886 EXPERIMENT STATION RECORD. IVol. 41 



The principles of the wheeled farm tractor, E. H. Hewitt {Oas Engine, 21 

 (1919), jS'o. 6, pp. 1S2-185, figs. 3). — The results of a large number of laboratory 

 and field expei-iments are suuiuiarized. 



A series of laboratory tests on full-sized wheels which checked very well 

 with work on the machine in the open showed that "(1) the maximum drawbar 

 pull is a definite function of the weight per inch of width. Weights used 

 varied from 10 to 200 lbs. per inch; the ratio of maximum possible drawbar 

 pull to total weight on the wheel was constant for that range. This was 

 found to be true whether the ground were wet or drj\ (2) On sandy ground 

 the drawbar pull available with a smooth metal wheel is about 30 per cent 

 of the weight on the wheel. (3) On damp, sandy ground the maximum draw- 

 bar pull is greater, being about 43 per cent of the weight, and under some con- 

 ditions even slightly higher. (4) Cleats increase the maximum drawbar pull 

 only in so far as the soil resists shearing ; that is, the cleat carries a section of 

 the top soil and slides it against the soil below the edge of the cleat. 



" Experiments indicated that this was practically independent of the depth 

 of the cleat, depending solely on the shearing strength of the soil at the depth 

 of the cleat edge. In some cases the shallower cleat pulled more than the 

 deeper cleat because the roots in the sod were not cut off and advantage was 

 taken of their shearing strength." In going uphill it was found that a cleat 

 inclined forward at an angle improved conditions somewhat, as the cleat en- 

 tered the soil almost horizontally, acting like a step and tending to lift the 

 weight off the wheel. "xVn inclination of about 30° was found to be the most 

 satisfactory on a 6-ft. wheel. This arrangement tends to self-clean to a great 

 extent. Setting the cleats at an angle of 30° to the axis of the wheel also 

 helps this cleaning efiiect by slipping action. The shearing strength of the 

 soils tested appeared to vary from 5 lbs. per inch of width in dry molding sand 

 to 75 lbs. in loam or sod." 



Studies of propelling power requirements on hard macadam road, dry short 

 clover, long grass sod on gravelly loam, and dry plowed land, in which a cart 

 with 46-in. wheels with 8-in. flat tires was towed by a motor truck, showed 

 that the drawbar pull increased rapidly with the weight per inch of width on 

 a regular curve on all but the hard macadam road, where the increase was 

 only slight. " If a tractor has too great weight per inch of width of wheel 

 and gets on soft ground, as it must do in harrowing, etc., the power consumed in 

 rolling friction becomes a lai-ge percentage of the total developed and the 

 drawbar pull is proportionately decreased. There is every advantage in in- 

 creasing the surface and reducing the weight. Tractors wath a weight of 

 over 200 lbs. per inch of width of wheels that are even 6 ft. in diameter be- 

 come very inefficient on soft ground." 



" By using antifriction bearings, good lubrication, and cut gears of good 

 design, it is possible to reduce the frictional losses from the engine to the rim 

 of the rear wheels to less than 16 per cent of the total power developed. . . . 

 It was found that on good sod with short grass the efficiency of the machine 

 was about 75 per cent, 1. e., the drawbar horsepower divided by the engine 

 horsepower was 0.75. On plowed ground, harrowing, or cross plowing, it was 

 about 65 per cent, and never fell below that figure." 



Average data of several years' laboratory tests of fuel efficiency and gear 

 changes with a number of engines indicate that for an ordinary engine the load 

 should not go below 60 per cent for maximum fuel economy. " On the best 

 ones under good conditions a fuel economy of 0.66 lb. per horsepower-hour can 

 be obtained at 50 per cent load. Tractor engines should not be used below this 

 point over long periods. Another point to be noted is the absolute necessity of 



