# Homework Help: Finding max torque

1. Jan 17, 2010

### 2slowtogofast

A cylindrical torsion bar made of steel (E = 209 GPa) with diameter of 20mm. The allowable shear stress for this material is 200 MPa what is the max torque that can be safely applied to this bar.

$$\tau$$ = Tr / J

$$\tau$$ = shear stress
T = torque
J= second polar moment of area

I was thinking of using this formula but I would not have used E = 209GPa in my solution is this wrong? If so could somebody point me in the right direction.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jan 17, 2010

### PhanthomJay

You are correct, you don't need E; the max torque is a function of the max stress and the geometry of the section. It is not related to the elasticity modulus. You would need the shear modulus G if you wanted to calculate the amount of rotation, not the stress.

3. Jan 18, 2010

thanks