Page:Motors and motor-driving (1902).djvu/238

206 with the two gear wheels and, which are carried upon the two halves of the divided shaft. The wheel, which is fast on the shaft driven by the motor, now drives wheel , which is in one with wheel , and rotates with it, and wheel , in its turn, drives wheel's and through it the transmission shaft and road wheels. Now it will be seen that is slightly smaller in diameter than, and  than , so that the speed of the road wheels is reduced in relation to the motor in the proportion of these differences. The third speed is obtained in a similar manner, but by swinging the other secondary shaft in its bearings and bringing wheels  and  in gear with wheels  and  on the main shaft. As the differences in diameter between these four wheels is greater than with the other four, it will be seen that the speed reduction is proportionately greater. Upon the centre of shaft will be seen a serrated clutch and two bevel pinions,  This is the reversing gear, which is put into operation by separating the serrated ends of the shaft and dropping a third bevel wheel not shown in the illustration into gear with the other two, which reverses the movement between the two halves of the shaft, and consequently drives the main shaft in the opposite direction to that in which the engine is running. With this gear an even greater amount of care is necessary in changing gear than with the last mentioned, as the teeth of the different gear wheels are not slidden sideways into each other, but the two rapidly moving toothed surfaces brought up against one another. In all these gear-driven devices the greatest care must be taken to see that full lubrication is provided. The gear-case should be kept sufficiently full of lubricant to enable the lower edges of the gear wheels to be constantly passing through it. It is important, too, that the bearings of the shafts should not be allowed to get too much worn before renewal. All bearings will wear and will require rebushing, i.e. relining with new metal surfaces, and this should be done when any very perceptible shake or side play is detected in them. This condition of things will generally make itself known by increased noise from the gear, and the extent of the wear can be