HYDRAULICS 1. What is hydraulic technology? In the hydraulic technology we transmit and control forces and velocities by transmitting and controlling pressure and flow. In nearly every kind of technology we use hydraulic drive and control techniques. A few examples are: mechanical engineering car technology agriculture technology earthmoving and mining technology ship building technology offshore-technology aircraft and spacecraft technology The principles of hydraulic technology are not new. In the 18 Th. century in London a hydraulic press was built and the Eifeltower was adjusted by water hydraulic jacks. About 200 years BC the Greek already used machines that were driven by water hydraulics. Pascals law Hydraulic systems operate according to Pascals law. The law of Blaise Pascal says: 'The pressure, in a static hydraulic fluid in a closed system is everywhere the same'. However, when the velocity of the flow is constant one may apply Pascals Law as well. Animation: When the man jumps on the small piston, he induces a pressure in the system. This pressure also works on the large piston: because of the large area of this piston, the force induced by the pressure is capable to lift the car. The pressure can be calculated with the formula: where: • p = pressure (psi) • F = force (pound) • A = area (square inch) The gearpump For simple systems with a relatively low level of pressure (about 140 to 180 bar or 14 to 18 MPa) the gearpump is the most used type of pump. The gearpump is a very simple, reliable, relatively cheap and less dirt sensitive hydraulic pump. The pump in the picture is driven in the indicated direction. As the gears rotate and the teeth at the suction side come clear of the meshing point, a vacuum is created and oil flows into the spaces between the theeth. The oil in the chambers is transported to the pressure side of the pump. There the teeth mesh and the oil is forced out the spaces between the theeth into the output port of the pump. The meshing of the teeth prevents the oil flowing back from the pressure to the suction side of the pump. So the oil is transported from the suction side to the pressure side along the housing side of the gear wheels! The pressure at the pressure side is determined by the resistance in the system. The most important resistance is the load on the hydraulic cilinder or hydraulic motor. In order to prevent cavitation, 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 (minimim absolute pressure: 0.8 bar or 80 kPa). The axial piston pump The axial piston pump with rotating swashplate. In hydraulic systems with a workingpressure above aprox. 250 bar the most used pumptype is the pistonpump. The pistons move parallel to the axis of the drive shaft. The swashplate is driven by the shaft and the angle of the swashplate 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 hydromotor. The axial piston pump with variable displacement The axial piston pump with variable displacement The animation shows how the displacement of an axial piston pump can be adjusted. In this example we use an axial piston pump with a rotating cilinder barrel and a static' swashplate. The cilinder barrel is driven by the drive shaft which is guided through a hole in the swashplate. The position (angle) of the swashplate determines the stroke of the pistons and therefore the amount of displacement (cm3/omw) of the pump. By adjusting the position of the swashplate the amount of displacement can be changed. The more the swashplate turns to the vertical position, the more the amount of displacement decreases. In the vertical position the displacement is zero. In that case the pump may be driven but will not deliver any oil. Normally the swashplate is adjusted by a hydraulic cilinder built inside the pumphousing. 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 cilinder or hydraulic motor. In order to prevent cavitation, 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 (minimim absolute pressure: 0.8 bar or 80 kPa). The axial piston pump with rotating barrel The axial piston pump with rotating barrel. This axial piston pump consists of a non rotating swashplate (green) and a rotating barrel (light blue). The advantage of this construction is that the pump can operate without valves because the rotating barrel has a determined suck and pressure zone. The animation shows the behaviour of only one piston; normally this pump has 5, 7, 9 or 11 pistons. The rotating barrel shifts at the right side over a so called port plate (yellow) . This port plate is mounted and locked in the housing. View A-A shows the port plate. When the angle of the swash plate is adjustable, the pump has a variable displacement and in that case the pump is often provided with a pressure or flow control or a combination of both ('Load Sensing' and pressure 'cut off') . The pump in the animation can also be applied as a hydraulic motor. The gearmotor For simple systems with a relatively low level of pressure (about 140 to 180 bar or 14 to 18 MPa) the gearmotor 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 oilflow. The pressure at the pressure side is determined by the load (torque) on the shaft of the hydraulic motor. The radial pistonmotor Interactive Place your mouse pointer on the animation in order to change the direction of rotation of the hydraulic motor! Radial piston motors are primarily applied there where high torques at a low speed are required, for example for a winch drive. Because of the low output speed in many cases a gearbox is not necessary. The animation shows how this hydraulic motor operates. The connecting rods of the five radial mounted pistons are 'pushing' on the eccentric part of the central shaft. A rotating sleeve valve, which is driven by the central shaft, is taking care for the proper oil supply to/from the cylinders. By changing the direction of oil supply to the motor the direction of rotation can be changed. An other type of radial piston motor is the one with the Internal radial pistons The internal radial piston motor Just like the radial piston motor of the 'star type', the internal radial piston motor is applied there where high torques are required. Of this type of hydraulic motor there are motors available with a displacement of 300 liter/rotation and an output torque of more than 1,400,000 Nm! For example, they are used to drive winches, shredders, wheels, bucket wheels. The animation shows how this hydraulic motor operates. The barrel with the eight radial mounted pistons rotates over a fixed shaft which has the function of a sleeve valve. At the right moment a piston is pushed outwards and the roller which is connected to the piston, has to 'follow' the curved and fixed mounted ring. This results in a rotation of the barrel; the barrel is connected to the output shaft of the motor and drives the load. By changing the direction of oil supply to the motor the direction of rotation can be changed. The radial piston motor as a wheel motor This radial piston motor has a static barrel and a rotating housing. It works just like the radial piston motor with the rotating cilinder barrel. The rotating housing is connected to a wheel so in fact this construction represents a wheel with integrated hydraulic motor. The animation shows how this hydraulic motor operates. The barrel with the eight radial mounted pistons is fixed; the housing and the central sleeve valve rotate. The central sleeve valve takes care for the distribution of the oil. At the right moment a piston is pushed outwards and the roller which is connected to the piston, pushes the housing with the curved ring aside. This results in a rotation of the housing with the wheel. By changing the direction of oil supply to the motor the direction of rotation can be changed. The limited angle rotary actuator The limited angle rotary actuator is applied when the shaft has to rotate over a limited angle. The animation shows how this simple actuator works: in this case the shaft can rotate over an angle of about 270 degrees. This type of actuator is, among others, used as a rotator actuator on (small) cranes and excavators. 