Improving the mobility performance of tracked vehicles in deep snow
Sammanfattning: Improving the mobility of tracked vehicles in deep snow requires an increase in the knowledge and understanding of the design parameters which influence the tractive performance. This relates particularly to the new vehicle requirements of increased payload to vehicle weight ratio, improved fuel economy and reduced life cycle costs. It has been the aim of the research presented in this thesis to improve the understanding of how different vehicle design parameters affect the tractive performance of tracked vehicles in deep snow. A new tracked vehicle was developed to enable experimental studies of different vehicle design parameters in deep snow. The vehicle was skid steered with a hydrostatic transmission and driven by a five-cylinder diesel engine. The vehicle design made it possible to vary several vehicle design parameters in the field, one at a time, without affecting the others. To be able to compare vehicle tests from different snow conditions and also to be able to simulate vehicle performance in snow, the strength properties of the snow are required. A bevameter was developed to characterise the pressure-sinkage relationship, the response to unloading- reloading and the shear stress-shear displacement of the snow. The idea of using the bevameter technique was to measure the terrain data under loading conditions similar to those encountered by an off-road vehicle. The bevameter developed consisted of a hydraulic cylinder with a piston of one metre stroke through the cylinder. A hydraulic motor with a planetary gear was attached to the upper end of the rod piston, which exerted the turning torque in the shear test. The cylinder exerted the normal force in both the pressure-sinkage and the shear test. The most important finding was that the drawbar pull strongly increased when the initial rear track angle was reduced. This was due to both an increased thrust between the tracks and the snow and to a reduced track motion resistance. The increased performance originated from a reduced load transfer from the last roadwheel to the others when the initial rear track angle was reduced. The tractive performance increased continuously with decreasing initial rear track angle. The tractive performance not only increased when the idler made contact with the snow, thus increasing the track contact length, but increased continuously with a decrease in the initial rear track angle prior to the idler contacting the snow. Another important finding was that the drawbar pull increased when the centre of gravity was moved forward, especially when the idler was in the elevated position. An increased belly ground clearance and a reduced initial angle of the vehicle belly both provided improved mobility performance of the vehicle. This was due to both increased thrust and reduced belly resistance, which in both cases, originated from a vertical load transfer from the rear of the belly to the tracks. The experimental tests of the influence of the initial rear track angle on the tractive performance were compared with corresponding simulations using the Nepean Tracked Vehicle Performance Model for Microsoft Windows, (NTVPMwin). The simulation also showed an increased drawbar pull when the initial rear track angle was reduced. However, there was difference in the absolute level of the drawbar pull with the simulation results showing a 6-10% higher drawbar pull.
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