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Differential Gears Explained and Illustrated.

27th July 1905, Page 14
27th July 1905
Page 14
Page 14, 27th July 1905 — Differential Gears Explained and Illustrated.
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Which of the following most accurately describes the problem?

The layman who contemplates the adoption of commercial motor vehicles as an adjunct to his business is sometimes hindered by an inability to follow descriptions of the parts of these vehicles. It is true that he leaves most matters to his engineer, but when the engineer reports to him that such and such a part is broken the name trequeutly does not convey any intelligible meaning, and he cannot judge whether the part is essential to the proper working of the motor vehicle, or whether it is a mere adjunct that could be dispensed with for a week or so. in fact, the term used conveys nothing to his mind. One puzzling term which presents a difficulty to the layman is that of the " differential gear." It is, therefore, proposed in this article to try to set forth a lucid explanation of the functions of the differential gear, and a description of its parts, to ensure their full appreciation. When a vehicle is moving in a perfectly straight line, both the wheels on one axle are revolving equally fast and following paths of equal length, but directly this vehicle

turns a corner the wheel on the inner side of tne curve follows a shorter path than that on the outer side of the curve To quote the extreme case, a van might be turning round so sharply that the inner wheel did not revolve at all, but became the pivot on which the vehicle turned. It viould then follow a path of no length whatever, whereas the outer wheel could follow a circular path, with the inner wheel as a centre. This is, of course, an example which is practically impossible with a motor vehicle, but which frequently happens with a horse-drawn vehicle when it is being manceuvred about in a yard. On an ordinary carriage or van designed to be drawn by a horse or by a motor, the axle is fixed, and the wheels are free to revolve on it. It does not matter, therefore, if one wheel follows a shorter path or revolves more slowly than the other, the wheels being independent of one another. On the other hand, in a railway wagon or tramcar, both wheels are keyed to the axle, and they must revolve together. When a tramcar turns a corner, and the inner wheel has to follow a shorter path

than the outer wheel, one wheel has to skid on the rails, as they must both revolve equally. In practice, it is always the inner wheel that skids, because centrifugal force throws a greater load on to the outer wheel where there is a flange, and, consequently, it controls the number of revolutions.

When considering a vehicle that has to be propelled through one pair of road wheels, the circumstances are entirely different from a pair of wheels which merely roll. Should the wheels be keyed rigidly to tha axle, the position would be the same as with a tramcar, except that road curves are sharper, and in turning a corner it would be necessary to skid one wheel, so as to allow for the difference in the length of the paths covered, But here the difficulty commences. It is found that instead of one of the driving wheels skidding, the steering wheels skid sideways and the vehicle refuses to turn the corner at all.

The earliest forms of self-propelled vehicles. excepting the pre-Victorian steam omnibuses, were traction engines. The first types of these were driven by only one wheel, the other wheel being free to revolve on the axle. This enabled the machines to turn corners readily, but as only one whecl was being driven, there was not sufficient grip on the road to enable them to draw heavy loads. The next stage was to key to the axle two driving blocks, as they were called, close U p to each wheel. The wheels were left free on the axle, but a pin could be passed through the wheel and the driving block so as to connect them up. The practice was to put the pins through both wheels when travelling along a straight road, but when a corner was reached the pin was taken out of the inner wheel, and the outer wheel alone was driving when rounding the corner. As soon as the turn had been completed, the pin was again inserted. This is still the practice with the majority of steam rollers, and it k frequently said that such steam rollers are driven by " pin gear," as distinct from those that are driven by" differential gear." It will now be seen that what is required is some arrangement that will enable both wheels to be driven, and yet will allow them to follow paths of different lengths as required. For instance, if a motor vehicle is threading its way through crowded London traffic, there is hardly a moment at which it is taking a straight path, and it may be said that Cie two driving wheels are never revolving at quite the same speed. The simplest form of differential gear is that known as the " bevel differential," which is illustrated in Figs. i and 2. Throughout the drawings, the differentiai gear is shown as though it were to be driven by a chain, but it must be understood that it can be driven by any suitable mechanism from the engine. The chain sprocket has been shown to make a distinction as to which part receives the driving impulse from the engine. In Fig. 2 three views of a bevel differential are shown. The centre view is looking from the end of the shaft, to which we may assume the road wheels are fixed ; the left-hand view is looking from the hack of the car with part of the gear in section, so as to show the inside; the right-hand view is exactly similar to the left one, except that all the parts have been separated so that they can be seen more distinctly.

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