Motors and Motoring
The Modern Motor Car.—II. The Gear-box -"— IN a previous article the principle of -construction of the ordinary gear-box was briefly described, and' here it is intended to refer to the reverse giSar;'and to the purpose of the box: These two articles have. been-.written.-preparatory to one describing free-wheel devices which are.fikely to come fnuch to the fore as affording a practical_ineans of really simplifying gear-changing and allowing appreciably greater economy in car maintenance while not calling for the scrapping of existing gear-boxes or for any large initial - outlay. In continuance of my simple explanation of how the -ordinary gear-box works, we come to the means of obtaining a reverse motion, so that while the engine continues to run the same way as when the car is driven, forwards, the vehicle may be reversed at will.
• It has been shown how a neutral gear position obtain- able—that is, how the car could be left stationary with the engine and clutch in operation ; also how the machine was driven forwards. We come -now to the reverse. Ina simple form, on the after end—that furthest from the permanent-mesh wheel—of the lay or lower shaft in the box is fixed x small pinion or gear wheel which meshes with an idle wheel having a separate support, in the box. In the ordinary way, when the lay shaft is running; this wheel is rotated, and as it form an intermediary and is normally doing no work, it is-known as an idle pinion. This is the reverse gear. To engage it so that the main shaft and the propellor shaft are turned in a reverse direction, the first or low speed pinion on the main shaft is slid into mesh with it. With the engine and clutch running, the upper permanent wheel turns clockwise, the engaged lower wheel anti-clockwise. As the whee_, on the lay shaft are fixed to it, it follows that the small gear wheel on the back end of the lay shaft also turns anti-clockwise; This rotates the idle or reverse gear clockwise, and in turns the low-speed pinion revolves anti-clockwise. Since this last wheel, although allowed to be slid horizontally along the castellated or squared main shaft, must rotate the shaft when it is moved, and the shaft is connected by a universal joint to the propellor shaft, it follows that the rear road wheels will be turned in a reverse or clockwise direction, and so drive the car backwards. There are various ways of arranging a reversing device, but the above is simple and illustrates the scheme. The whole point is clearly to introduce a third and intermediate wheel which will, of course, reverse the dire:ition to any obtained by two gear wheels in mesh. As to the matter of ratios and the purpose of the gear-box, it has already-been shown how we can obtain in practical form the drive for the car on different pairs of gear-wheels. If the two permanent wheels were the same size, and the respective engaging pinions on the lay and main shafts were also alike- in the number of teeth, no variation would be obtained, and for one revolution of the clutch the propellor shaft would turn once. Thus, all the reduction obtainable would be in the back :axle— namely, the difference between the driving bevel pinion or worm and the driven crown wheel or ,worm.wheel, according fo the type of gearing- used there. This would-1)e quite in- sufficient. The gear-box is there to provide the needed range of speed ratios between engine and back road ,wheels, We must remember that, -in- all--engineering, power and speed are inversely proportional. What one gains in power one loses in speed, and vice versa. On a car, for example, on the level a- driver wan* •more speed than power, while on a hill more power is required than speed, and botlyare of course limited. For this reason, while for ordinary work the eiigine may be made to turn at anything from four to five tithes to one revolution of the back road wheel, in order to be able to tackle a severe hill it is necessary to provide means whiireby
a driver can run his engine at anything from seven to twenty revolutions to one of the back wheels, while m estly
interme ratios are oT advantage. Here is the purpose of the gear-box, fiiid the wheels on the lay shaft and the sliding one on the main shaft are designed accordingly. -The two shafts are always run parallel to each other, and the respective pairs of wheels must of course always mesh pro- perly. Thus a small one on the lay shaft will be engaged*ith a large one on the main shaft, and so forth. To gear up, a large " driver " and a small " follower " are employed, and vice versa. It is just the same on a pedal bicycle. The "drOer " is the forward chain sprocket to which is attached the pedal cranks. If a rider's feet are -seen-, to -he moving round very fast and the machine is travelling comparatively slowly, one knows that the machine is low geared and good for power- -that is up hills—but poor for speed. If; on the other hand, his feet turn slowly and the bicycle travels fast it is Nigh geared,
the " driver " or forward sprocket being large in relation to the " follower " or driven sprocket at the back. In the gear- box, since the lay shaft wheels are the " drivers," as compared to the " followers " on the main shaft, the lowest gear ratio pair will consist of the smallest wheel on the lay shaft and the largest one on the main shaft, and so on with the other ratios. 'rake first or low gear, for instance, with the respective wheels engaged. It is clear that the one on the driving or lay shaft will make several revolutions, while the one turning the main shaft and the propellor shaft will make one, since the former has fewer teeth and less diameter than the latter. The lay shaft wheel is the one in connexion with the clutch and engine ; therefore, for one revolution of the main shaft wheel —that is the rear road wheels—the engine will turn a number of times. So power at the expense of vehicle speed will be obtained. The other ratios, two or three as the case may be, according to whether the gear-box is of the three or four-speed type, are correspondingly graduated up to top where the drive is direct from clutch to rear axle. On this speed the only reduction is that in the back axle drive, which in the normal car is about five to one. Here the bevel pinion being five times smaller than the crown wheel, it follows that since the " driver " is the smaller of the two the gearing will be down, and that the engine will make five turns to one by the road wheels.
Here the need for careful timing when changing gear with the ordinary box and where there is no free-wheel device will be seen, for when the speed lever is in neutral with the car in motion, the sliding wheels on the main shaft are being rotated by the road wheels—in other words by the vehicle speed—while the ones fixed on the lay shaft are governed by the engine and clutch speed. It is for this reason that if it is desired to change down early—that is, before the vehicle speed has slowed down to the approximate correct relative rate to engage the wheel on the lay shaft—it is needful to double- clutch, quickening up the lay shaft wheel by means of engine acceleration with the drive to the main shaft free. Changing early without this plan will mean that the respective gears which are to be engaged are revolving at such widely differing rates that the teeth will pass the gaps too quickly and grating without engagement will ensue. With speeds which approxi- mate it is clear that the wheels can be brought together and the teeth will be allowed to enter the spaces on the opposing wheel easily and without noise. In a rather similar way, when starting a car, if the clutch is not held out until it comes to rest, or nearly so, before engaging low speed, the big wheel on the main shaft, which is stationary, will be brought up for engagement to the small wheel on the lay shaft while it is in motion. Hence a grunt or scrape. If the clutch has been allowed to come to rest, the wheels to be meshed will both be stationary. It is, of course, possible that in that position tooth may meet tooth, and in that case the clutch should be momentarily let in to give a turn or two to the lay shaft, and then put out and permitted to come to rest. The chances arc that the teeth on one wheel will face spaces on the other and silent engagement can be made. Should tooth meet tooth again the procedure must be repeated. YOUR MOTORING CORRESPONDENT.
Previous page
