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ISO 22:1991 Belt drives — Flat transmission belts and corresponding pulleys — Dimensions and tolerances

Sudeshna

What is ISO 22:1991?

ISO 22:1991 is an international standard published by the International Organisation for Standardisation (ISO). It specifies:

  • Nominal dimensions of flat transmission belts.

  • Standardised profiles and dimensions for corresponding pulleys.

  • Permissible manufacturing tolerances.

  • Compatibility guidelines for ensuring reliable and efficient belt-drive systems.

The purpose of the standard is to ensure interchangeability, predictable performance, and global uniformity in flat belt drive design.

 

Why ISO 22:1991 Matters in Mechanical Design

 
1. Ensures Interchangeability

Machines manufactured in different countries must support standard belt sizes. ISO 22:1991 ensures belts and pulleys follow the same dimensional rules.

2. Supports Reliable Power Transmission

Correct dimensions and tolerances minimise slippage, uneven wear, and vibration, increasing system longevity.

3. Reduces Maintenance Costs

With standardised belts and pulleys, replacing components becomes easier, cheaper, and faster.

4. Enhances Safety

Precise tolerances prevent misalignment and belt failures that could damage machinery or cause accidents.

 

Key Components Covered in ISO 22:1991

ISO 22:1991 breaks down the specification into two major categories:

1. Flat Transmission Belts

The standard defines:

✔ Belt Widths

Common ISO-compliant widths include:
10 mm, 16 mm, 25 mm, 50 mm, 75 mm, 100 mm, and custom expansions for heavy industrial belts.

✔ Belt Thickness

Defined based on:

  • Material type (rubber, leather, fabric-reinforced, polymer).

  • Load-carrying capacity.

  • Flexibility requirements.

✔ Belt Length Tolerances

ISO defines strict tolerances for:

  • Effective length.

  • Endless belt length.

  • Belt stretch limits.

✔ Edge Quality and Straightness

The edges of flat belts must remain:

  • Smooth.

  • Parallel.

  • Free from warping.

This ensures stable alignment over long pulley spans.

 

2. Corresponding Pulleys

✔ Pulley Width and Diameter

The standard specifies:

  • Nominal pulley diameters

  • Width-to-belt ratio

  • Groove (if applicable) geometry

  • Face flatness requirements

✔ Crowned Pulley Profiles

To help belts self-centre, ISO 22:1991 includes guidance on:

  • Crown angle

  • Crown height

  • Symmetry across the pulley face

✔ Pulley Surface Finish

Surface roughness should prevent slippage while avoiding excessive friction.

✔ Hub and Shaft Interface

Standard tolerances ensure:

  • Proper mounting.

  • Load distribution.

  • Reduced vibration and noise.

 

Understanding ISO Tolerances in Belt Drives

1. Dimensional Tolerances

Applied to:

  • Belt width.

  • Belt thickness.

  • Pulley diameter.

  • Pulley width.

These tolerances guarantee consistency between manufacturers.

2. Geometrical Tolerances

Ensures:

  • Pulley runout control.

  • Belt straightness.

  • Parallelism of edges.

  • Concentricity between hub and rim.

3. Functional Tolerances

Designed to maintain performance under:

  • Dynamic loads.

  • Temperature variations.

  • Humidity and lubrication factors.

 

Applications of ISO 22:1991 in Industry

Flat transmission belts compliant with ISO 22:1991 are used in:

  1. Textile Machinery: High-speed, smooth belt operation is essential for precise weaving and spinning.
  2. Packaging Machines: Belts enable flexible, reliable, non-slip torque transmission.
  3. Printing Presses: Flat belts provide stable motion for sheet feeding and printing drums.
  4. Paper Mills: Long-distance, high-tension belt drives benefit from ISO-standardised pulley profiles.
  5. Industrial Automation Equipment: Standardised belts ensure compatibility across global OEMs.
 

Benefits of Implementing ISO 22:1991 Standards

  1. Improved Machine Efficiency: Optimized belt-pulley compatibility reduces power losses.
  2. Longer Belt and Pulley Life: Reduced stress concentrations and better alignment increase lifespan.
  3. Predictable Mechanical Performance: Every belt and pulley follows identical dimensional rules, leading to consistent performance.
  4. Simplified Design for Engineers: Engineers can confidently select from ISO-standardised belts and pulleys without custom calculations.
  5. Supports Global Manufacturing: Designs become universally accepted and interchangeable.
 

How Engineers Should Use ISO 22:1991

1. During Machine Design
  • Select belt widths and thicknesses based on load specifications.
  • Choose pulley diameters compatible with belt flexibility and speed.
2. During Procurement
  • Ensure all belts and pulleys follow ISO 22:1991 nomenclature.
  • Cross-check tolerances with vendor datasheets.
3. During Maintenance
  • Replace worn belts with ISO-dimension-compliant alternatives.
  • Re-align pulleys using ISO-based geometrical reference points.
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