External gear motors, Figure 1, consist of a pair of matched gears enclosed in one housing. 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, Figure 2. The inner gear has one less tooth than the outer. The shape of the teeth is such that all 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.

In Figure 2(a), pressure fluid enters the motor through the inlet port. Because the inner gear has one less tooth than the outer, a pocket is formed between inner teeth 6 and 1, and other 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, Figure 2(b).fig. 2. direct-drive gerotor motor has internal and external gear sets. both gears rotate during operation.

As the pair of inner and outer gears continues to rotate, Figure 2(c), a new pocket is formed between inner teeth 6 and 5, and outer socket G. 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 2(c), 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, Figure 3, consists of a set of matched gears, a coupling, an output shaft, and a commutator or valve plate. 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, Figure 3(a), tooth 1 of the inner gear is aligned exactly in socket D of the outer gear. 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 which rotates the inner gear counterclockwise and starts to seat tooth 2 in socket E. Tooth 4, at the instant shown in Figure 3(a), provides a seal between pressure and return fluid.fig. 3. orbiting gerotor motor has a stationary outer gear and a rotating inner gear. rotor and shaft turn in counter clockwise direction, but locus of point x is clockwise. commutator or valve plate, shown below illustration of each stage of motor rotation, provides pressure and tank passage for pressure fluid.

However, as rotation continues, the locus of point x is clockwise. As each succeeding tooth of the rotor seats in its socket, Figure 3(b), the tooth directly opposite on the rotor from the seated tooth becomes the seal between pressure and return fluid. 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 1/7 of a revolution, and point x has moved 6/7 of its full circle. In Figure 3(c), 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 3(a); 42 (or 6 X 7) tooth engagements or fluid cycles would be needed for the shaft to complete one revolution.

The commutator or valve plate, shown in Figures 3(d), (e), and (f), contains pressure and tank passages for each tooth of the rotor. 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.fig. 4. roller-vane gerotor motor incorporates rolling vanes that reduce wear.

A roller-vane gerotor motor, Figure 4, is a variation of the orbiting gerotor motor. 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 6-tooth rotor and 7-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.

Vane motors

Vane motors, Figure 5, have a slotted rotor mounted on a drive shaft that is driven by the rotor. Vanes, closely fitted into the rotor slots, move radially to seal against the cam ring. The ring has two major and two minor radial sections joined by transitional sections or ramps. These contours and the pressures introduced to them are balanced diametrically.fig. 5. vane motors (balanced type shown) have vanes in a slotted rotor.

In some designs, light springs force the vanes radially against the cam contour to assure a seal at zero speed so the motor can develop starting torque. The springs are assisted by centrifugal force at higher speeds. Radial grooves and holes through the vanes equalize radial hydraulic forces on the vanes at all times.

Pressure fluid enters and leaves the motor housing through openings in the side plates at the ramps. Pressure fluid entering at the inlet ports moves the rotor counterclockwise. The rotor transports the fluid to the ramp openings at the outlet ports to return to tank. If pressure were introduced at the outlet ports, it would turn the motor clockwise.

The rotor is separated axially from the side plate surfaces by the fluid film. The front side plate is clamped against the cam ring by pressure, and maintains optimum clearances as temperature and pressure change dimensions.

Vane motors provide good operating efficiencies, but not as high as those of piston motors. However, vane motors generally cost less than piston motors of corresponding horsepower ratings.

The service life of a vane motor usually is shorter than that of a piston motor. Vane motors are available with displacements of 20 in.3/rev; some low-speed/high-torque models come with displacements to 756 in.3/rev. Except for the high-displacement, low-speed models, vane motors have limited low-speed capability.