Determining Thread Length Engagement for Optimal Boss Length

  • Thread starter Thread starter araanandv2
  • Start date Start date
  • Tags Tags
    Length Thread
Click For Summary

Discussion Overview

The discussion revolves around determining the minimum thread length engagement required for a fastener in a product comprising two parts that are fastened together, with hydraulic fluid passing through. The focus is on ensuring that the engagement can sustain the load while minimizing the boss length due to assembly constraints.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant seeks assistance in calculating the minimum thread length engagement necessary to support a load with only 5 full threads engaged, emphasizing the need to minimize boss length.
  • Another participant expresses concern about the initial query, noting that calculations typically do not require a load due to reliance on empirical evidence from tests, and mentions the importance of thread tolerance and material differences.
  • A third participant questions the design choice of adding tabs for screwing the parts together, suggesting that brazing might be a more effective method for high-pressure applications.

Areas of Agreement / Disagreement

Participants present differing views on the necessity of considering load in the calculations, and there is no consensus on the best fastening method for the application.

Contextual Notes

Participants mention various factors that could influence thread engagement calculations, such as material differences, thread tolerances, and the specific application context (e.g., hydraulic pressure levels).

araanandv2
Messages
26
Reaction score
0
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.
 

Attachments

  • Thread.jpg
    Thread.jpg
    66.9 KB · Views: 578
Engineering news on Phys.org
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?
 
Last edited:
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
 

Similar threads

Screw and mating thread strength equations
  • · Replies 5 ·
Replies
5
Views
14K