Hydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic. A hydraulic pump is a mechanical source of power that converts mechanical power into hydraulic energy (hydrostatic energy i.e., flow, pressure). It generates flow with enough power to overcome pressure induced by the load at the pump outlet. When a hydraulic pump operates, it creates a vacuum at the pump inlet, which forces liquid from the reservoir into the inlet line to the pump, and by mechanical action this liquid is delivered to the pump outlet where it is forced it into the hydraulic system. Hydrostatic pumps are positive displacement pumps while hydrodynamic pumps can be fixed displacement pumps, in which the displacement cannot be adjusted, or variable displacement pumps, which have a more complicated construction that allows the displacement to be adjusted.
Most pumps work in open systems. The pump draws oil from a reservoir at atmospheric pressure. It is vital that there is no cavitation at the suction side of the pump. For this reason, the connection of the suction side of the pump is larger in diameter than the connection of the pressure side. In case of the use of multi-pump assemblies, the suction connection of the pump is often combined. It is preferred to have free flow to the pump. The body of the pump is often in open connection with the suction side of the pump.
In case of a closed system, both sides of the pump can be at high pressure. The reservoir is often pressurized with 6-20 bars boost pressure. For closed loop systems, axial piston pumps are generally used. As both sides are pressurized, the body of the pump needs a separate leakage connection.
Hydraulic pumps generally use less energy than other types of pumps while still completing the same work or transferring the same volume of fluid. It is widely believed that these pumps are 80% more efficient than pneumatic systems.
Careless idling of the hydraulic pump is unlikely due to the settings used. This type of idling (as in a pneumatic system) almost always causes damage and wastes valuable operator time during maintenance.
Total maintenance costs of hydraulic pumps are greatly reduced as they are self-lubricating and do not suffer from the problems associated with air-powered pumps such as moisture, oil-based cleaning, scale and rust. They also eliminate the problem of icing that occurs when the pump recirculation rapidly cools the exiting compressed air.