Evaluate cost and production using a 3-cy hoe to load wet gravel into 14-cy size trucks. Assume a bucket fill factor of 105% and that the hoe bucket swing cycle time is 24-sec. Use the information in Fig. 10.8 (p.322), a truck net-empty weight of 36,860 lb, and a rated payload of 40,000 lb. Dump time will be 1.3 min. The haul distance is 2.5 mi from the pit to the plant up a 3% grade. The rolling resistance of the haul road will be maintained at 3%. The hoe and operator cost $150 per hour and trucks cost $115 per hour (operated). What is the most cost-effective mix of bucket loads to use in loading the trucks and number of trucks to place on this job?

The excavator has a 3 cu-yd bucket and 24 sec. cycle time. The haul road from the pit to the plant is 2.5 miles up a 3% grade. Rolling resistance of the haul road is 3%.

Cost of hoe $150/hr., trucks $115/hr.

Step 1. Number of bucket loads.

The bucket fill factor for the hoe is 105%, from the problem statement. The hoe bucket volume will be 3.15 lcy (3 × 1.05). The heaped capacity of the truck is 14 lcy.

Balance Number of bucket loads 14 lcy/3.15 lcy = 4.44

The actual number of buckets should be an integer number, therefore two cases should be investigated, either placing 4 or 5 bucket loads on the truck.

MANUA Total resistance, percent of vehicle weight Rated GVW Rated NVW First Second Performance Chart Engine: GM 6-71 N at 239 hp Transmission: Fuller 5GT-1220 Axle ratio: 16.07:1 Tire size: 16.00-25 Third Fourth. Ro.Ra. 28.6 in. Fifth 150 mon 88229 2382 60 40 2947 Rimpull, Ib (x1000) 5 300 200 150 100 80 60 50 40 30 20 0 5 10 15 20 25 30 35 40 45 50 Vehicle weight, lb (x1000) mph FIGURE 10.8 Performance chart for a 22-ton rear-dump truck 25 20 2 15