A vane pump (Fig. 4.7) has a series of vanes that slide back and forth in slots. There are springs in these slots that push the vanes out until the tip contacts the cam ring. (Some designs port pressurized fluid into the slots to force the vanes out.) A chamber is formed between adjacent vanes and the cam ring. As the rotor turns, the chamber decreases in size. Fluid flows into this chamber when it is a maximum size and exits during some ?? of rotation when it is a minimum size. This change in chamber size provides the pumping action.
The principle of operation of a vane-type variable displacement pump is shown in Figs. 4.7 and 4.8. These illustrations are not to scale and are incomplete. Certain features are not shown. The cam ring is held in position in Fig. 4.7 with a threaded rod turned with a hand wheel. This ring will slide to the left when the hand wheel is turned. In the position shown in Fig. 4.6, the cam ring is centered on the axis of rotation. The chambers are equal size at the inlet and outlet, so no fluid is pumped (displacement is zero.) The rotor turns at the same speed, the vane tips are in contact with the cam ring, but no fluid is pumped.
The vane pump can be converted to a pressure-compensated pump by replacing the hand-wheel adjustment with a spring as shown in Fig. 4.9. A small cylinder, identified as the compensator, is placed on the opposite side. Outlet pressure acting on the compensator piston creates a hydraulic force that opposes the spring force. When the outlet pressure rises to a certain point, the hydraulic force becomes greater than the spring force, and the cam ring shifts to the left. As pressure continues to rise, the ring shifts more to the left until it eventually is centered on the axis of rotation (as shown in Fig. 4.8). At this pressure, known as the deadhead pressure, the pump displacement is almost zero. Some flow is produced to replace leakage.
It is now clear how the circuit in Fig. 4.6 can operate without a relief valve. The maximum pressure the pump can develop is limited by the compensator spring in the pressure-compensated pump.
A pressure-compensated pump can maintain deadhead pressure with very little energy input. Hydraulic power output is proportional to pressure × flow. If flow is zero, then hydraulic power output is zero. Some input energy is required to maintain deadhead pressure because of friction and leakage. The advantage of a demand-flow circuit (Fig. 4.6) as compared to a constant-flow circuit (Fig. 4.3) is that pressure is available at the instant the DCV is shifted; it does not have to build from zero.
Typical flow vs. pressure characteristics for a pressure -compensated variable displacement vane pump is shown in Fig. 4.10. When pressure reaches 2900 psi, identified as the cutoff pressure, the cam ring begins to shift, and the pump flow decreases. The rate of decrease (slope of the curve) is set by the spring constant of the compensator spring.
Figure 4.11 shows a pressure compensated variable displacement pump in an exploded view. The cam ring (pressure ring), vanes, and compensator are readily visible. Modern designs, like the one shown, do not use a compensator spring; rather, a specific pilot pressure on a bias piston that holds the pressure ring in place. As the pressure developed at the pump outlet rises, the force developed by the compensator piston eventually becomes large enough to equal the bias piston force and center the pressure ring.