Expensive repair or replacement of steel-cable belting on long load-out and field conveyors often is required when belts drift into their supporting steelwork. Clouds of smoke and sometimes hazardous damage can result. At the San Antonio quarry of Redland Stone Products, belt damage due to mis-tracking has ended.

While the conditions that caused Redland's tracking problems haven't gone away, the problems themselves have been eliminated with a new return-belt positioner. It works more forcefully and immediately than conventional designs by funnelling the belt into a centered position, rather than steering it there.

The quarry's railroad load-out conveyor runs at 500 fpm with a maximum capacity of 2,200 tph. It's an elevated, 48-in.-wide belt spanning the entire 1,400-ft distance from blending piles to railhead. This belt (the most critical conveyor on site) is the sole means of transporting custom-blended orders, usually in railcar-size batches of 100 tons each, to a pair of overhead silos straddling the tracks. Of Redland's total output-about 12 million tons in 1996-approximately 40% leaves on rails to material yards in Houston and other locations.

Tracking problems have hindered another key part of the operation at the opposite side of the plant. A 72-in. field conveyor carries minus-8-in. limestone from the 60-ft-high mobile primary crusher, once and possibly still the world's largest.

Because the quarry floor is 70 ft below plant level, the field conveyor operates in three end-to-end segments. The first two segments (600 and 760 ft long between end pulleys) are inclined at about 6 degrees. The third, essentially horizontal, spans 1,790 ft between end pulleys. Running at 500 fpm with a maximum capacity of 5,000 tph, this three-conveyor line is the sole means of feeding the Redland plant. Both load-out and field belts are 31/44 in. thick and operate under tensions of 350 to 400 piw (pounds per inch of belt width).

Automated operations Initial screening at the plant sorts minus 31/44-in. and minus 1 151/416-in. base sizes out of that incoming material and stores it in two surge piles built over underground reclaim conveyors for either load-out or further crushing. Larger rock goes to a third surge pile built over a reclaim conveyor that feeds secondary crushers and screens. That secondary plant produces and washes nine sizes of stone from plus 1 31/44-in. down to #4 yen #8 mesh, and screens the fines into two sizes of sand.

Finished stone is conveyed to stackers that maintain the nine sizes in separate piles, all lined up over a reclaim tunnel housing two parallel blend conveyors. This arrangement allows each blend conveyor to produce custom mixtures automatically by retrieving stone from any combination of size piles. These two 60-in.-wide belts separately feed a truck load-out and the rail load-out belt.

All stone-producing operations except the mobile crusher are controlled from a computer center perched atop a 60-ft tower in the middle of the plant to give operators a clear, full-circle view. From this post, operators monitor and balance production to keep various products adequately stockpiled.

Order specifications are programmed into the computer so everything from simple one-size orders to custom blends can be loaded out automatically on demand.

For truck shipments, drivers carry programmed plastic badges that identify their trucks. When the badge is passed across a reader at the load-out terminal, the computer calls up the load ordered for that truck.

For rail shipments, an operator at the load-out silos coordinates car positioning with the train crews, and uses a computer terminal to call up loads appropriate for each railcar. By anticipating each railcar's arrival, and keeping the twin silos alternately filled for the next car in line to be loaded, experienced operators can keep the rock flowing almost continuously.

Off-center loading Mistracking problems on the rail load-out conveyor derive from the way its belt receives loads from the blend conveyor, explains Plant Maintenance Superintendent Tim Waclawczyk. "These two lines meet at a 90 degrees transfer and the materials' momentum coming off the blend belt naturally throws it to the far side of the load-out belt. This pushes the load-out belt off centerline, causing the return to off-track in the same direction as it approaches the tail pulley directly below," said Waclawczyk. Close clearance between the two belts at the transfer point prevents use of a centering-chute.

Although the return idlers extend 4 in. beyond the width of the belt on either side, off-center loading force can distract the return belt farther than that. Belt edge shredding against the steel framework occurs here.

"The problem has gradually grown worse over the years as orders evolved into a greater variety of blends and load tonnages," Waclawczyk said. "We can have 600 tph coming through one minute, then 2,000 tph the next, and each different load has a different effect on belt tracking."

Redland's field conveyors experience similar return-side damage, often worse, but for a different reason: frequent relocation. The mobile crusher is moved close to the face every few days on hydraulic legs in order to minimize shuttle distance for the team of 13-cu-yd wheel loaders that feed it. As a result, field conveyor length and configuration change at least every six months, making permanent concrete or asphalt conveyor pads impractical.

