Two straights and two arcs
The belt runs straight between the pulleys and wraps an arc on each. The length is 2C cos(phi) + pi (D + d)/2 + phi (D - d), with sin(phi) = (D - d)/2C.
Reference tool — not a selection
Size a belt from its pulleys before it goes in an RFQ.
Belt length
Pulleys and centres.
Choose the belt type and enter the pulleys and the centre distance. The drawing and the RFQ line update with every change.
Belt
All 130 engineering tools
When the old belt is missing or its print worn, the drive itself gives the size: measure the two pulleys and the distance between their centres. For a V-belt the calculator gives the datum length the belt is ordered by; for a timing belt, the number of teeth and the whole-tooth belts either side with the centre distance each needs. The pulleys are drawn to scale with the belt running.
Use the datum diameters of V-belt pulleys, not the outside of the rim; for a timing belt, count the pulley teeth and read the pitch from the pulley or the old belt.
How the length is worked out
Two straights and two arcs.
Datum length for V-belts
V-belts are ordered by datum length, measured at the datum diameters of the pulleys; outside diameters give a longer, wrong length.
Whole teeth for timing belts
A timing belt has a whole number of teeth, so its length comes in steps of one pitch; the centre distance is set to the belt, not the other way round.
| Teeth | 5M (5 mm) | 8M (8 mm) | L (3/8 in) | H (1/2 in) |
|---|---|---|---|---|
| 12 | 19.1 | 30.56 | 36.38 | 48.51 |
| 14 | 22.28 | 35.65 | 42.45 | 56.6 |
| 16 | 25.46 | 40.74 | 48.51 | 64.68 |
| 18 | 28.65 | 45.84 | 54.57 | 72.77 |
| 20 | 31.83 | 50.93 | 60.64 | 80.85 |
| 24 | 38.2 | 61.12 | 72.77 | 97.02 |
| 28 | 44.56 | 71.3 | 84.89 | 113.19 |
| 32 | 50.93 | 81.49 | 97.02 | 129.36 |
| 36 | 57.3 | 91.67 | 109.15 | 145.53 |
| 40 | 63.66 | 101.86 | 121.28 | 161.7 |
| 48 | 76.39 | 122.23 | 145.53 | 194.04 |
| 60 | 95.49 | 152.79 | 181.91 | 242.55 |
Pitch diameter = teeth x pitch / pi.
Sources
Exact length and centre distance: York Industries, "Center distance and belt length". The short formula: calculate.co.nz, Firgelli and Autodesk Inventor’s V-belt help. Datum length and datum diameter: Continental’s V-belt measuring guide. Timing belt length as teeth times pitch: York Industries and B&B Manufacturing.
Before you send the request
Questions about belt length in an RFQ
How do I find a V-belt length without the old belt?
Measure the datum diameters of both pulleys and the centre distance, and the calculator gives the datum length. Then match it to the section in the V-belt identifier.
Why are there two timing belts in the answer?
A timing belt has a whole number of teeth. The calculator gives the belts just shorter and just longer than the drive needs, each with the centre distance it runs at.
What is the wrap angle?
The arc of belt contact on the small pulley. It falls below 180 degrees as the pulleys differ more and stand closer together.
What should the request include?
The pulley diameters or tooth counts, the centre distance, the belt section or pitch, the number of belts and the machine; a photograph of the drive helps.
Length found?
Send the line with a photograph of the drive.
A calculation is a cross-check, not a selection. Attach photographs of the drive and the pulleys, and Vessel Core checks them before an offer goes back.
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