ROLLERS.
Theoretically there is less friction in roller- than in ball-bearings, as there need be no sliding action whatever in the former if well made. But in actual practice no bearing can be made in which there is no tendency of the rollers to run together; and if we place them in a frame to hold them apart we shall have about as much friction as when they rub against one another. The most perfect plan is to place a small roller between each of the larger; with this arrangement the friction is practically nothing. The action of rollers upon the boxes is always a pure rolling friction, which cannot be the case with balls after the slightest groove is worn in the casing.
One reason for rollers being little used is that they tend to work out of line with the axle and box, which causes some ends to get a little in advance of the others, when they can no longer work perfectly. For an oscillating bearing,—that is, one that goes backward and forward, instead of continually around,—I have found rollers very good, since they cannot get much out of line; even when the bearing is a little imperfect, the rollers cannot multiply the imperfection, as they will in one that keeps going on in the same direction. The other great fault of the roller is its non-adjustability, although this can be rectified in the following way:
Roller construction.
The above cut shows a bearing and the construction lines that must be followed in its manufacture. The taper of the axle, roller, and box must all meet in a point, as at a; this arrangement is evident. The roller must be kept in proper position and roll around the large end in the same number of turns as the small end; hence the circumference of the small end of the roller must bear the same relation to the circumference of the larger as the relative ends of the axle and box bear to each other. The geometrical conditions are as follows: π being the relation of circumference to the diameter, referring to the diagram, we have b c : f g :: c d : g h :: b e : f i; hence π b c : π f g :: π c d : π g h :: π b e : π f i. Now, by virtue of the last formula, when the axle or box is revolved, each end of the roller will travel through exactly the same number of degrees around the axle and in the box, wherefore the axle rollers and box all keep straight.