How to Use the Shaft Allowable Torque & Twist Calculator
When designing a power transmission shaft, you need to know both the maximum torque it can carry without failing and how much it twists under load. This calculator uses shaft diameter and the material's allowable shear stress to find allowable torque, then factors in shear modulus and shaft length to calculate the angle of twist.
Allowable torque uses the standard solid circular shaft formula: T(lb-in) = (π/16) × d³ × τallow. Twist angle uses θ(rad) = T × L ÷ (G × J), where J is the polar moment of inertia (π×d⁴/32). Both are standard mechanics-of-materials formulas that apply to any material — just swap in the allowable shear stress and shear modulus (G) for the material you're using.
Selecting a material auto-fills a typical machine design reference value, but these figures assume a standard safety factor. You can edit them directly if your design standard calls for a different safety margin. Real shaft designs must also account for cyclic loading, shock loads, and stress concentration at keyways or shoulders, which all reduce the effective allowable stress — so have a mechanical engineer review the final design before committing to it.
Frequently Asked Questions
Selecting a material auto-fills a typical machine design reference value. This is an example figure that assumes a standard safety factor, so an engineer should re-verify it against actual load conditions and safety factors for a real design.
Common practice for general power transmission shafts limits twist to about 0.25 degrees per meter of length. If your result exceeds that, consider a larger diameter or a stiffer material.