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. [Illustration: 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. [Illustration] [Illustration: 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. [Illustration: 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. CHAPTER X LUBRICATION The most important thing in the care of a tractor is to oil it; every moving part should be lubricated, and the greatest care should be taken to assure a never-failing supply of oil and grease. Carelessness in lubrication is the principal cause of tractor trouble. There is nothing complicated or difficult about keeping a tractor properly oiled; yet more tractors break down from careless lubrication than from any other cause. Every tractor-maker issues an oiling diagram and oiling instructions, and there is no excuse for an operator whose machine does not get the right kind of lubricant in the right quantity at each place where lubrication is necessary. The cause of wear is friction; oil reduces friction and so reduces wear. No matter how smooth and highly polished two pieces of steel may be, there will be friction between them if they are rubbed together, and they will wear each other. If they are oiled, the particles of oil will keep the pieces from touching each other, and there will be no wear. Other substances than oil can be used; there are some kinds of machinery that are lubricated with water, for instance. For general use, however, oil and grease are the best, and are practically always used.