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Vane-type fixed-displacement pumps
The most common pump for industrial applications is the vane design shown in Figure 8-8. The left-hand cutaway view illustrates the original unbalanced design. Today, most vane pumps are of the balanced design shown on the right. Balanced vane pumps operate at higher pressures and have long bearing life. All vane designs compensate for wear, so their efficiency stays in the 90 to 95% range throughout their service lives. Vane pumps are efficient, quiet, and inexpensive. They have great longevity when supplied with clean fluid.
As a prime mover turns the rotor, centrifugal force slings the vanes outward. (Most manufacturers recommend a minimum speed of 600 rpm to make the vanes extend.) Now, as the vanes follow the off-center cam ring, a chamber is formed between the cam ring and the rotor. This chamber gets larger as the vanes extend, creating a negative pressure (vacuum) at the inlet port. Atmospheric pressure then forces fluid into these enlarging voids and fluid starts to move. As a vane passes the highest point on the cam ring, it is forced back into its slot and the chambers between the vanes decrease. As a chamber size decreases, fluid is forced out through the kidney-shaped openings to the outlet. Even though vane tips wear, they still touch the cam ring, so efficiency is not affected for a long time.
The other leakage and wear point is at the sides of the gears or rotors of these pumps. Most modern vane pumps have pressure-loaded floating plates that are hydraulically forced against the turning members. Hydraulic pressure tries to push the plates away from the gears or rotors in a certain area, but a slightly larger area on the opposite side of the plates pushes back under the same pressure. This keeps the side areas sealed without applying excess force against the turning members. (Some inexpensive low-pressure pumps may not have floating side plates but depend instead on manufacturing tolerances to control leakage.)
Note that the unbalanced vane pump in Figure 8-8 has pressure on one side of the rotor and vacuum on the other side. This pump has to have large bearings or operate at lower pressures. The balanced-design pump pictured on the right has pressure on opposite sides of the rotor. As a result, the bearing load is the same at 0 psi, 2000 psi, or any pressure at which the pump runs. The balanced design also produces twice the flow for the same overall package size.
Vane pumps are available with two or three pumps in one housing to give more flow or different rates of flow to satisfy the needs of some circuit designs. These pumps have a common inlet and separate outlets as required.
Typical circuits for fixed-volume pumps
Figure 8-9 shows a circuit using a fixed-volume pump in a simple, single-cylinder circuit. A tandem-center directional control valve routes all pump flow to tank at low pressure when the cylinder is idle. When the cylinder cycles, pressure never goes higher than necessary to do the work at hand, so energy waste is minimal. With an efficient pump, this circuit operates all day without a heat exchanger and fluid temperature never increases more than 10° or 15°F above ambient.
Figure 8-10 shows a multiple-cylinder circuit supplied by a fixed-volume pump. Here, the tandem-center valves are connected in series, so all pump flow can go to tank when the actuators are idle. This circuit works best when the actuators do not move simultaneously. When two or more actuators move at the same time, the pressure to make the cylinders move is additive and may exceed the relief valve setting. Also, downstream actuators only get fluid from the actuators upstream from them. As a result, stroke lengths may be limited.
Use caution when selecting directional valves for this circuit. Pay particular attention to pressure-drop charts because pressure drop is additive for each valve. This circuit could start up with a 200-psi drop at idle. With more valves in series, pressure drop at idle and running can cause sluggish operation and generate heat. Also, choose valves that are able to operate at tank line pressure. Every upstream valve sees pressure at pump and tank ports while a downstream actuator is working.
Figure 8-11 shows a multiple-cylinder circuit that uses a normally open solenoid-operated relief valve to unload the pump when the actuators are idle. Anytime an actuator cycles, a solenoid on its directional control valve and the solenoid on the normally open solenoid-operated relief must be energized at the same time. This circuit often requires flow controls -- and may need a heat exchanger to get rid of wasted energy.
The circuit in Figure 8-12 has a fixed-volume pump with an accumulator to store energy and allow the pump to unload when no fluid is required to do work. It is similar to a pressure-compensated pump circuit because there is only pump flow at pressure when the circuit calls for it. The pump-unloading-and-accumulator-dump valve sends pump flow to the circuit until pressure reaches its set level. After reaching set pressure, the valve opens fully and dumps all pump flow to tank at minimum pressure. When circuit pressure drops about 10 to 15%, this valve closes and again directs pump flow to the circuit. (A normally open solenoid-operated relief valve controlled by a pressure switch could be used in place of the pump-unloading-and-accumulator-dump valve.)