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Chapter 6: Dump Trucks

The most common hauling equipment used for mining and construction work are the 20 and 40 ton dump trucks

 USE

The 20 ton dump truck is primarily used for over the road hauling, while the 40 ton is used for off-road (in-pit) use.  The 20 ton can be increased by adding a separate trailer for an additional 20 ton capacity.

 

Figure 6-1.  20 Ton Dump Truck

 CAPACITY

 The capacity of hauling equipment is expressed in one of three ways:

  • gravimetrically by the weight of the load it will carry (in tons),  
  • by its struck rear-dump body volume (in cubic yards),
  • or by its heaped rear-dump body capacity (in cubic yards).

 The hauling capacity of dump trucks is normally expressed gravimetrically: 20-ton or 40-ton. Conversely, the capacity of loading equipment is normally expressed in cubic yards. The unit weight of the various materials to be transported may vary from as little as 1,700 pounds per LCY for dry clay, to 3,500 pounds per LCY for trap rock (see Table 1-2, in Introduction to Earthmoving, for weights of common materials). Always make sure that the volumetric load does not exceed the gravimetric capacity of the truck.

 OPERATION

 Loading

 For maximum efficiency, fill trucks as close to their rated hauling capacity as practical. Adjust the load size if haul roads are in poor condition or if the trucks must traverse steep grades. Overloading will cause higher fuel consumption, reduced tire life, and increased mechanical failures.

 Use spotting markers when trucks are hauling from a hopper, a grizzly ramp, or a stockpile. Spotting markers are also beneficial when excavators (such as a dragline, a clamshell, a loader, a backhoe, or a hoe) are used to load hauling equipment. They facilitate prompt and accurate vehicle spotting which improves loading efficiency.

 Spot trucks as close to the bank as possible when loading with an excavator. Ensure that the trucks are within the working radius of the dragline, the clamshell, or the hoe bucket. When using a loader, position the truck and loader so that the two machines form a V. This arrangement will reduce the loader cycle time (Figure 6-2).

Loading

Figure 6-2 Truck and Loader V-Positioning for Loading

Maintaining Proper Speed

 Haul at the highest safe speed and in the proper gear, without speeding. Speeding is unsafe and hard on equipment. When several trucks are hauling, it is essential to maintain the proper speed to prevent hauling delays or bottlenecks at the loading and dumping sites. Use separate haul roads to and from the dump site, if possible. Keep haul roads well maintained, with a minimum grade. Use one-way traffic patterns to increase efficiency.

 Dumping (Unloading)

 Always use spotters to control dumping operations. When dumping material that requires spreading, move the truck forward slowly while dumping the load. This makes spreading easier. Establish alternative dumping locations to maintain truck spacing when poor footing or difficult spotting slow the dumping operation.

 Preventive Maintenance

  Keep truck bodies clean and in good condition. Accumulations of rust, dirt, dried concrete, or bituminous materials hamper production. Consider the time spent cleaning and oiling dump bodies, particularly for asphalt or concrete hauling, when computing transportation requirements.

  •  Clean truck bodies thoroughly at the end of the day. When used to haul wet concrete mix, spray the dump beds with water before loading and clean them thoroughly as soon as practical after dumping.

  • Coat the walls and sides of truck bodies with diesel fuel or oil to prevent bituminous materials (plant-mix asphalt) from sticking

 PRODUCTION ESTIMATES

 The production capacity of the loading equipment is normally the hauling operation’s controlling factor. Never keep loading equipment waiting. If there are not enough trucks, there will be a loss in production.

 Number of Trucks Required

 Use the following formula to estimate the number of trucks required to keep loading equipment operating at maximum capacity:

        The numeral 1 in the formula is a safety factor against the necessity for closing down loading equipment due to lack of hauling equipment. If all operations are on schedule, one truck will always be standing byat the loader, ready for spotting.

        The truck cycle time is the time required for a truck to complete one cycle of operation. One complete cycle is the time a loaded truck takes to travel to the dump site, unload, return to the loading unit, and be reloaded.

        The loader cycle time is the time it takes the loading equipment to load the truck, plus any time lost by the loading equipment while waiting for the truck to be spotted.

 NOTE: After the job has started, the number of trucks required may vary because of changes in haul road conditions, reductions or increases in haul length, or changes in conditions at either the loading or unloading areas.

 Number of Standby Trucks Required

 Identify, based on the normal cycle time, the number of standby trucks that should be available to replace trucks that develop mechanical trouble. The number of standby trucks needed depends largely on the mechanical condition of the active trucks as well as the size and importance of the job. In the case of a small fleet and a single loading unit, the ratio of standby trucks to active trucks may be as high as 1:4. On larger jobs, the ratio is smaller. Standby trucks need not be idle; use them on lower priority tasks from which they can easily be diverted.

EXAMPLE

 How many 20-ton trucks (hauling 18 LCY per load) will it take to support a wheel loader having a 10-cubic-yard heaped-bucket capacity? The haul-unit cycle time is 20 minutes excluding loading time. The loader cycle time per bucket load is 0.5 minute. Consider a 60-minute working hour.

 Step 1. Determine the number of bucket loads required to fill a truck. 

 

Using only one bucket load would mean that the truck would only haul 10 LCY per trip. Using two bucket loads would mean that the truck would haul 20 LCY per trip and the extra material would spill out during the loading process.

 Step 2. Determine the loading time per haul unit.

 

Considering one bucket load per truck

 

 Considering two bucket loads per truck

 

 Step 3. Determine the number of hauling units needed to support the loading unit.

 Considering one bucket load per truck

Considering two bucket loads per truck

 Step 4. Determine the production based on the number of hauling units used. The loader will control the production because of the one extra truck added to the formula. Therefore, there is always a truck waiting at the loader.

Using one bucket load per truck will require 42, 20-ton dump trucks.

Using two bucket loads per truck will require 22, 20-ton dump trucks.

With an understanding of the effect of the different choices, determine the number of trucks to use on the haul and how many bucket loads to place on each truck. This illustrates that the capacity of both the loader and the trucks are set numbers. Therefore, there is a relationship between bucket loads and haul-unit capacity, which in practice must be an integer number.

MIke Albrecht, P.E.

o   40+ years’ experience in the mining industry with strong mineral processing experience in precious metals, copper, industrial minerals, coal, and phosphate

o   Operational experience in precious metals, coal, and phosphate plus in petrochemicals.

o   Extensive experience performing studies and determining feasibility in the US and international (United States, Canada, Mexico, Ecuador, Columbia, Venezuela, Chile, China, India, Indonesia, and Greece).

o    E-mail:  info@smartdogmining.com