Belt & Pulley Calculator
Belt length, center distance, ratio, wrap angle, belt speed and forces for timing, V and flat belt drives. Calculated with exact geometry formulas and shown in a moving belt simulation.
// Inputs
Check the inputs (the center distance may be too small for the pulleys)
Belt width / number of belts is selected from the manufacturer's power tables; the effective pull here is the input for that selection.
// Simulation
Frequently asked questions
How is timing belt length calculated?
Pulley pitch diameters are D = z·p/π and the belt length follows the exact formula L = 2C·cos θ + π(D₁+D₂)/2 + θ(D₂−D₁). The belt tooth count L/p must be a whole number; the tool also gives the nearest whole tooth count and the actual center distance with that belt.
How many teeth should be in mesh on the small pulley?
For timing belts at least 6 teeth in mesh on the small pulley are recommended. Fewer teeth lead to ratcheting under load and early wear; a larger center distance or smaller ratio increases the teeth in mesh.
What are the standard V-belt lengths?
ISO 4184 datum lengths are taken from the R40 preferred numbers (e.g. 1000, 1060, 1120, 1180, 1250 mm…). The tool shows the nearest standard length and the center distance needed with it. Check the manufacturer's available lengths before ordering.
Why does the wrap angle matter?
The power a belt can transmit decreases with the wrap angle on the small pulley. For V-belts this is taken into account with the factor c₁ (1 at 180°, about 0.82 at 120°). Below 120° for V-belts and 150° for flat belts is not recommended.
How are the tight and slack side forces of a flat belt found?
With the Euler (capstan) relation: F₁ / F₂ = e^(μα) and F₁ − F₂ = F_e = P / v, where α is the wrap angle on the small pulley and μ the belt–pulley friction coefficient. Centrifugal effects are neglected.
How are belt width or the number of belts chosen?
The power a single belt can transmit depends on profile, pulley diameter and speed and is given in the manufacturer's power tables. The tool provides the effective pull, speed and arc factor needed for that selection; choose width or number of belts from the manufacturer's catalogue.
When is a crossed belt used?
A crossed layout makes the shafts turn in opposite directions and increases the wrap angle. Because the belt rubs on itself it is only used with flat (and round) belts at low speeds, never with timing or V-belts.