Hydraulic Pumps & Motors

Introduction of hydraulic

1. Subject – Oil Hydraulics & Pneumatics Topic - Hydraulic Pumps & Motors Presented By – Dhruv Shah

2. The function of a pump is to convert mechanical energy into hydraulic energy. It is the heart of any hydraulic system because it generates the force necessary to move the load. Mechanical energy is delivered to the pump using a prime mover such as an electric motor. Partial vacuum is created at the inlet due to the mechanical rotation of pump shaft. Vacuum permits atmospheric pressure to force the fluid through the inlet line and into the pump. The pump then pushes the fluid mechanically into the fluid power actuated devices such as a motor or a cylinder. Function of hydraulic pump

3. Pumps are classified into three different ways as follow … I. Classification based on displacement: 1. Non-positive displacement pumps 2.Positive displacement pumps II. Classification based on delivery: 1. Constant delivery pumps. 2. Variable delivery pumps. III. Classification based on motion: 1. Rotary pump. 2. Reciprocating pump.

4. I. Classification Based on Displacement 1. Non-Positive Displacement Pumps Non-positive displacement pumps are primarily velocity-type units that have a great deal of clearance between rotating and stationary parts. Non-displacement pumps are characterized by a high slip that increases as the back pressure increases, so that the outlet may be completely closed without damage to the pump or system. Non-positive pumps do not develop a high pressure but move a large volume of fluid at low pressures. They have essentially no suction lift. Because of large clearance space, these pumps are not self-priming.

5. 2. Positive Displacement Pumps I. Classification Based on Displacement conti … Positive displacement pumps, in contrast, have very little slips, are self-priming and pump against very high pressures, but their volumetric capacity is low. Positive displacement pumps have a very close clearance between rotating and stationary parts and hence are self-priming. Positive displacement pumps eject a fixed amount of fluid into the hydraulic system per revolution of the pump shaft. Such pumps are capable of overcoming the pressure resulting from mechanical loads on the system as well as the resistance of flow due to friction. This equipment must always be protected by relief valves to prevent damage to the pump or system. By far, a majority of fluid power pumps fall in this category, including gear, vane and piston pumps.

6. II. Classification Based on delivery 1. Constant Delivery Pumps Constant volume pumps always deliver the same quantity of fluid in a given time at the operating speed and temperature. These pumps are generally used with relatively simple machines, such as saws or drill presses or where a group of machines is operated with no specific relationship among their relative speeds. Power for reciprocating actuators is most often provided by constant volume pumps. 2. Variable Delivery Pumps The output of variable volume pumps may be varied either manually or automatically with no change in the input speed to the pump. Variable volume pumps are frequently used for rewinds, constant tension devices or where a group of separate drives has an integrated speed relationship such as a conveyor system or continuous processing equipment.

7. III. Classification Based on Motion This classification concerns the motion that may be either rotary or reciprocating. It was of greater importance when reciprocating pumps consisted only of a single or a few relatively large cylinders and the discharge had a large undesirable pulsation. Present-day reciprocating pumps differ very little from rotary pumps in either external appearance or the flow characteristics.

8. Differences between positive displacement pumps and non-positive displacement pumps Positive Displacement Pumps Non-positive Displacement Pumps 1. The flow rate does not change with head. 1. The flow rate decreases with head. 2. The flow rate is not much affected by the viscosity of fluid. 2. The flow rate decreases with the viscosity. 3. Efficiency is almost constant with head. 3. Efficiency increases with head at first and then decreases.

9. Gear Pumps Gear pumps are less expensive but limited to pressures below 140 bar.It is noisy in operation than either vane or piston pumps. Gear pumps are invariably of fixed displacement type, which means that the amount of fluid displaced for each revolution of the drive shaft is theoretically constant. 1. External Gear Pumps External gear pumps are the most popular hydraulic pumps in low-pressure ranges due to their long operating life, high efficiency and low cost. They are generally used in a simple machine. The most common form of external gear pump is shown in Figs. 1.3and 1.4 It consist of a pump housing in which a pair of precisely machined meshing gears runs with minimal radial and axial clearence . One of the gears, called a driver is driven by a prime mover. The driver drives another gear called a follower. As the teeth of the two gears separate, the fluid from the pump inlet gets trapped between the rotating gear cavities and pump housing.

