There are various methods for making required changes in speed and direction during the transmission of power from the driving unit to the driven unit. In most of the installations with which you will be working, the power is transmitted by a drive mechanism consisting principally of gears and shafts. The process of transmitting engine power to a point where it can be used in performing useful work involves a number of factors. Two of these factors are torque and speed. The crankshaft of an engine supplies a turning force to the gears and shafts which transmit power to the driven unit.
Gears serve to increase or decrease torque.
For example, an engine may not produce enough torque to turn the shaft of a driven machine if the connection between the driving and driven units is direct, or solid. If the right combination of gears is installed between the engine and the driven unit, however, torque is increased, and the turning force is then sufficient to operate the driven unit. Another factor related to torque and to the transmission of engine power is engine speed. If maximum efficiency is to be obtained, an engine must operate at a certain speed. To obtain efficient engine operation in some installations, the engine may need to operate at a higher speed than that required for operation of the driven unit.