How this calculator works
A belt without slip has the same linear speed on both pitch circles, so diameter and RPM vary inversely. The transmitted power sets shaft torque and the difference between tight-side and slack-side tension. That tension difference is not either side's total tension. Open-belt length uses exact tangent geometry. Losses, belt stretch, minimum wrap and allowable tension need separate design checks.
Formula
D1 RPM1 = D2 RPM2; v = pi D1 RPM1/60; torque = 60P/(2 pi RPM); tight tension - slack tension = P/v.
Worked example
A 200 mm driver at 600 rpm drives a 400 mm pulley at 300 rpm. Belt speed is 6.283185 m/s; 100 W requires 1.591549 N m input torque. A 100 mm driver and 300 mm driven pulley at 100 rpm require 300 input rpm.
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Frequently asked questions
Why use pitch diameters?
The belt's working line, rather than the flange edge, determines travel per revolution.
Is effective tension the installation pretension?
No. It is only the running tension difference needed to transmit the entered power.
Can I use this for a crossed belt?
The speed magnitudes still follow the no-slip ratio, but the length output is for an open belt only.