It should be said here that the pin-roller gear, already described, as used on the Monarch and others, is free from any possible objection of “end-thrust,” as there is no tendency to shove the shaft either backward or forward.
THE PROBLEM OF EFFECT UPON BEARINGS.
A question arises as to the effect upon the bearings and the balls in them when any sort of shaft is used for chainless driving. Looking again at the cut of the shaft and adjacent parts, the reader will see that power applied on the pedal tends to roll the pinion directly away from the large gear, because if the pinion could roll clear away the axle would then be left free to turn; similarly, pressure at the back end tends to push apart those two pinions also. This pressure to separate comes upon the balls and the bearing surfaces, there being nothing else to take it. It is true that the draw of the chain, already explained, is thrown at once directly on the balls and the bearing surfaces, and that no large amount of trouble has been caused thereby, in all the last six years of use of chain driving. But it seems well settled that the old “League” bevel chainless did develop an experience of breaking balls and cups and cones, and it is claimed that bevel-gears produce a peculiar twisting strain on bearings. How much there is in this claim, and what precautions have been or can be taken by makers to meet the difficulty, time will show. Yet it should be said that the pin-roller gear, while free from any exposure to “end-thrust,” must take its chances with the other chainless models which have connecting shafts in respect to “side-thrust” on the bearings.
EFFECT OF “SIDE-THRUST” UPON THE FRAME.
A question arises as to the effect of “side-thrust” as distinguished from “end-thrust” upon the frame (as well as the bearings) in chainless driving. This “side-thrust” is under new conditions and in a somewhat different way; but it is not a new thing—it exists in chain driving, because it always exists. When power is applied to any structure consisting of several movable parts put together, the part to which the impulse is directly applied pushes first on the parts next to it; they pass the push to other parts, and so along the line (as if the several parts were links in a chain) until at last, all the parts having refused to either break or be shoved out of place, the load aimed at is moved. In case of the bicycle this load is the moving of the structure and its rider along the ground. If any of the parts involved could break more easily than the movement along the ground is accomplished that breakage would occur, instead of the movement intended. This is only one example of the law, already stated, that the yield is always in the direction, or at the place, of least resistance—the weakest thing gives up.
It was said just now that the pinion on the forward end of the shaft tries to roll away from the gear wheel which pushes it; the pinions at the rear also try to roll away from each other. Thus they put a side pressure on their bearings, as stated, but the same pressure comes on the frame which holds the bearings. At the crank axle this tends to crowd the fork sides toward or from each other, according as the teeth on the large gear face in one direction or the other; at the rear the tendency is to separate the forks. This tendency is to simultaneously crowd upon the balls, to spread open the forks and to press the teeth of the gears into closer contact.
Since the roller-geared and the bevel-toothed types must meet the same pressure on the bearings, they are alike in this pressure on the frames. As already remarked, each of them escapes entirely the heavy pressure which the pull of the chain puts on the axles and their bearings; in place of this they get other and different strains, as just described.
Observe that we do not say these strains will not be successfully resisted—that would be prediction. Some further strengthening of the frame might perhaps be had, and in fact the doubling of the fork on the chain side of chain-drivers, to get additional stiffness, is not unknown in present English practice. It might even be suggested as a fair question, whether a new or somewhat modified form of frame ought not to have been devised for chainless driving instead of applying it to a form never intended for it.
THE QUESTION OF THE GEAR TEETH.
The endurance of the gear teeth is also a question to be decided by use. It has been said that “the teeth are so designed as to be relatively stronger than the cranks and under excessive strain the cranks will break first;” also that “the individual parts are stronger than the elementary parts of the chain.” We have seen cranks tested, in regular shop routine, by samples taken out of each small lot, under a measured load of 1,000 pounds, and have seen them show their quality by returning to the straight line when the load was removed. The cross-section of an average crank is three to five times that of a bevel or radial tooth. In practice, cranks do not break; some other part, less strong, breaks when something must, and so the statement that the bevel tooth is stronger than the crank which is to be measured against it under load seems rather too forcible. The comparatively slight tooth must bear the same strain which comes on other parts and the very small though real bit of elasticity or “give” which the chain possesses, by virtue of being made up of many parts joined together, is lacking in gears of any kind; the strain on those is “solid” and unrelieved. The fact that breakage of a sprocket (unless by some collision or extraordinary fall) is a mishap almost unheard of does not insure the gear tooth in the least—the two are not the same case. The sprocket tooth is very thick in the direction of the strain, and the pull of the chain comes on not less than five teeth at once on the rear sprocket and twice as many on the front, thus dividing the load; the gear teeth, on the contrary, are thin, and the strain is concentrated on not more than two at a time, practically upon one. Yet we must distinguish here the bevel and the spur-gear tooth from the peculiar teeth on the pin-roller gear; the latter are so thick that no doubt of their strength need be raised.