The basic principle.
A hydraulic pump converts shaft power into fluid power. Positive-displacement pumps repeatedly enclose a volume of fluid and transfer it from inlet to outlet. The system’s load and resistance determine the pressure needed.
Gear, screw and vane pumps use different mechanisms. Displacement describes volume per revolution. For a fixed-displacement pump, theoretical flow is displacement multiplied by speed; internal leakage reduces delivered flow.
Flow from a rotating pump.
A 10 cm³/revolution pump at 1,500 rpm has a theoretical flow of 15 litres per minute. Actual delivery depends on operating conditions and leakage.
Follow the relationship.
- Motor turnsSupplies shaft power.
- Pump displaces fluidTransfers volume from inlet to outlet.
- Circuit resists flowPressure rises to meet the demand.
Connect it to the lift.
In a conventional hydraulic lift power unit, the pump supplies the lifting circuit for upward travel. Some arrangements vary motor speed to control flow. A running motor does not alone prove correct delivery: the inlet conditions, pump and valve paths also matter.
A closer look
Poor inlet conditions can cause cavitation when local pressure falls sufficiently for vapour cavities to form. Air entering the fluid is a different phenomenon, although both can affect noise and performance.
Check your understanding
Does a fixed-displacement pump deliver the same flow at every shaft speed?
Reveal the explanation
No. Its theoretical flow changes with speed; real delivery also depends on leakage and conditions.