10. The trapped fluid is then carried around the periphery of the pump casing and delivered to outlet port. The teeth of precisely meshed gears provide almost a perfect seal between the pump inlet and the pump outlet. When the outlet flow is resisted, pressure in the pump outlet chamber builds up rapidly and forces the gear diagonally outward against the pump inlet. When the system pressure increases, imbalance occurs. This imbalance increases mechanical friction and the bearing load of the two gears. Hence, the gear pumps are operated to the maximum pressure rating stated by the manufacturer. 1. External Gear Pumps conti …

11. 2. Internal Gear Pumps Another form of gear pump is the internal gear pump. They consist of two gears, An external gear and an internal gear. The crescent placed in between these acts as a seal between the suction and discharge.) When a pump operates, the external gear drives the internal gear and both gears rotate in the same direction. The fluid fills the cavities formed by the rotating teeth and the stationary crescent. Both the gears transport the fluid through the pump. The crescent seals the low-pressure pump inlet from the high-pressure pump outlet. The fluid volume is directly proportional to the degree of separation and these units may be reversed without difficulty.

12. 3. Ge -rotor Pumps Gerotor pumps operate in the same manner as internal gear pumps. The inner gear rotor is called a gerotor element. The gerotor element is driven by a prime mover and during the operation drives outer gear rotor around as they mesh together. The gerotor has one tooth less than the outer internal idler gear. Each tooth of the gerotor is always in sliding contact with the surface of the outer element. The teeth of the two elements engage at just one place to seal the pumping chambers from each other. On the right-hand side of the pump, pockets of increasing size are formed, while on the opposite side, pockets decrease in size. The pockets of increasing size are suction pockets and those of decreasing size are discharge pockets. Therefore, the intake side of the pump is on the right and discharge side on the left.

13. 4. Lobe Pumps The operation of lobe pump is similar to that of external gear pump, but they generally have a higher volumetric capacity per revolution. The output may be slightly greater pulsation because of the smaller number of meshing elements. Lobe pumps, unlike external gear pumps have both elements externally driven and neither element has any contact with the other. For this reason, they are quieter when compared to other types of gear pumps. Lobe contact is prevented by external timing gears located in the gearbox. Pump shaft support bearings are located in the gearbox, and because the bearings are out of the pumped liquid, pressure is limited by bearing location and shaft deflection. They do not lose efficiency with use. They are similar to external gear pumps with respect to the feature of reversibility.

14. 5. Screw Pumps These pumps have two or more gear-driven helical meshing screws in a closefitting case to develop the desired pressure. These screws mesh to form a fluid-type seal between the screws and casing. A two-screw pump consists of two parallel rotors with inter-meshing threads rotating in a closely machined casing. The driving screw and driven screw are connected by means of timing gears. When the screws turn, the space between the threads is divided into compartments. As the screws rotate, the inlet side of the pump is flooded with hydraulic fluid because of partial vacuum. When the screws turn in normal rotation, the fluid contained in these compartments is pushed uniformly along the axis toward the center of the pump, where the compartments discharge the fluid. Here the fluid does not rotate but moves linearly as a nut on threads.

15. 6. Vane Pumps There are two types of vane pumps: Unbalanced vane pump: Unbalanced vane pumps are of two varieties: i . Unbalanced vane pump with fixed delivery. ii. Unbalanced vane pump with pressure-compensated variable delivery. 2. Balanced vane pump.

