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large farms to advantage. These machines require two men to handle them, one to run the engine and the other to handle the load of ploughs. While costly to purchase and operate, they have such a large capacity that they are economical if operated at capacity.

The very large machines have some disadvantages. Owing to their weight they tend to pack the ground and it is only in ploughing that their full capacity can be used to advantage. Many operators of large farms prefer to divide the initial investment required for one of these massive machines and purchase a number of the medium sized tractors, even though it takes more help to operate them, and do the same amount of work that the large machine will perform. It is safer to have the work divided among a number of machines rather than employing an extremely large one, because failure of one of several tractors will not prove so serious as when the one large machine refuses to operate. Except in unusual cases, even the operators of very large farms prefer the medium class tractor to the heavy types.

Parts of Typical Tractors Outlined.—The plan view presented at Fig. 15 shows clearly the various parts of a typical gas tractor and their relation to each other. The machinery is carried on a frame made of structural steel supported at the front end by an axle of the pivoted type, having steering knuckles to which the front wheels are attached. In this construction the axle does not move, but the wheels can assume the angle required for describing a curve, because the tiebar which joins the steering arm actuates both wheels in unison. The front wheels of a tractor of conventional design are always smaller than the rear members because they are generally used only for steering, and do not carry a large proportion of the tractor weight in most constructions.

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Fig. 15.—Parts of the Pioneer Tractor Outlined.

5.-Parts of the Pioneer Tractor Outlined. Mounted at the front end of the frame are cooling fan and radiator. The latter is used to carry a supply of water for cooling the engine eylinders and also to cool the heated water from the engine that is pumped through it. This member forms a very important part of the cooling system. The air fan placed back of the radiator is positively driven by gearing from the engine and serves to draw a blast of air through the openings of the cooler, and abstract the heat from the walls of the tubes through which the water circulates.

The power plant in this case is carried about midway of the frame and is composed of a four-cylinder gasoline engine, having opposed cylinders horizontally disposed, the crankshaft of the motor being at right angles to the frame. Beside the motor fly-wheel a driving pinion is mounted, this communicating with the main shaft of the transmission by an intermediate drive pinion, which meshes with the driving pinion on the engine crankshaft and with a large gear on the transmission mainshaft. A belt-pulley is mounted outside of the frame on the crankshaft extension, this enabling one to obtain power from the engine for running various machines. A clutch is provided on the engine shaft, so the driving pinion may be put in action or so the belt pulley may be clutched to the shaft.

When the driving pinion is turning with the shaft it produces motion of the transmission mainshaft which is a square member, carrying three sliding gears actuated by a common shifter member. When the smaller of these gears is meshed with the medium sized gear attached to the differential gear case a slow speed ahead is obtained. When the gears are in the position shown a reverse motion is obtained. The gear on the main shaft is driving an intermediate pinion at the bottom of

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