Chapter 9: Off-Road Hauling
November 1, 2007 By: P&Q Staff Pit & QuarryThe heavy lifting, so to speak, in a quarry operation is performed by haul trucks. The main job of a haul truck in a quarry environment is transporting material from the quarry face to the primary crusher.
There are two main kinds of haul trucks: Rigid frame and articulated. Rigid frame haulers are typically used when there is a haul road in place, or a smooth path between the muckpile and the crusher. Artics are used when the terrain is uneven, the ground is soft or conditions are usually muddy.
Today's haul trucks are much different than those of years gone by. With increased payloads, horsepower, torque and fuel efficiency, these trucks not only increase productivity but also can lower an operation's cost-per-ton.
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Savvy production managers know that haul trucks must be matched to job specifications. It is worth reviewing your operation and equipment fundamentals just to make sure your assumptions are correct.
It is also good practice to match haul trucks to the machine that loads them. That means:
- The right number of trucks and the right capacity to serve the crusher and/or stockpiles without making the loader wait or causing a line of trucks waiting on the loader. Both situations hurt productivity and drive up cost.
- Truck-bed size and a configuration that makes a good loading target, allowing quick spotting of the bucket and even load distribution. Optimum load time is about two minutes, or three to four passes with no partial bucketloads.
When thinking about a haul-truck purchase, it is wise to make today's purchase with tomorrow in mind. How many trucks of what size will you need two years from now? Five years? Will you replace or add crushers? This could change the needs of your hauling operation.
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Many aggregates plants are considering the elimination of as many haul trucks as possible, due to increases in fuel costs. Those that are planning to stick with haul trucks, are moving to larger loading and hauling units, able to handle the same tonnage with fewer machines. The objective is increased productivity, not necessarily higher total production. In other words, a producer that was using four 35-ton units might switch to three 50-ton units.
Production remains about the same in this scenario, but with a substantial improvement in tons-per-manhour and lower maintenance costs. Over the years, there have been a number of progressive improvements to trucks, including:
- New cabs. Cabs are no longer just a place to sit, but have become the truck operator's office. Typical features include not only heating and air conditioning, but radio mount, and well-placed controls. Greater window capacity offers better visibility. These cabs are built for sound reduction and come equipped with suspension seats.
- Horsepower ratings. Gross horsepower ratings may be misleading. Net horsepower is also an important rating for comparison, since this more closely measures the power that can be applied to moving the truck.
- Retarding systems. Two types of retarding systems became available: One acts on the driveline, the other works through the brakes. The brake-retarding system spares drivetrain components from tremendous reverse-torque loads and helps prolong machine life. In addition, the brake-retarding system takes advantage of advanced brake systems. The most effective brakes today are oil-cooled, multi-disc type which provide up to five times the life of shoe brakes and never need adjusting. Multi-disc brakes are not to be confused with caliper-disc brakes, which are similar to automotive-type disc brakes and rely on a pliers-like squeeze to slow the wheel. The multi-disc design multiplies the braking surface, but eliminates metal-to-metal contact. A pressurized flow of oil separates discs and carries away friction-generated heat. One system combines retarding, emergency, service and parking functions in a dual-piston design.
- Frame design. Basically, the life expectancy of a truck comes down to chassis strength. Frames should be box-section construction with high weld penetration in the manufacturing process and castings at high stress points. Frames should also be weldable to allow field repairs without high-technology weld procedures.
- Electronic controls. Modern electronics allow substantial improvement in the control of various machine functions and components. Electronic transmission control, for example, assures smoother shifts to minimize torque spikes. The result: greater transmission and engine life and greater fuel efficiency. Electronic monitoring systems for vital machine functions are not only more reliable than mechanical gauges, they give the operator both visual and audible alarms. Future applications will contribute to further improvements in machine reliability, performance, and fuel economy.
- Traction control. If your haul road is subject to periodic slippery conditions, a traction-assist feature on the rear axle may be worthwhile, not only for improved control and handling but also for reduced tire wear. Two types are available: 1) Mechanical systems similar to automotive-type, limited slip differentials; and 2) Electronic controls that monitor wheel rotation on each side independently and, when one exceeds the other by a set limit, applies the brakes, transferring torque to the tire with better traction. Electronic traction assist does not interfere with normal differential action and places no increased stress on the driveline.
- Hydraulic, rather than air-over, hydraulic brakes. The result is superior reliability, response, control, serviceability and modulation. The parking brake is integrated into the gear selector for easy operation.
- Increased engine-service intervals. For some trucks, 500 hours is common – twice that of previous truck models. On-board engine oil evacuation and pre-lube can reduce oil-change labor time by as much as 50 percent.
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