Pumps

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Slide 1:

PUMPS Prepared by M.Ganesh Murugan Senior Training Officer

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PUMPS : Hydraulic Pumps convert mechanical energy from a prime mover (engine or electric motor) into hydraulic (pressure) energy. The pressure energy is used then to operate an actuator. Pumps push on a hydraulic fluid and create flow. Pump Classifications : All pumps create flow. They operate on the displacement principle. Pumps that discharge liquid in a continuous flow are nonpositive-displacement type. Pumps that discharge volumes of liquid separated by periods of no discharge are positive-displacement type.

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a) Nonpositive-Displacement Pumps. With this pump, the volume of liquid delivered for each cycle depends on the resistance offered to flow. A pump produces a force on the liquid that is constant for each particular speed of the pump. Resistance in a discharge line produces a force in the opposite direction. When these forces are equal, a liquid is in a state of equilibrium and does not flow. If the outlet of a nonpositive-displacement pump is completely closed, the discharge pressure will rise to the maximum for a pump operating at a maximum speed. A pump will churn a liquid and produce heat. Figure 3-1 shows a nonpositive-displacement pump. A water wheel picks up the fluid and moves it.

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b. Positive-Displacement Pumps. With this pump, a definite volume of liquid is delivered for each cycle of pump operation, regardless of resistance, as long as the capacity of the power unit driving a pump is not exceeded. If an outlet is completely closed, either the unit driving a pump will stall or something will break. Therefore, a positive-displacement-type pump requires a pressure regulator or pressure-relief valve in the system. Figure 3-2 shows a reciprocating-type, positive-displacement pump.

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Figure 3-3 shows another positive-displacement pump. This pump not only creates flow, but it also backs it up. A sealed case around the gear traps the fluid and holds it while it moves. As the fluid flows out of the other side, it is sealed against backup. This sealing is the positive part of displacement. Without it, the fluid could never overcome the resistance of the other parts in a system.

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c. Characteristics. The three contrasting characteristics in the operation of positive- and non positive-displacement pumps are as follows: Non positive-displacement pumps provide a smooth, continuous flow; positive displacement pumps have a pulse with each stroke or each time a pumping chamber opens to an outlet port. Pressure can reduce a non positive pump's delivery. High outlet pressure can stop any output; the liquid simply recirculates inside the pump. In a positive-displacement pump, pressure affects the output only to the extent that it increases internal leakage. Non positive-displacement pumps, with the inlets and outlets connected hydraulically, cannot create a vacuum sufficient for self-priming; they must be started with the inlet line full of liquid and free of air. Positive displacement pumps often are self-priming when started properly.

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Pumps are usually rated according to their volumetric output and pressure. Volumetric output (delivery rate or capacity) is the amount of liquid that a pump can deliver at its outlet port per unit of time at a given drive speed, usually expressed in GPM or cubic inches per minute. Changes in pump drive affect volumetric output, Pumps are sometimes rated according to displacement, that is the amount of liquid that they can deliver per cycle or cubic inches per revolution. Pressure is the force per unit area of a liquid, usually expressed in psi. (Most of the pressure in the hydraulic systems is created by resistance to flow.) The pressure developed in a system has an effect on the volumetric output of the pump supplying flow to a system. As pressure increases, volumetric output decreases. This drop in output is caused by an increase in internal leakage (slippage) from a pump's outlet side to its inlet side. Slippage is a measure of a pump's efficiency and usually is expressed in percent. Some slippage is designed into pumps for lubrication purposes. I f pressure increases, more flow will occur through a leakage path and less from an outlet port. Any increase in slippage is a loss of efficiency. PERFORMANCE

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a. External. Figure 3-6 shows the operating principle of an external gear pump. It consists of a driving gear and a driven gear enclosed in a closely fitted housing. The gears rotate in opposite directions and mesh at a point in the housing between the inlet and outlet ports. As the teeth of the two gears separate, a partial vacuum forms and draws liquid through an inlet port into chamber A. Liquid in chamber A is trapped between the teeth of the two gears and the housing so that it is carried through two separate paths around to chamber B. As the teeth again mesh, they produce a force that drives a liquid through an outlet port. GEAR PUMPS:

