The flow output from a pump can be used to drive a motor where the rotary speed of the motor is determined by its displacement. Hydrostatic motors are a class of actuator in that they convert flow and pressure into velocity and torque on a continuous basis but they can also be used in some applications where only limited movement is required.

However, rotary actuators are available that have a limited rotation angle and offer a reduction in cost because of their relative mechanical simplicity. These can also avoid the need for a holding brake because of their low leakage. Hydrau­lic cylinders are normally used for providing linear motion, which can have zero leakage so that, with blocked ports, stationary loads will be held indefinitely. The use of a rack and pinion gear drive allows the linear movement to be converted to rotary motion whilst retaining the stalled characteristics of the hydraulic cylinder.

Linear actuators, or cylinders, are extensively used in all of the major engi­neering fields and the system designer needs to be aware of the different types of construction that are available, the various mounting methods and the influence that these have on their load carrying characteristics.