Minimum Shaft Diameter for Torsion
For a solid circular shaft transmitting pure torque, the minimum safe diameter is found by rearranging the torsion shear-stress formula τ = 16T/(πd3) to solve for d: d = (16T / (πτallow))1/3, where T is the transmitted torque and τallow is the material’s allowable shear stress.
This gives the theoretical minimum diameter based on torsional shear alone; real shaft designs typically add a safety factor and also check for bending loads, fatigue, keyway stress concentrations, and deflection limits, all of which can require a larger diameter than this baseline calculation.
Last reviewed August 2026