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Homework Help: Bending moment

  1. Aug 16, 2006 #1
    Calculate the bending moment in the centre of the axle??
    What limit stress is valid for this loading case ?

    may i know how to form the moment eqn??
    F=300N

    picture - http://files.filefront.com//;5410274;;/

    pls help
    thanx
     
  2. jcsd
  3. Aug 16, 2006 #2
    What do you think? Try to form the equation on your own and we will inspect it.
     
  4. Aug 16, 2006 #3
    i have try it many times but i don't know.
    if in mechanics,this have to use the heaviside function,but this is machine design.therefore ,i don't know what to use.

    i assume it is F(1) <x-175>^0 + F(1)<x-825^0>...
    but i think this is incorrect
     
  5. Aug 16, 2006 #4
    Well, heaviside functions aren't needed here.

    Step 1: Determine the value of [tex]F_{1}[/tex] using equilibrium condition along the vertical (y-axis).

    Step 2: By considering half part of the rotating machine (cut machine into two at the centre), write down the bending moment equation of either side using [tex]\sum\vec{F}l=\vec{M_{b}}[/tex]. It will be the same for the other half since it is symmetrical.
     
    Last edited: Aug 16, 2006
  6. Aug 16, 2006 #5
    if i cut into the half,then i'll get F * 325mm = M(b) rite??am i correct??

    if it is correct,in order to find limit stress = bending moment / W , where W= pi (d^3) / 64 right??
    how can i find the diameter and which shaft should i consider??
     
  7. Aug 16, 2006 #6
    Bending_Moment.jpg

    Your answer isn't correct. Try writing down the moment at C. Then we discuss the rest of the question.
     
    Last edited: Aug 16, 2006
  8. Aug 16, 2006 #7
    then it should be M + (F1*325) - (1/2F * 500) =0 right??am i correct??

    if it is correct,in order to find limit stress = bending moment / W , where W= pi (d^3) / 64 right??
    how can i find the diameter and which shaft should i consider??
     
  9. Aug 16, 2006 #8
    Bingo.

    How did you obtain the W? The correct axial section modulus W is [tex]W=\frac{\pi*d^3}{32}[/tex], assuming that the cross section of the axle is not hollow.

    Setting [tex]\sigma_{all}=\frac{\sigma_{lim}}{n_{s}}=\frac{M_{b}}{W_{by}}[/tex] and solve for the diameter d as you have found [tex]M_{b}[/tex] in the first part, [tex]\sigma_{lim}[/tex] can be found in a strength data table, depending on the material given and the safety factor should be, well, leave the diameter in terms of [tex]n_{s}[/tex] if it isn't stated.
     
    Last edited: Aug 16, 2006
  10. Aug 16, 2006 #9
    ok......thanx...pls help me in the other question in the engineering side...thanx
     
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