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Homework Help: Torsion problem for strenght of materials help

  1. Mar 16, 2009 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

    I got the torque of the motor as 210,085lb-in
    Then I converted the 4* to .698 rad

    From there I plugged it into the equations. I solved for T from tao=Tc/J then plugged that T into the theta=TLJG and put everything else in there.

    When I did that I got a diameter of 3.44" or 1.72" radius. I don't get what I'm doing wrong?
  2. jcsd
  3. Mar 17, 2009 #2


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    duffman1278: Conversion mistake; 4 deg is not 0.698 rad. Also, generally always maintain four (or
    five) significant digits throughout all your intermediate calculations, then round only the final answer
    to three significant digits, unless the final answer begins with 1, in which case round the final answer
    to four significant digits.

    Initially compute T from the given data, at which point T is known, not an unknown. Do not solve for
    T in the stress nor deflection equation; solve for d. The unknown is d. Try it again.
  4. Mar 17, 2009 #3
    The 4 degree's I put on here was just a type-o this is what I got when I redid it.

    [tex]\phi[/tex]= .069813
    T(shaft)= 52,521.1312 lb-in
    G= 12x106
    [tex]\tau[/tex]= 12,000psi
    L= 120"

    I solved for T as you said in which I got


    I then plugged that T into the [tex]\phi[/tex]=[tex]\stackrel{TL}{JG}[/tex]

    That then gave me [tex]\phi[/tex]=[tex]\stackrel{L\tau}{Gc}[/tex]

    I solved for "c" which would be the radius and it gave me 1.72 or 3.43in still.
  5. Mar 17, 2009 #4


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    The answer for T is already given in the third line of post 3. Don't solve
    for T again after that; just use it in your other equations thereafter to
    solve for d (or c).
  6. Mar 17, 2009 #5
    I love you!! omfg this stupid problem was so easy the entire time.
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