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  • cotter pins shearing strength

    Hi
    I am helping to build a Robot for a local high school.Need to know the shear strength of a 3/32" cotter pin.
    ? The load is ~ 10-15 lbs. and the pin is held on both ends between two # 25 chains ~ 0.875" apart.ie Chains running parallel(~0.875" apart) with the cotter pin joining the two chains.The pin is being used to draw back a spring.
    Do you think it will hold?
    It will load up the spring every 4-10 seconds and have a cycle of 2 minutes.

    Eddie
    please visit my webpage:
    http://motorworks88.webs.com/

  • #2
    I can't really visualize what the setup is, but here's some ideas that may help you.

    "Mild steel" has a tensile strength of around 50,000 psi, give or take. Something 3/32" in diameter has a cross-section of about 0.007 square inches. 0.007 times 50,000 is 350, which is way more than your 10-15 pound load.

    Now, that is REALLY a crude approximation, as tensile strength and shear strength are two different things, and the way in which the load is applied matters, and a 3/32" cotter pin isn't a solid 3/32" in diameter but is in two halves, who knows whether the tensile strength of the cotter pin is 50,000 psi, and on and on...but I think, given the great disparity between the load you have and the sort-of strength approximation guesstimate I just figured out, I doubt you need to worry.

    If you can, run your setup (or create a test setup) with about 4x the actual expected load for a while and see what happens. Nothing like practical testing.....
    ----------
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    • #3
      The Shear Strength of all steels is 75% of Tensile strenght.
      ( see "Strength data for iron and steels" in Machinery's Handbook)
      The problem is that yield strength is lower, meaning that some stretching (thinning)may occur before failure. You see this usually with loads over a period of time.such failures occur at lower forces (longer time however)
      Shear is a function of sharp edges as well.
      A "shear pin works at predetermined failure points,"until" the hole edges get rounded, then it takes more force to "Fail"


      I think SGW spelled it out, however don't forget in most cotter pin installtions, TWO points are in shear (both ends !) so double the ultimate failure point

      have fun

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      • #4
        The cotter pin shouldn't be relied upon to support the load. The malleable nature of the pin will cause it to thin on each cycle until it fails.
        Spence

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        • #5
          Eddie
          I am not sure I quite visulize what you mean, do you have a picture or drawing?

          If not when in doubt, over do it.

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          • #6
            Thanks for all the help.
            Robot did great.We did not break any pins!!
            And,we got 3 second and one third place.
            Got second in best design!!
            Not too bad for a small high school in Newfoundland up against all of Canada and some teams from the US.
            please visit my webpage:
            http://motorworks88.webs.com/

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