August 1, 2012
Select the right machine for the job and maintain it according to plan to achieve the lowest owning and operating costs and the lowest cost per ton.
W heel tractor-scrapers have great utility in quarry and aggregate operations due to their ability to move high volumes of material over long distances without the need for traditional loading tools. These machines frequently deliver the lowest cost per yard when moving materials that aren’t rocky.
Wheel tractor-scrapers efficiently remove soils and other non-rock overburden and stockpile it far away from the mining face for use in reclamation. The same machines haul those materials back and spread them during reclamation. In some sand and gravel operations, scrapers function as primary overburden removal tools. In large operations, a self-loading elevating scraper often works with the haul road construction and maintenance fleet to carry materials to road construction and repair sites and to pick up windrows created by motor graders during blading activities.
Selecting the optimum wheel tractor-scraper
Having the right machine for the application is critical to keep maintenance costs down. Because wheel tractor-scrapers are available in several different configurations, mine operators have the opportunity to boost production and lower costs by selecting the right machine.For example, Caterpillar offers single- and dual-engine machines, as well as a push-pull arrangement that teams tandem-powered scrapers. Loading configurations include open bowl and elevating. The best scraper for the job depends on the site conditions.
When grades are gentle and underfoot conditions are good, a single-engine scraper can operate at maximum speed and lowest cost. With the help of a push tractor, the single-engine scraper loads quickly and hauls to the fill, while carrying minimum iron and using minimum fuel. The push tractor provides the primary loading power, but loading cost still remains lower than systems used to load trucks.
If the site presents grades steeper than 5 percent on the haul and 12 percent when returning empty, tandem-powered scrapers are the answer. The higher horsepower-to-weight ratio allows twin-engine scrapers to climb grades as much as 35 percent. Also, four-wheel drive powers such scrapers through underfoot conditions that stop two-wheel-drive machines. The ability to operate in poor underfoot conditions can assure more workdays per year from the twin-engine scrapers. Tandem-powered scrapers can cut cycle times, because they have better gradeability and higher usable rimpull. Acceleration is faster out of the cut and away from the fill. Even though higher owning and operating costs partially counter the increased production, tandem-powered scrapers may still produce the lowest cost per yard.
Push-pull arrangements allow two twin-engine scrapers to hook up and assist each other in loading. A pair of scrapers can operate as a team or they can separate and work individually with a pusher. The push-pull system offers high production and the lowest cost, because the two scrapers can load in less time than two standard scrapers working with a pusher.
Elevating scrapers provide work-alone capabilities and eliminate the need for push loading. They are most economical on hauls with lower rolling resistance and in favorable material. The elevating mechanism breaks up chunks, which eases dumping and enhances compaction in the fill.
All wheel tractor-scrapers are built for durability and long life, and the advanced age of many scrapers still operating in fleets in North America is testimony to that. But their capabilities — loading, hauling, and spreading materials — require many moving parts that must be maintained properly to ensure reliable operation and high productivity.
The operation and maintenance manual for each particular model of wheel tractor-scraper should be the primary source of recommended maintenance procedures and of the frequency with which each procedure must be performed. Because there are several types of scraper configurations — open bowl, elevating, auger, single engine, twin engine, push-pull — maintenance items vary considerably.
The operator can help keep small problems from becoming big ones by conducting a thorough pre-shift inspection of the machine (see Safety & Maintenance Checklist). In addition to regular fluid level checks, the operator should pay particular attention to the components that do the most work. The operator should check for damage, cracked welds, and distortion on the bail, ejector, ejector rollers, and cutting edge. The hitch pins and stops also should be scrutinized for any damage, and tires should be checked for damage and proper inflation.
Any debris blocking airflow through the radiator or wrapping around moving parts should be removed. Of course, operators should report, immediately, any damage that threatens safety or the health of the machine.
Most wheel tractor-scraper models have daily service recommendations that specify components to be greased. Auger equipped scrapers and elevating scrapers have several additional lubrication requirements that open-bowl machines don’t have. Daily maintenance procedures can be performed by properly trained and equipped operators or by service technicians. Similarly, the fuel system water separator and fuel tank water and sediment should be drained daily.
Every 50 service hours or weekly, the bowl lift cylinder bearings and the hitch should be lubricated. At 250 service hours, oil samples — engine, transmission, and hydraulic — should be taken. Prompt oil analysis and reporting can help catch a developing problem before components are damaged. At 500 service hours, a cooling system coolant sample should be taken and analyzed.
Five hundred hours marks the point of several filter changes. Engine oil and filter are to be changed at that interval, as is the fuel system filter. The hydraulic oil filter and transmission oil filter also are to be replaced at 500 hours. A less common, but no less critical, preventive maintenance function at 500 hours is the cushion hitch (accumulator) pressure. The cushion hitch is filled with nitrogen and, effectively, dampens bouncing and loping as the machine travels.
At 1,000 service hours, the transmission oil must be changed. At 2,000 hours or one year of service, the oils should be changed in the differential and final drives, the hydraulic system, and the wheel bearings.
A sometimes overlooked aspect of scraper operation and maintenance is the selection and maintenance of Ground Engaging Tools (GET). Several options are available to optimize the machine for different material types and loading conditions. GET options include standard, serrated, and abrasion-resistant material. Most are reversible to provide long life and reduced operating costs.
With proper equipment selection, attention to detail during inspections, and strict adherence to preventive maintenance guidelines, operations can achieve low-cost, high-production results from their wheel-tractor scrapers.
Mark Sprouls is a mining writer who has worked with and within the industry for more than 35 years. He is based in Tucson, Ariz. Information for this article is courtesy of Caterpillar Inc.
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