Handle to the Left. When the handle is moved to the left, the piston and rod assembly also move to
the left. The inlet check valve now closes, preventing the fluid in the left chamber from returning to the
reservoir. At the same time, the pistonhead check valve opens, allowing the fluid to enter the right
Fluid Into the System. The pump produces pressure on both strokes because of the difference in
volume between the right and left chambers. The piston rod takes up a good share of the space in the
right chamber. Therefore, the excess fluid is forced out of the pump and into the hydraulic system,
creating fluid pressure.
PUMP-DRIVEN HYDRAULIC PUMPS
Power-driven pumps receive their driving force from an external power source, such as the aircraft
engine. This force is converted into energy in the form of fluid pressure. The four basic types of power-
driven hydraulic pumps are gear, vane, diaphragm, and piston. Of these, the piston type is most
commonly found in Army aircraft. The reason for this is that it operates more efficiently at higher
pressures and has a longer life than any of the others. Piston pumps are further categorized as either
constant delivery or variable delivery.
Pumps are coupled to their driving units by a short, splined coupling shaft, commonly called a drive
coupling. As shown in Figure 1-5, the shaft is designed with a weakened center section called a shear
section, with just enough strength to run the pump under normal circumstances. Should some trouble
develop within the pump causing it to turn unusually hard, the shear section will break. This prevents
damage to the pump or driving unit.
Figure 1-5. Pump Drive Coupling.
Constant-delivery piston pumps deliver a given quantity of fluid per revolution of the drive coupling,
regardless of pressure demands. The quantity of fluid delivered per minute depends on