Belt-drive fan service
HVAC V-Belt Length Calculator
Estimate open-drive pitch length and sheave wrap angles before matching a standard replacement belt.
Estimate belt pitch length
Enter pitch diameters and the center-to-center shaft distance in one unit.
How to use this calculator
- Identify the belt section and both sheave pitch diameters.
- Measure center distance with the drive stationary and guarded work area made safe.
- Use the calculated pitch length to locate a standard belt and verify the available take-up range.
Formula
Small-sheave wrap = 180° - 2 sin-1((D - d) ÷ (2C))
Variables
| Symbol | Meaning | Required convention |
|---|---|---|
| D | Larger pitch diameter | Same length unit as all inputs |
| d | Smaller pitch diameter | Use pitch/effective diameter |
| C | Shaft center distance | Open-belt geometry |
| L | Approximate belt pitch length | Convert to manufacturer belt designation |
Worked example
With 4- and 8-inch pitch diameters and 20 inches between shafts, the estimated pitch length is about 59.04 inches. The small-sheave wrap is about 168.5°.
Result interpretation
Belt labels can use inside, outside, datum, or effective length depending on belt type and standard. Do not order only from the calculated number. Match the belt manufacturer’s length convention, groove profile, number of matched belts, power rating, center-distance adjustment, and equipment documentation.
Frequently asked questions
Why is the calculated length not always a belt part number?
The equation estimates pitch length. Commercial belt designations and catalog tables may use another reference length.
What does wrap angle tell me?
It describes belt contact on a sheave. Capacity and minimum acceptable wrap depend on the specific belt, sheave, load, and manufacturer design method.
Can I use outside sheave diameter?
Not without confirming the manufacturer’s conversion. The formula is based on pitch or effective diameters.