16. i . Unbalanced Vane Pump with Fixed Delivery The main components of the pump are the cam surface and the rotor. The rotor contains radial slots splined to drive shaft. The rotor rotates inside the cam ring. Each radial slot contains a vane, which is free to slide in or out of the slots due to centrifugal force. The vane is designed to mate with surface of the cam ring as the rotor turns. The cam ring axis is offset to the drive shaft axis. When the rotor rotates, the centrifugal force pushes the vanes out against the surface of the cam ring. The vanes divide the space between the rotor and the cam ring into a series of small chambers. During the first half of the rotor rotation, the volume of these chambers increases, thereby causing a reduction of pressure. This is the suction process, which causes the fluid to flow through the inlet port. During the second half of rotor rotation, the cam ring pushes the vanes back into the slots and the trapped volume is reduced. This positively ejects the trapped fluid through the outlet port. In this pump, all pump action takes place in the chambers located on one side of the rotor and shaft, and so the pump is of an unbalanced design. The delivery rate of the pump depends on the eccentricity of the rotor with respect to the cam ring.

17. i . Fig. of Unbalanced Vane Pump with Fixed Delivery

18. ii. Balanced Vane Pump with Fixed Delivery A balanced vane pump is a very versatile design that has found widespread use in both industrial and mobile applications. The rotor and vanes are contained within a double eccentric cam ring and there are two inlet segments and two outlet segments during each revolution. This double pumping action not only gives a compact design, but also leads to another important advantage: although pressure forces acting on the rotor in the outlet area are high, the forces at the two outlet areas are equal and opposite, completely canceling each other.

19. Piston pumps are of the following two types: Axial piston pump : These pumps are of two designs: i . Bent - axis type piston pump. ii. Swash - plate-type piston pump. 2. Radial piston pump. 7. Piston Pumps

20. i . Bent-Axis-Type Piston Pump It contains a cylinder block rotating with a drive shaft. However, the centerline of the cylinder block is set at an offset angle relative to the centerline of the drive shaft. The cylinder block contains a number of pistons arranged along a circle. The piston rods are connected to the drive shaft flange by a ball and socket joints. The pistons are forced in and out of their bores as the distance between the drive shaft flange and cylinder block changes. A universal link connects the cylinder block to the drive shaft to provide alignment and positive drive. The volumetric displacement of the pump depends on the offset angle No flow is produced when the cylinder block is centerline. It can vary from 0 deg. to a maximum of about 30 degree. For a fixed displacement, units are usually provided with 23 deg. or 30 deg. offset angles.

21. ii. Swash-Plate-Type Piston Pump The cylinder block and drive shaft are located on the same centerline. The pistons are connected to a shoe plate that bears against an angled swash plate. As the cylinder rotates, the pistons reciprocate because the piston shoes follow the angled surface of the swash plate. The outlet and inlet ports are located in the valve plate so that the pistons pass the inlet as they are being pulled out and pass the outlet as they are being forced back in. This type of pump can also be designed to have a variable displacement cap ability. The maximum swash plate angle is limited to 17.5° by construction.

22. Pump Performance The performance of a pump is a function of the precision of its manufacture. An ideal pump is one having zero clearance between all mating parts. Because this is not possible, working clearances should be as small as possible while maintaining proper oil films for lubrication between rubbing parts. 1. Volumetric efficiency ( η v) : It is the ratio of actual flow rate of the pump to the theoretical flow rate of the pump. Volumetric efficiency ( η v) = Actual flow rate of the pump / / Theoretical flow rate of the pump 2. Mechanical efficiency ( η m) : It is the ratio of the pump output power assuming no leakage to actual power delivered to the pump. 3. Overall efficiency ( η o): It is defined as the ratio of actual power delivered by the pump to actual power delivered to the pump.

