Quote by Andyhnc
Thanks again for replying, so I work out the value for J by setting Di to zero, then I use this in the calculation for stress using a value of radius that makes the stress the same as it was for the hollow shaft, and that value is my solid shaft radius? Just to confirm? Or, can the equation for stress be transposed to get radius. Thanks agen!

You can transpose the equation to solve for radius. Aside from getting students familiar with computing stress in shafts due to torque, this problem will demonstrate that hollow shafts are nearly as strong as solid shafts due to the 4th power in the calculation for J. Hollow shafts have considerably higher natural frequencies than solid ones and this is quite important in the design of spinning shafts.