The Gasoline Motor

by Harold Whiting Slauson

As was stated above, the shaft may be easily loosened from the babbitt metal after the latter has cooled, and this would form a satisfactory type of bearing were it not advisable that some means be supplied by which the wear could be taken up without renewing the entire babbitt lining. The bearing boxes of the crank shaft are each made in two halves, the lower portion being cast integral with the crank case, while the upper half is in the form of a separate cap that may be held in place by two or four bolts. In this case, it is necessary that the boxes shall be in two sections, for the shape of the crank shaft prevents it from being slid into place lengthwise, and consequently it must be placed on its bearing from the top. In some designs of motors the bearing caps form the lower half of the box, but as in this case the base of the motor must be inverted in order to remove the crank shaft, the caps will still be considered as the "top" halves of the boxes.

There may be dove-tail grooves cut in the inside of the halves of the boxes to retain the babbitt metal after it has been poured in place. Consequently, in order to remove the cap after renewing the babbitt lining, the babbitt metal must be cut in two at the joint between the two halves of the box. The two halves of the box, instead of fitting closely together, are separated by thin strips of copper or fiber known as "shims" that serve to relieve the shaft from the pressure of the bolts when the bearing cap is screwed in place. In other words, the two halves of the box must be held tightly in place by means of the bolts and nuts, but none of this pressure should rest on the revolving shaft, as this would bind it and prevent it from turning easily. Consequently by "building up" the space between the two halves with these thin shims the proper adjustment may be obtained.

These shims provide the method of taking up the wear in the babbitt that will eventually result. By loosening the box retaining bolts and removing the required number of shims, the halves of the box will be brought closer together. When the bearing cap is screwed securely in place, the shaft should be able to revolve freely without binding, and yet the fit should be sufficiently tight to prevent any "play" at right angles to the length of the shaft.

The pressure of a shaft should not be concentrated in one place, but should be distributed over as large a surface of the babbitt metal as is possible. A few years ago, when renewing or repairing a bearing, it was considered sufficient to pour in the molten metal or to remove the proper number of shims—and the bearing was then said to be ready for its work. But even though no play was apparent, it was possible that the shaft rested on only a few portions of the bearing surface; and the increased attention that is now paid to the details of automobile construction is no better exemplified than in the fact that nearly all bearings are "scraped" in. This operation is simple and consists merely in removing any slight excess babbitt metal so that the lining fits the shaft throughout its entire length and circumference. The babbitt is sufficiently soft to enable it to be peeled or scraped with a sharp tool provided for the purpose, and no great degree of skill is necessary in obtaining the required fit.

In order to determine at exactly what portions of the babbitt lining the pressure is too great, a dye or paint known as "blueing" is used. The bearing portion of the crank shaft is painted with this, and the cap is then screwed in place. If the crank shaft is then turned and the cap removed, it will be found that the blueing has been transferred from the bearing to the portions of the babbitt metal on which the pressure is the greatest. These portions should then be shaved with the tool mentioned above, and the same test repeated. As the excess metal is removed, it will be found that the blueing gradually is deposited over a larger area of the babbitt, but it is not to be supposed that the fit can be made so perfect that the color will be distributed evenly over the entire surface. Care should be taken to screw the bearing cap onto the shims as tightly as possible each time the blueing test is to be made.

There is nothing that will heat a bearing so quickly as a poor alignment of the shaft supported by it. For this reason gasoline engine crank shafts are made exceptionally strong and heavy, especially those that are supported only at their extremities, or at these points and in the center of their length. A shaft that is bent or twisted to even the slightest degree will soon "burn out" all of its bearings, regardless of the amount of oil that may be fed to them. This is because of the unequal pressures on the different sides of the bearing that allow no room for the admission of the film of oil or other lubricant that is necessary in all cases to prevent a "hot box."

On the other hand, the bearings must all be in perfect alignment, for to set one slightly "off" would produce the same result as though the shaft were bent. It will be seen that the use of babbitt produces a "self-aligning" bearing, for the straight shaft may be set in its proper position and the molten metal poured around the interior of the boxes.

As it is highly important that the cap screws or nuts holding the bearing cap in place should remain set as tightly as possible, precautions must be taken to prevent any of these from working loose. This may be done by means of a cotter pin that passes through a hole in each bolt and through a pair of corresponding notches cut in the top of opposite faces of the nut. A notch is generally cut in the top of each face of the nut in order that the latter may be held securely in place in any position. A continuous wire passing through all of the bolts and nuts is sometimes used instead of the individual cotter pins.

Many modern automobile motors are designed with the crank shaft running in ball bearings. The type generally used consists of a row of balls set between the inner and outer edges of two concentric rings. The inside of the outer and the outside of the inner ring are grooved, constituting the ball "race" which forms the surface upon which the balls roll and which, at the same time, serves to hold them in place. Each ball of the same bearing must be made of exactly the same size as its companions—or at least within one or two ten-thousandths of an inch—and each one must be large enough and of sufficient strength to withstand, by itself, the entire pressure in that bearing. The inner ring slips over the bearing portion of the shaft with a comparatively tight fit, while the outer ring remains stationary in its bed in the crank case.

The inner ring turns with the shaft, thus causing the balls to roll in their race. Each ball rolls about its own axis, and the entire series describes a circular motion in the same direction as that taken by the shaft, but considerably slower. Consequently there is no rubbing in such a bearing, all the motion being of the rolling type, and as this reduces friction to a minimum, the balls may be run without oil, although lubrication of the proper kind would certainly not harm the bearing. Ball bearings are adapted only for a two-bearing crank shaft, for inasmuch as the rings must be slipped over the shaft, it would be manifestly impossible to provide a ball bearing in the center, or in any other portion beyond a crank.