Calculate the minimum required diameter for a solid shaft based on power, speed, and allowable shear stress.
Last updated: March 2026 | By ForgeCalc Engineering
Shaft sizing is the process of determining the minimum diameter required for a rotating shaft to transmit power safely. The shaft must be strong enough to resist the torsional stress (shear) caused by the transmitted torque.
This calculator uses the ASME code for shaft design, which focuses on the allowable shear stress of the material. It also incorporates a service factor to account for shock loads, vibrations, and start-up conditions.
Result in lb-in
τ (tau) is allowable stress
A service factor is a multiplier applied to the design load to account for uncertainties. For example, a motor with heavy shock loads might need a service factor of 2.0, while a smooth electric motor might only need 1.0.
No. This calculator assumes pure torsion. If the shaft also carries heavy lateral loads (like from a pulley or gear), you must use the combined stress formula which accounts for both bending and torsion.
Allowable stress is the maximum stress a material can safely handle. It is usually a fraction of the material's yield strength (e.g., 18% of yield for shafts with keyways).
Hollow shafts are more efficient (higher strength-to-weight) but require a different formula: d_outer = [ (16 × T) / (π × τ × (1 - k⁴)) ]^(1/3), where k is the ratio of inner to outer diameter.
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