The valve-actuating mechanisms are those parts that transform the rotary motion of a drive mechanism into reciprocating motion. Reciprocating motion is used for the operation of engine cylinder valves. The camshaft is the

Figure 4-28.Engine barring device.

principal part of a valve-actuating mechanism. Rotary motion of the cams on the camshaft is changed to reciprocating motion, and power is transmitted to the various cylinder valves (intake, exhaust, fuel injection, and air start) by means of rocker arm or tapped assemblies. These assemblies make contact with the cams by the use of cam followers.

The process of transmitting the power developed in an engine to useful energy involves the motion of many parts. The major parts are the piston, connecting rods, and crankshaft; however, many related parts must be considered in the process of changing reciprocating motion to rotary motion. For example, a piston, which may be of the trunk type or the crosshead type, receives the force of combustion and transmits it to the crankshaft through a connecting rod.

To accomplish their functions, pistons are fitted with piston rings, such as compression and oil control rings. Piston rings function to maintain a gastight seal between the piston and cylinder wall, to assist in cooling the piston, and to control cylinder wall lubrication.

Trunk-type pistons are fastened to the connecting rods by pins which may be stationary, semifloating, or full-floating. Side thrust created by combustion and the motion of the moving parts is received by the cylinder wall through a trunk-type piston. The crosshead assembly absorbs the side thrust in engines fitted with the crosshead type of piston.

The crankshaft, the largest of the moving parts of an engine, receives the power impulses from all cylinders of the engine, transforms the motion of the pistons and connecting rods into rotary motion, and transmits the resulting torque to the flywheel or driven unit.

Other important parts which must be considered in connection with moving parts of an engine are bearings. Bearings may be of the antifriction type or friction type. Both types can be found on diesel installations. Antifriction bearings are commonly used in the engine accessories while friction bearings are commonly used within the internal structure of the engine.

As in the case of the principal stationary parts of the engine, you should be thoroughly familiar with all of the major moving and actuating components of an engine. You should know the functions and operating principles of these parts and how these parts are related to the stationary parts. You should be able to associate each component, as discussed separately, with other related components of the engine and how each part or assembly is related to the cycle of engine operation. If you are uncertain concerning any of these areas, go back and review this information before proceeding to the next chapter.