Hydraulic pumps convert mechanical energy into fluid power energy. All hydraulic pumps are positive displacement which means the outlet flow is sealed from the inlet flow. A small amount of fluid is designed to leak internally to lubricate and cool the internal components of the pump. The only function of the pump is to produce flow in a system. The amount of resistance and or load induced on the system determines pressure. If a pump flow is directed back to the reservoir without restrictions, no pressure will be developed. If a load or restriction is introduced to the pump flow, pressure will rise to overcome the resistance. Hydraulic pumps can be grouped into two classes: fixed and variable displacement.
A fixed displacement pump will generate a constant flow at a given rpm. When designing a fixed displacement system, low pressure flow returning to the reservoir is needed when the system is idle. Some form of relief is always required with this type of pump.
A variable displacement pump can change flow internally depending upon system pressure created by an external signal. Flow from these pumps can be blocked provided the signal to the pump will automatically limit the pressure below the lowest maximum pressure of any component in the system.