Work out keyway size instantly with clear inputs, formula shown and shareable results.
The key carries torque as a tangential force F = 2T/d at the shaft surface. Shearing across the key gives required width as F / (allowable shear x length). Standard proportions take width as a quarter of the shaft diameter and height as a sixth, so the check is usually whether the standard key is long enough rather than wide enough.
Tangential force
F = 2T / d
Key shear
Width required = F / (allowable shear x key length); standard width = d / 4
Preliminary estimate only. Key and keyway design must be verified against the relevant dimensional standard and checked by a qualified engineer for both shear and bearing stress.
F = 2T / d. The key carries torque as a tangential force F = 2T/d at the shaft surface.
Yes. Bearing stress on the key side, F divided by half the height times the length, often governs before shear does, especially with short keys in soft hubs.
This calculator takes 4 inputs: Shaft diameter, Transmitted torque, Key length, Allowable shear stress. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.