A wheel-tractor scraper is operating on a level grade. Assume no power derating is required for equipment condition, altitude, temperature, and so on. Use the scraper specifications in Table 8.1 and the “performance charts” in Figs. 8.10 and 8.11. Disregarding traction limitations, what is the maximum value of rolling resistance (in pounds per ton) over which the empty scraper can maintain a speed of 20 mph? What minimum value of coefficient of traction between the tractor wheels and the traveling surface is needed to satisfy the requirements of part a of the question? Solution Since rolling resistance is given by R = Required tractive force (pound) / Gross weight of vehicle (ton); so use the said tables to get weight of vehicle and maximum tractive force, to get maximum rolling resistance (doesn\'t matter what the speed is, since acceleration is zero, this means total traction force = total resistance force). And coefficient of traction is given by c = Usable force / Force applied by engine So, use maximum tractive force as usable force and find force applied by engine for this force (just before the slippage occurs) from tables, to get minimum coefficient of traction..

1. A wheel-tractor scraper is operating on a level grade. Assume no power derating is required for
equipment condition, altitude, temperature, and so on. Use the scraper specifications in Table 8.1
and the “performance charts” in Figs. 8.10 and 8.11.
Disregarding traction limitations, what is the maximum value of rolling resistance (in pounds per
ton) over which the empty scraper can maintain a speed of 20 mph?
What minimum value of coefficient of traction between the tractor wheels and the traveling
surface is needed to satisfy the requirements of part a of the question?
Solution
Since rolling resistance is given by R = Required tractive force (pound) / Gross weight of vehicle
(ton); so use the said tables to get weight of vehicle and maximum tractive force, to get
maximum rolling resistance (doesn't matter what the speed is, since acceleration is zero, this
means total traction force = total resistance force).
And coefficient of traction is given by c = Usable force / Force applied by engine
So, use maximum tractive force as usable force and find force applied by engine for this force
(just before the slippage occurs) from tables, to get minimum coefficient of traction.