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b. Internal. Figure 3-7 shows an internal gear pump. The teeth of one gear project outward, while the teeth of the other gear project inward toward the center of the pump. The two gears mesh on one side of a pump chamber, between an inlet and the discharge. On the opposite side of the chamber, a crescent-shaped form stands in the space between the two gears to provide a close tolerance. The rotation of the internal gear by a shaft causes the external gear to rotate. Since the two are in mesh. Everything in the chamber rotates except the crescent, causing a liquid to be trapped in the gear spaces as they pass the crescent. Liquid is carried from an inlet to the discharge, where it is forced out of a pump by the gears meshing. As liquid is carried away from an inlet side of a pump, the pressure is diminished, and liquid is forced in from the supply source. The size of the crescent that separates the internal and external gears determines the volume delivery of this pump. A small crescent allows more volume of a liquid per revolution than a larger crescent.

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c. Lobe. Figure 3-8 shows a lobe pump. It differs from other gear pumps because it uses lobed elements instead of gears. The element drive also differs in a lobe pump. In a gear pump, one gear drives the other. In a lobe pump, both elements are driven through suitable external gearing.

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In a vane-type pump, a slotted rotor splined to a drive shaft rotates between closely fitted side plates that are inside of an elliptical- or circular-shaped ring. Polished, hardened vanes slide in and out of the rotor slots and follow the ring contour by centrifugal force. Pumping chambers are formed between succeeding vanes, carrying oil from the inlet to the outlet. A partial vacuum is created at the inlet as the space between vanes increases. The oil is squeezed out at the outlet as the pumping chamber's size decreases. T he normal wear points in a vane pump are the vane tips and a ring's surface, the vanes and ring are specially hardened and ground. A vane pump is the only design that has automatic wear compensation built in. As wear occurs, the vanes simply slide farther out of the rotor slots and continue to follow a ring's contour. Thus efficiency remains high throughout the life of the pump. VANE PUMPS: Unbalanced Vane Pumps : U nbalanced design, (Figure 3-9), a cam ring's shape is a true circle that is on a different centerline from a rotor's. Pump displacement depends on how far a rotor and ring are eccentric. The advantage of a true-circle ring is that control can be applied to vary the eccentricity and thus vary the displacement. A disadvantage is that an unbalanced pressure at the outlet is effective against a small area of the rotor's edge, imposing side loads on the shaft.

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Balanced Vane Pumps. In the balanced design (Figure 3-10), a pump has a stationary, elliptical cam ring and two sets of internal ports. A pumping chamber is formed between any two vanes twice in each revolution. The two inlets and outlets are 180 degrees apart. Back pressures against the edges of a rotor cancel each other. Recent design improvements that allow high operating speeds and pressures have made this pump the most universal in the mobile-equipment field.

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Vane-type double pumps : (Figure 3-11) consist of two separate pumping devices. Each is contained in its own respective housing, mounted in tandem, and driven by a common shaft. Each pump also has its own inlet and outlet ports, which may be combined by using manifolds or piping. Design variations are available in which both cartridges are contained within one body. An additional pump is sometimes attached to the head end to supply auxiliary flow requirements. Double pumps may be used to provide fluid flow for two separate circuits or combined for flow requirements for a single circuit. Separate circuits require separate pressure controls to limit maximum pressure in each circuit.

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TWO-STAGE PUMPS: Two-stage pumps consist of two separate pump assemblies contained in one housing. The pump assemblies are connected so that flow from the outlet of one is directed internally to the inlet of the other. Single inlet and outlet ports are used for system connections. In construction, the pumps consist of separate pumping cartridges driven by a common drive shaft contained in one housing. A dividing valve is used to equalize the pressure load on each stage and correct for minor flow differences from either cartridge.

