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.
Fig. 68.—Tractor Arrangement
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.
In D, the drive is entirely enclosed within the rear axle housing, and the rear part of the frame is formed by the axle housing and the housing of the change speed gear.
E has a 1-cylinder horizontal engine with a single chain drive, while F has a similar engine but drives to both wheels.
G has no frame, its place being taken by the crank case of the engine and the housings of the parts of the transmission. G and H have 4-cylinder vertical engines, G driving through an enclosed rear axle and H through bull gears.
Fig. 70.—“Gray” Tractor
Figure 70 has one broad wheel instead of two narrower ones, this being placed inside of the frame instead of outside. It has a 4-cylinder vertical engine placed across the frame, and drives through two chains.
Fig. 71.—Types of Front Axles
The front axle of a tractor is almost always attached to the frame by a pivot, so that the wheels will follow uneven ground. Some of the forms of front axles are shown in Figure 71.
Fig. 72.—Spring Support
The first is a plain bar, while the second is arched to raise the front of the frame in order to permit the steering wheels to cut under. In the third the wheel axles are mounted on springs, which take up some of the vibration and act as shock absorbers.
The fourth axle shown is built of steel bars riveted together to form a truss, and the fifth is similar, with the frame pivot carried on springs. The sketches at the bottom indicate the extent to which the pivoted front axle may swing.
Figure 72 shows a spring support for the axles, front and rear. The axle bearing is in a block sliding in guides, the weight being supported by a heavy spring.