"Production demands don't often give us time to prepare the conveyor route very well," Waclawczyk said, "and a good rainstorm can wash out what preparation we have." The resulting misalignment, compounded by conveyor length, leads to trouble.

Damage detection and repair Control room computers monitor zero-speed sensors, motor amperage and other inputs to detect problems in belt movement. When problems occur, this system triggers an audible alarm and a visual alert on the control room screen, and then documents it. Any downstream belt stoppage shuts down upstream production.

"These safeguards can't totally protect the belts from damage," Waclawczyk said. "Even if they could, we'd still want to avoid the wear and tear of frequent stopping and starting,"

Mistracking has been bad enough for the field conveyor steelwork to tear into a belt and snag a cable. With 600-hp drives and high belt speeds, snags can result in 1,000-ft slits.

Repairing a minor rip to the 48-in. belt requires an outside contractor with vulcanizing equipment, costs around $1,500 and shuts the process down for about eight or nine hours.

"If we have to cut out a piece and resplice the belt, we're looking at $5,000 and maybe 12 to 13 hours of downtime. When the damage is so big that we have to splice in a new section of belt, then we have two splices with twice the cost and downtime, plus the cost of the belting," Waclawczyk said. Totally replacing the belt could run around $150,000, he said.

In the past, conventional trackers, steered the Redland belt with a pivoting roller, guided by upright side rollers that react to the belt position. "These worked OK, especially when new," he said, "but they couldn't put enough pressure on the belt to overcome the more severe forces we sometimes see here, especially at the blend-belt transfer. They also needed a fair amount of maintenance; we had to keep those pivots clean and lubricated, and those little upright side rollers can wear out pretty quickly on a bad-tracking belt."

Track down a tracker In 1996, Waclawczyk discovered a new belt positioner being introduced by Flexco under the name Persuader. It uses a pair of short rollers angle-mounted across the belt in fixed position, pressing downward against the return-belt's bottom cover along the outer areas of belt width. These rollers funnel the belt into a centered position as it approaches the tail pulley so it will be in correct position to receive its load. With no pivots to wear out, seize or need maintenance, its only moving parts are the rollers, which operate like inverted idlers.

The unit installs as a single piece, using an adjustable-width mounting bracket that attaches to the conveyor framework with two bolts on each end. A sliding, bolt-tightened adjuster allows the user to determine the amount of roller pressure applied to the belt. This design makes the device easy to reposition when conveyor length changes, or when conveyor relocation causes optimum positioner locations to migrate to other areas along the belt.

"Naturally, the first place we tried them was on the railroad load-out conveyor," Waclawczyk said. "One [belt positioner] mounted 25 ft forward of the tail pulley was all it took to end the mistracking damage. I watched it through three or four different loadouts and never saw the belt move more than 2 in. off pulley center."

Additionally, by holding the return belt firmly in position, the unit helped minimize the sideways drift where the load hit the belt.

No more edge damage After initial success, 10 more units were ordered for the field conveyors. On the longest field section, four were installed along the return. One placed 25 ft forward of the tail pulley, and the other three 895, 1,450 and 1,700 ft away. The two shorter field conveyor sections each were given three return-side positioners-one 25 ft forward of the tail pulley, one near the head pulley and one approximately midway between.

This funnelling approach to belt positioning soon demonstrated two main advantages. Firstly, it forces the belt into center position immediately, while the steering effect of the pivot type devices needs some distance, sometimes 60 to 80 ft of belt travel for belt guidance. Secondly, it changes the cross-section shape of the belt from flat to an inverted-trough channel, which exerts a straightening influence on the belt that extends about 50 ft in both directions.

"Conventional pivot-type troughing trainers work well on the topsides to help keep the loaded belt down in its trough," Waclawczyk said , "but on the returns, where the worst belt damage occurred, it's the only belt positioner we've found that can apply enough force to keep tracking right."

When the new field belt positioners were installed, two-position tracking switches were added to those conveyors to monitor belt performance and minimize the potential for damage. These switches trigger the computer room alarm if the belt strays to within 1 11/42 in. of the steelwork, and shut the conveyor down if the belt moves far enough to contact the steelwork.

"That was done in early '97," Waclawczyk said. "Since then we haven't had any automatic shutdowns, and only a few warning alarms when rainy weather makes it easier for the belt to slip sideways on the drums.

"On the rail load-out conveyors, if we hadn't found [the positioner], that belt probably would have been destroyed by now. We haven't been seeing the usual smoke signals, so from the performance we've gotten so far, I expect the new positioners are going to cut our yearly belt repair and replacement costs by at least 30%."