AGRICULTURAL TRACTORS Tractors were usually rated according to the number of 14 in. plough bottoms they would pull. In 1927, of 77 models on the American market pulling two or more bottoms, 9 were two-plough tractors; 28, three-plough; 22, f our-plough; and the remainder or 18, were capable of pulling five or more ploughs, up to ten. How ever, the tractors in the two-plough class were produced on the largest scale. Of the total of 178,074 wheeled tractors produced in the United States in 1926, were one-plough and two plough, while 45,523 had a rating of three-ploughs or two-to-three ploughs. Table I. shows the production of tractors in the United States.
In 1912 the large tractors of b to 12 plough capacities reached their zenith, and thereafter declined rapidly. The first popular small tractor was produced the following year and in 1914 3,000 tractors of this small design were sold. Although smaller, it was still of the same crude design as the larger machines, with exposed gears, plain bearings and similar features. About 1916 a number of engineers with motor-car experience entered the tractor field, and as a result of their efforts tractor design was greatly improved.
The tractor industry was hard hit by the heavy slump in prices of agricultural products late in 1920. Prices of tractors and other agricultural machinery did not drop nearly as rapidly nor as much as those of farm products, and the result was a "buyers' strike," the effect of which is clearly reflected in the production figure for 1922. A very keen price competition set in, and many tractor manufacturers went out of business during the next two years. With the improvement in the economic position of the American farmer the demand for tractors increased again, but the production record set in 1920 had not been equalled up to the end of 1926. During that year the exports of tractors from the United States amounted to 27% of the total production on a numerical basis and to nearly 25% on a value basis, the actual value of exports amounting to $29,561,023. A survey made by the U.S. department of agriculture in 1927 showed that there were then 29 concerns in the country manufacturing wheeled agricultural tractors and five manufacturing the crawler type.
It was of the frameless or backbone type, the crankcase of the engine and the cases of the change-speed gear and rear axle forming the supporting structure. The engine had four vertical cylinders of 4 in. bore and 5 in. stroke, which were cast in a single piece with the upper half of the crankcase, the lower half of this case being a separate casting. To the rear of the crankcase was bolted a housing which enclosed the flywheel, the friction clutch and the change gearing, and also formed the rear axle housing. There was no direct "through" drive, the power being always transmitted from the upper to the lower of two parallel shafts in the gearset, and an extension of the lower shaft carried the worm which meshed with the worm wheel on the rear axle differential gear.
Changing of gear was effected not by meshing the gears, which remained in mesh continuously, but by securing one or another of the gears to the shaft by positive clutches. The rear wheels were in. in diameter and the tractor was geared to give a ploughing speed of 24 m.p.h. with an engine speed of 5,000 r.p.m. In addi tion to the ploughing speed there were provided two other forward speeds and a reverse motion. Over the engine was mounted a double-compartment fuel tank holding 1 gal. of petrol and 16 gal. of kerosene, the petrol serving to start and run the engine until the carburetter was hot enough to vaporize the kerosene. Air was taken in through an air washer and the carburetter in which the kerosene, after having been sprayed into a stream of air to form a very rich mixture, was vaporised by being passed between two stampings of sheet steel, the outer faces of which were in contact with the hot exhaust gases. Additional air was mixed with this rich mixture close to the inlet valves. Ignition was by a flywheel magneto, engine lubrication by the circulating splash system, and circulation of the cooling water by the gravity or thermo-siphon system. Unlike most others, this tractor was not fitted with an engine governor by the manufacturer, but several speciality manu facturers offered governors specially designed for it and many owners of the tractor fitted them. The Fordson tractor had a wheelbase of 63 in. and weighed 2,562 lb. complete. The front axle was swivelled at its centre on a saddle casting bolted to the front of the engine and was braced near its ends by radius rods with flexible joints extending to lugs cast on the bottom of the engine crankcase. A belt pulley for power work was provided and ran at engine speed.