Minimum thread length for forged eye bolt

Discussion in 'High Power Rocketry (HPR)' started by snrkl, Aug 9, 2018.

  1. Aug 9, 2018 #1

    snrkl

    snrkl

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    Wondering if anyone can give me guidance on just how much thread is needed for secure MMT attachment?

    This is for my 4” deadpool (38mm)..
    https://www.rocketryforum.com/threads/new-102mm-project-chimichunga-i.145203/


    I was pondering doubling the front MMT CR but I’m now thinking it might not leave enough thread for a secure attachment.

    IMG_9577.jpg

    Am I over thinking / engineering this by doubling the front CR?

    Thoughts?
     
  2. Aug 9, 2018 #2

    krislhull

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    In the aviation industry, the standard practice is to have 2-3 threads showing. I follow this rule of thumb on my builds. For rocketry, though, what you have is probably fine. I would put a dab of epoxy over the nut to seal it just to make sure.
     
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  3. Aug 9, 2018 #2

    krislhull

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    EDIT: Double post... sorry!
     
  4. Aug 9, 2018 #3

    dr wogz

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    that's actually a mechanical design standard!

    so, +1 on 2-3 threads showing.. and loc-tite (or epoxy, CA, or some other method of locking that nut in place..)

    What about counter-boring to inset (then epoxy in) the washer?
     
  5. Aug 9, 2018 #4

    rharshberger

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    There are definitely mechanical design standards, about one for every type/application of bolt/nut. However as Kris stated the amount of engagement in this instance is probably okay for this application, usually I like to see 1 to 1.5 diameter length of thread engagement on my projects.
     
  6. Aug 9, 2018 #5

    snrkl

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    Thanks for the advice - much appreciated.

    Do people think I’m over engineering by doubling the CR?

    The single CR is a 3mm 5ply aircraft grade plywood.

    The one I’m thinking of using to double is a 5mm 3ply outdoor grade ply...
     
  7. Aug 9, 2018 #6

    Steve Shannon

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    There’s nothing wrong with doubling that particular centering ring. Double check what it does to your Cg.

    I would do a modification on what Rich suggested. I would sink the shoulder of the Eyebolt into the forward CR, thus allowing the threads to protrude further through, giving you enough room for your washer (very important) and nut and still allow a couple of threads to be exposed.
     
  8. Aug 9, 2018 #7

    Garrace

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    You can also use a thin large area washer and looks like from the picture that would put the threads flush with the end and that is the most strength your going to get. The rule about having 1and 1/2 threads exposed is only to make sure if using a lock nut the fastener is fully engaged in the locking device. I would definitely lock tight or epoxy the nut though. Countersinking the shoulder wouldn’t significantly weaken it either as long as you’re using a large area washer on the nut side where the stress is.
     
  9. Aug 9, 2018 #8

    boatgeek

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    I would use a large diameter (aka fender) washer with a single CR. With a normal thickness fender washer, the edges of the washer will be pretty close to the glue line between the CR and body tube/MMT so there's little risk of pulling through the plywood.
     
  10. Aug 9, 2018 #9

    G_T

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    I'd prefer a fender washer on BOTH sides. Tightening the nut will otherwise pull the eyebolt through the plywood. The plywood is the weak link here. I'd use an eyebolt with a longer thread, and cut off the excess.

    Gerald
     
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  11. Aug 9, 2018 #10

    rharshberger

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    +1 to washer on both sides.
     
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  12. Aug 9, 2018 #11

    blackjack2564

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    Or just save that one for use on G-10 & go buy another longer one.....couple bucks and ya got the right one.
    You can't ever have enough loose e-bolts....quick-links....threaded rod laying around, if your going to be in rockets for any length of time.
     
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  13. Aug 10, 2018 #12

    snrkl

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    Thanks all.

    Hardware fixings that aren’t high volume items can be a pain to find in Australia - we only really have one major hardware store chain. This coupled with my desire to support our Local Rocketry Vendor (LRV) meant that I ordered the eye bolts he had, which are 12mm long..

