BIZOMAT MAKEOS · Power Transmission
Timing (Synchronous) Belt Length and Teeth
How many teeth does the timing belt need, and what centre distance results?
Pitch length and tooth count of a synchronous (timing) belt for two pulleys at a centre distance, the exact centre distance for the chosen belt, and teeth in mesh on the small pulley.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Belt pitch | mm | Belt pitch (e.g. 5 mm for HTD 5M, 5.08 mm for XL, 8 mm for 8M). |
| Small pulley teeth | teeth | Teeth on the small pulley. |
| Large pulley teeth | teeth | Teeth on the large pulley. |
| Approximate centre distance | mm | Wanted centre distance. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Belt pitch
- What it is
- The distance between the centres of neighbouring belt teeth, measured along the pitch line. It is set by the belt profile and must match the pulleys.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Imperial profiles have inch-based pitches; convert to millimetres exactly (1 inch = 25.4 mm).
What this means: Small pulley teeth
- What it is
- The number of teeth on the smaller pulley.
- Units and format
- Enter a number in teeth: a whole number of teeth. The form accepts 8 teeth to 500 teeth. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 teeth as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Fewer teeth means fewer teeth in mesh and a tighter bend for the belt; check the belt maker’s minimum.
What this means: Large pulley teeth
- What it is
- The number of teeth on the larger pulley.
- Units and format
- Enter a number in teeth: a whole number of teeth. The form accepts 8 teeth to 1000 teeth. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 teeth as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Approximate centre distance
- What it is
- The distance you want between the two pulley shaft centres. The tool picks the nearest whole-tooth belt and then gives the exact centre distance for it.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 200 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The belt length grows by about twice any increase in centre distance.
- Get it right
- Allow adjustment on one shaft for fitting and tensioning the belt.
Method and formulas
- Pitch diameters D = z p ÷ π.
- L = 2C + π(D + d)/2 + (D − d)² ÷ 4C; teeth = L ÷ p rounded to the nearest whole tooth.
- Exact C from the open-belt relation by iteration; teeth in mesh = z₁ × wrap ÷ 360°.
- Example: 20/40 teeth at about 200 mm need a 110-tooth belt (550 mm), giving C = 199.36 mm.
Limitations
- Pitch-line geometry; stock belt lengths come in steps set by the maker.
- Allow centre adjustment for installation and tension.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Select a stock timing belt length
- Set the centre distance for a chosen belt
- Check teeth in mesh on the small pulley
Who it is for
- Design engineers
- Maintenance engineers
- Automation engineers
What you get
- Belt pitch length and tooth count
- Exact centre distance for the rounded belt
- Teeth in mesh on the small pulley
Interpreting the result
- Keep at least six teeth in mesh on the small pulley for most belt types.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2473–2535. Topic reference; no source tables or recommended settings are reproduced.
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Open the Timing (Synchronous) Belt Length and Teeth calculator