Research Bulletin (Iowa Agriculture and Home Economics Experiment Station)


The investigation was confined to a study of the force and motion relations of rigid-cylindrical transport wheels and of the soil on which they operate. The analytical methods of theoretical mechanics, and laboratory and field experiments were used.

Equations were developed for the relative effect of speed, diameter and effective obstruction height on impact. The mechanics of a wheel rolling on a non-elastic friable medium was developed and the fact of slippage established both rationally and experimentally. The nature of the soil motions and soil displacements caused by a rigid wheel was studied. It was found that the soil adjacent to the track left by a rigid wheel is moved ahead and that this change in position of soil particles is attained by curved rather than straight line motion.

The effect of area upon the supporting capacity per unit area was studied for four conditions. A wide variation was found for this relationship. Apparently this variation was caused by differences in the cohesion and internal friction of the soils.

An apparatus for studying the rolling resistance and slippage of individual transport wheels was designed and built. With this apparatus the possibility of using variable load trials was studied. The results obtained by the use of this method were unreliable. The effect of speed was investigated for two conditions, meadow and tilled soil. For these soil conditions and speeds up to 5 miles per hour, the effect of speed appeared to be of minor importance compared to the effects of diameter and width. The effect of repeated trials in the same track was investigated, and the rolling resistance was found to vary approximately as the -0.2 power of the number of the trial for trials on meadow and as the -0.5 power on tilled soil.

The effects of load (300 to 1200 lb.), diameter (16 to 60 inches) and width (2.5 to 20 inches) on rolling resistance and slippage were investigated for three surfaces-meadow, tilled soil and a layer of dry loose sand on concrete. The rolling resistance was found to vary approximately as the 0.6 to 1.3 power of the load, the -0.5 to -0.7 power of the diameter and the -0.5 to 0.5 power of the width. These variations are explained qualitatively by certain combinations of wheel dimensions and soil conditions.

The association of soil moisture, volume weight and resistance to penetration with rolling resistance was studied. A very high positive correlation, 0.97, was obtained between penetration readings of the "penetrometer" used in these studies and rolling resistance, and a high negative correlation, -0.87, was obtained between volume weight and rolling resistance.



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