Thread Pressure - Thread Stripping

  • #1
cmarentes89
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TL;DR Summary
At what forces and thread engagement will my threads shear or strip.
Hello all,

So I've attached two images, one is the spread-sheet that I'm working on and the other is an image of the formulas that I'm using. I'm a bit confused on how length of engagement works for those formulas. So for 3 1/4-8UN-2A threads, with a shear strength of 44.5 KSI, the minimum length of engagement should be about 2.68 inches. From that, I understand that my engagement length should be 2.68 inches to avoid any thread stripping and/or shearing. But what if I use 1.00 inches of thread engagement engagement? The formulas shows that it would not strip when I compare the forces required and actual to strip the threads.... My ultimate goal is to determine a shear safety of factor and the minimum length of engagement at any applied pressure.
 

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  • #2
cmarentes89 said:
From that, I understand that my engagement length should be 2.68 inches to avoid any thread stripping and/or shearing. But what if I use 1.00 inches of thread engagement?
Then, on some occasion the thread may strip, and you would, if you survived, be questioned by the coroner.

The formulas have a tolerance allowance for imperfections.

The length of the nut, or engagement, determines the area of thread in pressure contact and in shear. The tensile strength of the threaded portion of the bolt, must be balanced against the shear strength of the thread.

The formulas appear to be not specific to the material.
The ratio of shear strength to tensile strength in the material used, will be an important parameter.
 
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  • #3
Baluncore said:
Then, on some occasion the thread may strip, and you would, if you survived, be questioned by the coroner.

The formulas have a tolerance allowance for imperfections.

The length of the nut, or engagement, determines the area of thread in pressure contact and in shear. The tensile strength of the threaded portion of the bolt, must be balanced against the shear strength of the thread.

The formulas appear to be not specific to the material.
The ratio of shear strength to tensile strength in the material used, will be an important parameter.
Would definitely would never try it in real life lol.

I see, but in a ratio comparison, wont my shear strength to tensile strength always be the same? 1/2 of the tensile strength?
 
  • #5
Baluncore said:
Not always, shear strength of steel is usually between 60% and 70% of tensile strength, other materials may differ.
Cast iron may range from 0.55 to 1.25 .
https://www.engineersedge.com/materials/material_tensile_shear_and_yield_strength_15798.htm
Thank you so much for the data, it will be very helpful. So I guess my question is, I'm I looking at the wrong formulas for what I'm trying to accomplish?

I figured my material shear strength multiplied by my shear area would have give me my actual force on threads. Then compare that to my actual loading pressure multiplied by my shear area again would give me a factor of safety based on shear. But in this method, all I would have to compare is my material shear strength to my actual loading pressure.... which eliminates the shear area, so this doesn't seem right.
 
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