Background
An open-cut coal mine in Queensland, Australia was experiencing recurring belt cover damage on its main run-of-mine conveyor β 1,400mm wide ST1600, 1,800 meters long. The damage pattern was unusual: diagonal burn marks appearing at irregular intervals along the top cover, spaced approximately 1.2 meters apart and 30β40mm wide.
The pattern repeated every 1.2 meters β exactly the standard idler spacing on the conveyor. This spacing-linked pattern is diagnostic of idler-related damage.
Diagnosis
The 1.2-meter spacing of the burn marks corresponded exactly to the carrying-side idler set spacing. This pattern occurs when one or more idler rolls seize (stop rotating) while the belt continues to move over them.
A seized idler roll acts as a stationary friction surface against the moving belt. The relative velocity between belt and idler generates heat through friction. Depending on the severity: - Mild seizure: gradual cover abrasion at contact point - Severe seizure: enough heat to scorch and melt the rubber cover - Extreme: sufficient heat to ignite coal dust accumulated on the seized roller
The diagonal orientation of the marks (not perpendicular to belt direction) indicated the belt was tracking slightly off-center, causing contact at the junction between the center roll and the wing roll β a known high-stress point for idler bearings.
Seized Idler Investigation
A full survey of the conveyor idlers found: - 23 carrying-side idler rolls that did not rotate freely when tested by hand - Of these, 14 were completely seized (zero rotation) - 9 had stiff bearing rotation indicating early-stage seizure
The failed idlers were examined: - Bearing seal failure was the dominant cause β 18 of 23 failed idlers had damaged or deteriorated bearing seals allowing coal dust and moisture into the bearing - Inadequate lubrication intervals β the bearing grease had dried out in 5 idlers that had not been re-greased on schedule
The coal at this operation was fine and slightly moist β it penetrated seals more aggressively than dry coarse coal.
Corrective Actions
Immediate: - Replace all 23 seized and stiff idlers - Inspect and replace covers on affected belt sections where burn damage exceeded 3mm depth - Adjust belt tracking to reduce lateral force on center/wing junction bearings
Systemic: - Upgrade to sealed-for-life bearing idlers (triple-lip seal design) on the most coal-dust-exposed sections of the conveyor - Reduce greasing interval from 6 months to 3 months for this conveyor - Add seized idler detection to operator walkdown checklist β a seized idler that is hot can often be detected by smell (burning rubber) or visual inspection (smoke or discoloration)
Belt: The scorched sections were vulcanized-patched where damage was 2β4mm deep. Where damage exceeded 4mm (approaching the carcass), those sections were cut out and fresh belt vulcanized in.
Outcome
Following the idler replacement and bearing specification upgrade, the burn-mark damage pattern stopped. At the 12-month inspection, no new diagonal burn marks were observed. The sealed-for-life idlers in the most exposed section were inspected at 18 months and showed no bearing deterioration.
Key Points
Diagonal burn marks at regular intervals = seized idlers. This pattern is diagnostic. Identifying it quickly allows the problem to be found and fixed before the belt is destroyed.
Idler seizure causes heat, not just wear. A fully seized idler against an ST1600 belt running at 4.5 m/s generates significant friction heat β enough to damage the cover, ignite coal dust, or in extreme cases, start a conveyor fire.
Fine wet coal is aggressive to bearing seals. Standard bearing seals designed for dry conditions fail faster in fine wet coal environments. Specify idlers with appropriate sealing for the material being conveyed.
Seized idler inspection should be a routine walkdown item. Most seized idlers can be identified by touch (hot bearing housing) or sound (rattling or squealing) before they cause significant belt damage. Early detection is cheap; late discovery is expensive.
Elephant Rubber supplied the replacement belt sections and vulcanizing patch materials for this project.