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PISTON PUMPS: Piston pumps are either radial or axial. a. Radial. In a radial piston pump (Figure 3-14), the pistons are arranged like wheel spokes in a short cylindrical block. A drive shaft, which is inside a circular housing, rotates a cylinder block. The block turns on a stationary pintle that contains the inlet and outlet ports. As a cylinder block turns, centrifugal force slings the pistons, which follow a circular housing. A housing's centerline is offset from a cylinder block's centerline. The amount of eccentricity between the two determines a piston stroke and, therefore, a pump's displacement. Controls can be applied to change a housing's location and thereby vary a pump's delivery from zero to maximum.

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Figure 3-15 shows a nine-piston, radial piston pump. When a pump has an uneven number of pistons, no more than one piston is completely blocked by a pintle at one time, which reduces flow pulsations. With an even number of pistons spaced around a cylinder block, two pistons could be blocked by a pintle at the same time. If this happens, three pistons would discharge at one time and four at another time, and pulsations would occur in the flow. A pintle, a cylinder block, the pistons, a rotor, and a drive shaft constitute the main working parts of a pump.

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INTERNAL RADIAL PISTON MOTOR: 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. By changing the direction of oil supply to the motor the direction of rotation can be changed.

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THE RADIAL PISTON MOTOR AS A WHEEL MOTOR: 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. By changing the direction of oil supply to the motor the direction of rotation can be changed.

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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:

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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 pump in the animation can also be applied as a hydraulic motor.

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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 c y linder barrel and a static' swashplate. The c y linder 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 c y linder built inside the pumphousing. THE AXIAL PISTON PUMP WITH VARIABLE DISPLACEMENT:

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BENT-AXIS AXIAL PISTON PUMP: Pumping action is the same as an in-line pump. The angle of offset determines a pump's displacement, just as the swash plate's angle determines an in-line pump's displacement. In fixed-delivery pumps, the angle is constant. In variable models, a yoke mounted on pintles swings a cylinder block to vary displacement. Flow direction can be reversed with appropriate controls.

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PUMP OPERATION : The following graphs address some of the problems that could occur when a pump is operating: a. Overloading. One risk of overloading is the danger of excess torque on a drive shaft. ( You may need a larger pump ) b. Excess Speed. Running a pump at too high a speed causes loss of lubrication, which can cause early failure. Excess speed also runs a risk of damage from cavitation. ( use a higher displacement pump )

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c . Operating Problems. There are common operating problems in a pump. (1) Pressure Loss. Pressure loss means that there is a high leakage path in a system.(relief valve, cylinders, motors , & A badly worn pump). (2) Slow Operation. This can be caused by a worn pump or by a partial oil leak in a system. Pressure will not drop, however, if a load moves at all. Therefore, hp is still being used and is being converted into heat at a leakage point. (3) No Delivery. If oil is not being pumped, a pump- Could be assembled incorrectly. Could be driven in the wrong direction. Has not been primed. The reasons for no prime are usually improper start-up, inlet restrictions, or low oil level in a reservoir. Has a broken drive shaft. (4) Noise. If you hear any unusual noise, shut down a pump immediately. Cavitation noise is caused by a restriction in an inlet line, a dirty inlet filter, or too high a drive speed. Air in a system also causes noise. N oise can be caused by worn or damaged parts, which will spread harmful particles through a system, causing more damage if an operation continues.

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d. Cavitation. Cavitation occurs where available fluid does not fill an existing space. 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 . cavitation can be caused by: acceleration of the oil flow behind a throttle / when the oil contains water or air high fluid temperature a resistance in the suction part of the system a suction line which is to small in diameter a suction hose with a damaged inside liner a suction filter which is saturated with dirt (animation) high oil viscosity insufficient breezing of the reservoir

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Thank you M.Ganesh Murugan 9715447621

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