Pump Disp., HP, GPM & RPM


Hydraulic pumps convert mechanical energy into fluid power energy. All hydraulic pumps are positive displacement which means the outlet flow is sealed from the inlet flow. A small amount of fluid is de- signed to leak internally to lubricate and cool the internal components of the pump. The only function of the pump is to produce flow in a system. The amount of resistance and or load induced on the system determines pressure. If a pump’s flow is directed back to the reservoir without restrictions, no pressure will be developed. If a load or restriction is introduced to the pump’s flow, pressure will rise to overcome the resistance. Hydraulic pumps can be grouped into two classes: fixed and variable displacement.

A fixed displacement pump will generate a constant flow at a given RPM. When designing a fixed displacement system, low pressure flow returning to the reservoir is needed when the system is idle. Some form of relief is always required with this type of pump.

A variable displacement pump can change flow internally depending upon system pressure created by an external signal. Flow from these pumps can be blocked provided the signal to the pump will automatically limit the pressure below the lowest maximum pressure of any component in the system.


Gear pumps use two gears that mesh together to produce flow. The drive gear is connected to the input shaft and the idler gear is turned by the drive gear. These fixed displacement pumps create flow by rotating the gears together. As the gears rotate and separate, a vacuum is formed that draws fluid into the suction port (this can vary from pump to pump). The fluid travels around the outside of the gear to the pressure side of the pump. As the gears mesh together the fluid is forced out of the pressure port. Due to their rugged construction and ability to handle higher levels of contaminants, they are widely used in tougher applications.


Fixed displacement vane pumps use a set of vanes that are inserted into a rotor. The vanes extend out to a cam ring that remains stationary. As the rotor and vanes turn, the area expands on the suc- tion side of the pump, creating a vacuum. The vacuum draws the fluid into the pump. When the fluid rotates to the pressure side of the pump, the area is reduced and forces the fluid out of the pump through the pressure port.


A variable displacement pressure compensated piston pump uses a set of pistons parallel to the in- put shaft, which are inserted into a cylinder block. One end of the piston is attached to a swash plate which can be set on an adjustable angle, perpendicular to the input shaft. As the shaft rotates and turns the pistons and cylinder block around the shaft, the piston will reciprocate in the cylinder block bore as it rides up and down on the slope of the swash plate. If the swash plate angle is set to zero, fluid will not enter or exit the pump. As the angle of the swash plate increases, the piston travels a longer stroke within the bore and displaces more fluid. The compensator mounted on the pump con- trols the pressure at which the pump strokes to a near zero displacement.

Horsepower to Drive Hydraulic Pump Pump Displacement
Desired FlowGPM
System PressurePSI
Horsepower RequiredHP
Calculated for electric motors. Double this figure for gas engines.
Desired FlowGPM
Operating SpeedRPM
Displacement (in3/rev)in3/rev

Pump Flow Rate (GPM) Pump Speed (RPM)
Pump Displacementin3/rev
Pump Flow RateGPM
Flow RateGPM
Pump Displacementin3/rev
Pump SpeedRPM

Hydraulic Gear Pump Sizing
Gear Widthinches
Bore Diameterinches
Distance Across Gear Chambers - Lengthinches
Pump Displacementin3/rev