Speed ratio
- Pulleys: Ddriver × Ndriver = Ddriven × Ndriven
- Gears: Tdriver × Ndriver = Tdriven × Ndriven (T = teeth)
- Ratio = driven ÷ driver (size or teeth). A bigger driven pulley or gear means slower speed and more torque.
- For compound drives, multiply the ratios of each stage.
- Every gear mesh reverses rotation. An idler changes direction but not the ratio.
Open belt length
For an open (uncrossed) belt on two pulleys, where C is the centre distance, D the large pulley and d the small pulley (all in the same units):
L ≈ 2C + π(D + d) ÷ 2 + (D − d)² ÷ (4C)
Belt makers measure V-belts on a pitch length, so use pitch diameters and check the maker's chart.
⚠️ Reference only. Always verify against the manufacturer's specs, your jurisdiction's code and your site's procedures.