CHAPTER 3

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.

3-1. Pump Classifications. All pumps create flow. They operate on the displacement principle. Fluid is taken in and displaced to another point. 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.

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.
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.
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.
c. Characteristics. The three contrasting characteristics in the operation of positive- and nonpositive-displacement pumps are as follows:

3-2. Performance. 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. Because 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 covered in this manual is created by resistance to flow.) Resistance is usually caused by a restriction or obstruction in a path or 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 pumps have greater internal slippage than others; some pumps are rated in terms of volumetric output at a given pressure.

3-3. Displacement. Displacement is the amount of liquid transferred from a pump's inlet to its outlet in one revolution or cycle. In a rotary pump, displacement is expressed in cubic inches per revolution and in a reciprocating pump in cubic inches per cycle. If a pump has more than one pumping chamber, its displacement is equal to the displacement of one chamber multiplied by the number of chambers. Displacement is either fixed or variable.

a. Fixed-Displacement Pump. In this pump, the GPM output can be changed only by varying the drive speed. The pump can be used in an open-center system-a pump's output has a free-flow path back to a reservoir in the neutral condition of a circuit.
b. Variable-Displacement Pump. In this pump, pumping-chamber sizes can be changed. The GPM delivery can be changed by moving the displacement control, changing the drive speed, or doing both. The pump can be used in a closed-center system-a pump continues to operate against a load in the neutral condition.

3-4. Slippage. Slippage is oil leaking from a pressure outlet to a low-pressure area or back to an inlet. A drain passage allows leaking oil to return to an inlet or a reservoir. Some slippage is designed into pumps for lubrication purposes. Slippage will increase with pressure and as a pump begins to wear. Oil flow through a given orifice size depends on the pressure drip. An internal leakage path is the same as an orifice. Therefore, if 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.

3-5. Designs. In most rotary hydraulic pumps (Figure 3-3), the design is such that the pumping chambers increase in size at the inlet, thereby creating a vacuum. The chambers then decrease in size at the outlet to push fluid into a system. The vacuum at the inlet is used to create a pressure difference so that fluid will flow from a reservoir to a pump. However, in many systems, an inlet is charged or supercharged; that is, a positive pressure rather than a vacuum is created by a pressurized reservoir, a head of fluid above the inlet, or even a low-pressure-charging pump. The essentials of any hydraulic pump are-

3-6. Gear Pumps. Gear pumps are external, internal, or lobe types.

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. Both sets of teeth project outward from the center of the gears. 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.
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. One gear wheel stands inside the other. This type of gear can rotate, or be rotated by, a suitably constructed companion gear. An external gear is directly attached to the drive shaft of a pump and is placed off-center in relation to an internal gear. 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.
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.

3-7. Vane Pumps. 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.

Because the 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.
a. Characteristics. Displacement of a vane-type pump depends on the width of the ring and rotor and the throw of the cam ring. Interchangeable rings are designed so a basic pump converts to several displacements. Balanced design vane pumps all are fixed displacement. An unbalanced design can be built in either a fixed- or variable-displacement pump. Vane pumps have good efficiency and durability if operated in a clean system using the correct oil. They cover the low to medium-high pressure, capacity, and speed ranges. Package size in relation to output is small. A vane pump is generally quiet, but will whine at high speeds.
b. Unbalanced Vane Pumps. In the unbalanced 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. Thus there is a limit on a pump's size unless very large hearings and heavy supports are used.
c. 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.
d. Double Pumps. 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. Combining pump deliveries does not alter the maximum pressure rating of either cartridge. Separate circuits require separate pressure controls to limit maximum pressure in each circuit.
Figure 3-12, shows an installation in which double pumps are used to provide fluid flow for operation of a cylinder in rapid advance and feed. In circuit B, two relief valves are used to control pumping operation. In circuit A, one relief valve and one unloading valve are used to control pumping operations. In both circuits, the deliveries of the pump cartridges are combined after passing through the valves. This combined flow is directed to a four-way valve and to the rest of the circuit.
In circuit B, an upper relief valve is vented when a cylinder rod reaches and trips a pilot valve. A vented relief valve directs the delivery of a shaft-end pump cartridge freely back to a tank. Another relief valve controls the maximum pressure of a circuit. An unloading valve and a relief valve in circuit A do the same operation. The output of both pump cartridges combines to supply fluid for a rapid advance portion of a cycle. When the output of one circuit returns to the tank, after reaching a certain point in the cycle, the other circuit completes the advance portion of a cycle. Both pump outputs are then combined for rapid return.
e. 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.
In operation, developing fluid flow for each cartridge is the same as for single pumps. Figure 3-13 shows fluid flow in a vane-type, two-stage pump. Oil from a reservoir enters a pump's inlet port and passes to the outlets of the first-stage pump cartridge. (Passages in a pump's body carry the discharge from this stage to an inlet of the second stage.) Outlet passages in the second stage direct the oil to an outlet port of the pump. Passage U connects both chambers on the inlet side of a second-stage pump and assures equal pressure in both chambers. (Pressures are those that are imposed on a pump from external sources.)
A dividing valve (see Figure 3-13) consists of sliding pistons A and B. Piston A is exposed to outlet pressure through passage V. Piston B is exposed to the pressure between stages through passage W. The pistons respond to maintain a pressure load on a first-stage pump equal to half the outlet pressure at a second-stage pump. If the discharge from the first stage exceeds the volume that can be accepted at the second stage, a pressure rise occurs in passage W. The unbalanced force acting on piston B causes the pistons to move in such a manner that excess oil flows past piston B through passage Y to the inlet chamber of a first-stage cartridge. Fluid throttling across piston B in this manner maintains pressure in passage V.
If the discharge from a first-stage pump is less than the volume required at a secondstage pump, a reduced pressure occurs at piston B. An unbalanced force acting on piston A causes the pistons to move so that oil flows past piston A into passages X and W to replenish a second-stage pump and correct the unbalanced condition. Passages Z and Y provide a means for leakage around the pistons to return to the inlet chamber of a first-stage pump. Pistons A and B always seek a position that equally divides the load between the two pumping units.

