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Differential Gears Explained and Illustrated.

3rd August 1905, Page 16
3rd August 1905
Page 16
Page 17
Page 16, 3rd August 1905 — Differential Gears Explained and Illustrated.
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Which of the following most accurately describes the problem?

It will be noticed (Fig. 2) that the shaft is separated into h.% t) halves, and that each road wheel is fixed to a separate half of the main axle. Keyed to each half axle is a bevel spur. wheel, through which the drive is transmitted to the shaft. The centre part of the differential, which is called in this case the " spider," is quite free to revolve on the ends of the two half-shafts, and carries with it two bevel pinions, which are free to revolve radially on pins fixed into the " spider." An examination of the right-hand view will show all these parts distinctly. Consider now for a moment the

left-hand view, where the whole differential gear is shown together. The drive is being transmitted—in this case through a chain—and, if the vehicle is going in a perfectly straight line, the resistance to motion is equal in both road wheels, and consequently the bevel pinions find an equal resistance against each of the bevel spur-wheels. There is, therefore, no tendency for the bevel pinions to revolve. They act as though they were quite rigid with the " spider," and they drive the two bevel spur-wheels equally. Consider next what would happen if the right road wheel were held so that it was unable to revolve at all. The result would be

that the right half-shaft would be unable to turn. The effect of turning the " spider." wouid.then be for the bevel pinions to move on the right bevel spur-wheel as though that were a rack, and the teeth on the left edge of the bevel pinions would be turning the left bevel spur-wheel twice as fast as the" spider "was revolving. This operation will frequently be seen in a garage when the driver is testing the differential : he lifts one wheel off the ground by means of a jack,

leaving the other stationary on the ground; then, when the engine is turning, the wheel which is in the air is seen to be revolving very fast ; in point of fact, twice as fast as the "spider." Similarly, when the vehicle is going round a corner, the effect of slowing the inner wheel is to aid the outer wheel to turn proportionately faster ; in fact, the " differential gear" differentiates according to the paths taken by the two wheels, and drives the gear equally well, whether on a straight path or on a curved route. Fig. z is taken from a photograph of the actual parts of a bevel differential

as used in a steam wagon. To the right will be seen the "spider," which in this case is driven by the gear, and, therefore, has teeth on the outer edge to mesh with another toothed wheel instead of to run in a chain, The two pinions can be clearly seen on this "spider." At the back will be seen one of the road wheels with its bevel spur wheel attached, and below it will be seen the shaft with the other bevel spur-wheel. In this particular case the differential gear is not placed in the centre of the shaft, but is near to one end, and, instead of the shaft being cut into two halves, the one road wheel and its bevel are carried by a sleeve which is free to turn on that end of the shaft The other road wheel is keyed to the shaft, and revolves with it. The shaft, therefore, drives one road wheel but only carries the other by the sleeve. This has the advantage of giving a plain shaft for the "spider" to revolve on, instead of ends of two half-shafts. It will be noticed in the photograph that the shaft is left rather larger where the " spider" revolves on it. The bevel differential is probably the simplest form, for there are other ways of attaining the same result. Fig. 3 shows a " crypto differential," and, similarly, there are three views, the centre one of which shows the three parts separately. To the left half-shaft, which drives the left road wheel, is keyed a large spur-wheel with the teeth on the inside instead of the outside. To the right half-shaft, which drilies the right road wheel, is keyed a smaller spur. wheel with the teeth on the outside. The " spider " now becomes a "cage," and revolves freely on the right halfshaft. It carries two pinions, which are free to revolve on cylindrical projections which are part of the" cage." When the vehicle is taking a straight path, the gear all revolves together, as though it were one solid piece, and the teeth of the pinions do not move on the teeth of either of the spur-wheels, but directly the path of the vehicle hecongs curved, and one wheel is checked in its motion, the pinions begin to revolve, and make the one spur-wheel move faster in proportion to the amount that the other spur-wheel is checked. Fig. 4 shows another form of differential, which is called the " planetary differential." Here again plain spur-wheels are used instead of bevels, and the " spider " has again become a " cage," part of which revolves on each half-shaft. The planet pinions have the teeth turned off for ene-third of their length, and they are arranged so that each pinion gears with one of the spur-wheels and with the neighbouring pinion, whilst the neighbouring pinion gears with the opposite spur-wheel. The effect, therefore, of slowing one spur-wheel is to make the pinion, which is in gear with that spur-wheel, revolve, and so drive its neighbouring

pinion, which again drives the other spur-wheel proportionately faster. In many high-class cars planetary differentials are used in preference to bevel differentials, because bevel wheels always produce a thrust outwards, and this necessitates some arrangement to take up the thrust. With a planetary differential there is no thrust outwards, and the bearings of the shaft may be consequently simplified.

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