Auxiliary drive mechanisms are used in internal combustion engines to maintain a fixed and definite relationship between the rotation of the crankshaft and the camshaft. This is necessary in order that the sequence of events necessary for the correct operation of the engine may be carried out in perfect unison. Timing and the rotation of various auxiliaries (blowers, governor, fuel and lubricating oil pumps, circulating water pumps, overspeed trips, etc.) are accomplished by a gear or chain drive mechanism from the crankshaft. (Some small engine auxiliaries may be belt-driven.)

GEAR MECHANISMS

The principal type of power transmission for timing and accessory drives in most diesel engines is a system of gears similar to those shown in figure 3-24. In some of the larger engines, there may be two separate gear trains, one for driving the camshaft and the other for driving certain accessories.

The type of gear employed for a particular drive depends upon the function it is to perform. Most gear trains use single helical spur gears, while governor drives are usually of the bevel type; reverse and reduction gear units employ double helical gears to balance fore and aft components of tooth pressure.

Small gears are usually made from a single forging, while larger ones are quite often built up in split sections. (See the crankshaft gear in figure

Figure 3-24.Relative arrangement of the gears in an auxili-ary drive mechanism.

3-24.) Most gears are made of steel, although cast iron, bronze, or fiber are sometimes used.

The timing gear train shown in figure 3-24 is used on some two-stroke cycle diesel engines. The camshafts rotate at the same speed as the crankshaft. Note that two idler gears are necessary to transfer crankshaft rotation to the camshaft gears. The idler gears are used because the cam-shafts and crankshaft are displaced a considerable distance. If idler gears were not used, the crankshaft and camshaft gears would have to be considerably larger.

A similar timing gear train may be found in some four-stroke cycle engines, except that the camshaft gear or gears will have twice as many teeth as the crankshaft gear to permit the cam-shaft to rotate at one-half the crankshaft speed. A different type of drive gear mechanism is used for a four-stroke cycle, V-type gasoline engine. The camshaft gears are driven through a train of bevel gears from the crankshaft. This ar-rangement serves to drive not only the camshaft but also other accessories, such as a magneto, or distributor, a fuel pump, and a tachometer. An additional gear, called the oil and freshwater pump drive gear, meshes with the crankshaft gear.

The causes of gear failure (improper lubrica-tion, corrosion, misalignment of parts, torsional vibration, excessive backlash, wiped gear bearings and bushing, metal obstructions, and improper manufacturing procedures) are basically the same as the causes of similar troubles in other engine parts. The best method of prevention is to adhere to the prescribed maintenance procedures and follow the instructions given in the manufacturers technical manual.

Maintenance and repair of gear trains involve a thorough check (for scoring, wearing, pitting, etc.) of the gear shafts, bushings and bearings, and gear teeth during each periodic inspection. Be sure that the oil passages are clear, and that the woodruff keys, dowel pins, and other lock-ing devices are secured to a tight fit in order to prevent longitudinal gear movement. It is essen-tial that all broken or chipped parts be removed from the lubrication system before new gears are installed.

An engine must not be barred over while the camshaft actuating gears are removed from the train. Should the engine be barred over, there is

Figure 3-25.Checking clearance of positive displacement blower lobes.

danger that the piston will strike valves that may be open and extending into the cylinder. Make certain that any gears removed are replaced in the original position. Special punch marks, or numbers (figure 3-24), are usually found on gear teeth that should mate. If they are not present, make identifying marks to facilitate the correct mating of the gears later.

Bearing, bushing, and gear clearances must be properly maintained. If bushing clearances exceed the allowable value, the bushings must be re-newed. The allowable values for backlash and bushing clearances should be obtained from the instruction manual for the engine involved. Usually, a broken or chipped gear must be replaced. Care should be exercised in determin-ing whether a pitted gear should be replaced.