PUTTING ROLLERS BETWEEN LOAD AND ROAD
As intimated in Chapter I the forerunner of the wheel was probably the roller. It was much easier to move a heavy object on rollers because rolling friction was substituted for sliding friction, but the rollers would not stay under the object; they traveled only half as fast as the load they carried. To make them keep up with the load they had to be mounted on axles which were fastened either directly to the load or to a cart body on which the load was supported. Thus the wheel came to be invented, but except for the fact that it stays by its load and does not roll out from under it a common wheel is not to be compared with a roller for efficiency. To be sure, it substitutes rolling friction for sliding friction where it contacts with the road, but the friction at the axle is sliding rather than rolling. However, drawing an object on wheels is a decided improvement over sliding it along the road, for two reasons: the sliding friction at the axle is reduced to a minimum by choosing materials that will slide upon each other with comparatively little resistance, by polishing them smooth and by lubricating them. But even if these precautions were not taken there would be a distinct advantage in the use of wheels because of the relatively shorter travel at the axle than at the rim of the wheel. If a wheel is thirty inches in diameter and turns on an axle one inch in diameter, it will travel thirty times as far at the rim as it does at the axle; hence the sliding friction at the axle is far less than it would be at the point of contact with the ground, were the wheel locked so that it could not turn. But it is not necessary to have any sliding friction at the axle if we revert to the old roller system that prevailed before the day of the wheel. The axle may be considered the load and the axle bearing the road. We can then put rollers between load and road. Because the road is a circular one that travels with the load we can line it with rollers throughout its length, and the load will never lack for rollers to roll upon. Thus we have the roller bearing which is so widely used in modern vehicles. Ball bearings operate on the same principle except that the balls furnish less contacting surface and are not so suitable for supporting heavy loads, as are rollers. Sliding friction is almost completely eliminated and unless heavily loaded a wheel on ball bearings will not heat even when the bearings are not lubricated.
REDUCING ROAD FRICTION
While there is little friction between a wheel and the roadway upon which it travels, the roughness of the road is a very important factor. Every time a wheel goes over a stone the entire vehicle must be lifted; this represents so much wasted energy. The advantage of the pneumatic tire lies in the fact that it absorbs small inequalities without making it necessary for the entire vehicle to rise over them. This means less load lifted and hence less work done. Large wheels are better than small ones because they do not sink so deeply into depressions and because they surmount small obstacles more readily. On a steel track the size of the wheel does not make so much difference as it does on a road because the track surface is smooth and is but little depressed by the wheel. It has been estimated that a horse can pull ten times as great a load on rails as on an ordinary macadamed road. Some years ago broad steel tracks were laid in some of our city streets for the use of horse-drawn trucks. They served very well as far as the vehicles were concerned. The road friction was reduced considerably, but the fact was overlooked that the horses needed a good friction surface under their feet. Two horses could not pull a truck along the track without walking on the track and the smooth steel made such slippery footing that the tracks had to be torn out and replaced with common paving.
INCREASING TRACK FRICTION
The difference between sliding and rolling friction is well illustrated in a locomotive. The driving wheels are turned around by steam power. They must either roll or slide on the track and the load they will pull without slipping is a measure of the excess of sliding friction over rolling friction. To increase the traction or the adhesion of the locomotive to the track it is provided with a number of driving wheels. In some of our largest locomotives driving wheels are placed under the tender so as to obtain a maximum of traction.
ANCIENT LINEAGE OF THE AUTOMOBILE
When it was first proposed to substitute steam propulsion for the horse it was not realized that a rail would furnish enough traction to permit of hauling heavy loads, and some of the early locomotives that ran on rails were provided with toothed wheels that engaged in racks alongside the rails. In fact, the earliest locomotives were not built to run on rails but on ordinary roads; in other words, they were automobiles. The motor car can therefore boast of a more ancient lineage than the railroad engine. Joseph Cugnot of France is said to have built a three-wheeled steam carriage in 1769 which was so top-heavy that it upset when making a sharp turn at three miles per hour. Several steam carriages were built in England in the eighteenth century, but they were not successful. The real father of steam traction was Richard Trevithic, of Camborne, Cornwall, whose first steam carriage, built in 1801, carried eight passengers. His third machine, built in 1804, was the first to run on rails. This was strictly a locomotive intended to haul cars. It ran with its load at the astonishing speed of five miles an hour. Trevithic was the first to exhaust the steam from the cylinders into the smoke-stack and thereby increase the draft through the furnace and generated steam at higher pressure.
All the early locomotives used toothed gears to turn the driving wheels until George Stephenson introduced connecting rods to drive the driving wheels direct from the pistons. George Stephenson’s “Rocket,” built in 1829, won a prize of 500 pounds in a speed contest when it established a record of 24 1-6 miles per hour. It also established the doubtful honor of being the first mechanical speed monster to exact the toll of human life. On its prize run it ran over a man and killed him.