A 30-meter conveyor belt wandered off-center after installation. The pulleys were aligned to within 0.5 mm. The belt was a PVG solid woven, rated for 800 kN/m tension. The tension was set to the manufacturer’s spec. The belt tracked straight for the first 100 meters, then curled to the right side. The customer re-aligned the pulleys. It tracked straight for a day, then wandered again. The belt was rubbing the side frame. Edge wear started after a week. The problem wasn’t the pulley alignment. It was the idlers.
The idler alignment is where belts wander
A conveyor belt tracks based on the total alignment of every component — pulleys, idlers, and the belt itself. One misaligned idler pulls the belt sideways. The effect is cumulative over 30 meters. The pulleys are aligned to 0.5 mm. But the 40 return idlers under the belt are installed to “close enough.” One idler is off by 2 degrees. That idler pulls the belt 5 mm sideways at that point. The next idler, also slightly off, adds another 3 mm. After 10 idlers, the belt has moved 30 mm off-center. It curls against the frame.
The customer spent a day aligning the pulleys. They spent zero time checking the idlers. The pulleys were perfect. The idlers were sloppy. The belt wandered anyway.
The fix
Every idler was checked with a laser alignment tool. The laser projected a line along the conveyor centerline. Each idler was adjusted so its center aligned with the laser within 1 mm. The front idlers (the ones carrying the load) were adjusted first. The return idlers were adjusted second. It took two people four hours to align 80 idlers on a 30-meter conveyor. The belt tracked straight. It didn’t wander again.
Also, the carry idler spacing was checked. The manufacturer recommends 1.2 m spacing for a 600 mm belt. The conveyor was built with 1.5 m spacing. The sag between idlers was too much. The belt dipped between idlers and caught the edge. Tightening the idler spacing to 1.2 m reduced the sag. The belt ran flatter. The tracking improved.
The belt that’s built wrong
Sometimes the belt itself is the problem. If the belt was spliced off-center (the two ends aren’t cut square), the belt tracks to one side regardless of pulley alignment. The splice must be cut at 90 degrees to the belt edge. A 1-degree error in the splice causes the belt to wander 30 mm over 30 meters. The customer checked the old splice — it was off by 1.5 degrees. The new splice was cut with a template. The tracking improved immediately.
Also, the belt tension matters. Too tight and the belt stretches unevenly. Too loose and it sags. The correct tension is the minimum tension that prevents slip on the drive pulley. Set it there. Don’t over-tension because “more tension means better tracking.” It doesn’t — it just wears the belt faster.
The training idlers
For long conveyors (over 20 meters), install self-aligning training idlers every 10 meters. These are idlers that pivot on a central bearing. If the belt wanders, it contacts a side roller, which pivots the idler and steers the belt back. They’re not a substitute for proper alignment — they’re a backup. But they catch small tracking errors before they become edge damage. The training idler costs $80. The belt edge repair costs $300.
The maintenance check
Every quarter, check the idler alignment. Vibration loosens idler brackets. Dust builds up on idler rolls, making them spin unevenly. A frozen idler roll (the bearing is seized) causes the belt to drag on that side — it wanders. Spin every idler by hand. Replace the frozen ones. Clean the dust off. The belt will track straight. The 15-minute quarterly check prevents the 4-hour realignment job.
Conveyor belts wander because the idlers are misaligned, not because the pulleys are off. Check every idler with a laser, tighten the spacing to spec, and cut the splice square. Training idlers are a backup, not a cure. The perfect pulleys won’t save a belt that’s riding on sloppy idlers.
