A truck tractor unit, operating in its fourth gear range and at its full-rated rpm., is observed to maintain a steady speed of 7.5 mph, when operating under the conditions described herein. Ambient air temperature is 60°F. Altitude is sea level. The tractor is climbing a uniform 5.5% slope with a rolling resistance of 55 lb per ton, and it is towing a pneumatic-tired trailer loaded with fill material. The single-axle tractor has an operating weight of 66,000 lb. The loaded trailer has a weight of 48,000 lb. The weight distribution for the combined tractor trailer unit is 55% to the truck drive axle and 45% to the trailer rear axle. a. The manufacturer, for the environmental conditions just described rates the tractive effort of the new tractor at 330 rimpull horsepower. What percentage of this rated rimpull hp does the tractor develop? i. In performing your calculations, it will be 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 will be acceptable. ii. You may also disregard the power required to overcome wind resistance and to provide acceleration. iii. Assume traction is not a limiting factor. b. What is the value of the coefficient of traction if the drive wheels of the tractor are at the point of incipient slippage for the conditions just described?

Traffic and Highway Engineering

5th Edition

ISBN:9781305156241

Author:Garber, Nicholas J.

Publisher:Garber, Nicholas J.

Chapter21: Pavement Management

Section: Chapter Questions

Problem 10P

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A truck tractor unit, operating in its fourth gear range and at its full-rated rpm., is observed to maintain a steady speed of 7.5 mph, when operating under the conditions described herein. Ambient air temperature is 60°F. Altitude is sea level. The tractor is climbing a uniform 5.5% slope with a rolling resistance of 55 lb per ton, and it is towing a pneumatic-tired trailer loaded with fill material. The single-axle tractor has an operating weight of 66,000 lb. The loaded trailer has a weight of 48,000 lb. The weight distribution for the combined tractor trailer unit is 55% to the truck drive axle and 45% to the trailer rear axle. a. The manufacturer, for the environmental conditions just described rates the tractive effort of the new tractor at 330 rimpull horsepower. What percentage of this rated rimpull hp does the tractor develop? i. In performing your calculations, it will be 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 will be acceptable. ii. You may also disregard the power required to overcome wind resistance and to provide acceleration. iii. Assume traction is not a limiting factor. b. What is the value of the coefficient of traction if the drive wheels of the tractor are at the point of incipient slippage for the conditions just described?

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