Hydraulic motors are rotary actuators that convert hydraulic, or fluid energy into mechanical power. They work in tandem with a hydraulic pump, which converts mechanical power into fluid, or hydraulic power. Hydraulic motors provide the force and supply the motion to move an external load.
Three common types of hydraulic motors are used most often today—gear, vane and piston motors—with a variety of styles available among them. In addition, several other varieties exist that are less commonly used, including gerotor or gerolor (orbital or roller star) motors.
Hydraulic motors can be either fixed- or variable-displacement, and operate either bi-directionally or uni-directionally. Fixed-displacement motors drive a load at a constant speed while a constant input flow is provided. Variable-displacement motors can offer varying flow rates by changing the displacement. Fixed-displacement motors provide constant torque; variable-displacement designs provide variable torque and speed.