Figure 64 shows one of the large bevel gears of a differential, with the three small gears, the other large bevel gear being removed. A differential in section is shown in Figure 65. A tractor with only one driving wheel has no differential. Such tractors usually have two wheels, but one of them runs loose on the axle, and serves only to support the tractor. The rear axle construction of a tractor with a 1-wheel drive is shown in Figure 66, which should be compared with the 2-wheel rear construction shown in Figure 65. [Illustration: FIG. 65.—“CASE” REAR AXLE] There are a number of methods used for transmitting power to the driving wheels. In Figure 64 a chain is used; there are tractors with but one chain, and others with a chain for each driving wheel. [Illustration: FIG. 66.—“OIL-PULL” REAR AXLE] The most usual method is by a _master gear_, or _bull gear_, which is a large and heavy gear attached to the driving wheel, as shown in Figures 65 and 66. In some tractors this gear is nearly the size of the wheel, and is fully exposed; in others it is smaller, and enclosed in an oil-tight housing. [Illustration: FIG. 67.—DRIVING WORM] The small gears that drive the bull gears are on the ends of the cross shaft, called the _jack shaft_, that carries the differential. In the Fordson tractor the differential is built into the axle, as it is in an automobile, and power is applied by a _worm_. The worm is driven by the change speed gear, and is a screw meshing with a gear on the differential, whose teeth are cut at the proper angle to make them fit the threads of the worm. A worm, which is shown in Figure 67, is always enclosed, and runs in oil. CHAPTER IX TRACTOR ARRANGEMENT The uneven ground over which tractors must work requires the weight to be kept low, to prevent capsizing, and they are also built wide, for the narrower they are the more easily they tip over. They cannot be broad in front, however, for if they are the steering wheels cannot be swung enough to permit them to turn in the small circle that is desirable. To give a small turning circle some tractors are built with the front of the frame raised enough to permit the wheels to cut under. Others use small steering wheels, but this is not desirable because small wheels will not run over rough ground as readily as large ones, and steering is difficult. [Illustration] [Illustration: FIG. 68.—TRACTOR ARRANGEMENT] [Illustration] [Illustration: FIG. 69.—TRACTOR ARRANGEMENT] Types of tractors are indicated in Figures 68 and 69. A has a 4-cylinder vertical engine in front, driving both wheels by bull gears, while B is a 2-cylinder horizontal engine in the center, driving both wheels by chains. C has a 4-cylinder vertical engine set across the frame. These three types have riveted steel frames, to which the parts are attached.