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All types of hydraulic motors have these common design features: a driving surface area subject to pressure differential; a way of timing the porting of pressure fluid to the pressure surface to achieve continuous rotation; and a mechanical connection between the surface area and an output shaft.fig. 1. external gear motors have one driving gear and one idler gear enclosed in single housing. output torque is a function of pressure on one tooth because pressure on other teeth is in hydraulic balance.

The ability of the pressure surfaces to withstand force, the leakage characteristics of each type motor, and the efficiency of the method used to link the pressure surface and the output shaft determine the maximum performance of a motor in terms of pressure, flow, torque output, speed, volumetric and mechanical efficiencies, service life, and physical configuration.

Motor displacement refers to the volume of fluid required to turn the motor output shaft through one revolution. The most common units of motor displacement are in.3 or cm3 per revolution.

Displacement of hydraulic motors may be fixed or variable. A fixed-displacement motor provides constant torque. Speed is varied by controlling the amount of input flow into the motor. A variable-displacement motor provides variable torque and variable speed. With input flow and pressure constant, the torque speed ratio can be varied to meet load requirements by varying the displacement.

Torque output is expressed in inch-pounds or foot-pounds, and is a function of system pressure and motor displacement. Motor torque ratings usually are given for a specific pressure drop across the motor. Theoretical figures indicate the torque available at the motor shaft assuming no mechanical losses.

Breakaway torque is the torque required to get a stationary load turning. More torque is required to start a load moving than to keep it moving.

Running torque can refer to a motor's load or to the motor. When it refers to a load, it indicates the torque required to keep the load turning. When it refers to the motor, running torque indicates the actual torque which a motor can develop to keep a load turning. Running torque considers a motor's inefficiency and is a percentage of its theoretical torque. The running torque of common gear, vane, and piston motors is approximately 90% of theoretical.

Starting torque refers to the capacity of a hydraulic motor to start a load. It indicates the amount of torque which a motor can develop to start a load turning. In some cases, this is considerably less than the motor's running torque. Starting torque also can be expressed as a percentage of theoretical torque. Starting torque for common gear, vane, and piston motors ranges between 70% and 80% of theoretical.

Mechanical efficiency is the ratio of actual torque delivered to theoretical torque.

Torque ripple is the difference between minimum and maximum torque delivered at a given pressure during one revolution of the motor.

Motor speed is a function of motor displacement and the volume of fluid delivered to the motor.

Maximum motor speed is the speed at a specific inlet pressure which the motor can sustain for a limited time without damage.

Minimum motor speed is the slowest, continuous, uninterrupted rotational speed available from the motor output shaft.

Slippage is the leakage through the motor — or fluid that passes through the motor without performing work.