1. The problem statement, all variables and given/known data the statement: You are taked to design a 2m long shaft which is to be manufactured either as a solid circular bar or a circular tube. the shaft is required to transmit a torque of 100Nm without exceeding the alloqable strength of material. Find the required outer diameter of the tube if the thickness of the tube is specified as one-eigth of the outer diameter? shear stress = 320Mpa 2. Relevant equations T/J = [itex]\tau[/itex] /r J= [pi(D^4 - d^4)] / 32 3. The attempt at a solution thickness is 1/8 1/8 + 1/8 = 1/4 J= pi( D^4 - 0.75^4 D^4) / 32 work it out in sections 0.75^4 = 81/256 >>> 1 - 81/256 = 175/256 >>> 175/256 x pi = 2.14 >> 2.14 / 32 = 0.067D^4 then using the formula T/J = t/r 1000/0.067D^4 = 320x10^6 / D/2 get D on one side i get 1000 / 2 x 320 x 10^6 = 0.067D^4 / D worked all that out then cube root it to get 0.028 which i then x1000 to get 28mm I feel i'm on the right path ( i hope) but the whole 1/8 of the diameter has confused me and i'm wondering whether ive done that right. finding the "J" has to be my main concern i feel ive got the method right for finding D later on. If anyone can help my go through it and check everything is correct or if its wrong, advice on how to correct it would be fantastic. thanks.