    That being said, I managed to find 80mm M6 forged eye bolts at my local HW store (I got the last 3) so this will fix the problem. For now anyway.

    Your point on saving this for g10 is valid - the reason the LRV only carries 12mm eye bolts is that he usually recommends g10 bulkheads for ebays - I haven’t worked with fibreglass before, and was reluctant to add too many new material types while building my first DD...

    Chalk this one up to: listen to the LRV and take the subtle hints next time!!
     
  14. Aug 10, 2018 #13

    lakeroadster

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    What you have there is OK-AS-IS for the given application. Loctite it, no worries.
     
  15. Aug 10, 2018 #14

    snrkl

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    Problem solved (and likely over built!)

    IMG_9583.jpg
    IMG_9584.jpg
    IMG_9585.jpg
     
  16. Aug 10, 2018 #15

    OverTheTop

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    T-nuts are another good option for threading into plywood.


    index.jpg
     
  17. Aug 10, 2018 #16

    snrkl

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    I’ve had a devil of a time finding those in any hardware store in Australia… Especially in small metric sizes… I ended up ordering a whole bunch of eBay and waited the required six weeks for them to be delivered…

    How have you gone finding them in Melbourne?
     
  18. Aug 10, 2018 #17

    OverTheTop

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    I can find some in my workshop :). Want me to send you some? What size you after and I'll see what I have. Purchased on eBay somewhere.
     
  19. Aug 10, 2018 #18

    snrkl

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    I’m good now, thanks for the offer...

    I ended up ordering a massive bag of m4 tee nuts and m4 gnurled captive nuts for use in nosecone ebays for use in attaching the NC Bulk plate to the NC shoulder CR...
     
  20. Aug 10, 2018 #19

    dr wogz

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    Washer and split / spring type lock washer? I, personally, would do away with the lock washer, and go glue (CA or epoxy) or loc-tite..

    The other thought I had, was to double up (or triple up) the area where the eye-bolt is to be located. Then tap the wood, soak the threads in CA, then re-tap again. Then just nut & washer on the backside. I've seen some pretty strong threads cut into wood..

    Now, the real question here is (in my opinion) is how much 'pull thru' force is needed to rip a 1/4" nut & bolt thru a 1/4 piece of .. 5-ply plywood? Especially since it is also being glued in place. The nut (and washer) are on a small portion of the CR, which is supported all around. Does this not exceed the breaking strength of the shock cord?
     
    Last edited: Aug 10, 2018
  21. Aug 10, 2018 #20

    jqavins

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    I'd remove those split washer spring type lock washers. Not only because, as others have said, you probably don't need them if you use thread locker (be it LocTite or epoxy) but also because they don't work. We've all seen it and cursed it: "Damn, why did that loosen up? It's got a lock washer!"

    I'm only a dumb sparky, but several mech. eng. associates have told me that the only lock washers that are worth spit are wedge type, commonly called nord locks for their most prominent brand. And buying nord locks is certainly overkill.
     
  22. Aug 10, 2018 #21

    snrkl

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    Not split washers, just a smaller diameter as the larger washers are some “almost right” imperial measurement “things”, and not a proper metric m6 washer (the m6s washers I could find were all too small for what I wanted on the OD)...
     
  23. Aug 10, 2018 #22

    lakeroadster

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    You are really other thinking this. Using these steel fasteners, on plywood, means the fastener is so much stronger than the base material that conventional bolting standards and practices don't apply.

    Go back to your original forged eyebolt, locktite it and be done.

    If the lack of threads sticking out of the nut really bothers you, use a jam nut.

    If somebody calls you on it.. they don't understand machine design. The weakest link is the failure path... and it won't be the steel fasteners.
     
  24. Aug 16, 2018 at 3:57 AM #23

    Not Quite Nominal

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    The weak point is probably the plywood.

    A nut requires 2 or 3 threads, or half a diameter, to achieve rated strength. With a couple threads short, you'll not get full strength - but you'll pull that nut clear through the plywood long before you pull the bolt out of the nut.
     

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