Not every pulley in a belt drive transmits power. Idler pulleys route the belt, increase wrap angle on drive pulleys, or maintain tension — but they are specified differently from drive pulleys. Confusing the two leads to undersized bearings, wrong groove type, or belt tracking problems on conveyors and compressor sets.
Drive pulleys
Drive pulleys (motor and driven) transfer torque through belt friction. They must be grooved to match belt profile, sized for load and speed ratio, and mounted securely on keyed or taper lock bores.
Motor pulley is typically the smaller diameter in reduction drives; driven pulley is larger. Both must be aligned in the same plane to prevent belt edge wear.
Idler pulleys
Idlers do not carry primary drive torque — they rotate freely on bearings, redirecting belt path. They may contact the back of the belt (flat idler) or the grooved face (grooved idler) depending on layout.
Used to increase wrap angle on small motor pulleys, route belts around obstructions, or apply controlled tension in tensioner assemblies.
Flat vs grooved idlers
| Type | Contacts | Use when |
|---|---|---|
| Flat idler | Belt back (flat side) | Routing, backside wrap |
| Grooved idler | Belt V-face | Tensioning V-belt drives |
| Crowned idler | Flat belt centre | Tracking correction (flat belts) |
| Tensioner idler | Grooved or flat | Automatic/manual tension |
Sizing idlers
Idler diameter should not be so small that belt flexing exceeds minimum bend radius — undersized idlers crack belts prematurely.
Bearings in idlers must suit belt tension side load and RPM. Sealed bearings are standard for dusty environments.
- Specify belt profile even for idlers contacting grooved face
- Note whether idler is adjustable (slot mount) or fixed
- Share belt span and tension if requesting engineering input
Common mistakes
- Using drive pulley spec for idler without bearing rating check
- Flat idler on V-belt loaded face — wrong contact pattern
- Idler diameter too small for belt section
- No alignment between idler and drive pulleys