Page 116 - Fluid Power Engineering

90 Cha pte r F o u r

exit line and is acted on by the pressure necessary to over-
come the exit line resistance.
4. The delivery stroke ends when the volume of the chamber
reaches its minimum value. Afterward, the chamber is sepa-
rated from the delivery line.

This process is repeated continuously as the pump-driving shaft
rotates. In addition to the fluid displacement, the pump should act on
the fluid by the pressure required to drive the load, or overcome the
system resistance.
The function of the displacement pumps is explained by describ-
ing the construction and operation of the single-piston pump, shown
in Fig. 4.1. The piston (4), driven by a crank shaft (5), reciprocates
between two dead points. During the suction stroke, the piston moves
to the right and the oil is sucked from the tank (1) through a check
valve (2) of very low cracking pressure. The cracking pressure is the
minimum pressure difference needed to open the check valve. Then,
during the delivery stroke, the piston moves to the left, displacing the
oil to the exit line through the check valve (3). The pump acts on the
oil by the pressure, P, needed to drive the load. Therefore, the pump
drive should act on the piston by the force needed to produce this
pressure, and the crank shaft should be acted on by a torque propor-
tional to this force. The cylinder (6) retracts under the action of the
loading force by opening the shut-off valve (7).

FIGURE 4.1 Operation of a single-piston pump.