Gear Motors

A gear motor develops an output torque at its shaft by allowing hydraulic pressure to act on gear teeth. A gear motor consists basically of a housing with inlet and outlet ports, and a rotating group made up of two gears. One gear is attached to a shaft that is connected to a load. The other gear is the driven gear. The various commonly used gear motors are shown in Fig.2.16.

In a gear motor, the imbalance necessary for motor operation is caused by gear teeth unmeshing. The inlet is subjected to system pressure and the outlet is at return line pressure. As the gear teeth unmesh, all teeth subjected to system pressure are hydraulically balanced except for one side of one tooth on one gear tooth. The larger the gear tooth or the higher the pressure,the more torque is produced.

An internal gear motor consists of one external gear which meshes whith the teeth on the inside circumference of a larger gear. A popular type of internal gear motor in industrial systems is the gerotor motor. This motor is an internal gear motor with an inner drive gear and an outer driven gear which has one more tooth than the inner gear. The inner gear is attached to a shaft which is connected to a load. The imbalance in a gerotor motor is cuased by the difference in gear area exposed to hydraulic pressure at the motor inlet.

Fluid pressure acting on these unequally exposed teeth results in a torque at the motor shaft. The larger the gear or the higher the pressure, the more torque will be developed at the shaft. Fluid entering the rotating group of a gerotor motor is separated from the fluide exiting the motor by means of a port plate with kindey-shaped inlet and outlet ports.

When selecting a gear motor, an important consideration is the degree of gear service and gear life based on the load conditions to which the motor will be subjected. Gear motors are divided into three classes. Each class uses different gear sizes to handle specific load conditions. Each class gives about the same life for the gears. The American Gear Manufacturer's Association has defined three operating conditions commonly found in industrial service and has established three standard gear classifications to meet these conditions:

Class I    For steady loads within the motor rating of 8 hours per day duration, or for intermittent operation under moderate stock conditions.
Class II   For 24-hour operation at steady loads within the motor rating, or 8-hour operation under moderate shock conditions.
Class III  For 24-hour operation under moderate shock condition, or 8-hour operation under heavy shock conditions.

For conditions that are more severe than those covered by Class III gears, a fluid drive unit may be incorporated in assembly to cushion the shock to an acceptable value.

To achieve multiple speeds, separte units are available with a transmission comparable to that of an automobile. These units must be assembled with the motor and the driven machine. Because the amout of power lost in gearing is very small, the multiple drive has essentially constant horsepower. In other words, as the output speed is decreased, the torque is increased. Generally, this means that larger shaft sizes are needed for the output side.