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 prescribted maximum housing pressure (often 2 bar or 0,2 MPa) should not be exceeded. To prevent this problem hydraulic pumps and motors generally are equiped with a drain port. This drainport should be connected directly to the reservoir and the pump/motor should be mounted in that position that the drainport 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 unsufficient 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. 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 pumpflow 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 coverted into heat.For that reason longtime operation of the pressure relief valve should be avoided. The pilot operated pressure relief valve The pilot operated pressure relief valve is applied in systems with a considerable amount of flow. It's 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 bar. 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) 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 maine 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 pumpflow will be drained towards the reservoir by the main valve. he 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 motorcircuit. This may cause cavitation at the left side of the motor. In this system however the circuit is protected against cavitation by the check valves (suction valves). The diagram on this page forms a basic diagram for most motor circuits. The direction control valve Symbol of a 4/3-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 cilinder. 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 pumpflow 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 hydraulicly locked in its position. By shifting the spool to the left the cilinder will make its outward stroke. The oil flows from Port P to A to the cilinder and the oil from the rodside of the cilinder flows via port B to T back to the reservoir. The flowcontrol Simulation of a simple system with a flowcontrol In order to control the velocity of a hydraulic motor or cilinder you have to control the flow. In this example the flow to the cilinder is controled by a simple flowcontrol. . The pressure behind the flowcontrol is determined by the load on the cilinder and is in this case 80 bar. De flowcontrol is adjusted on a flow of 8 l/min. The hydraulic pump delivers 12 l/min so a part of the pumpflow, 4 l/min flows through the pressure relief valve back to the reservoir. The pressure before the flowcontrol is determined by the pressure relief valve, in this case 120 bar. The pressure drop in the flowcontrol (40 bar) and in the pressure relief valve (120 bar) is converted into heat. This kind of flowcontrol is relatively cheap but has a low energy effeciency. 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 control The flow through a flow control is determined by: a) The area of the flow control: a larger area means a higher amount of flow and b) 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 checkvalve; at the right: application of a pilot operated checkvalve on the cilinder of the outrigger legs of a crane A pilot operated checkvalve is used to keep a part of the system free from internal leakage for example a hydraulic cilinder or motor. A very good example is the application of the pilot operated checkvalve on the cilinder of the outrigger legs of a crane. The cilinder is connected to port B of the checkvalve. When oil is supplied to port A the oil can flow freely towards port B and to the cilinder. When the leg has to be retracted oil is supplied to te rodside of the cilinder. The pressure at the rodside of the cilinder is used as pilot pressure on port Z for opening the checkvalve. Now the oil can flow back from port B to port A. The pressure at port Z needed to open the checkvalve 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 checkvalve . An important and major difference between these two valves is: - the opening pressure of a pilot operated checkvalve 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 balancevalve 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 checkvalve which is integrated in the housing of the balancevalve. 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 balancevalve and the oil at the bottom side of the cylinder flows through the balancevalve 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 balancevalve will decrease and the spring moves the balancevalve 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 nitrogyn 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 manufacterer 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 gaspressure 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! The cilinder with end position cushioning Animation of a cilinder with end position cushioning When a cilinder 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 cilinder head or cilinder cap. Its capacity to absorb this energy depends on the elastic limit of the material. If the kinetic energy exceeds this limit the cilinder 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 cilinder cap the fluid must exhaust from the piston chamber by means of the adjustable throttle valve. This throttle valve regulates the degree of cushioning Cavitation An undesired phenomena in hydraulic system is cavitation. Most of the time cavitation occurs in the suction part of the system. When cavitation takes place the pressure in the fluid decreases to a level below the ambient pressure thus forming 'vacuumholes' in the fluid. When the pressure increases, for example in the pump, these 'vacuumholes' implode. During this implosion the pressure increases tremendously and the temperature rises to about 1100 degrees Celcius. The high pressure in combination with the high temperature, causes a lot of damage to the hydraulic components. A cavitating pump might be completely damaged in several hours and the wear parts may cause damage in the system. cavitation 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 reservo 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 bar . 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 bar. 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. THE ESCHER CILINDER The image at the top shows an impossible construction called the 'Escher cilinder'. I named this cilinder after Maurits Escher (1898-1972) who was a Dutch artist in graphics; he is the designer of impossible constructions like on the image at the left.
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