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Homework Help: Finding max torque

  1. Jan 17, 2010 #1
    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.

    [tex]\tau[/tex] = Tr / J

    [tex]\tau[/tex] = shear stress
    T = torque
    r = radius
    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. jcsd
  3. Jan 17, 2010 #2


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    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.
  4. Jan 18, 2010 #3
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