The Gasoline Motor

by Harold Whiting Slauson

The expert will judge of the condition of a motor by the wear that has occurred in the bearings rather than by any exhibition of temporary power that it may develop in a short test, and it is for this reason that the "general public" runs a risk whenever it buys a second-hand car that has not been thoroughly overhauled by a reputable factory or inspected by a competent engineer. The bearings are in reality the vitals of the motor, and when these are worn beyond the point of easy adjustment or renewal, the repairs necessary to place the machine in good condition would oftentimes cost more than the entire engine is worth. But even in a badly-worn motor, the bearings may be "taken up" and "doctored" so that, for a while at least, the engine will seem to run perfectly and develop its full power. This will not be for long, however, and soon the motor will begin to pound, knock, and rattle until an examination will bring to light the true condition of the bearings.

In no machine are the bearings subjected to more severe usage than in the automobile motor. In order that the motor car power plant shall be light in weight and occupy but a small amount of space, the power must be transmitted at high speeds. In many an automobile motor, the pressure imparted to a single bearing during a certain portion of its revolution may frequently be well over two tons, and in this same bearing, the "speed of rubbing" may approach eight or nine hundred feet per minute. In other words, at normal speeds of the motor, about a sixth of a mile of steel surface will rub over a certain point in each crank shaft bearing during every minute that the engine is running.

When properly lubricated, an iron or steel shaft will run in almost any kind of a metal bearing that is sufficiently strong to carry the weights and pressures imposed upon the shaft. The friction generated between two different metals that rub against each other, however, varies according to the composition of those metals, and consequently it is advisable to employ some material for a bearing that will offer a minimum resistance to the turning of the shaft. Friction must be reduced between all moving surfaces in order that the mechanical efficiency of the machine shall be high, and it is in the bearings that a large amount of power may be absorbed.

But even between the best-lubricated surfaces, employing the most efficient metal as a bearing, some wear is bound to occur. The crank shaft of a four- or a six-cylinder motor is forged or sawed from one piece of steel, and with the accurate machining, finishing, and grinding to which it is subjected, it becomes an expensive part of the engine. Consequently it is advisable that the wear of bearings of such parts shall be restricted to the "boxes" or surrounding stationary metal in which the shaft revolves at these points. In order that all wear shall occur here, rather than in the shaft, the boxes are made of or lined with a softer metal. If the crank shaft is of hard steel, the bearing metal may be of brass or bronze, but it has been found that babbitt metals give the most satisfactory service for such conditions—particularly as a sufficiently hard crank shaft is difficult to produce commercially.

Not only is a babbitt metal softer than the steel of the shaft and consequently receives practically all the wear of the bearing, but it has the added advantage of melting at comparatively low temperatures. At first thought, this may seem like a doubtful advantage, but in case of a failure of the oil supply to that bearing, this characteristic may be the means of saving the crank shaft, and possibly the crank case, cylinders, and connecting rods, from rack and ruin.

The purpose of lubrication is to reduce friction between the two surfaces in contact. Friction generates heat, and consequently the temperature of a bearing to which a sufficient supply of oil is not delivered will be raised to a very high point. This high temperature will cause both parts of the bearing to expand, with the result that the fit becomes very tight and the shaft binds or "seizes" in its box. This is the familiar "hot box," so often the bane of railroad men, and if the shaft is still run under these conditions, the bearing material will be torn out and the surface of the shaft, axle, or whatever the revolving portion happens to be, will be cut and abraded, oftentimes beyond the possibility of repair. It is such accidents as these that are prevented by the use of an easily-melted babbitt metal.

If the oil supply becomes insufficient so that the temperature of the bearing is raised above a certain point, the babbitt metal will be melted and will run out of its container before any damage can be done to the shaft. Efficient running cannot, of course, be obtained with the bearing "burned out" in this manner, but the babbitt is quickly and easily renewed and serves as a sort of fusible safety valve that saves many an expensive crank shaft replacement.

Babbitt metals may be of various compositions and proportions and many contain lead, but those which have been found to give the best results for use on the crank shafts of automobile motors are composed only of tin, antimony, and copper. If lead is used at all for this purpose, it should not appear in proportions above one per cent of the total composition. Inasmuch as a babbitt metal will fuse at a comparatively low temperature and is much softer than steel, it is obvious that such a material will not withstand heavy pressures unless reinforced and is unsuited for structural purposes. Consequently the babbitt is placed in the bearing box in the form of a thin lining within which the shaft revolves.

When the shaft is "lined up" in the box, the hot babbitt metal may be poured in until the space is entirely filled. When the babbitt cools, the shaft may be turned, and when lubricant has been introduced in the oil grooves which should have been provided for the purpose, the new bearing will be ready for use. It is not to be expected that the majority of motor car owners will rebabbitt the crank shaft bearings themselves, but it is necessary to understand the general principles of such bearing design in order to inspect the motor intelligently and to determine upon the repairs needed.

The above method of renewing "burned out" bearings applies to babbitts in general, but the severe usage that automobile engine crank shaft and connecting rod bearings are called upon to withstand necessitates the exercise of a certain amount of additional care. It is necessary that the box shall fit the shaft perfectly, so that there can be no "play," and yet the shaft must be allowed to turn easily within its surrounding babbitt metal.