Motors and Motoriti
The Modern Motor Car.—I. The Gear-Box
I HAVE written an article on simpler gear-changing with special reference to -free wheel devices both as to their advan- • tages in the actual control of a car as well as in its maintenance. On reading the article through, it struck me that some readers might be puzzled and lose the value of most of the information because, although the matter is simply and more or less untechnically treated, I had to assume that the functions of an ordinary gear-box were familiar. I am, therefore, holding the article over pending this explanation of how the normal gear-box works and the purpose it serves. It is, perhaps, needless to add that gear-changing is a subject in motor design and handling which has caused more interest than any other feature of a car for the last thirty years.
Manufacturers throughout the world have attempted to improve on the principles which Levassor incorporated in his chassis when cars first started, and to show how deep the problem is it is only necessary to realize that practically every car made to-day has a gear-box and one in which the original ideas are in the main largely relied upon. Gear-changing has always been a pitfall and a nuisance to the inexperienced, in fact one may say to the bulk of motor drivers. Any change of real importance in this part of a chassis is therefore of widespread concern. In order to make clear the description of the purpose of the free wheel as applied to cars, it will be best, to help those who have not much knowledge of the inside working of the ordinary gear-box, to refer primarily to the latter. First, then, as to how the gear-box works. Gear-boxes on different makes of car vary considerably in. detail, as to change-speed actions, as to the number of speeds, and as to the type of change. These diversities, however, are really matters of detail, and the broad principle which has been so long retained is adopted on 99 per cent. of the cars made throughout the world. -- For -purposes of clearness I will deal with the reverse gear later on.
There are two horizontally placed shafts in a gear-box, the one called the main or primary shaft, the other the lay or secondary. Until four or five years ago, these shafts were almost universally placed side by side in a horizontal plane, but the whole modern tendency is to place one shaft above the other. This makes a deeper box but a narrower one. This, however, concerns detail and not principle. Let us consider two horizontal shafts, the main being above and parallel to the lay shaft. On these two shafts are set gear-wheels of different sizes, that is with varying diameters and number of teeth. At the firward—namely, engine—end of the top or main shaft there is a•wheel which is connected with the. shaft coming from the clutch but which is free to revolve irrespectively of the rest of the main shaft and its gear-wheels. The arrangement by which this gear- wheel can revolve without turning the main shaft and the other wheels is quite simple, and usually means that the forward end of the main shaft is made circular. It is spigoted in to the centre of the clutch shaft, which carries the gear-wheel in question and which is fixed to it. Suitable bearings are, of course, employed. In this way we have a gear wheel wtdch will be revolved when the clutch is in motion, yet will not turn the main shaft in the box. Take, now, the lay shaft below. All the gear wheels here are rigidly fixed to the shaft, and just below the wheel at the forward end of the upper shaft and meshing with it is a gear-wheel on the lay shaft. These two wheels are always in mesh with each other, and are known as the, permanent.or-constant-mesh wheels for that reason. We have then arrived so far that whenever the clutch is turned the lay shaft, through 'the; permanent wheels, is rotated, and in consequence all the gear-wheels attached to it. Thus the engine and clutch dan tie running, but /be car is ugt driven because pa power is delivered to the
main gear-box shaft to the back end of which is attached the propellor shaft leading to the rear axle.
To return to the upper or main shaft in the box. This shaft is generally castellated—that is, it is cut with sharp . . .
, grooves along its length so that the -gear wheels on it can be slid horizontally but when turned will revolve the shaft. If ,=it` is simpler to imagine, it may be assumed. that the shaft is square in section and that the centre of each gear wheel on this shaft has a square hole. Again, this will enable the wheels to be moved horizontally at will, but when they are rotated the shaft must lolloW suit. It will be seen that if no one of the gear wheels on the main shaft is in mesh or engagement with any one on the lay shaft below, a neutral position is obtained and the engine can be run with the clutch engaged while allowing the car to remain stationary. If, however, by a suitable arrangement of levers one of the main shaft wheels is engaged with a wheel on the lay shaft and the clutch is running, it will be clear that the ear will be driven. The clutch shaft is revolving the upper permanent wheel which is rotating the lower permanent wheel, and this in turn is running the lay shaft and all its wheels. One of these wheels is turning the engaged gear which drives the main shaft, and so the propellor shaft and rear road wheels. Here, then, is the operation, briefly put, of the ordinary gear-box. Top gear, or as it may be referred to, top speed, high gear, or direct drive, is obtained by connecting up the main shaft with the clutch shaft. This plan is clearly advan- tageous, for, except for the permanent-mesh wheels, no other gears on the lay shaft need be engaged. The result is greater quietness and reduced loss of power. There are two- general means of making this connexion, one by dog clutches and the other by internal gear. With the former, dog clutches, which are really like deep notches, are formed on the sliding wheel on the castellated or square main shaft nearest the permanent mesh wheel on the clutch shaft. On this wheel are corres- ponding dog clutches. Thus, if the main shaft wheel is slid up to the permanent wheel they will become rigidly connected, and if the clutch is, revolved it will turn directly the main gear-box shaft and the propellor shaft behind. The other method is similar, except that in place of dog clutches the main shaft wheel carries a small toothed wheel on the inside of it, nearest the clutch shaft. The inside of the permanent wheel on the clutch shaft is hollowed out with internal teeth to corres- pond with the opposing small wheel. The reverse plan can also be used. If, therefore, the main shaft wheels be brought up to the permanent wheel, the small one will enter the latter and whenever it is turned by the clutch the main gear-box shaft will be revolved and the car driven. The sliding wheels on the main shaft and the fixed one.on the lay or lower shaft are made of different sizes so that various gear ratios are obtained. With this subject, with the reverse gear, and with the general purpose of the box it is proposed to deal in the next article. A subsequent one will discuss fully free wheeling arrangements. YOUR MOTORING CORRESPONDENT.
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