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, the pressure will rise to overcome the resistance. Hydraulic pumps can be grouped into two classes: fixed and variable displacement.