Understanding Belt Idlers
Learn where belt idlers come in handy, and how to select and implement the right idler for your application.
Placement, Purpose, and Best Practices
In most belt drive designs, adjustable centers are preferred for installation and tensioning. But when adjustable centers aren’t possible, idlers provide a practical solution. They not only help achieve proper belt tension but also serve other purposes in complex drive layouts.
What Are Belt Idlers?
Idlers are pulleys or sheaves used to:
- Maintain or adjust belt tension
- Clear obstructions in the drive path
- Redirect belts to turn corners
- Reduce belt whip and vibration on long spans
- Increase arc of contact on critically loaded pulleys
- Enable clutching in certain drive systems
However, idlers add extra bending stress to the belt, which can shorten its lifespan. Correct placement is key to minimizing negative effects.
Key Considerations for Idler Selection and Placement
- Inside Idlers: Should be grooved sheaves, sprockets, or flat pulleys. They reduce arc of contact on both pulleys, so position them for equal arc on each.
- Outside Idlers: Must be flat pulleys (except for double V-belts or dual-sided synchronous belts). Position close to the small sheave to increase arc of contact.
- Diameter Rules:
- Outside idlers should be at least 1.5× the diameter of the smallest sheave in the drive.
- Inside idler diameter should match the smallest sheave diameter.
- Width & Flanges:
- Flat outside idlers may be flanged (flange height ≥ 25% of belt thickness) and should be ~15% wider than grooved pulleys.
- Inside idlers should not be flanged; unflanged width should be 25% wider than grooved pulleys.
- Reverse Bending Risks: Outside idlers cause reverse bending, which is particularly damaging to 3V, 5V, and 8V belt cross-sections.
Preferred Idler Positions
- Inside Slack Side – Least harmful to belt life
- Outside Slack Side
- Inside Tight Side
- Outside Tight Side – Most harmful due to reverse bending
Idlers on the slack side cause lower stress peaks than those on the tight side.
Service Factor Add-Ons for Idler Placement
When calculating drive requirements, add the following service factors to account for reduced belt life:
- Inside Slack: +0.0
- Outside Slack: +0.1
- Inside Tight: +0.1
- Outside Tight: +0.2
Minimum Diameters for Flat Outside Idlers
Example minimums:
- A, AX belts – 4.3 in
- B, BX belts – 7.7 in
- C, CX belts – 12.5 in
- D belts – 18.1 in
- 3V belts – 4.0 in
- 5V belts – 12.0 in
- 8V belts – 18.0 in
A full chart is available for synchronous, ribbed, and miniature belt types.
Best Practice for Drive Layouts with Idlers
Always lay out the drive to scale or use CAD/software to:
- Check extreme idler positions (no engagement vs. maximum engagement)
- Verify nominal belt length
- Ensure installation and take-up allowances are achievable
