fixed displacement pumps Vs Variable Displacement pumps

I have been designing and building hydraulic power units and control systems for many years, and their respective use boils down to system efficiency and cost. Fixed displacement pumps are usually much less expensive than variable displacement pumps. Also, variable displacement pumps get very rare at flow rates less than 3-4gpm.

The use of a fixed displacement pump depends on the application. While the pump is turning it will always be outputting flow which has to go somewhere, and if it is under load (the system is dead headed against a load, such as when a cylinder is fully extended) that flow has to be dumped back to tank via a relief valve. This will heat the oil up at a rate almost equal to the input horespower (i.e. the hp of the prime mover turning the pump), which is undersireable. This heat has to be removed from the system, with a heat exchanger, but the stress to the oil will degrade it before its time.

A variable displacement pump can be set to 'compensate', or unload, under a dead head condition, only outputting enough flow to maintain the desired system pressure. This will still generate heat, but since the input horsepower is so much less it can be better managed.

If the expense can be spent and the flow rate is high enough, use a variable displacement. NOTE: some applications MUST use one or the other, making my simple rules of thumb not so simple. If you have to use a fixed displacement, then make sure the flow is unloaded properly back to the reservoir with minimum resistance (pressure)....or just shut the motor off.

Any experienced hydraulic circuit designer should be able to properly apply either type of pump to most any applcation.