Calculate Max Permissible Torque in a Shaft

Any help with this would be much appreciated.

I have a metal shaft - steel - it is of a certain diameter. How can I work out the max permissible torque for this geometry and diameter before the shaft fails?

many thanks for anyone who can help.

Welcome to EE!

First, you need to determine the material characteristics (Young's modulus, Poissons ration and Modulus of rigidity).

Next, determine the appropriate applied load with a FOS (Factor of Safety) - as you don't want to load the bar at yield or even close to yield.

Then, see or use one the following calculators or videos..

[URL="http://www.engineersedge.com/material_science/torsion-calc-round.htm"]Shaft Stress and Deflection under Torque[/URL]

[URL="http://www.engineersedge.com/video/Torsion-Formula-Mechanics-of-Materials_v1253"]Torsion Formula - Mechanics of Materials[/URL]

[URL="http://www.engineersedge.com/video/Shaft-Shear-Stress-due-to-Torsion_v1252"]Shaft Shear Stress due to Torsion[/URL]

With respect to the above please note that:

1. When used for a rotating shaft the determination of the Factor of Safety or maximum allowable shaft must include a factoring for fatigue stresses due to varying shaft loadings and start/stop loadings if applicable.

2. The above equations only apply to shafts in pure torsion with no applied bending forces from mounted gears, pulleys or sprockets for which the bending stresses and their inherent shaft bending fatigue stress must be included as a part of the shaft stress calculatiions and the selection of an acceptable maximum allowable stress for the shaft material.

Also, see:

[URL="http://www.engineersedge.com/calculators/hole_pattern_calculation.htm"]Mass Moment of Inertia Equations and Calculators[/URL]