Introduction to Hydraulic Pumps


In this lesson we will differentiate between fixed and variable displacement and compare the different designs of gear, vane and piston pumps.

Pump Designs

At first glance understanding hydraulic pumps can seem like a daunting task.

But when you take a closer look, there are only three basic designs that can function in only one of two different modes.

Pump Designs:




Displacement Modes:



"But," you may very well be asking yourself, "how can there be more pump designs than displacement modes?"

Gear pumps are strictly fixed displacement, and piston pumps are commonly variable displacement.

But as you can see, it just so happens that vane pumps can come in one of two configurations.

They can be either fixed or variable!

Let's take a closer look at the two displacement modes!

Fixed Displacement Pumps

A fixed displacement pump, as the name suggests, does not have a control to change its displacement.

Assuming that it is receiving a full charge of fluid, it will always displace the same amount of fluid per revolution.

If you want to change the flow rate, the only way to do so is to change the pump RPMs by changing the speed of the prime mover.


Flow rate is a measurement of volume over time, such as liters per minute, (LPM), or gallons per minute, (GPM).

The most common fixed displacement pumps are gear pumps.

There are also some vane pump designs that are fixed displacement.

Variable Displacement Pumps

Variable displacement pumps, on the other hand, can change displacement while continuing to turn at the same RPMs.

The most common variable displacement pumps are piston pumps.

There are also some types of vane pumps, called unbalanced vane pumps, that are variable displacement as well.

Now let's go ahead and take a closer look at each individual pump design.

Gear Pumps

Gear pumps come in either an external or internal gear design, both of which are fixed displacement pumps.

External Gear Pump
Internal Gear Pump

Vane Pumps

There are two main designs for vane pumps.

  • Balanced - Fixed displacement
  • Unbalanced - Either fixed or variable displacement (pressure compensated)

Fixed Displacement Vane Pumps

Balanced Vane Pump
Unbalanced Vane Pump

Variable Displacement Vane Pump

Unbalanced, Variable Displacement, Pressure Compensated Vane Pump

Piston Pumps

Piston pumps mainly consist of the variable displacement type.

Variable Displacement, Pressure Compensated Piston Pump


Gear pumps are:

Vane pumps are:

Piston pumps can be:


  • Fixed displacement pumps do not have controls to change the displacement and will always displace the same amount of fluid per revoultion, without changing the RPMs.
  • Variable displacement pumps have controls to change the displacement, while continuing to turn at the same RPMs.
  • There are three main pump designs: gear, vane and piston.

We hope you enjoyed Introduction to Hydraulic Pumps

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