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MODERN ENGINEERING PRACTICE - 12


Agricultural tractor































IN the early days of the mechanization of the farm there might occasionally be seen two portable steam engines, one at either end of a field, connected by a system of wire ropes. A plough would be attached to these ropes and dragged from side to side, the engines being moved slightly forward as each furrow was cut. Since the coming of the petrol engine, however, the plough, the harrow or the harvesting machine is attached directly to a tractor which has the same degree of mobility as the horse. The tractor illustrated above will do the work of eight to twelve horses, according to the nature of the ground, the “rated” drawbar pull being 13.5 horse-power.


It is a four-wheeled machine, the plain front pair of wheels being steered and the rear pair driving the tractor, for which purpose they are provided with spade lugs for gripping the ground. The main frame consists of a single casting through which the rear axle passes; this casting rests on the swivelling forecarriage of the front wheels. The frame has a recess at the front into which the engine is fitted. The engine has four cylinders (replaceable) with a bore of 4¼ in and a stroke of 5 in. It runs at a speed of 1,000 revolutions a minute. It is similar to a normal motor car engine with overhead valves. A part of the camshaft and the tappet rods for two of the valves are seen near the centre. The camshaft is driven by the gear wheels seen at the right hand and the governor, magneto and oil pump are driven from the camshaft. The fan, which keeps the radiator cool, is driven by a belt from the crankshaft.


As in a motor car, the engine is kept running and the tractor is started or stopped by the engagement or disengagement of a plate friction clutch actuated by a pedal from the driver’s seat. The first motion shaft, that is, the one in line with the crankshaft, passes into a gearbox and, by means of secondary shafts and gears, the power is transmitted to the rear axle, which is fitted with the usual differential gear to allow the rear wheels to turn at different speeds when the tractor is going round a curve. The first motion and secondary shafts are provided with sliding gears which permit three speed changes in a forward direction, viz. 2¼ miles, 3 miles, and 4¼ miles an hour and a single speed of 2½ miles an hour in reverse. The changes are made by appropriate movements of a single lever by the driver.


The shaft seen above the first motion shaft is one that does not exist in a motor car and has two purposes. It is driven from the gearbox and at the forward end, drives, through bevel gears, a pulley at the side of the tractor. This pulley runs at a speed of 645 revolutions a minute and can be used, with the tractor stationary, to drive a chaff cutter, turnip cutter or any other farm machine, by means of a belt. The “rated” power obtainable from the tractor when using the belt drive is 21.6 horse-power. Another form of drive can be obtained from the auxiliary shaft, as this can be prolonged from the point marked “power take off” in the illustration, by a shaft running out at the back of the main frame at the point closed in by a small domed cover. This shaft rotates at a speed of 543 revolutions a minute and is used for operating tractor-drawn and tractor-mounted machines with power transmitted direct from the tractor engine. The rear direct drive or the pulley drive is engaged as required by a clutch lever.


[From part 21, published 20 July 1937]



You can read more on “Power and the Plough”, “Romance of Motor Car Making” and “Story of the Motor Car” on this website.

Agricultural Tractor