Stripping overburden in the Illinois coal belt, the Dusty Coal Company uses 270-horsepower, track-type tractors (with direct-drive transmissions) and drawn scrapers. The overburden is a very soft loam that weighs 2,800 lb/yd (loose). Estimated RR factor for the haul road is 300 lb/ton. If the scraper weighs 35,000 lb (empty) and carries 25 loose cubic yards per trip, what is the RR of the loaded unit? What operating gear and speed do you estimate for the loaded machines on level ground? (See Table 15.4.)

Table 15.4

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Gear 1 2 23 I TABLE 15.4 Speed and Drawbar Pull (270 hp) (Track-Type Tractor) Drawbar Pull Forward 3 4 5 6 Forward mph 1.6 2.1 2.9 3.7 4.9 6.7 km/h (2-6) (3-4) (4.7) (6.0) (7.9) (10.8) Reverse mph 1.6 2.1 2.9 3.8 4.9 6.8 At Rated rpm lb kg 52,410 (23,790) 39,130 (17,760) 26,870 (12,200) (6.1) 19,490 (8,850) (7.9) 13,840 (6,280) (10.9) 8,660 (3,930) km/h (2.6) (3.4) (4.7) Maximum at Lug kg (28,990) (21,760) (15,080) lb 63,860 47,930 33,210 24,360 17,580 11,360 (11,060) (7,980) (5,160)