A pump converts mechanical energy, typically produced by an electric motor, into hydraulic energy.

The pump does not generate pressure, only flow. Pressure occurs when the flow from the pump meets a resistance, such as a cylinder attached to a load.

The mechanical action of the pump creates a vacuum at the inlet port, which allows atmospheric pressure to force liquid from the reservoir into the pump. The mechanical movement of the pump then forces the liquid into the hydraulic system.

Positive Displacement Pumps

Most hydraulic pumps are 'positive displacement' pumps. 'Non-positive displacement' pumps used in other applications generate slippage, causing the output to decrease as a load is applied.

A positive displacement pump has an extremely close fit between the pumping element and the pump case, resulting in slippage that is negligible compared to the pump's output. If the output port were plugged, the pressure would immediately rise to the point that the pumping element, case, rotor, or driving motor would fail.

Types of Pump

The displacement of a pump can either be fixed or variable. Variable displacement permits the output during a pumping cycle to change. This is accomplished by altering the geometry of the displacement chamber with an electrical or fluid control system.

Pumps are classified by the type of element that transmits the liquid. The most common elements are gears, pistons and vanes. By their nature, piston pumps lend themselves to variable as well as fixed displacement operation.

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