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  • #16
    I will third what JC said.
    Calculating shear loads is very involved.
    Check your Machinery's Handbook under "Strenght of Material"
    to get an idea of what you want. They have a section on taper pins and shear.


    Basically what you need to know is that all :
    Steels shear at 75% of tensile (T)
    Aluminum is about 75%
    Brass is about 100%

    So the cross-sectional area at both ends of the pin- times 2 (ends) - times tensile strength - times 75% = the shear load.
    so a steel pin (soft) 1/4 inch in diameter has :
    (.05) x 2 x 60,000 (T) x 75%= 4,415 pounds to shear.
    Now if the shaft is 1 inch it will take "X" amount of torque to shear
    But if the shaft is 2 inches at shear point, you double the required force to shear. So you see, the points of shear,
    relative to diameter have a dramatic effect as well. Diameter, material, and distance from center of rotation are all needed to figure it out.

    I am not trying to be contentious here, but I have never heard of a roll pin used for shearing, only as coupling devices.
    The problem with roll pins is that you cannot calculate the shear point of the pin, because of FOUR variables.
    First, the orientation of the 'layers" will affect shear
    (when a roll pin is in the hole, it does not have a equal numjber of layers all the way around )
    Second, "shear" for a flexible coupling is very difficult for calculation
    ( tables for roll pin use will express load "capacity" which is the MINIMUM capability of the pin.)
    No engineering manuals exist that express how to calculate it
    Third, not all roll pins are of the same spring material , which means 'T" is a unknown.
    Fourth. It is very poor practice to use shear pins that allow, or even promote scoring/galling of the mating surfaces.
    A hardened pin will tear up the coupling surfaces and make it impossible to remove the broken pin.

    Do they use harden pins ? you ask ?
    Yes, when the designer fails to allow a big enough pin diameter, or a installer
    thinks it isn't strong enough. Both are mistakes in my opinion, having had the experience of fixing those issues in the past.
    If you find one in an application, it is a mistake.

    Shear pin users and all standard references all point to straight or taper pin use, using standard material for ease of calculation.
    The simple method pointed out earlier by someone is to use aluminum, and if it shears, go to brass, and then to mild steel.
    When that fails, you need to ream to a larger size, not screw up the couplings

    Rich
    Green Bay, WI

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    • #17
      Reason I asked what colour was is because Brass is yellow, Alumin. is whiteish and Steel is grey. Rich has solved the problem for you, if the 3mm pin won't work bore it out to say 6mm or even 10mm and you'll never have problems with it again. Original designer must have not allowed for someone putting excess load on the system.

      Sadly in the REAL world, not every design uses the best material for the job.

      Regards Ian.
      You might not like what I say,but that doesn't mean I'm wrong.

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      • #18
        Rich, that was an informative precis. Thanks!

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        • #19
          Machined a 4mm shear pin out of brass and put it in. After just 2 operations, the pin sheared.

          I machined it out of mild steel, and this time I over sized by 0.02mm and put a slit down the middle so that it will be held in there firmly. Pressed it in and now it's working

          I'm glad that I took the conservative approach first. Thanks everyone. I can now enjoy the sun.

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          • #20
            Originally posted by juergenwt
            I second JC., Shear pins are from brass or aluminum. Hardened Roll pins are used as drive pins, never as shear pins. If you have a tapered pin it serves as a locating or as a drive pin. Never as a shear pin
            Excpetion taken to the last sentance. The SHEAR pin on the Clausing/
            Metosa lead screw is TAPERED. It is brass though. I've had to make a
            few replacements for the school shop. :-)
            ...lew...

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            • #21
              I'm glad that I took the conservative approach first. Thanks everyone. I can now enjoy the sun.
              Hmmm keep a few spares in the car.

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