A V-belt that walked off the sheave. The drive was a 7.5 kW motor, 4-pole, driving a fan through two SPB belts. After 6 months, one belt slipped off the sheave. The customer installed a new belt. It walked off again after 2 weeks. The sheaves looked fine — but the groove walls were worn. The belt no longer sat at the bottom of the groove. It rode on the side walls, which caused it to walk off under vibration. This is about sheave groove wear and when to replace sheaves along with belts.
How a V-belt works
A V-belt wedges in the sheave groove. The sides of the belt contact the groove walls. The friction between belt and groove transmits torque. The belt sits at a specific depth in the groove — not at the bottom. The correct belt sits with its top surface flush with (or slightly below) the sheave top. The sides carry the load.
When the groove wears, the angle opens up. A new SPB sheave has a 34-degree groove angle. After 6 months, the wear rounds the groove walls to 38 degrees. The belt rides deeper in the groove. The effective diameter drops. The belt slips. Worse, the worn groove walls are smooth and polished — the friction coefficient drops. The belt walks off under vibration.
What was changed
1. Replaced both sheaves. The motor sheave and fan sheave were replaced. The groove angle was restored to 34 degrees. The new belts sat correctly. The belt never walked off. The sheaves cost $200 each. The customer had been replacing belts for $50 every 2 weeks. The sheaves paid for themselves in 2 months.
2. Checked the groove wear with a gauge. A sheave groove gauge (a go/no-go template) checks the groove angle and depth. A worn groove fails the gauge. The gauge costs $30. Every belt replacement should include a groove check. If the groove fails, replace the sheave. A new belt on a worn sheave lasts 2 weeks. A new belt on a good sheave lasts 2 years.
3. Aligned the sheaves. The sheaves were also misaligned by 2 mm. Misalignment causes the belt to track to one side and walk off. A straightedge across both sheaves corrected the alignment. The belt ran centered. The edge wear stopped.
The belt replacement rule
Always replace all belts as a set. A new belt on a multi-belt drive should be installed with the other old belts. The belts stretch differently. A new belt is shorter than an old one. It carries more load. It wears faster. The correct procedure: replace all belts at the same time, matched to the same belt set (manufactured from the same batch). Never mix old and new belts on the same drive.
The groove wear table
| Groove condition | Belt life | Action |
|---|---|---|
| New (34° angle, sharp) | 2-3 years | None |
| Slight wear (36°, polished) | 1 year | Monitor, replace at next change |
| Worn (38°, rounded bottom) | 2 weeks | Replace sheave now |
| Cracked or broken groove wall | Immediate | Replace sheave immediately |
The belt tension revisit
After replacing sheaves, the belt tension must be re-set. The new groove has a different geometry. The belt sits at a different depth. The tension changes. The frequency method (pluck the belt, measure Hz) sets the tension. Too tight: bearings fail. Too loose: belt slips and walks off. The correct tension is critical. A V-belt that walks off is usually either too loose or on worn sheaves — rarely both.
The sheave rule: always check the groove with a gauge when replacing belts. The walking belt wasn’t defective — the groove angle wore from 34 to 38 degrees, the belt rode deep, and walked off. Replace sheaves at the first groove failure. Replace all belts as a matched set. Align the sheaves. A $200 sheave replacement saves $500/year in belt replacements and downtime.
