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Shaft radius from shear modulus

  1. Sep 12, 2016 #1
    Hi. I am trying to determine the size of a shaft for a motor im designing. The thing is, I cant seem to find shear stress alone for steel. I can find shear modulus easy though... But I don't really know the angle of twist. I know torque and I am trying to figure out radius.

    Should I just set angle of twist really low and get it that way? What is a reasonable angle of twist for a motor shaft.... 0.5 degrees? I was just going to find similar torque motors and plug their shaft radius into the torsion equation but I would like to know how they determine it from the Shear modulus from scratch. I am guessing guess the angle...
     
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  3. Sep 12, 2016 #2

    Nidum

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  4. Sep 12, 2016 #3

    Nidum

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    Just to be clear - is your problem with doing the calculations or with finding material properties for the steel that you are using ?
     
  5. Sep 12, 2016 #4
    It is with finding material properties. I never thought about angle of twist for a rotor shaft. I don't really know what acceptable margins of this are. I was trying to find a maximum shear stress of various types of steel but could only find a shear modulus. To use the shear modulus you need to know the angle of twist (I think).
    What metric would tell me the maximum angle of twist before the shaft reaches a plastic zone? That way I could plug that angle into the shear modulus to get the maximum shear stress.
     
  6. Sep 13, 2016 #5
    Try "Shear Strength".
     
  7. Sep 13, 2016 #6

    Nidum

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    For ordinary low/medium carbon steels and approximate calculations you can assume shear yield stress to be 0.6 times tensile yield stress .

    What grade of steel are you actually using ?
     
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