“What I claim, and wish protected by Letters Patent is,—
“In bearings for bicycles, tricycles, or carriages, the combination of hardened conical or curved surfaces, hardened spherical balls, and the means, substantially as shown and described, of adjusting or setting up the parts, for the purposes set forth.
“Joseph Henry Hughes.”
Other forms, such as the disk pattern with an annular groove upon its face, have their special uses.
As to friction, ball-bearings may be said to reduce this to nothing, since in mathematical calculations, rolling friction on hard surfaces is usually neglected, as compared with sliding friction. In actual practice this would not quite hold good, since oil and dirt will make a difference. The balls, in the ordinary bearings in the market, roll upon conical, spherical, or cylindrical surfaces. In either of the last two cases the radius of curvature of the box is so much greater than that of the ball that the effect is the same as upon the cone, and in all cases where a bearing is well constructed the action is the same as that of a ball rolling upon a flat surface. True, some friction results from the contact of the balls with each other, but as there is no force driving them together, it is very slight.
Annular, ball-bearing.
So long as the bearings are new and properly made, each ball touches and rolls along what may be considered a mathematical line, and there is, in fact, no friction worthy of consideration. Nevertheless there is some, and in time a small groove is worn, or rolled, into the bearing, which groove just fits the ball. The friction is greater now than before, and increases with the deepening groove until, finally, when the depth of the groove equals the radius of the ball, the friction reaches its maximum and would be at that time nearly equal to one-fourth of the amount of friction engendered if the ball actually slid in the groove. The ball would then roll on lines along the groove through points c, c thirty-eight and one-fourth degrees around from E towards D, as shown in the annexed diagram. ([Fig. 1].)
Fig. 1.
Rolling Lines, ball-bearing.
The reader can form a tolerably clear idea of the amount of friction caused by the ball sliding without rolling; let this then be the unit. Also let the radius of the ball be the unit depth of groove. The following table gives roughly in these units the frictions for the groove depths expressed in tenths.