Efficiency drives an earthmover's decision-making process. "The basic idea of successful mass excavation is to work as efficiently as possible," says Tim Doucette, marketing manager, Liebherr Construction Equipment. "The higher the efficiency, the higher the production and profitability. Machinery standing idle (i.e., excavators waiting for trucks), inefficient cycle times or trucks waiting to be loaded reduces production and profitability."
Size excavators to the task
Mining operations place a premium on sizing excavators to maximize production with each truck load. "In mining, three passes is optimal," says Mike Stec, hauler/loader sales engineer, Volvo Construction Equipment North America. "Construction markets may not be as critical, dissecting every single penny. Typically, four to six passes or under 90-second loading is acceptable."
The goal is to maximize the total cycle time. "The fewer passes that are made result in shorter cycle times, which in turn will increase your production," says Brad Van De Veer, field coordinator/senior application consultant, Caterpillar. "If you six pass, you increase cycle times and reduce production. The quicker you can get that truck loaded and on the road, the better — provided you have enough trucks.
"If you're limited on trucks and the excavator is waiting on trucks anyway, you can get away with six passes," says Van De Veer. "It also depends on the haul distance."
There are major downfalls to loading in too few or too many passes. "If loading is under four passes, you have an oversized bucket in proportion to the truck it is loading," says Carl Heggen, product manager, hydraulic excavators, Komatsu America Corp. "Dumping a large amount of material at one time can add more wear and tear to the truck. Under sizing the excavator (in excess of six passes per truck) can result in long-term loss of productivity due to increased total cycle time." This includes the time to load the truck, travel to the dump site, dump the load and return to the loading site.
"If it is taking six passes or more to load a truck, and other trucks are waiting in line, the trucks' total cycle time is unnecessarily increased and productivity will suffer," says Heggen.
The trick is to match the bucket capacity to the truck capacity so you can load in four to six passes. "Expected excavator bucket fill factor and material density must also be taken into account," says Heggen. "If you were running 40-ton trucks and material density is 3,000 lbs./cu. yd., a Komatsu PC800LC-8 with a 6.0-cu.-yd. bucket with 95% bucket fill would load the truck in five passes. If you moved up to a PC1250LC-8 with an 8.5-cu.-yd. bucket at a 95% bucket fill, 40-ton trucks would be loaded in three passes and the excavator would probably be waiting between trucks.
"With material density the same (3,000 lbs./cu. yd.), a PC600LC-8 with a 4.5-cu.-yd. bucket would easily load 30-ton trucks in five passes," says Heggen. "The above is based on excavators equipped with standard boom, short stick and a standard counterweight."
In situations requiring longer reach, a longer boom and arm would need to be installed, with a necessary downsizing of the bucket. "This will increase the number of passes per truck, possibly increasing excavator cycle time, and production will decrease," Heggen says. "Every jobsite has different conditions, so you really want to work closely with your distributor to ensure the best configuration for maximum productivity."
The choice is not always clear cut. "Material density plays a large part in determining the size," says Doucette. "Hard, dense material would require a large excavator, while lighter, less dense material may allow for the use of a smaller excavator.
"In a perfect world, the material density determines the size of the bucket," says Doucette. "A specific excavator can handle a specific load at the end of the attachment (measured in tons). If you require more tons per bucket, you would ideally select a larger machine. In practice, the size of the truck and the desired number of buckets to load it determine the bucket/excavator size."