If gear units had no losses, the torque at the output shaft of a gear motor would equal the motor torque multiplied by the reduction of the gear train. Since all gearing does have some internal losses, the actual output torque will be less than the calculated no-loss value by a factor known as gearing efficiency. The efficiency of a gear train will vary with the number of gears. For rough approximation, a value of 7% to 10% loss per gear may be assumed. When precise calculation is necessary for a specific gear reduction, values should be obtained from the factory.
When high gear reductions are used, the torque at the output shaft may exceed the strength of the gearing. For this reason a maximum torque value, a gear torque rating, must be specified to prevent gearing damage. Gear torque ratings are based upon a uniform, steady torque load. Dynamic or shock loads impose stresses which can exceed gearing strength or cause early fatigue failure. To some extent this can be taken into consideration in a reduced gear torque rating. However, dynamic braking with inertial loads or locking of the output shaft can force the gearing to absorb destructive amounts of kinetic energy stored in the momentum of the load or the rotor with immediate or rapid failure. As an example, permanent magnet motors with gear trains should not be stalled since the inherent pulsating nature of the stall torque can cause gearing damage in a short period of time.