The terms SPEED RATIO and GEAR RATIO are frequently used in descriptions of gear-type mechanisms. Both ratios are determined by dividing the number of teeth on the driven gear by the number of teeth on the driving gear. For example, assume that the crankshaft of a particular engine is fitted with a driving gear that is half as large as the meshing, driven gear. If the driving gear has 10 teeth and the driven gear has 20 teeth, the gear ratio is 2 to 1. Every revolution of the driving gear will cause the driven gear to revolve through only half a turn. Thus, if the engine is operating at 2,000 rpm, the speed of the driven gear will be only 1,000 rpm; the speed ratio is then 2 to 1. This arrangement doubles the torque on the shaft of the driven unit. The speed of the driven unit, however, is only half that of the engine.

On the other hand, if the driving gear has 20 teeth and the driven gear has 10 teeth, the speed ratio is 1 to 2, and the speed of the driven gear is doubled. The rule applies equally well when an odd number of teeth is involved. If the ratio of the teeth is 37 to 15, the speed ratio is slightly less than 2.47 to 1. In other words, the driving gear will turn through almost two and a half revolu-tions while the driven gear makes one revolution.

The gear with the greater number of teeth, which will always revolve more slowly than the gear with the smaller number of teeth, will produce the greater torque. Gear trains that change speed always change torque. When speed increases, the torque decreases proportionally.

NOTE: The mechanical force of any driven unit will always be somewhat LESS than that of the driving unit due to power losses caused by such things as friction and lost motion. Therefore, out-put horsepower will always be equal to input horsepower minus any losses.

TYPES OF DRIVE MECHANISMS

You have just learned that the torque or the speed of an engine may need to be changed to satisfy the torque and speed requirements of the driven mechanism. The term INDIRECT DRIVE, as used in this chapter, describes a drive mechanism that changes both speed and torque. Drives of this type are common to many marine engine installations.

When the speed and the torque of an engine do NOT need to be changed to drive a machine satisfactorily, the mechanism used is a DIRECT DRIVE. Drives of this type are commonly used when the engine furnishes power for the operation of auxiliaries, such as generators and pumps.