PRINCIPLES OF NAVAL ENGINEERING 



consideration lias been given to the source of 

 power or to the mechanisms which cause these 

 parts to operate. 



In many cases, the mechanism which trans- 

 mits power for the operationof the engine valves 

 and blower may also transmit power to parts 

 and accessories which are components of vari- 

 ous engine systems. For example, such items 

 as the governor; fuel, lubricating, and water 

 pumps; and overspeed trips, are, in some en- 

 gines, operated by the same mechanism. Since 

 mechanisms which transmit power to operate 

 specific parts and accessories may be related 

 to more than one engine system, such operating 

 mechanisms are considered here before the 

 remaining engine systems are discussed. 



The parts which make up the operating mech- 

 anisms of an engine may be divided into two 

 groups: the group which forms the drive mech- 

 anisms and the group which forms the actuating 

 mechanisms . The source of power for the oper- 

 ating mechanisms of an engine is the crank- 

 shaft. 



As used in this chapter, the term drive 

 mechanism identifies the group of parts which 

 takes power from the crankshaft and transmits 

 that power to various engine parts and acces- 

 sories. In engines, the drive mechanisms 

 does not change the type of motion, but it may 

 change the direction of motion. For example, 

 the impellers or lobes of a blower are driven 

 or operated as a result of rotary motion which 

 is taken from the crankshaft and transmitted to 

 the impellers or lobes by the drive mechanism, 

 an arrangement of gears and shafts. While the 

 type of motion (rotary) remains the same, the 

 direction of motion of one impeller or lobe is 

 opposite to that of the other impeller or lobe 

 as a result of the gear arrangements within 

 the drive mechanism. 



A drive mechanism may be of the gear, 

 chain or belt type. Of these, the gear type is 

 the most common; however, some engines are 

 equipped with chain assemblies, A combination 

 of gears and chains is used as the driving 

 mechanism in some engines. 



Some engines have a single drive mechanism 

 which transmits power for the operation of en- 

 gine parts and accessories. Inother cases, there 

 may be two or more separate mechanisms. 

 When separate assemblies are used, the one 

 which transmits power for the operation of the 

 accessories is called the accessory drive . Some 

 engines have more than one accessory drive. 

 A separate drive mechanism which is used to 



transmit power for the operationof engine valves 

 is generally called the camshaft drive or timing 

 mechanism . 



The camshaft drive, as the name implies, 

 transmits power to the camshaft of the engine. 

 The shaft, in turn, transmits the power through 

 a combination of parts which causes the engine 

 valves to operate. Since the valves of an en- 

 gine must open and close at the proper moment 

 (with respect to the position of the piston) and 

 remain in the open and closed positions for 

 definite periods of time, a fixed relationship 

 must be maintained between the rotational speeds 

 of the crankshaft and the camshaft. Camshaft 

 drives are designed to maintain the proper 

 relationship between the speeds of the two shafts. 

 In maintaining this relationship, the drive causes 

 the camshaft to rotate at crankshaft speed in a 

 2-stroke cycle engine; and at one-half crank- 

 shaft speed in a 4-stroke cycle engine. 



The term actuating mechanism, as used in 

 this chapter, identifies that combination of parts 

 which receives power from the drive mechanism 

 and transmits the power to the engine valves. 

 In order for the valves (intake, exhaust, fuel 

 injection, air starter) to operate, there must be 

 a change in the type of motion. In other words, 

 the rotary motion of the crankshaft and drive 

 mechanism must be changed to a reciprocating 

 motion. The group of parts which, by changing 

 the type of motion, causes the valves of an en- 

 gine to operate is generally referred to as the 

 valve actuating mechanism . A valve-actuating 

 mechanism may include the cams, cam fol- 

 lowers, push rods, rocker arms, and valve 

 springs. In some engines, the camshaft is so 

 located that the need for push rods is eliminated. 

 In such cases, the cam follower is a part of the 

 rocker arm. (Some actuating mechanisms are 

 designed to transform reciprocating motion into 

 rotary motion, but in internal combustion en- 

 gines most actuating mechanisms change rotary 

 motion into reciprocating motion.) 



There is considerable variation in the design 

 and arrangement of the parts of operating mech- 

 anisms found in different engines. The size of 

 an engine, the cycle of operation, the cylinder 

 arrangement, and other factors govern the de- 

 sign and arrangement of the components as 

 well as the design and arrangement of the mech- 

 anisms. Three types of operating mechanisms 

 are shown in figures 22-24, 22-25, and 22-26. 



The mechanisms which supply power for the 

 operation of the valves and accessories of gaso- 

 line engines are basically the same as those 



570 



