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.
Figure 3-9. Unbalanced vane pump
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 second stage 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.
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 crescents haped 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.
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—
• A low-pressure inlet port, which carrys fluid from the reservoir.
• A high-pressure outlet port connected to the pressure line.
• Pumping chamber(s) to carry a fluid from the inlet to the outlet port.
• A mechanical means for activating the pumping chamber(s).
Pumps may be classified according to the specific design used to create the flow of a liquid. Most hydraulic pumps are either centrifugal, rotary, or reciprocating.
a. Centrifugal Pump. This pump generally is used where a large volume of flow is required at relatively low pressures. It can be connected in series by feeding an outlet of one pump into an inlet of another. With this arrangement, the pumps can develop flow against high pressures. A centrifugal pump is a nonpositive-displacement pump, and the two most common types are the volute and the diffuser.
(1) Volute Pump (Figure 3-4). This pump has a circular pumping chamber with a central inlet port (suction pipe) and an outlet port. A rotating impeller is located in a pumping chamber. A chamber between the casing and the center hub is the volute. Liquid enters a pumping chamber through a central inlet (or eye) and is trapped between the whirling impeller blades. Centrifugal force throws a liquid outward at a high velocity, and a contour of a casing directs a moving liquid through an outlet port.
(2) Diffuser Pump (Figure 3-5). Similar to a volute type, a diffuser pump has a series of stationary blades (the diffuser) that curve in the opposite direction from whirling impeller blades. A diffuser reduces the velocity of a liquid, decreases slippage, and increases a pump’s ability to develop flow against resistance.
b. Rotary Pump. In this positive displacement-type pump, a rotary motion carries a liquid from a pump’s inlet to its outlet. A rotary pump is usually classified according to the type of element that actually transmits a liquid, that is, a gear-, vane-, or piston-type rotary pump.
c. Reciprocating Pump. A reciprocating pump depends on a reciprocating motion to transmit a liquid from a pump’s inlet to its outlet. Figure 3-2, shows a simplified reciprocating pump. It consists of a cylinder that houses a reciprocating piston, Figure 3-2, 1; an inlet valve, Figure 3-2, 2; and an outlet valve, Figure 3-2, 3, which direct fluid to and from a cylinder. When a piston moves to the left, a partial vacuum that is created draws a ball off its seat, allowing a liquid to be drawn through an inlet valve into a cylinder. When a piston moves to the right, a ball reseats and closes an inlet valve. However, the force of a flow unseats a ball, allowing a fluid to be forced out of a cylinder through an outlet valve.
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.
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.
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.
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.
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.
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.
Characteristics. The three contrasting characteristics in the operation of positive- and non positive-displacement pumps are as follows:
• Nonpositive-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 nonpositive 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.
• Nonpositive-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.
Figure 2-2, shows a power-driven pump operating a reversible rotary motor. A reversing valve directs fluid to either side of the motor and back to the reservoir. A relief valve protects the system against excess pressure and can bypass pump output to the reservoir, if pressure rises too high.
An electro-hydraulic stepper motor (EHSM) is a device, which uses a small electrical stepper motor to control the huge power available from a hydraulic motor (Figure 4.10).
It consists of three components:
1. Electrical stepper motor
2. Hydraulic servo valve
3. Hydraulic motor.
These three independent components when integrated in a particular fashion provide a higher torque output, which is several hundred times greater than that of an electrical stepper motor.
The electric stepper motor undergoes a precise, fixed amount of rotation for each electrical pulse received. This motor is directly coupled to the rotary liner translator of the servo valve. The output torque of the electric motor must be capable of overcoming the flow forces in the servo valve. The flow forces in the servo valve are directly proportional to the rate of flow through the valve. The torque required to operate the rotary linear translator against this axial force is dependent on the flow gain in the servo valve.
The hydraulic motor is the most important component of the EHSM system. The performance characteristics of the hydraulic motor determine the performance of the EHSM. These are typically used for precision control of position and speed. These motors are available with displacements ranging from 0.4 cubic in. (6.5 cm3) to 7 cubic in. (roughly 115 cm3). Their horsepower capabilities range between 3.5 hp (2.6 kW) and 35 hp (26 kW). Typical applications include textile drives, paper mills, roll feeds, automatic storage systems, machine tools, conveyor drives, hoists and elevators.
A limited rotation hydraulic motor provides a rotary output motion over a finite angle. This device produces a high instantaneous torque in either direction and requires only a small amount of space and simple mountings.
Rotary motors consist of a chamber or chambers containing the working fluid and a movable surface against which the fluid acts. The movable surface is connected to an output shaft to produce the output motion.
Figure 4.1 shows a direct acting vane-type actuator. In this type, fluid under pressure is directed to one side of the moving vane, causing it to rotate. This type of motor provides about 280° rotation.
Rotary actuators are available with working pressures up to 350 kg/cm3 (4978 psi). They are typically foot mounted, flanged or end mounted. Most designs provide cushioning devices. In a double vane design similar to the one depicted in the figure above, the maximum angle of rotation is reduced to about 100°. However in this case, the torque-carrying capacity is twice that obtained by a single vane design.