# Homework Help: Torsion of circular shafts

1. May 27, 2017

### DevonZA

1. The problem statement, all variables and given/known data

A solid and hollow shaft of the same material must transmit the same maximum torque. The diameter of the solid shaft is 203mm and the outside diameter of the hollow shaft is to be 216mm.
a) Determine the inside diameter of the hollow shaft. (answer = 138.7mm)
b) What percentage saving in mass would be obtained? (answer 33.46%)

2. Relevant equations

J=π*(D^4)/32

J=π*(D^4-d^4)/32

T/J = G * theta/l = τ/R

3. The attempt at a solution

I figured because I am only given diameters I need to use the polar second moment of area equations

For the solid shaft:
J=π*(D^4)/32
= π*(.203^4)/32
= 1.667x10^-4 m^4

For the hollow shaft (I dont think I can make the value of J the same for both the solid and hollow shaft but I don't know what else to do):
J=π*(D^4-d^4)/32
1.667x10^-4= π*(0.216^4-d^4)/32
d = 214mm

Obviously this is incorrect but I don't know where I have gone wrong

2. May 27, 2017

### haruspex

Your algebra is all correct, even if it was guesswork. You seem to have gone wrong in the final calculation. If you cannot find the error, post all your steps there.

(You could have cancelled the π and the /32 since they occur both sides of the equation.)