Hydraulic Pumps

The positive displacement hydraulic pump is a device that converts mechanical energy into hydraulic energy. When driven by its motor (prime mover), it basically performs two functions. Firstly, it creates a partial vacuum at the pump inlet port. This vacuum enables the atmospheric pressure to force fluid from the reservoir (tank) into the pump. Secondly, the mechanical action of the pump traps this fluid within the pumping cavities, transports it through the pump, and forces it into the hydraulic system.

It is often assumed that pumps create pressure, but the sole purpose of pumps is to create flow. Pressure is force per unit area and is created by resistance to flow. A pump is a mechanism designed to produce the flow necessary for the development of pressure within the device. But it cannot of itself produce pressure, since it cannot provide resistance to its own flow.

To illustrate this principle, the leaking actuator shown in fig. 54 does not create any pressure loss since the remaining fluid within the actuator still raises the piston. Pressure would only be lost, if the entire flow from the pump escaped past the piston. The piston leak does however, affect the piston speed, since 2/3 of the pump flow is returning to tank without performing any work. Thus, the intended piston speed is also reduced by 2/3.