BEARINGS, bearings are bearings involving the principle of rolling friction, as distinguished from sliding friction. An ordinary shaft turning in a plain journal slides around on a layer of some lubri cating substance. If the lubricant is good and properly applied, little energy or power is lost in the heat produced by rubbing friction. If not, then much heat is produced, often to such an extent that the oil or grease is set on fire, dried up, thereby causing a so-called shot box' or journal. The starting friction of a plain journal differs very greatly from the friction of motion. The reason is that while at rest under a heavy load, the film of the lubricant is penetrated, and contact of metal with metal is established. To overcome this contact, until the bearing has moved far enough to drag the film of oil between the points of contact again, requires much more power. Careful tests show the coefficient of rest to be from .09 to .13 as compared with .05 to .08 for the co efficient of motion. Bearings involvi.ng rolling friction are entirely different in this respect. Slight hibrication, largely to prevent rusting, is needed. Between the shaft and the wheel or other bearing is interposed either a series of balls or rollers of hardened metal, usually steel, or a steel alloy, arranged to revolve be tween the two surfaces..
Ball bearings came into general use with the advent of the modern bicycle; they have Veen highly developed in automobile practice, and have come into general use in light machin ery to ;educe friction, being. markedly superior to plain bearings, especially in case of starting. The balls are made ui a great number of sixes, and positioned in bearings in numerous ways. Since small pieces of metal heat and cool rapid ly, it is a difficult matter to temper steel balls exactly as desired. To have a long life each ball must be uniformly hardened as deeply as possible. Much ingenuity has been displayed in securing accurate tempering, and an exceed ingly good average of balls are marketed at surprisingly low cost. In addition to hardness, uniformity of size and surface polish are essen tial. The only way to know precisely what is the structure and strength of a particular ball is to break it up. However, by microphotog raphy experts are able to judge very accurately the character of balls.
It is apparent that a series of balls, traveling . in a cir'cular raceway in a machinery bearing must occasionally strike each other, jam and rub the surfaces in contact in opposite direc tions. Except for this, sliding friction is wholly eliminated in a properly designed ball bearing, and the balls simply roll on the surfaces. To produce a. good bearing it is essential that the balls shall be as nearly perfect as can be, and in practice good balls will show variations of only 1-10,000 of an inch in diameter. If the balls in a bearing vary in size, the larger balls must bear nearly all the strains and the sooner crystallize and break. The surfaces against which the balls bear are sometimes flat, but usually at least one surface must be curved to keep the balls in position; often both sur faces are curved. The radius of curvature of
the raceway against which the balls bear must always be less than the curvature of the ball, else there will be sliding friction. The less number of balls there are in a bearing the less will be the number of contact points, and the less the friction. But if there is a heavy load to carry, it is necessary to have many balls, and thus distribute the load. It has been demonstrated that the s.peed of rotation has little effect on the carrying capacity. Marked and numerous variations in speed and load, as in automobile.use, reduce the carrying capacity of a ball bearing, hence they are designed with a large margin or surplus of strength. A good ball bearing will have a coefficient of friction of approximately 0.0015.
Ball be.arings are made to carry ordinary load or side-friction, to resist end-thrust of a shaf t, and sometimes for angular load, the positioning of the bearing surfaces determin ing how they.operate. In designing a ball bear ing, the engineer must consider the load or stress .to be.placed on it constantly, and also in exceptional instances; he must also bear in ntind the different speeds of rotation to which it will be subject. If it is a bearing on a vehicle axle, carrying a wheel, he will fix the inner raceway firmly to the axle, but mount the outer raceway loosely on the wheel with a uslip-fit.)) By properly shaping his raceway he can make the bearing carry the end-thrust of the axle as well as the supported load.
Designs of ball bearings have been made with alternate small idle balls between the larger working balls, with a view to reducing back rotation or jamming, but these have not been widely adopted. Another arrangement is to stagger the balls — that is arrange them this 0000 waY:00000• This is accomplished by build ing parallel raceways that slightly impinge. There are some advantages in such a construc tion.
For heavy. loads roller bearings are superior to ball beanngs. Hardened steel rollers of uniform size are mounted in a cylindrical case, that positions them so that they are kept slightly apart. They may then be introduced into the journal box by slipping over the shaft. By ex tending the length of the journal box, and using a sufficient number of rollers, almost any load can be carried with a minirhurn of friction. A 24-inch diameter shaft, provided with a 36-inch journal-box, carrying 38 one-and-a-half inch rolls, was made to carry a load of 575.,000 pounds. Since 1900 the use of roller bearings has become very common. Both ball bearings and roller bearings require lubrication, though very much less than plain bearings need. It is a mistake to supposg that they can be run absolutely without oil. It is very necessary that such bearings be kept free from grit or dirt, as these will cut the balls or rollers in a short time.
Other anti-friction bearings are made by the use of anti-friction metals, that is, soft alloys, as babbitt metal, which is placed in the bearing to carry the steel or iron shaft.