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Using 2 bolts M12 or 4 bolts M6?

  1. Aug 19, 2010 #1
    Hi there,

    I'm not sure that using the same type of 2 bolts M12x1.5 (L=30mm) or 4 bolts M6x1 (L=25mm) & place equidistant in a circle as picture below, which can bear force better & why?

    Thanks many in advanced!

    [PLAIN]http://img824.imageshack.us/img824/7971/123ok.jpg [Broken]
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Aug 20, 2010 #2
    Is this coursework?

    If so, here's a hint.
    Think about what is acting on the joint besides force. Think in 3D.
     
  4. Aug 20, 2010 #3
    hi Studiot, certainly this's not coursework, just a small problem that i design it for a gearbox. First time i try to use 4 M6 as mentioned above & my boss agreed with solution but later i found a guide from catalogue they using 2 M12 in stead as picture attached, just a bit wondering that the total diameter of these two solutions is almost the same but not sure which is stronger :grumpy:

    While looking for an answer i made a test on Inventor software & the result is that: M6 appeared in failure with axial force 750N but M12 is compliance with 1500 N axial force (even if 2500N is still fine). by the way not understand essence of situations.
    P/S: one more test
    one M12 can bear an axial force P = 2500N & 7 bolts M6 can bear an approximately axial force (P = 2500N)

    Thanks many in advanced & have nice weekend!
     
    Last edited: Aug 20, 2010
  5. Aug 20, 2010 #4
    OK what I am talking about is moment.

    A single bolt cannot cope with moments due to forces applied across the joint or slight angular misalignments.

    Two bolts can cope with moments in one plane ie along one axis

    3 or more bolts can cope with moments applied in two planes at right angles and therefore all such moments.

    Does this help?
     
  6. Aug 20, 2010 #5

    Mech_Engineer

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    Think about it this way- a bolt's strength is based on it's shank's cross-sectional area (not diameter)... so what has more cross-sectional area, 2 12mm bolts, or 4 6mm bolts?
     
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