The Hydraulic Pumps converts mechanical energy into hydraulic energy. When the hydraulic pump is running, it performs two functions. First, its mechanical action creates a vacuum at the pump inlet, causing atmospheric pressure to force liquid from the reservoir to the pump inlet line. Secondly, its mechanical action transports this liquid to the outlet of the pump and presses it into the hydraulic system.

The pump produces liquid movement or flow: it does not generate pressure. It generates pressure to produce the required flow rate, which is a function of the resistance to fluid flow in the system. For example, for a pump that is not connected to the system (load), the fluid pressure at the pump outlet is zero. In addition, for pumps delivered to the system, the pressure will only rise to the level needed to overcome the load resistance.


All pumps can be classified as positive displacement or non-positive displacement. Most pumps used in hydraulic systems are positive displacement pumps.

Non-positive displacement pumps produce continuous flow. However, since it does not provide a reliable internal seal to prevent slippage, its output will vary significantly with changes in pressure. Centrifugal pumps and propeller pumps are examples of non-positive displacement pumps.

If the output port of the non-positive displacement pump is blocked, the pressure will rise and the output will decrease to zero. Although the pumping element will continue to move, the flow will stop due to slippage inside the pump.

In positive displacement pumps, the slip is negligible compared to the volumetric output flow of the pump. If the output port is blocked, the pressure will immediately increase to the point where the pump element or its casing will fail (it may explode if the drive shaft does not break first) or the pump's prime mover will stall.


A positive displacement pump is a pump that discharges (delivers) the same amount of liquid in each rotation cycle of the pumping element. Due to the tight fit between the pumping element and the pump housing, it can be delivered constantly in every cycle. That is, the amount of liquid sliding through the pumping element in the positive displacement pump is minimal and negligible compared to the theoretically highest possible delivery. Regardless of the pressure changes during pump operation, the delivery volume per cycle remains almost constant. Please note that if the fluid slips severely, the pump is not working properly and should be repaired or replaced.

Positive displacement pumps can be fixed displacement or variable displacement. The output of the fixed displacement pump is kept constant at a given pump speed during each pumping cycle. The output of the variable displacement pump can be changed by changing the geometry of the displacement chamber.

Other names describing these pumps are hydrostatic pumps for positive displacement and hydrodynamic pumps for non-positive displacement. Hydrostatic pressure means that the pump converts mechanical energy into hydraulic energy with a smaller liquid volume and speed. In a hydrodynamic pump, the speed and movement of liquid are large; the output pressure actually depends on the speed at which the liquid flows.



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