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Transmit rotation with an open belt

Pulley sizes and centre distance determine speed ratio and geometric belt length. Start by distinguishing pitch diameter from outside diameter.

Open-belt front view with exact external tangent geometry, pitch diameters, centre distance and same-direction rotation arrows.n₁n₂D₁D₂C
Front view of a noncrossed open belt. Both pulleys rotate in the same direction. Blue n₁ is the driver and red n₂ the driven speed. D₁/D₂ and the belt line represent ideal pitch geometry.

What you will understand

  • Pitch diameters and open-belt rotation
  • Speed ratio, ideal length and wrap
  • Tension difference versus bearing load and product ratings

Blue: defined conditions · Red: calculated quantities. Diagrams are schematic.

  • D₁, D₂: driver and driven pitch diameters
  • C: shaft centre distance
  • n₂: driven speed, L: ideal pitch-line length

The examples below are illustrative. Each example button loads its fixed values, units and mode. Ordinary calculator links do not load an example.

Define arrangement and pitch geometry

This is a two-shaft, noncrossed, no-slip open-belt model. For toothed pulleys verify pitch diameter from pitch and tooth count. Do not substitute outside diameter or belt outside circumference.

  • Shaft positions and pulley/flange/belt clearance
  • Compatible tooth profile/pitch, width and environment

Check driven speed and belt speed

An open belt turns both shafts in the same direction. Doubling driven pitch diameter halves driven speed. A speed ratio does not establish torque or tension capacity.

n₂=n₁D₁/D₂, v=πD₁n₁/60 (D₁ in m, n₁ in rpm)

Calculate length and wrap

Geometric length is two tangent spans plus pulley arcs. Reduced small-pulley wrap calls for attention to engaged teeth and contact conditions.

Example: drive 160 mm from 80 mm

Inputs

  • D₁=80 mm, D₂=160 mm, C=300 mm
  • n₁=1000 rpm, noncrossed belt without slip

Results

  • n₂=500 rpm, belt speed v≈4.189 m/s
  • Ideal L≈982.332 mm; small wrap≈164.675°

982.332 mm is not automatically an orderable catalog length. Check the product length convention, tooth count, centre adjustment and tensioning provision. Wrap alone does not establish capacity.

Load the fixed values, units and mode for this illustration. Then edit them for your conditions.

Separate transmitted torque and shaft load

For pitch diameter D, transmitted torque T requires tension difference ΔF=2T/D. Bearing load is the vector sum of span tensions, not their difference. Review pretension and each span tension separately.

For practical review

The geometry calculation does not assess belt width, tooth strength, ratcheting, stretch or tension split. Product selection requires actual speed, transients, load variation, shaft stiffness, alignment and profile-specific ratings and engagement.

Keep a review checklist

This checklist is a personal reminder, not a safety score or an automatic approval. It is not sent or saved.

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Reference information and illustrative calculations. This content does not guarantee suitability or safety of a design. Confirm the actual materials, component specifications and operating conditions.