Positive & Non-Positive Pumping


  • Describe the difference in function between positive & non-positive displacement pumps
  • Categorize common models of pump as positive or non-positive
  • Explain the advantages of both styles of pump

Key Concept

There are similarities and important differences between positive & non-positive displacement systems. It’s very important to be aware of which type of system you are dealing with!

Positive Displacement Pump

A hydraulic pump is a positive displacement pump, which means that for every revolution of the pump shaft, the same amount of fluid should be displaced regardless of the system pressure.

Positive Displacement Pump:
The pump will displace the same amount of fluid on every revolution, regardless of system pressure.
Identical systems with one hardly workin' (left) and one workin' hard (right). Both systems still displace the same amount of fluid.

Non-Positive Displacement Pumping

A pump that is used to circulate coolant around an engine block, or a centrifugal pump used in an industrial plant to move fluid from one holding tank to another, is usually a non-positive displacement pump.

This type of pump allows for slippage to take place inside the pump whenever the system pressure starts to increase.

Non-Positive Displacement Pump

A non-positive displacement pump's output flow rate changes in response to restriction (pressure) on the outlet.

Pump running at high efficiency

Pump running against a strong restriction

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