1. Hey everyone, i'm brand new to this site and am requiring some help and a push in the right direction. Here's my problem, I have a solid circular shaft of 200mm diameter, and it is to be replaced with a hollow shaft having a diameter ratio of 2. I need to determine the dimensions of the hollow shaft if the maximum stress is to be the same as for the solid shaft. I also need to know what the percentage saving in weight of the hollow shaft compared with the solid one. 2. I think the relevent equations are: Tau(max) = T/J x r(max) J = Pi(r^4)/2 J = Pi(ro^4 - ri^4)/2 3. I have used the above equations to establish tau of the solid shaft, but then do I use a the second J equation to find the inner radius given that I know the diameter ratio is 2 ( meaning 400mm diameter?? is this correct?). Do i also assume torque is the same for both the hollow and solid shaft? Can anyone give me a gentle shove in the right direction?