In this lesson we will differentiate between fixed and variable displacement and compare the different designs of gear, vane and piston pumps.
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.
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!
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, 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.
Gear pumps come in either an external or internal gear design, both of which are fixed displacement pumps.
There are two main designs for vane pumps.
Piston pumps mainly consist of the variable displacement type.