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Thread: Need Assistance on Shoulder Bolt Design

  1. #1

    Need Assistance on Shoulder Bolt Design

    I am developing a pivot point for a recreational wind driven vehicle using an elliptical stride path to generate force. The stride length is approximately 18-26" in length and the max weight could be anywhere up to 300 lbs. I would like to be consistent with applicable ISO7379, ANSI B18.3.3M and DIN 9841 specifications as applicable, but I am looking at a design slightly out of those specifications, and I want to get some input on whether I may be setting up for a tensile stress failure.

    Ideally the bolt would be
    M20 x 80mm M14 x 25mm (Overall length 80mm with 25 mm of threads), alloy steel. Additionally, instead of the ten threads, I would have thread one, a flattened surface where threads 2-4 are and threads 5-10 for complete insertion. I was planning on a set screw in the 2-4 thread position to ensure there would be no loosening.

    1. Is the shoulder drop form M20 to M14 too great i.e. will it reduce the the tensile strength and create a stress point that would not be there in a M20 to M16 bolt?
    2. Is M14, particularly M20 to M14 covered by any ISO or ANSI specifications?
    3. Do I gain an advantage using set screws or am I potentially creating another stress point?
    4. Would it increase strength to add greater thread length e.g. 35mm

    I would like this to use ISO specs, but from what I see, the M20 to M14 transition and thread length may prevent this. Any assistance or personal experience would be appreciated.


  2. #2
    Lead Engineer RWOLFEJR's Avatar
    Join Date
    Mar 2011
    Rochester Pennsylvania
    Hi and welcome to the forum...

    I'n not really clear on what your doing but from what I've gathered it seems that maybe your biggest concern is losing some threads off the end of your stripper bolt? Would you be able to drill... tap... counterbore for snug fit to shoulder diameter. Then use the shoulder that is in the c'bore to prevent the bolt backing out so you won't lose any of your threads? You could put a slight v groove in the shoulder bolt at the location of your set screw. Or just throw a little loctite on it?
    It's nice to put any shear or radial load on the shoulder instead of the relief cut at the start of the threads.

    But like I said... I'm not real sure what you're after.
    Good Luck,

  3. #3
    Technical Fellow
    Join Date
    Feb 2011
    I am with Bob, your request is not clear at all.

    If I think what Bob is thinking then his answer is the solution. Machine a small flat n the shoulder section of the bolt and then have a set screw locate on that flat. That way there would be no loss of strength as the threaded portion is the weakest part of the bolt and it will be untouched.

    A good rule of thumb also, in case you are not aware, is that the thread engagement length should be no less than 1-1/2 diameters. Or in the case of a shoulder bolt, the full thread length.

    Finally, you asked for personal experience, now, some of that I can offer too. If you are building a prototype do not worry about building to ISO or any other formal standards at this point. My experience (40+years) has been that until the final prototype, things will change drastically over iterations. Any and all the effort put into working standards during the prototype journey is wasted effort. Also, and it may because I am inept as an Engineer, I have never ever developed and built a device with more than about ten-parts, in less than three prototypes. In fact, one project I started many years back is at prototype stage 132 and I am still not happy with it, but I just can't let go of it.

    Personally, I believe it is naive to think that one prototype will be enough.

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