3-8. 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.
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.
(1) Pintle. A pintle is a round bar that serves as a stationary shaft around which a cylinder block turns. A pintle shaft (Figure 3-16) has four holes bored from one end lengthwise through part of its length. Two holes serve as an intake and two as a discharge. Two slots are cut in a side of the shaft so that each slot connects two of the lengthwise holes. The slots are in-line with the pistons when a cylinder block is assembled on a pintle. One of these slots provides a path for a liquid to pass from the pistons to the discharge holes bored in a pintle. Another slot connects the two inlet holes to the pistons when they are drawing in liquid. The discharge holes are connected through appropriate fittings to a discharge line so that a liquid can be directed into a system. The other pair of holes is connected to an inlet line.
(2) Cylinder Block. A cylinder block (Figure 3-17) is a block of metal with a hole bored through its center to fit the pintle's and cylinder's holes that are bored equal distances apart around its outside edge. The cylinder's holes connect with the hole that receives a pintle. Designs differ; some cylinders appear to be almost solid, while others have spokelike cylinders radiating out from the center. A cylinder's and pintle's holes are accurately machined so that liquid loss around a piston is minimal.
(3) Pistons. Pistons are manufactured in different designs (see Figure 3-18). Diagram A shows a piston with small wheels that roll around the inside curve of a rotor. Diagram B shows a piston in which a conical edge of the top bears directly against a reaction ring of the rotor. In this design, a piston goes back and forth in a cylinder while it rotates about its axis so that the top surface will wear uniformly. Diagram C shows a piston attached to curved plates. The curved plates bear against and slide around the inside surface of a rotor. The pistons' sides are accurately machined to fit the cylinders so that there is a minimum loss of liquid between the walls of a piston and cylinder. No provision is made for using piston rings to help seal against piston leakage.
(4) Rotors. Rotor designs may differ from pump to pump. A rotor consists of a circular ring, machine finished on the inside, against which the pistons bear. A rotor rotates within a slide block, which can be shifted from side to side to control the piston's length of stroke. A slide block has two pairs of machined surfaces on the exterior so that it can slide in tracks in the pump case.
(5) Drive Shaft. A drive shaft is connected to a cylinder block and is driven by an outside force such as an electric motor.
b. Axial Piston Pumps. In axial piston pumps, the pistons stroke in the same direction on a cylinder block's centerline (axially). Axial piston pumps may be an in-line or angle design. In capacity, piston pumps range from low to very high. Pressures are as high as 5,000 psi, and drive speeds are medium to high. Efficiency is high, and pumps generally have excellent durability. Petroleum oil fluids are usually required. Pulsations in delivery are small and of medium frequency. The pumps are quiet in operation but may have a growl or whine, depending on condition. Except for in-line pumps, which are compact in size, piston pumps are heavy and bulky.
(1) In-Line Pump. In an in-line piston pump (Figure 3-19, diagram A), a drive shaft and cylinder block are on the same centerline. Reciprocation of the pistons is caused by a swash plate that the pistons run against as a cylinder block rotates. A drive shaft turns a cylinder block, which carries the pistons around a shaft. The piston shoes slide against a swash plate and are held against it by a shoe plate. A swash plate's angle causes the cylinders to reciprocate in their bores. At the point where a piston begins to retract, an opening in the end of a bore slides over an inlet slot in a valve plate, and oil is drawn into a bore through somewhat less than half a revolution. There is a solid area in a valve plate as a piston becomes fully retracted. As a piston begins to extend, an opening in a cylinder barrel moves over an outlet slot, and oil is forced out a pressure port.
(a) Displacement. Pump displacement depends on the bore and stroke of a piston and the number of pistons. A swash plate's angle (Figure 3-19, diagram B) determines the stroke, which can vary by changing the angle. In a fixed angle's unit, a swash plate is stationary in the housing. In a variable unit's, it is mounted on a yoke, which can turn on pintles. Different controls can be attached to the pintles to vary pump delivery from zero to the maximum. With certain controls, the direction of flow can be reversed by swinging a yoke past center. In the center position, a swash plate is perpendicular to the cylinder's, and there is no piston reciprocation; no oil is pumped.
(b) Components. The major components of a typical, fixed-displacement in-line pump are the housing, a bearing-supported drive shaft, a rotating group, a shaft seal, and a valve plate. A valve plate contains an inlet and an outlet port and functions as the back cover. A rotating group consists of a cylinder block that is splined to a drive shaft, a splined spherical washer, a spring, nine pistons with shoes, a swash plate, and a shoe plate. When a group is assembled, a spring forces a cylinder block against a valve plate and a spherical washer against a shoe plate. This action holds the piston shoes against a swash plate, ensuring that the pistons will reciprocate as the cylinder turns. A swash plate is stationary in a fixed-displacement design.
(c) Operation. A variable-displacement in-line pump operates the same as a fixed angle except that a swash plate is mounted on a pivoted yoke. A yoke can be swung to change a plate angle and thus change a pump's displacement. A yoke can be positioned manually with a screw or lever or by a compensator control, which positions a yoke automatically to maintain constant output pressure under variable flow requirements. A compensator control consists of a valve that is balanced between a spring and system pressure and a spring-loaded, yoke-actuating piston that is controlled by a valve. A pump's compensator control thus reduces its output only to the volume required to maintain a preset pressure. Maximum delivery is allowed only when pressure is less than a compensator's setting.
(2) Wobble-Plate In-Line Pump. This is a variation of an in-line piston pump. In this design, a cylinder barrel does not turn; a plate wobbles as it turns, and the wobbling pushes the pistons in and out of the pumping chambers in a stationary cylinder barrel. In a wobble-plate pump, separate inlet and outlet check valves are required for each piston, since the pistons do not move past a port.
(3) Bent-Axis Axial Piston Pump. In an angle- or a bent-axis-type piston pump (Figure 3-20), the piston rods are attached by ball joints to a drive shaft's flange. A universal link keys a cylinder block to a shaft so that they rotate together but at an offset angle. A cylinder barrel turns against a slotted valve plate to which the ports connect. 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.

