Determining Thread Length Engagement for Optimal Boss Length

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The discussion focuses on determining the minimum thread length engagement necessary for a product with two parts fastened together, where hydraulic fluid flows through. The user seeks to understand if five full threads are sufficient for load sustainability or if more engagement is required, considering assembly constraints. Responses emphasize that calculations for thread engagement do not need to factor in load, as they are based on empirical tests, and suggest reviewing relevant standards like FED-STD-H28 and SAE paper 770420 for deeper insights. Additionally, the importance of material differences between the threaded components and the potential for using brazing instead of fasteners for high-pressure applications is highlighted. Overall, the conversation underscores the need for thorough analysis and consideration of design alternatives.
araanandv2
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Hi,

Please help me in finding thread length engagement.

Please find attaché image

I have a product comprising of 2 parts fastened together.
Hydraulic fluid is passing thru it.

My inputs- load, size of the fastener used (ex: M8 X 1.5).
My constraints are-the boss length should be kept to the minimum (because of assembly constraints.

I need to find the minimum thread length engagement of the threads i.e. whether it can sustain the load when only 5 full threads are engaged or more threads need to be engaged.
By finding the engagement length I can decide the length of the boss.

In the process visited the site
http://www.engineersedge.com/thread_strength/thread_minimum_length_engagement_fed-std-h28.htm.


In the formula the load parameter is not considered.

Please mail me a solution.
 

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Oy. This has me worried.

First off, the calculations you see do not require a load because they are based on years of tests and empirical evidence. They are assuming failure in either shear, tensile or combined loading. Also, the class of tolerances on the threads will have an effect on the final number as well. One big thing to consider is the difference in materials between the external thread member and the internal thread member.

I would highly recommend getting a copy of FED-STD-H28 to get a background before attempting to do these calculations. An even better item for you to get your hands on would be SAE paper 770420, Analysis and Design of Threaded Assemblies.

What kind of pressures are you dealing with? Hydraulic fluid implies that it will be high pressure, but it could be on the return side which is usually pretty low pressure. I also didn't see any face seals between the passages. Is this a complete drawing or is this just to illustrate what you are doing with the fasteners?
 
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I'm not really sure why you're adding the tabs for screwing the two parts together. I doubt this is for heat exchanger design, but typically, reservoirs, heat exchangers, etc are all BRAZED together. I would recommend you investigate this as an option - it can usually handle fairly high burst and proof pressures.
 
Thank you
 

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