23. Function hydraulic motor Hydraulic motors are rotary actuators. However, the name rotary actuator is reserved for a particular type of unit that is limited in rotation to less than 360 degree. A hydraulic motor is a device which converts fluid power into rotary power or converts fluid pressure into torque. Classification of Hydraulic Motors 1. Gear motors. 2. Vane motors. 3. Piston motors: i . Axial piston-type motors. ii. Radial piston-type motors

24. 1. Gear Motors: A gear motor develops torque due to hydraulic pressure acting against the area of one tooth. There are two teeth trying to move the rotor in the proper direction, while one net tooth at the center mesh tries to move it in the opposite direction. In the design of a gear motor, one of the gears is keyed to an output shaft, while the other is simply an idler gear. Pressurized oil is sent to the inlet port of the motor. Pressure is then applied to the gear teeth, causing the gears and output shaft to rotate. The pressure builds until enough torque is generated to rotate the output shaft against the load.

25. 2. Vane Motors : There is an eccentric rotor carrying several spring or pressure-loaded vanes. Because the fluid flowing through the inlet port finds more area of vanes exposed in the upper half of the motor, it exerts more force on the upper vanes, and the rotor turns counterclockwise. Close tolerances are maintained between the vanes and ring to provide high efficiencies. The displacement of a vane hydraulic motor is a function of eccentricity. The radial load on the shaft bearing of an unbalanced vane motor is also large because all its inlet pressure is on one side of the rotor. The radial bearing load problem is eliminated in this design by using a double-lobed ring with diametrically opposite ports. Side force on one side of bearing is canceled by an equal and opposite force from the diametrically opposite pressure port. The like ports are generally connected internally so that only one inlet and one outlet port are brought outside. The balanced vane-type motor is reliable open-loop control motor but has more internal leakage than piston-type and therefore generally not used as a servo motor.

26. Piston Motors Piston motors are classified into the following types: 1. According to the piston of the cylinder block and the drive shaft, piston motors are classified as follows: i . Axial piston motors. ii. Radial piston motors. iii. Radial Piston Motors 2. According to the basis of displacement, piston motors are classified as follows: i . Fixed-displacement piston motors. ii. Variable-displacement piston motors.

27. i . Axial Piston Motors In axial piston motors, the piston reciprocates parallel to the axis of the cylinder block. These motors are available with both fixed-and variable-displacement feature types. They generate torque by pressure acting on the ends of pistons reciprocating inside a cylinder block. Drive shaft and cylinder block are centered on the same axis. Pressure acting on the ends of the piston generates a force against an angled swash plate. This causes the cylinder block to rotate with a torque that is proportional to the area of the pistons. The torque is also a function of the swash-plate angle. The inline piston motor is designed either as a fixed- or a variable-displacement unit. The swash plate determines the volumetric displacement.

28. ii. Bent-Axis Piston Motors This type of motor develops torque due to pressure acting on the reciprocating piston. In this motor, the cylinder block and drive shaft mount at an angel to each other so that the force is exerted on the drive shaft flange. Speed and torque depend on the angle between the cylinder block and the drive shaft. The larger the angle, the greater the displacement and torque, and the smaller the speed. This angle varies from 7.5° (minimum) to 30° (maximum). This type of motor is available in two types, namely fixed-displacement type and variable-displacement type.

29. iii. Radial Piston Motors In radial piston-type motors, the piston reciprocates radially or perpendicular to the axis of the output shaft. The basic principle of operation of the radial piton motors. Radial piston motors are low-speed high-torque motors which can address a multifarious problem in diverse power transfer applications.

30. Performance of Hydraulic Motors The performance of hydraulic motors depends upon many factors such as precision of their parts, tolerances between the mating parts, etc. Internal leakage between the inlet and outlet affects the volumetric efficiency. Friction between mating parts affects the mechanical efficiency of a hydraulic motor. 1. Volumetric efficiency : The volumetric efficiency of a hydraulic motor is the ratio of theoretical flow rate to actual flow rate required to achieve a particular speed. 2. Mechanical efficiency: The mechanical efficiency of a hydraulic motor is the ratio of actual work done to the theoretical work done per revolution. 3. Overall efficiency: The overall efficiency of a motor is the ratio of output power to input power of the motor.