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Homework Help: Torque of a Shaft

  1. Dec 12, 2013 #1
    1. The problem statement, all variables and given/known data
    The question wants me to find the maximum torque.
    I do not understand why the solution uses the radius of the smaller part of the shaft rather than the radius of the thicker part of the shaft. Please see the attached image.

    2. Relevant equations

    shear stress = (max torque)(maximum radius)/J where J = polar moment of inertia.

    J = π/2c^4 where c = max radius of the rod

    3. The attempt at a solution

    I want to use 37.5mm as the radius (radius of larger part of the shaft), but it is not correct. The formula above directly relates torque T and c (radius), which is why I would assume it would be correct to use the largest c value possible. I would appreciate any insight. Thanks.

    Attached Files:

  2. jcsd
  3. Dec 12, 2013 #2

    Simon Bridge

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    Probably because the diagram shows the torque being applied to the thin part of the shaft.

    It's shear stress you are calculating - the bigger part of the shaft may get a torsional (corkscrew) and a centrifugal stress - but to get a shear stress, something has to be pulling on the surface.
    I suspect you are just taking your equation too literally - go back to the definition of a shear force and see how that applies to a torque. i.e. what if you applied the torque to the inner surface of a hollow shaft?
  4. Dec 12, 2013 #3
    Should have seen that, haha. Thank you.
  5. Dec 12, 2013 #4


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    Staff Emeritus
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    The smaller radius shaft must transmit the same torque as the larger radius part.
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