Shear pins bending, not shearing

Discussion in 'High Power Rocketry (HPR)' started by Kelly, Apr 24, 2019.

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  1. Apr 24, 2019 #1

    Kelly

    Kelly

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    I've got a new build, and using shear pins for the first time. Fuselage is 7.5" sonotube, and I made a custom glassed nosecone. The nosecone fit is pretty loose, but I'm using pins, so no problem, I figured. I ran some calcs to show that 3, 2-56 nylon screws should work well. I drilled 3, 1/4" holes through body tube and NC, filled them with epoxy, and drilled 2-56 tappable holes in the body and clearance holes in the NC.

    In the first ground test, the NC deployed exactly as intended, but upon inspection I see that the 3 screws have not sheared, but simply bent away. Probably the undersize shoulder diameter on the NC contributed to this.

    How big of a problem is this? Should I try to somehow build up the shoulder size, around the pins, in order to get a tighter fit and a better shearing action? Or just be happy that the NC is deploying properly?

    Kelly
     
  2. Apr 24, 2019 #2

    Tim51

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    In my experience a clean cut seems to depend on two factors (1) an effective cutting edge at the mating surfaces (such as brass shim) (2) the length of the nylon pin itself. Longer pins tend to bend or pivot as the charge blows - shorter pins tend to shear as intended.
     
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  3. Apr 25, 2019 #3

    Banzai88

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    The shear surfaces need to be a close tolerance fit. I wouldn't do anything else until you've solved the clearance issue.

    I've found that threading BOTH holes works best. YMMV.
     
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  4. Apr 25, 2019 #4

    GalantVR41062

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    I sharpened the edge of a staple and installed that in the nose cone at the bottom of the 2-56 hole for nylon screws. The staple is stuck threw the cone and glued in, nice clean shears.

    ~John
     

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  5. Apr 25, 2019 #5

    jimzcatz

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    Your problem isn't long pins it's loose fit. Plus it's cardboard. Even though glassed, you need a knife edge to cut the nylon.
     
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  6. Apr 25, 2019 #6

    OverTheTop

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    Shearing, rather than bending and/or stretching, creates a very predictable force for holding on the nc. Any other deformation adds uncertainty to the force required.

    You might be able to improve the clearances (less) near the pins to improve the outcome.
     
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  7. Apr 29, 2019 #7

    Kelly

    Kelly

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    Thanks, all. I glued some 15mil brass shim stock to the inside of the body and tapped for 2-56. On the NC, I built up the appropriate areas with some epoxy, and then filed down by trial and error to get a good fit. Pins started shearing perfectly, and the rocket got me my L2 yesterday.

    The epoxy is not a good solution, though - it is brittle and tends to crack occasionally. I'll be replacing that with some more durable - probably more brass.

    Thanks again!
     
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  8. Apr 29, 2019 #8

    Tim51

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    Glad you got it working ok. Congratulations and welcome to the Level 2!
     
  9. Apr 30, 2019 #9

    OverTheTop

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    Well done on the L2!
     

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