3-9. Pump Operation. The following paragraphs 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. Torque is circular force on an object. An increase in pressure/pump displacement will increase the torque on a shaft if pump displacement/pressure remains constant. Often in a given package size, a higher GPM pump will have a lower pressure rating than a lower GPM pump. Sometimes a field conversion to get more speed out of an actuator will cause a pump to be overloaded. 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. If a needed delivery requires a higher drive speed than a pump's rating, use a higher displacement pump. Excess speed also runs a risk of damage from cavitation.
c. Cavitation. Cavitation occurs where available fluid does not fill an existing space. It often occurs in a pump's inlet when conditions are not right to supply enough oil to keep an inlet flooded. Cavitation causes the metal in an inlet to erode and the hydraulic oil to deteriorate quicker. Cavitation can occur if there is too much resistance in an inlet's line, if a reservoir's oil level is too far below the inlet, or if an oil's viscosity is too high. It can also occur if there is a vacuum or even a slight positive pressure at the inlet. A badly cavitating pump has oil bubbles exploding in the void. The only way to be sure a pump is not cavitating is to check the inlet with a vacuum gauge.
To prevent cavitation, keep the inlet clean and free of obstructions by using the correct length of an inlet's line with minimum bends. Another method is to charge an inlet. The easiest way to do this is to flood it by locating the reservoir above the pump's inlet. If this is not possible and you cannot create good inlet conditions, use a pressurized reservoir. You can also use an auxiliary pump to maintain a supply of oil to an inlet at low pressure. You could use a centrifugal pump, but it is more common to use a positive-displacement gear pump with a pressure-relief valve that is set to maintain the desired charging pressure.
d. Operating Problems. Pressure loss, slow operation, no delivery, and noise are common operating problems in a pump.
(1) Pressure Loss. Pressure loss means that there is a high leakage path in a system. A badly worn pump could cause pressure loss. A pump will lose its efficiency gradually. The actuator speed slows down as a pump wears. However, pressure loss is more often caused by leaks somewhere else in a system (relief valve, cylinders, motors).
(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. To find this point, feel the components for unusual heat.
(3) No Delivery. If oil is not being pumped, a pump-
(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. Air will severely damage a pump because it will not have enough lubrication. This can occur from low oil in a reservoir, a loose connection in an inlet, a leaking shaft seal, or no oil in a pump before starting. Also, noise 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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