The Gasoline Motor

by Harold Whiting Slauson

Next in importance to the main bearings of a crank shaft are those by which the connecting rods communicate their motion to the cranks. These are known as the crank pin bearings or the "big end" of the connecting rod bearings. But inasmuch as the upper, or smaller, end of the connecting rods are termed the wrist-pin bearings, the other end may be called simply the connecting rod bearing.

The connecting rod bearings are similar to the main bearings described in the foregoing pages and are renewed and adjusted in the same manner. It is probable, however, that these receive a greater amount of wear than do the main bearings, inasmuch as the former obtain the direct impact of the force of each explosion. Furthermore, the box of the connecting rod bearing describes a complete circle with each revolution of the crank shaft, in addition to the "internal rotation" of the crank, while an alternate push and pull is delivered to it by the connecting rod on its various strokes.

Consequently it is the connecting rod bearings that will become loose and require "taking up" before any attention need be bestowed on the main bearings. The wear will increase in the connecting rod bearing as the play becomes greater, and if matters are not remedied, the box may eventually be broken, with the result that the end of the connecting rod thus freed will start on the "rampage" and will punch several pieces out of the bottom of the crank case.

Brass or bronze bearings may be used at the big end of the connecting rods, but the large majority of motor car engines are provided with babbitted bearings at these points. It is especially necessary that these bearings should be scraped to a perfect fit and that the shims should be adjusted properly so that no side play will be apparent when the connecting rod is moved transversely to the length of the crank shaft. When renewing the babbitts of connecting rod bearings care should be taken to allow the connecting rod to swing free before the molten metal is poured in. If this is not done, the connecting rod may be forced slightly to one side or the other and will be held permanently in this position when the babbitt cools. This will induce a slight side thrust in the connecting rod, which will be communicated to the piston, with the result that the side of the latter and of the portion of the cylinder wall against which it moves will be scored and worn unduly.

Inasmuch as the connecting rod bearings are subjected to such a variety of strains, and as looseness at these points will result in serious wear, it is doubly necessary that the nuts and bolts holding the bearing caps in place should be securely wired or held tightly by means of the previously-mentioned cotter pins. It is evident that the base of the large end of the connecting rod forms the upper half of the bearing box, while the cap constitutes the lower end and is attached from the bottom.

The connecting rod bearings on some motors are hinged at one side so that the cap may be turned away from the crank shaft when it is desired to remove the connecting rod. In this case the hinge replaces the one or two bolts or nuts on one side of the box and is held in the proper position by those on the other side. While it may be easier to adjust a bearing provided with such a cap, the results obtained can hardly be expected to be as satisfactory for high-grade service, as is the case when the shims may be used on both sides of the two halves of the bearing.

The wrist-pin bearing is located at the upper, or small, end of each connecting rod, and, although it also carries the full force of each explosion, it is not subjected to as great wear as is the bearing at the other end of the connecting rod. The reason for this is that this bearing does not revolve and its friction surface is reduced to the comparatively small arc through which the connecting rod swings. Wear can occur here, however, and because this bearing is more inaccessible than is the crank shaft or connecting rod bearing, trouble at the wrist pin is often overlooked.

The wrist pin can only be reached by the removal of the piston and connecting rod. In the majority of designs the wrist pin is placed in the sides of the piston and is held stationary by small keys or by set screws. In this case, the bearing surface is formed by the wrist pin and the small end of the connecting rod, at which point the greatest wear occurs. This bearing is never babbitted, but in order to reduce the wear on the wrist pin—which is generally made of hardened steel—the circular opening in the upper end of the connecting rod is lined with a bronze or brass bushing that forms a bearing fit over the wrist pin. It is this lining, or bushing, that will wear rather than the hardened steel wrist pin, but as the former is easily removed and is not expensive to replace, the renewal of this bearing is a comparatively simple matter.

In other types of wrist pin bearings, the pin is held stationary in the connecting rod opening and turns with it as the connecting rod swings through its arc on each stroke of the piston. With such a design, the bearing surface is formed by each end of the wrist pin and the openings in the sides of the piston walls in which the wrist pin rests. In order to form an easily-replaced bearing surface, these openings in the piston walls are lined with brass or bronze bushings that receive the major part of the wear, as has been described in connection with the bushings fitted to the opening at the small end of the connecting rod.

There is nothing complicated or mysterious connected with the renewal or repair of bearings, but the man who makes such replacements or adjustments must be an accurate and careful worker, and while he need not be a "born machinist," he must at least possess the "knack" of handling tools properly. And he must, above all, realize that the designers and manufacturers of his motor have been dealing in measurements of the thousandth part of an inch and that too great care cannot be taken in the repair of bearings to obtain a perfect fit.