# Maximum torque on thin walled aluminium shaft

I am trying to determine the maximum torque that can be applied to a 6061 Aluminium tube with O.D. = 55 mm and I.D. = 53 mm. The yield strength of 6061 Aluminium I have taken as 55 MPa and using the formula $$T = \frac{tJ}{r}$$ and the polar moment of inertia of a hollow cylinder $$J = \pi\frac{D^4 - d^4}{32}$$

This comes out to 247 Nm. I'm highly skeptical of this number as it seems quite absurd that an aluminium tube with a 1 mm thick wall can handle that much torque.

It's been quite a few years since I've done mechanics so I'm sure I'm missing something (or several things).

I suspect a thin walled shaft like that will fail through buckling long before it reaches the shear stress limit. Especially if it's long and doesn't have thicker butting at the ends.

There may be some guidelines/rules of thumb you can look up to get an idea what performance is reasonable in this case, but certainly the shear limit is only useful as the theoretical maximum.

• It's only about 150mm long and the 1mm wall only goes for about 40mm of the length. Realistically the maximum torque that will be applied will be around 2.5Nm. – pulse89 Jun 2 '16 at 0:51
• One option I am looking at is I can create a 1.5mm step about 8mm from the face, bringing the diameter out to 65mm for that 1.5mm length. – pulse89 Jun 2 '16 at 1:03