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How to calculate how many threads I can use?

  1. Apr 27, 2015 #1
    Hi guys,

    Here is my problem / task: I have a 232 bar pressure cylinder (not sue what material it is made from) with a brass fill valve at one end. I am trying to design a new shorter valve so that it takes up less space inside the cylinder, and thus increasing the capacity.

    How would I calculate the minimum amount of threads I need to use before the pressure causes the thread to strip and the fitting comes flying out the end of the cylinder?

    The original fitting has a thread engagement length of 10mm or 0.393 inches and uses an M25 x 1.0mm pitch thread. I have tried to use the following formula however it gives me an answer of 0.92904 inch / 23.59mm which is over twice the amount of the original fitting.


    I understand that the first 6 threads take all the load, but If i use a stronger material for the fitting, say 303 stainless steel, can I get away with less threads?

    I am trying to get the thread engagement length down to 5mm however I just dont know how to calculate it.

  2. jcsd
  3. Apr 28, 2015 #2
    What is the actual tension load on the bolt?

    If you don't know the pressure cylinder material then changing from brass to steel bolts won't guarantee anything.
  4. Apr 29, 2015 #3
    I have no idea what tension load is, the first time I have ever dealt with designing threads.

    The cylinder is some sort of hardened steel, possibly a form of gun steel, although there is no way I can know for sure. The cylinder threads can easily take the pressure its the threads on the fitting that I am unsure about.
  5. Apr 29, 2015 #4
    How do you know the cylinder threads can easily take the pressure?
  6. Apr 29, 2015 #5


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    Staff: Mentor

    This sounds pretty dangerous. What is your background (education and design experience)? Why were you assigned this task if you have no experience with this sort of thing?
  7. Apr 29, 2015 #6
    Because thats the pressure it currently runs at :s, its an already made pressure cyclinder on a product...all im doing is trying to increase the capacity.

    I wasnt assigned it its a personal project. My experiance is aerospace/electronics/manufacturing engineering/machining/mechanics.

    Im not some dum secondary school kid that knows nothing about engineering, im just trying to do something and seeking a bit of help.

    Someone an a different engineering forum has explained what i need to do...what i need to take into account and given me the formulas with good explanations.
  8. Apr 29, 2015 #7


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    Staff: Mentor

    I never claimed you were dum [sic]. The PF rules prohibit dangerous discussions, and depending on your background and level of experience, what you are asking about can be dangerous. Hope the advice you got was good -- please be careful and stay safe.
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