Output-shaft and bush failure on main conveyor drives: repair, shaft manufacture and re-commissioning

Equipment

A conveyor drive gearbox reduces motor speed to pulley speed through two or three gear stages in a cast or fabricated housing. The output shaft carries the highest torque and the lowest speed; it is supported on heavy bearings and often runs in a bush at the coupling end. The input shaft, intermediate shafts and gears share the housing and the oil.

Failure modes

Output shafts crack at keyways and shoulders under torsional fatigue; bushes wear oval and score the shaft; bearings pit and spall; gears pit, scuff or lose teeth; housings crack at the feet from misalignment or soft foot; seals fail and the oil is lost or contaminated.

Common causes

Shock loads from belt start-up and blocked chutes; misalignment between motor, gearbox and pulley; overloading beyond the rated service factor; contaminated or degraded oil; and running on after a bearing has begun to fail.

Inspection

Trend vibration and oil temperature on the drive; sample oil for wear metals and water; listen for changes at the output end; check coupling alignment; look for oil leaks at the output seal, which is often the first sign of a worn bush. When a failure is suspected, strip and measure every component against the drawing before deciding on the repair scope.

Maintenance

Change oil at the manufacturer’s interval or on condition; keep breathers clean and seals sound; align couplings on installation and after any disturbance; control start-up sequences and chute blockages; record vibration and temperature trends so a developing fault is caught before the shaft goes.

Repair

A stripped gearbox is assessed component by component. Bearings, seals and worn gears are replaced; the housing is checked for cracks and bearing-housing bores are measured and re-machined or sleeved where worn. A failed output shaft is replaced with a new shaft manufactured to drawing, or reverse-engineered from the failed shaft and mating parts where no drawing exists, with the specified material and heat treatment. A new bush is machined to suit.

Replacement

Replace the whole unit when the housing is cracked beyond repair, when gears of a discontinued design are unobtainable, or when the gearbox is under-rated for the duty. Otherwise a rebuilt gearbox with a new output shaft returns to service faster and cheaper than a new unit on long lead time.

Umbusso capability

Umbusso repairs gearboxes in its Benoni South engineering workshop under a 15-ton overhead crane, with shafts and bushes manufactured on Umbusso-owned CNC machines. The reference project is the repair of two main coal conveyor gearboxes at an Eskom power station in 2017, including material treatment and manufacture of a new output shaft and bush.

This article is general technical guidance. Intervals, standards and methods must be confirmed against the current regulation, the manufacturer’s documentation and the site standard before they are applied.