle valve. This throttle valve regulates the degree of cushioning.

The closed loop system with main pump on zero displacement

The closed loop system with main pump on zero displacement and electric motor is running

Closed loop system with main pump activated

The closed loop system with activated main pump

Cavitation

An undesired phenomenon in hydraulic system is cavitations. Most of the time cavitations occur in the suction part of the system. When cavitations takes place the pressure in the fluid decreases to a level below the ambient pressure thus forming 'vacuum holes' in the fluid. When the pressure increases, for example in the pump, these 'vacuum holes' implode. During this implosion the pressure increases tremendously and the temperature rises to about 1100 degrees Celsius. The high pressure in combination with the high temperature causes a lot of damage to the hydraulic components. A cavitations pump might be completely damaged in several hours and the wear parts may cause damage in the system.

cavitations can be caused by:

o acceleration of the oil flow behind a throttle or when the oil contains water or air o high fluid temperature

o a resistance in the suction part of the system

o a suction line which is to small in diameter

o a suction hose with a damaged inside liner

o a suction filter which is saturated with dirt (animation)

o high oil viscosity

o insufficient breezing of the reservoir

Compressibility of fluids

Many people think that a fluid is incompressible. However, fluids are, like any material, in a certain amount compressible. In calculations the amount of compressibility of fluid is considered to be 1 volume-% per 100 bars. This means that for example when there is fluid supplied to a 200 litre oil drum which already is completely filled with fluid (see animation), the pressure increases with 100 bar for each 2 litre of extra supplied fluid. When we supply 3 litre of extra oil the pressure increases with 150 bars. The compressibility of fluid plays a key role in for example fast hydraulic systems like servo-systems of a flight simulator. To obtain a maximum dynamic performance, the compressibility should be as less as possible. This is achieved by mounting the control valves directly on the hydraulic motor or cylinder. In that case the amount of fluid between the control valve and the motor/cylinder is as less as possible.