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Selecting the correct gearhead for a specific purpose means considering many requirements. The most obvious are the physical dimensions and torque capacity, with manufacturers placing great emphasis on the performance figures such as backlash. However, there are many more considerations.

The most important reason for using a gearhead is to increase torque.

Large electric motors are expensive, so it is cheaper to fit a gearbox to a fast rotating motor to produce greater torque than it is to increase the size of the motor. Also, gearheads can improve the dynamic response of a servo system if the inertia between the load and motor is correctly matched.

The first performance criterion considered is usually the backlash of the gearhead; this is the amount by which the width of a gear tooth exceeds the thickness of the engaging tooth, measured as the angle at the pitch circle of the gear.

For many applications, such as a car gearbox, backlash is of little concern and is in fact necessary to allow space for lubrication of the teeth. However, when it occurs in a servo system it will lead to positioning and repeatability errors unless compensated for, particularly if direction is reversed.