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Pulley and O-Ring Helper

Tell us what you need. We’ll tell you what to measure, what size part or O-ring to use, and give you the printable STL.

Runs in your browser with Rust and WebAssembly. Your measurements stay on your device.

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How to measure

  1. Wheel size: Measure straight across the outside edge, through the centre.
  2. Axle size: Measure straight across the metal axle.
  3. Space between axles: Measure from the centre of one axle to the centre of the other.
  4. O-ring thickness: Measure the thickness of the rubber cord, not the whole ring.
Pulley words, if you want them

The wheel attached to the motor is the driver. The other wheel is the driven pulley. Engineers calculate with pitch diameter, an imaginary circle through the middle of the belt. You don’t need to measure that: enter the outside size and O-ring thickness, and this tool estimates it for you.

A larger wheel away from the motor turns more slowly and provides more ideal turning force. A smaller one turns faster and provides less.

About the STL

The downloaded model has a raised centre, a reinforced outer edge, and an optional rounded O-ring groove. The axle hole includes a small amount of extra space for printing. Measure the printed hole before assembly because printers and materials can change its size. The model does not add a keyway, set screw, or bearing seat.

Choosing an O-ring

The answer gives you the O-ring’s inside opening and cord thickness. It includes the stretch setting under “Belt fit options.” The default 5% stretch is only a starting estimate, so check the O-ring supplier’s allowed stretch.

Large speed changes

If one wheel would be too large for your printer, the tool checks whether two belt stages will fit instead. Enter both axle-to-axle distances and your printer’s largest wheel size. The default limit is 220 mm.

A valid two-stage plan gives you two files: one compound wheel for the middle axle and one wheel for the final axle. If the wheels would overlap or the small wheel would be too weak around the axle hole, the tool does not offer an unsafe STL.

Build safely

A ratio calculator cannot certify a rotating part. Check shaft speed, belt profile, alignment, balance, print orientation, material strength, heat, load, guards, and safe failure containment. Do not use a printed prototype where failure could injure someone.