TRANSMISSION OF ENGINE POWER

The main components of an engine have been covered in the preceding chapters of this manual. If the power developed by an engine is to be used to perform useful work, there must be some way to transmit the power from the engine (driving unit) to such loads as the propeller of a ship or boat or the drive shaft of a generator, a compressor, or a pump. This chapter provides general information on how the force available at the crankshaft of an engine is transmitted to a point where it will perform useful work. The combination of devices used to transmit engine power to a driven unit is commonly called a DRIVE MECHANISM.

After reading the information in this chapter, you should be able to identify various components associated with the drive mechanism, such as clutches, gears, couplings, and bearings, in terms of the ways in which they function to transmit the power from the engine to the driven unit. You should also be able to recognize the more common problems associated with these drive mechanisms, some general procedures you should perform for preventive maintenance, and the purpose and content of the Engineers Bell Book.

FACTORS RELATED TO THE TRANSMISSION OF ENGINE POWER

The basic characteristics of an internal-combustion engine make it necessary, in many cases, for the drive mechanism to change both the speed and the direction of shaft rotation in the driven mechanism. 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. You will better understand the way a drive mechanism transmits power if you review chapters 1 through 9 of Basic Machines, NAVPERS 10624-A1. Chapter 6 will be especially helpful to you; it describes the basic types of gears and discusses how gears are used to change the direction of rotation and the speed of a shaft. 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.

TORQUE

Torque, or turning force, is the force that tends to cause rotational movement of an object. 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.

SPEED

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. In other installations, the speed of the engine may need to be lower than the speed of the driven unit. Through a combination of gears, the speed of the driven unit can be increased or decreased so that both the driving and the driven units operate at their most efficient speeds; that is, so that the proper speed ratio exists between the units.