TYPES, ADJUSTMENTS AND REPAIRS Main engine bearings may broadly be divided into two classes; namely, adjustable and non-adjust able. The former, of course, embodies the addition of a wedge and, as applied to the vertical type engine, is outlined in Figs. 7 and 23.
Adjustable Bearings for Vertical bearing design is found mainly on the older engines, such as the American Diesel. It follows gas engine practice of fifteen years ago. At that time the rapid wear of the main bearings seemed to demand some form of adjustment. It is now apparent that the trouble lay in excessive bearing pressures. With more liberal bearing area this difficulty of rapid wear has largely disappeared.
In the hands of an experienced operator the adjustable feature has its attractions. It is well-nigh impossible to have three or four bearings with babbitt liners of uniform characteristics. The babbitt first run out of the ladle is of a density considerably higher than the bearing that is poured last. The natural result of this non-uniform density is the variation in the bearing wear. To the engineer who is versed in the finer adjustments of a Diesel engine the realignment of the bearings is not a difficult task. In readjusting the bearings the first step is to remove the bearing tops, then thoroughly wipe off the shaft and bearings after the shaft has been raised by a jack set at each end of the frame. After the shaft is lowered on to the bottom bearing, it should be aligned by using a spirit level. The two end bearings are adjusted until the shaft is level. The inside bearings are then brought up snug against the shaft by means of the-wedges. The connecting rod big-ends are next made fast and the clearance of each cyl inder measured. Often, when the engine has been badly out of alignment, the readjustment of the bearings alters the clearance of one of the end cylinders. This, of course, must be taken care of by varying the thickness of the shims between the connecting rod and its big-end.
In making corrections for misaligned bearings, it is highly important that the bearing bolt nuts be tightened up very snugly and a lock nut provided. The same applies to the side set-screws of both the wedge and the bearing where they are used, as in Fig. 7. If either the bolt or set-screw works loose, the bear ing will shift sidewise, allowing the shaft to bend at each impulse. Many shafts have been fractured because of this unrestrained flexure. Each time the crankcase is opened the bearings should be examined and the bolts tightened.
Adjustable Bearings on Horizontal Engines.—The direction of the bearing pressure, due to the explosion in the cylinder, makes the adjustable bearing well-nigh imperative on horizontal engines. The cylinder pressure against the piston head may be
said to act in three directions. Part acts in a vertical direction against the cylinder walls; the remainder has its direction along the connecting rod. This latter force is separated into two com ponents, one acting tangentially to the crank-pin, producing rotation, while the second component acts against the lower bear ing in a direction dependent on the crank position; but at all points on the outward or power stroke its direction falls between the horizontal and vertical plane of the lower half of ,the bearing. If the bearing be of two-piece construction, the wear resulting from this bearing pressure cannot be taken up. The bearing wears oblong, and the engine soon pounds.
Means must be provided for compensation for this wear, and the quarter-box designs of bearing are the natural selection. The adjustable or quarter-box design may follow either of two types; namely, a three or four-piece bearing. In actual opera tion the three-piece is as satisfactory as is the four-piece bearing. The pressure is against the bottom and the front quarter, con sequently there is practically no wear on the rear or cylinder side of the bearing. The wear on the bottom is usually insignificant since it receives but little pressure other than that due to the weight of the flywheel and shaft. The front quarter experiences the greatest wear and must be given constant attention. Ordi narily it will be found that the two main bearings do not wear at a uniform rate. While the wear on each bearing should be taken up as it develops, it should not be forgotten that the lower shell and the rear quarter also require . attention. Periodically the shaft should be realigned and, if it is proven out of line, the lower and rear shells shimmed or wedged up the proper amount. Since the clearance space between the cylinder head and the pfston is small in the Diesel engine, there is a danger of excessive compres sion pressureS if the wear on both front and back quarters is taken up by the front quarter only, for this tends to throw the shaft toward the cylinder. Usually the horizontal engine em ploys an extension shaft and outboard bearing. This outer bearing along with the main bearing adjacent to the flywheel carries the weight of the wheel. The flywheel weight on this main bearing causes more rapid wear on the bottom quarter than on the other main bearing. Realizing this the operator should see that the bearing does not become low.