The pistons are designed with large hollow centers. The centers are intersected by cross-drilled
relief holes that open into the pump case. Each piston is equipped with a movable sleeve, which can
block the relief holes. When these holes are not blocked, fluid displaced by the pistons is discharged
through the relief holes into the pump case, instead of past the pump check valves and out the outlet
When full fluid flow is required, the sleeves are positioned to block the relief holes for the entire
length of piston stroke. When zero flow is required, the sleeves are positioned not to block the flow
during any portion of the piston stroke. For requirements between zero and full flow, the relief holes are
uncovered or blocked accordingly.
The sleeves are moved into their required positions by a device called a pump compensator piston.
The sleeves and compensator piston are interconnected by means of a spider. Fluid pressure for the
compensator piston is obtained from the discharge port (system pressure) through a control orifice.
Stroke-Reduction Principle. The stroke-reduction principle (Figure 1-9) is based on varying the
angle of the cylinder block in an angular pump. This controls the length of the piston's stroke and thus
the volume per stroke.
The cylinder block angle change is achieved by using a yoke that swivels around a pivot pin called a
pintle. The angle is automatically controlled by using a compensator assembly consisting of a pressure-
control valve, pressure-control piston, and mechanical linkage that is connected to the yoke.
As system pressure increases, the pilot valve opens a passageway allowing fluid to act on the control
piston. The piston moves, compressing its spring, and through mechanical linkage moves the yoke
toward the zero flow (zero angle) position. As system pressure decreases, the pressure is relieved on the
piston, and its spring moves the pump into the full flow position.
The purpose of a hydraulic accumulator is to store hydraulic fluid under pressure. It may be used to--:
Dampen hydraulic shocks which may develop when pressure surges occur in hydraulic systems.
Add to the output of a pump during peak load operation of the system, making it possible to use
a pump of much smaller capacity than would otherwise be required.