The most reliable means for changing shaft speed remains a mechanical variable-speed transmission.

These drives are used to provide a variable output speed from a constant-speed power source (as in a machine tool driven by a constant-speed ac motor) or to provide torque increase for a variable-speed power source (as in an automobile). Variable mechanical drives are less costly than competing electrical variable-speed drives and their control is much simpler. But mechanical drives often are not as durable and cannot be controlled as precisely as electrical drives. And except for gear drives, the mechanical type generally cannot transmit as much power as electricaldrives when variable speed is essential.

The basic types of adjustable-speed drives are geared transmissions (which provide only specific fixed-speed ratios), variable-pitch belt and chain drives (which provide infinitely variable speeds), traction drives (also infinitely variable), and fluid drives.


These are the most durable, rugged, and efficient of all adjustable-speed drives. But they are capable of providing only a specific number of fixed gear ratios. Normally chosen for applications involving heavy loads or requiring long, trouble-free life. Generally, more expensive than belt or chain drives. Gear drives are commonly classified according to end use:

Automotive transmissions: Used as main transmissions in cars, trucks, farm machinery, and earth-moving equipment. Usually provide from four to 10 speeds.

Auxiliary transmissions: Usually installed behind the main transmission to increase available ratios.

Transfer cases: Provide additional power outlets (as in a four-wheel-drive vehicle) or provide offset from normal drivelines.

Power takeoffs: Usually mounted beside main transmission and driven by an additional gear in that transmission. Similar to a transfer case.

Marine gears: Transmissions carrying power to the propeller on a marine drive. Differ from other transmissions in that they generally provide single forward and reverse speeds and use friction-type shifting clutches.

Hydraulic drives: Gearboxes connecting power source and hydraulic pumps in hydrostatic drives.

Industrial transmissions: A broad category applying to any transmission powering machinery other than that described above. Many have integral power packages, such as electric or hydraulic motors, or they may be an integral part of driven components.

Differentials: A set of gears with three independent, rotating members with a speed and torque relationship to each other. This definition creates two application types.

The first consists of one input and two outputs. The automobiledifferential is the best example here. The important factor in this type of application is the two outputs are connected mechanically. In automobiles, the connection is the road. The differential automatically balances speeds and torques between the two wheels because only the sum of the wheel speeds is defined by the input speed. That means each wheel will rotate at the speed required to maintain the predetermined torque relationship of 1:1 between them.

The second application type has two inputs and one output. Less well known than the first, this technique solves industrial problems when the superimposition of one motion relative to another is required, such as timing of cutoff, registration control on printing presses, tension control, and phase shifting on textile industry equipment.

Differential efficiency is a function of the relative speed of the three elements. As relative speeds increase, the inherent losses due to basic gear efficiency, seals, and bearings also increase; thus, efficiency decreases.