All types of hydraulic motors have 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.

The ability of the pressure surfaces to withstand force, the leakage characteristics of each type of 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. Hydraulic motor displacement may be fixed or variable. A fixed-displacement motor provides constant torque. Controlling the amount of input flow into the motor varies the speed. A variable-displacement motor provides variable torque and variable speed. With input flow and pressure constant, varying the displacement can vary the torque speed ratio to meet load requirements.

Torque output is expressed in inch-pounds or foot-pounds. It 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, it indicates the actual torque that 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 that 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 that 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 the fluid that passes through the motor without performing work.

Gear motors

External gear motors consist of a pair of matched gears enclosed in one housing (Fig. 1). Both gears have the same tooth form and are driven by pressure fluid. One gear is connected to an output shaft. The other is an idler. Pressure fluid enters the housing at a point where the gears mesh. It forces the gears to rotate and follows the path of least resistance around the periphery of the housing. The fluid exits at low pressure at the opposite side of the motor. Close tolerances between gears and housing help control fluid leakage and increase volumetric efficiency. Wear plates on the sides of the gears keep the gears from moving axially and help control leakage.

Internal gear motors fall into two categories. A direct-drive gerotor motor consists of an inner-outer gear set and an output shaft (Fig. 2). The inner gear has one fewer tooth than the outer. The teeth are shaped so all of the teeth of the inner gear are in contact with some portion of the outer gear at all times. When pressure fluid is introduced into the motor, both gears rotate. The motor housing has integral kidney-shaped inlet and outlet ports. The centers of rotation of the two gears are separated by a given amount known as the eccentricity. The center of the inner gear coincides with the center of the output shaft.

Pressure fluid enters the motor through the inlet port (Fig. 2a). Because the inner gear has one fewer tooth than the outer, a pocket is formed between inner teeth 6 and 1 and outer socket A. The kidney-shaped inlet port is designed so that just as this pocket’s volume reaches its maximum, fluid flow is shut off, with the tips of inner gear teeth 6 and 1 providing a seal (Fig. 2b).

As the pair of inner and outer gears continues to rotate, a new pocket is formed between inner teeth 6 and 5 and outer socket G (Fig. 2c). Meanwhile, the pocket formed between inner teeth 6 and 1 and outer socket A has moved around opposite the kidney-shaped outlet port, steadily draining as the volume of the pocket decreases. The gradual, metered volume change of the pockets during inlet and exhaust provides smooth, uniform fluid flow with a minimum of pressure variation (or ripple).

Because of the extra tooth in the outer gear, the inner gear teeth move ahead of the outer by one tooth per revolution. In Figure 2c, inner tooth 4 is seated in outer socket E. On the next cycle, inner tooth 4 will seat in outer socket F. This produces a low relative differential speed between the gears.

An orbiting gerotor motor consists of a set of matched gears, a coupling, an output shaft, and a commutator or valve plate (Fig. 3). The stationary outer gear has one more tooth than the rotating inner gear. The commutator turns at the same rate as the inner gear and always provides pressure fluid and a passageway to tank to the proper spaces between the two gears.

In operation, tooth 1 of the inner gear is aligned exactly in socket D of the outer gear (Fig. 3a). Point y is the center of the stationary gear, and point x is the center of the rotor. If there were no fluid, the rotor would be free to pivot about socket D in either direction. It could move toward seating tooth 2 in socket E or, conversely, toward seating tooth 6 in socket J.

When pressure fluid flows into the lower half of the volume between the inner and outer gears, if a passageway to tank is provided for the upper-half volume between the inner and outer gears, a moment is induced that rotates the inner gear counterclockwise and starts to seat tooth 2 in socket E. Tooth 4, at the instant shown in Figure 3a, provides a seal between pressure and return fluid.

However, as rotation continues, the locus of point x is clockwise. As each succeeding tooth of the rotor seats in its socket, the tooth directly opposite on the rotor from the seated tooth becomes the seal between pressure and return fluid (Fig. 3b). The pressurized fluid continues to force the rotor to mesh in a clockwise direction while it turns counterclockwise.

Because of the one extra socket in the fixed gear, the next time tooth 1 seats, it will be in socket J. At that point, the shaft has turned one-seventh of a revolution, and point x has moved six-sevenths of its full circle. In Figure 3c, tooth 2 has mated with socket D, and point x has again become aligned between socket D and point y, indicating that the rotor has made one full revolution inside of the outer gear. Tooth 1 has moved through an angle of 60° from its original point in Figure 3a; 42 (or 627) tooth engagements or fluid cycles would be needed for the shaft to complete one revolution.

The commutator or valve plate contains pressure and tank passages for each tooth of the rotor (Fig. 3d, e, and f). The passages are spaced so they do not provide for pressure or return flow to the appropriate port as a tooth seats in its socket. At all other times, the passages are blocked or are providing pressure fluid or a tank passage in the appropriate half of the motor between gears.

A roller-vane gerotor motor is a variation of the orbiting gerotor motor (Fig. 4). It has a stationary ring gear (or stator) and a moving planet gear (or rotor). Instead of being held by two journal bearings, the eccentric arm of the planetary is held by the meshing of the six-tooth rotor and seven-socket stator. Instead of direct contact between the stator and rotor, roller vanes are incorporated to form the displacement chambers. The roller vanes reduce wear, enabling the motors to be used in closed-loop, high-pressure hydrostatic circuits as direct-mounted wheel drives.