The 5-minute method
- Ask: how long did the belt last? What changed (new motor, new pulley, VSD, new product, season start)? Does it fail at start-up or while running?
- Look at the old belt: sides, underside, back, cords, length compared with a new one.
- Look at the pulleys: gauge the grooves; check for rust, oil, damage, wobble.
- Check alignment and tension before fitting new belts.
- Check the design: run the drive designer. Is the drive simply under-belted for the load?
Photograph itWhatsApp us photos of the old belt (sides, underside, any break) and the pulley grooves. Most problems can be diagnosed from good photos.
Noise
| Symptom | Likely causes | Fix |
|---|
| Squeal at start-up or under load | Low tension; overload; high starting torque; worn grooves; oil or water on belts | Measure and set tension; check design power and service factor; gauge pulleys; remove contamination; consider soft start |
| Chirp or squeak at each revolution | Misalignment; dirt or rust in grooves; a nick in the belt | Align; clean grooves; replace a damaged belt |
| Slapping or thumping | Low tension; belts of different lengths in a set; long span whipping | Tension; matched set; banded belt or idler |
| Whine (timing belts) | Too much tension; misalignment; belt rubbing a flange | Reduce tension to spec; align to ¼° |
| Grinding or rumble | Bearing failure (often from over-tension); belt rubbing the guard | Check bearings and guard clearance; check tension |
Slip and heat
| Symptom | Likely causes | Fix |
|---|
| Glazed, shiny, hard sides | Slip: low tension, overload, worn grooves, oil | Tension; check design; gauge pulleys; eliminate oil |
| Burn marks, smell, very hot belts | Heavy slip or stalls; jammed machine; pulley too small | Find the stall; tension; bigger pulley or more belts |
| Driven machine running slow | Slip; wrong pulley sizes; worn grooves (belt sinks, effective diameter falls) | Check speed with a tacho; run the speed calculator; gauge pulleys |
| Belts hard and cracked with heat | High ambient (enclosed guard, hot engine room); small pulleys | Ventilate the guard; raw-edge cogged or heat-resistant belts; larger pulleys |
Belt wear patterns
| What you see | Cause | Fix |
|---|
| Sides worn evenly, belt sits low in groove | Normal wear after long life, or worn grooves, or dust/abrasives | If life was short: gauge pulleys; guard against dust |
| Sides worn on one side only | Misalignment | Align to ½° |
| Underside (bottom) worn or shiny | Belt bottoming in worn or wrong grooves; wrong section | Replace pulleys; check section vs groove |
| Cover worn through at the corners | Belts rubbing guard or each other; misalignment; foreign object | Clearance; align |
| Swollen, soft, sticky belt | Oil or grease contamination | Fix the leak; clean pulleys; replace belts; oil-resistant belts |
| Back of belt worn or cracked | Back-side idler; belts rubbing guard | Move idler inside on slack side, or enlarge; clearance |
| One belt in a set worn more | Mixed old and new belts; unequal lengths; one worn groove | Replace whole set with a matched set; gauge grooves |
Breakage and cracking
| What you see | Cause | Fix |
|---|
| Clean break soon after fitting | Belt prised on with a bar (cords broken); foreign object; shock load | Fit by hand; guard; banded belts or higher service factor |
| Ragged break after some time | Overload, jam, under-belted drive | Check design power; protection on the driven machine |
| Cracks across the underside | Pulleys below minimum diameter; heat; age; back-bending | Larger pulleys; cogged belts; ventilation; correct idler |
| Fine surface crazing | Ozone and UV (outdoor drives), age in storage | Guard from sun; store correctly; rotate stock |
| Belt stretched beyond take-up | Overload, under-belted drive, low quality | Re-design; matched quality belts |
Turnover and jumping off
| Symptom | Likely causes | Fix |
|---|
| Belts turn over in the grooves | Worn or wrong grooves; misalignment; pulsating loads; flat idler; broken cords | Gauge pulleys; align; banded belts; V-grooved idler |
| Belts jump off | Shock loads; low tension; misalignment; debris | Tension; align; banded belts; guard against debris |
Vibration and whip
- Span whips or flaps: low tension, long centres, pulsating load (compressors, crushers). Fix: tension; banded belts; idler on the slack side.
- Vibration at pulley speed: unbalanced or bent pulley, bore off-centre, bent shaft. Fix: check run-out with a dial gauge; balance.
- Vibration at belt-pass frequency: a damaged or uneven belt, or mismatched set. Fix: replace with a matched set.
- Resonance: a span with a natural frequency near a forcing frequency. Changing tension slightly, or adding an idler, moves it.
Bearing and shaft failures
Repeated motor or machine bearing failures on a belt drive usually mean over-tension or a pulley mounted too far out on the shaft. A shaft broken at the shoulder behind the pulley is classic fatigue from the same cause. Measure tension, move pulleys close to the bearings, and check the shaft load with the shaft load calculator.
Timing belt failures
| What you see | Cause | Fix |
|---|
| Teeth sheared off | Shock load; fewer than 6 teeth in mesh; low tension (ratcheting) | Check teeth in mesh; tension; wider belt |
| Worn tooth flanks / land | Over-tension; worn pulleys; misalignment | Tension to spec; replace pulleys |
| Belt edge wear, belt climbing flange | Misalignment; flange bent | Align to ¼°; repair flange |
| Snapped across, clean | Belt kinked or crimped before fitting; debris; prised on | Handle correctly; guard |
| Cracks in the back | Back-side idler too small; cold; age | Larger idler; correct compound |
Round PU belt failures (packlines)
- Breaks at the weld: poor weld (cold, contaminated, not square, undercut). Use a proper welding blade and clamp; let it cool before stretching.
- Belts slack after a few days: not enough stretch at installation for that cord diameter and hardness. Shorten; follow the stretch figure.
- Belts jumping off: worn or misaligned grooved pulleys; too little stretch.
- Wear on one side: pulley misalignment along the line.
Still stuck?
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