Performance of Soils under Track Loads. Report 3. Track Mobility Number for Coarse-Grained Soils
ARMY ENGINEER WATERWAYS EXPERIMENT STATION VICKSBURG MS
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Dimensional analysis was used to produce a listing of 28 dimensionless Pi terms from which to develop a comprehensive description of straight-line, level-ground, single-track performance in soil. Five of these terms--P.W, PTW, MWr, zl, and theta include dependent performance variables pull P, towed force PT, torque M, sinkage z, and trim angle theta, respectively. The remaining 23 Pi terms contain only independent variables 15 of these terms are needed to describe track operation in air-dry sand. Among the independent variables included in these 15 terms, seven were determined from laboratory model-track tests to have significantly more effect than the rest on track pull at 20 percent slip load, sand penetration resistance gradient, track width, nominal trakc-ground contact length hard-surface road-bogie deflection, horizontal at-rest center of gravity, and spacing between road wheels. A single dimensionless predication term, sand-track mobility number was developed that takes all seven of these independent variables into account. Road-wheel spacing was found to influence track performance significantly only for those values of road-wheel diameter larger than those ordinarily used or recommended for prototype tracked vehicles. Track pull is increased slightly by 1 locating the drive sproket at the rear rather than the front of the track 2 maintaining high tension in the track belt and 3 using a decreasing, rather than an increasing, front-to-rear pattern of road-bogie cylinder pressure, i.e. by decreasing the ability of the road bogies to resist deflection from fron to rear of track.
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