The first operation in vulcanizing a tire is to cut a round hole in the tire where the puncture is, making the hole as small as possible; then the rubber cover which is around the hole is peeled off, so that the canvas of the tire for about three-quarters of an inch around the hole is exposed. Then the inside, as well as the outside of the tire is cleaned thoroughly with a clean rag saturated with naphtha; the naphtha is allowed to evaporate, and then the vulcanizing cement is used in the same way. The cement is allowed to dry, and when dry the cementing operation is repeated, so as to give the surfaces two coats of cement. It is important that both coats of cement are thoroughly dry before inserting the rubber patch, and before doing so soapstone is rubbed on the underside of the patch. Care should be taken not to have any soapstone on the part of the patch which is to be united to the tire. The best way to insert the patch is to roll it around a stiff wire about one-eighth inch thick, holding it closed with the fingers, and when in this position it should be dipped into the naphtha and slipped into the hole quickly; as soon as it enters the hole the patch will open. The patch remains in the tire, and the hole in the tire is sewed up. After this the upper side of the tire is pressed firmly against the patch (which lies on the inside of the tire) with the fingers, or better still, as is done in rubber factories, with a stitcher. This tool resembles a pinker without teeth, and is about one-sixteenth of an inch on its periphery. Then the surface to be vulcanized is covered with uncured gum, flush with the tire and no more. Naphtha should always be used to clean the uncured gum, as well as the part of the tire to which it is to be applied, but before this is done it is cemented as before and the cement allowed to dry. Soapstone is then rubbed over the patched part and then “baked.” The utmost care should be taken to have the proper degree of heat and the time required. A thermometer which registers the heat correctly should be employed, and not a pressure gauge, as with the latter there may be fifty pounds pressure, but not the required heat. If a thermometer is used, the heat can always be ascertained as a rule. Three hundred and ten degrees of heat for twenty minutes will vulcanize a tire. The patch to be inserted in a tire is made up as follows: (1) a layer of uncured gum; (2) one or two layers of frictioner or coated duck; (3) another layer of uncured gum. The surfaces to be united should always be wiped with naphtha, and care should be taken that no air is enclosed in the patch when preparing it. Many repairers do not sew the puncture to be vulcanized, and in such a case at least two layers of canvas should be used, or three is still better. It is not always desirable to vulcanize a valve stem. The better way to repair a defective valve stem is as follows: A new hole is cut, a brass or shoe valve is inserted, and the old hole is plugged up the same as a puncture is repaired.
CHAPTER IX.
BEARINGS AND POINTS OF CONTACT.
Friction gives us a grip on the earth, and is indispensable for propulsion, but it is not in the least wanted in cycle bearings or in any other bearings, and one of the problems of mechanics is how to reduce it as much as possible in places where it consumes power as well as produces wear.
No material thing, however polished, is quite smooth; every surface may therefore be considered as covered with irregular hooks or teeth, however flat and smooth it appears to the unassisted eye, and these catch and hold one another, producing the hang or drag called friction. Oil, being a fluid, fills up the spaces between these invisible teeth and levels off the surface; the office of lubrication is, therefore, to get between the contact surfaces and keep the hooks or teeth thereon from touching. When surfaces are desired to slip and slide on each other easily, oil is helpful; when the hooks or teeth are to catch into each other, as between locomotive driving wheels and the rail, grease is out of place, whether it is oil or grasshoppers, for it spoils the “adhesion.”
The earliest mode of reducing friction is doubtless as old as the Tower of Babel, for the idea must have occurred to the primitive man. It is simply to put a roller underneath and convert sliding into rolling motion. This is in principle equivalent to mounting the weight on wheels, and it is the solitary and final way of dealing with the problem of friction. The common grindstone bearing is a familiar example; the axle of the stone rests on the rims of a pair of small wheels which stand so as to lap past each other. Here the axle rolls the wheels as it turns, and their motion at their centres is so slight that friction is nearly eliminated.
EVOLUTION OF THE BEARING.
In cycle construction, the first bearing was the “plain” one in common use elsewhere; then a nicely fitted and hardened sleeve was added, and this was known as the “parallel” bearing. Rollers were also tried, but rollers have a determined habit of going askew, one end moving faster than the other, and as soon as they get out of parallel thus, they set up a great resistance. To meet this difficulty, the ends were sometimes made so as to overlap and match into one another, or the ends were loosely passed through thin rings, which revolved with the rollers around the axle; but the rollers still tried to run askew, and the efforts of the rings to prevent them caused another friction, so that the roller was abandoned. About the same time, the “adjustable cone” was tried. This was a male cone, threaded on the axle and fitting into a female coned space in the hub. The character of the rubbing action was not changed by this device, which was called a device to make wear in order that wear might be taken up, but the parts could obviously be kept in contact (though not in nice fit) by screwing the cone further in.
The next and final step was to interpose steel balls between these coned faces; and as the ball is a very short roller, with ends rounded off, it can go in any direction it pleases. The principle of lateral adjustment by moving a coned surface to or from another coned surface opposed thereto, with balls placed between, was patented more than twenty years ago and is still in universal use; yet, as just remarked, this is the adjustable cone modified. It is to be borne in mind that the only possible service of the cone, as before, is still to take up wear, and also that the retention of the cone for adjustment introduces new difficulties. Note also that on the old high “[Ordinary]” the large wheel had its bearing cases fixed and the axle revolving, because the power was applied to the axle, while the rear wheel had its axle fast and the wheel hubs revolved around it. On the modern bicycle the method reverses, both wheels revolving around fixed axles, while the crank shaft, which is the part receiving the driving power, revolves within a fixed bearing-case.