Wheel loaders, excavators and haul trucks are the primary movers of material from the pit face to the plant at aggregate operations. Short loading cycle times and proper loader-truck and truck-crusher size matching are critical to maximizing their efficiency. But haul speed also significantly impacts productivity.

Support equipment, therefore - such as motor graders to keep haul roads smooth and well drained - can be important production tools. In a 1996 article in Rock Products ("Tips for turning haulers into money-makers," March 1996, page 50), Caterpillar's Don Stretch listed the benefits of a good haul road:

- lower repair costs;

- better tire life;

- more up-time;

- more efficient operators; and

- safer operating environment.

Rough haul roads create high rolling resistance and force operators to slow down. Decreasing average haul speed by as little as 11/42 mph could cost an operation 50,000 tpy in lost production, according to Stretch.

In addition, proper grading of haul road surfaces improves drainage, helping maintain the road's structural integrity and eliminating water puddles, he said. Keeping truck tires dry can avoid some repair costs because wet tires are more susceptible to cuts.

Rough haul roads also increase the risk of tire and vehicle damage caused by rock that has fallen off the bed of a bouncing truck. A smooth road allows full truck loads to reach the crusher without scattering rock along the way.

Wheel loaders can accomplish some haul road maintenance, but, because that is really outside their design application, it is not the most efficient use of that production loading tool. For heavily used haul roads, a motor grader provides the most effective maintenance.

At least five companies market medium to large motor graders suitable for a quarry environment. Table 2 lists the smallest and largest grader models offered by each company and some selected specifications.

Caterpillar manufacturers 10 grader models, including the largest machine available. Cat's 24H, designed for large mine haul road maintenance, has a standard 24-ft blade and is powered with a 500-hp diesel engine.

Champion, which traces its grader roots to the 1870s, is now part of the Volvo Construction Equipment Group. Its Series VI 700 models, introduced in 1999, feature an improved blade-control systems that allows positioning of the blade up to 90 vertically on either side of the grader and up to 21 downward for profile cuts. The blade can be lowered 32 in. below ground level for deep cuts.

Komatsu America markets a line of smaller Galion motor graders - 144 to 204 hp - plus two larger models, up to 280 hp. The company recently introduced a new line of graders called Laterra. The three Laterra models - GD555A-3, GD655A-3 and GD675A-3 - range in operating weight from 30,525 to 34,390 lb and in horsepower from 140 to 180 hp.

The Laterra models have a dual-mode transmission that allows the operator to move from torque converter to direct drive mode by the flick of a switch. They also have variable-displacement pumps and load-sensing systems to provide better speed and responsiveness from hydraulic components, the company said.

Graders lope, or bounce, because of natural vibration. Grader design, tire size and air pressure, weight distribution on each axle, road surface profile and speed all determine the vibration frequency, according to Bridgestone/Firestone Off-Road Tire Co. (Rock Products, "Maintaining smooth haul roads: the ups, downs, ins and outs of road grader loping," Nov. 1996).

BFOR conducted tests, independently and with equipment manufacturers, that identified several causes of loping and some ways to reduce the problem.

- Out-of-round wheel assemblies - deflate tire, rotate 180 on rim and remount.

- Flat spots on tires - warm up tires by running unloaded for a short time before starting work. If flat spot is cause by tire damage, rotate or replace the tire.

- Uneven tire wear - rotate or replace tires.

- Improper weight distribution - field tests indicate that graders should carry 30% of their weight on the front tires; 70% on the rear. Check weight distribution with attachments and readjust to 30/70, if possible.

- Improper tires - match tire size and type to equipment and conditions. Larger tires at lower inflation pressures may reduce loping, but can change the effective transmission ratio.

- Tire pressure not matched to load - match cold tire inflation pressures to specifications for each tire's actual load.