As a project manager of Very Good Construction you want to plan renting a fleet of 25 cu yd tractor-scrapers and have them hauling between the pit and a road construction job. The haul road is a rutted dirt road that deflects slightly under the load of the scraper. There is a slight grade of 2.5% from the pit to the fill location. Rolling resistance factor is assumed as 100 lbs/ton. The return road is effectively level. The haul distance to the dump location is 0.50 miles and the return distance is 1.50 miles. The scrapers are push loaded in the pit. The cycle time for the pusher is 1.50 minutes and the cycle time for the scrapers is 8 minutes. Assume that the weight on the wheels is 75 tons (full) and 50 tons (empty).

a. What are the rolling resistances and grade resistance?
b. What are the effective grades?
c. How many scrapers you recommend to be rented? Explain.
d. What is the production of the system in case 4 scrapers would be rented?

Explanation:

a. Rolling resistance factor = 100 lb/ton for hauling as well return path Grade for haul route = 5%. Grade for return route = 0 From Table 14.2 – rutted dirt road – rolling resistance factor = 100 lb/ton, Rolling resistance on haul route = 100(75) = 7,500 lbs Rolling resistance on return route = 100(50) = 5,000 lbs Grade resistance, GR = % grade 20lb/ton/%grade weight on wheels (tons) Grade resistance on haul route = 5 × 20 × 75= 7,500 lbs

b. Effective (Equivalent) Grade = RR% + GR% To convert rolling resistance, RR into rolling resistance factor, RR%, divide rolling resistance by 20 (see section 14.5 in the text book) and therefore; Effective grade = + grade Effective grade while hauling = + 5 = 10% Effective Grade while returning = + 0 = 5%.

c) Considering the cycle time of the pusher and the scrapers, Scrapers to be used = = 6.4 The motivation is to keep the pusher busy all the time instead of waiting for the scraper to return If 6 scrapers are used, the pusher will be idle and the cost will be lower (since we will have 6 scrapers and 1 pusher). So is the productivity. If 7 scrapers are used, the scrapers will be idle and the productivity will be higher. So is the cost.

d)The productivity will depend on the scrapers (the pusher with idle time) which will be:

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