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Thread: Thread engagment rules of thumb vs. published standards

  1. #1
    Associate Engineer
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    Thread engagment rules of thumb vs. published standards

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    I've searched high and low on the matter of min. length of thread engagement. What I find are rules of thumb but no actual citations that refer to an ASME, ISO, ANSI, or other recognized authority on the matter. The problem is that I'm wanting to tell a customer that there should be more threads in a fitting but I don't want to send them rules of thumb as authortative back up for my claim, I want to send definitive rules or published standards. For example, I can say there should be a minimum of 3 full threads as a rule of thumb, but this isn't stated anywhere that I can find as a definitive design standard. Similarly, a statement like "Basic design rules state that the screw should break before the threads strip." To be clear, I'm not arguing the validity of the rules of thumb, I agree with them. What I'm seeking is a more credible, authoritative, reference that I can use to give a customer. How does one bridge the gap between a generally accepted rule of thumb and a published standard from an internationally recognized institution?

  2. #2
    Technical Fellow Kelly_Bramble's Avatar
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    To my knowledge there is not an ASME, ISO or other industry standard that embraces thread engagement rule-of-thumb design for minimum thread count.

    As I understand design and engineering industry practices the rules of thumb for thread engagements are based on design analysis of actual thread engagement designs. Most correctly, all thread engagement designs should be analyzed for full tensile strength however this is not practical nor required for all design and engineering activity. It is for these reasons that our rules of thumb for minimum thread engagement are used throughout industry. The rules I'm most familiar with is a minimum of 1.5 x Major diameter thread engagement for static loading and 2.0 x major diameter size for dynamic loading.

    For your "3 full threads as a rule of thumb," statement - that sounds risky for most any end item based on the variations in manufacturing and assembly

    When in doubt - an analysis under worst case loading + a factor of safety is done.

    The following is what ASME has to say..

    Per. ASME B1-13M, appendix B, page 50

    "In general the length of engagement of mating threads is selected to utilize full tensile strength of a bolt prior to shearing and nut threads. Other applications may require internal thread shear prior to failure of theoretically threaded part. ..."
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
    Technical Fellow jboggs's Avatar
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    Haven't you heard? The customer is always right!

    I have a "rule of thumb" I work with (1.5D) as I'm sure Kelly does and every other experienced engineer. But each application is so different I don't know how any "standards" could really apply. For example, you use the term "fitting" rather than "screw". To me that implies some kind of fluid connection using pipe threads. Well, that's a whole different animal than screw threads.

    If you do mean screw threads, you didn't provide the whole picture of use. Sizes? Loading? Vibration? Environment? Repetitive disassembly? Materials? Safety factor? etc.

    I would say you should try to meet all of the customer's requirements if at all possible. If some of those requirements make you uneasy, you should clearly express your recommendation to them in writing. If the only thing you have to base it on is your own professional judgment, so be it. That's why they hired an engineer. They should understand that if they choose to go a different way they do so at their own risk.

  4. #4
    Technical Fellow Kelly_Bramble's Avatar
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    Here is a webpage that defines the recommended practice on the length of screw threads engagement.

    The reference is ASME B1.13M-2005 (Revision of the ASME B1.13M-2001) per. ISO 965-1.

    Length of Screw Thread Engagement per. ISO 965-1 Calculator, Formula and Table
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

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