A truck tractor unit, operating in its second gear range and at its full rated rpm, is observed to maintain a steady speed of 1.5 mph, when operating under the conditions described here. Ambient air temperature is 60°F, altitude is sea level. The truck is climbing a uniform 6.5% slope with a rolling resistance of 65 lb per ton, and it is towing a pneumatictired trailer loaded with fill material. The two-axle truck tractor unit has a total weight of 60,000 lb, 55% of which is distributed to the power axle. The loaded trailer has a weight of 75,000 lb.
a. The manufacturer, for the environmental conditions just described, rates the tractive effort of the new truck at 60 rimpull horsepower. What percentage of this rated rimpull does the truck actually develop? In performing your calculations, it is acceptable to assume that the component of weight normal to the traveling surface is equal to the weight itself (i.e., cos 0 taken as 1.00, where 0° is the angle of total resistance). The “20 lb of rimpull required per ton of weight per % of slope” approximation is acceptable. You may also disregard the power required to overcome wind resistance and to provide acceleration. Assume traction is not a limiting factor. (87.7%)
b. What is the value of the coefficient of traction if the drive wheels of the truck are at the point of incipient slippage for the conditions just described? (0.4)
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