Belt pulleys formed a part of the equip ment of practically all tractors, being used for operating threshing machines, wood saws, silo fillers, etc. Many tractors built after 1924 were equipped with power take-offs or auxiliary drives through gearing and shafting, which were used on the farm when hauling grain binders, corn pickers and spraying outfits ; by saw mill owners for operating cut-off saws and edgers, and by road builders for operating concrete mixers. It was customary to apply a double horsepower rating; for instance, a three-plough tractor was often rated as a 12-25 h.p. machine. This signified that the tractor could develop 25 h.p. on the belt and 12 h.p. on the draw bar, the difference of 13 h.p. being required for moving the tractor itself over the field.
The crawler (British term "caterpillar") tractor had one or two continuous-chain tracks on which it ran. The outside tread of the chain track in contact with the ground had transverse projecting bars or depressions which caused it to grip the ground firmly, while the inside tread of the chain formed a smooth track on which rolled idler wheels mounted on the frame and carrying the weight of the tractor. The power from the engine was transmitted through a suitable reduction gear to a pair of toothed wheels which engaged with teeth or rollers on the inner side of the track and, when power was applied by letting in the clutch, the tractor rolled ahead on the track and the track at the same time rolled forward. The power was applied to the two chain wheels through a differential gear, and in order to steer a tractor of this kind, one track was held stationary by a brake on the shaft of its chain wheel, while all the power was applied to the other chain wheel.
The Cletrack tractor was one of the smallest tractors of this type. Much of the development work in connection with crawler tractors was done in California, where the Holt, the Best and the Yuba, all tractors made in large sizes, originated. In 1927 many of these large crawler tractors were used for other than agricultural purposes—for example, in oilfields, in lumbering and in road-building. The city of New York in 192o purchased a large number for use in clearing the streets of snow. The lumber ing or logging work done was mainly in swampy districts where horses cannot work except in winter when the ground is frozen, whereas the crawler tractors make logging in such districts possi ble the year round. The weight on the track of a crawler trac
tor of the 1927 type was as low as 5 lb. per sq.in., and such a tractor could go into boggy places where no wheeled vehicles could follow.
The three chief items of cost in tractor ploughing are fuel, depreciation and labour. At the tractor trials held at Lincoln, England, in the autumn of 1919, the fuel con sumption per acre averaged almost exactly 4 imp. gal. for plough ing in heavy clay soil, and 3 gal. for ploughing on cliff lands. In the corn belt of the United States, where the soil is comparatively light, it has been customary to reckon on a fuel consumption (either petrol or kerosene) of 2.5 U.S. gal. (2 imp. gal.) per acre. On the other hand, in a Bulletin of the U.S. dept. of Agriculture on "The Gas Tractor in Eastern Farming," a fuel consumption of 31 U.S. gal. per acre is made the basis of cost calculations for the eastern section, and in the tractor trials held at Harrisburg, Pa., in 1919, the average fuel consumption of all tractors using kero sene worked out at 3.28 U.S. gal. (2.62 imp. gal.) per acre. The fuel consumption in tractor ploughing varies both with the char acter and condition of the soil and with the depth of ploughing. The average depth of ploughing at Lincoln was
inches. The resistance of the soil (drawbar pull) averaged 11.5 lb. per sq.in.
for the heavy clay soil and 9 lb. per sq.in. for the cliff land. In the corn belt of the United States the soil resistance generally varies between 5.5 and 7 lb. per sq.in., and this explains the low fuel consumption in ploughing there. The estimate here given of the cost of ploughing one acre is based on the results of an inquiry by the U.S. dept. of Agriculture among 400 tractor farmers in the Dakotas concerning their experiences in 1917 and 1918. Only one change is made from the estimate of the department, namely, the assumption of a tractor life of seven instead of nine years.
Other factors on which the estimate is based are the following : cost of petrol, $0.276 per U.S. gal. ; kerosene, $0.152 per gal.; lubricating oil, So.5o per gal. ; grease, $o.io per lb. ; repairs, 4% of the first cost per year; depreciation on a 7-year basis; man labour at $4.00 per day; interest at 6% on the average investment (one-half of total investment). Such items as housing, insurance and taxes are neglected.