Thread Engagement for 105 KSI HHCS in 40 KSI Body 3

[aggieengineerrmb]

What are some well accepted standards for industrial and oilfield equipment (pressure containing vessels) for thread engagement of HHCS's. In this particular case, I have a 1/2-13 Grade B7 (105 KSI yield) HHCS threaded into a cast steel (WCB with 40 KSI yield) part. How far should the thread engage into the part to insure the threads will not pull out under stress? Ideally, I'd like the HHSC to fail under tensile stress before the threads pull out of the mating body surface.

 

[CoryPad]

thread725-35222

A rough calculation shows that you should have ~ 0.65 inches of full thread engagement between these parts.

 

[arto]

How about "Helicoil by Design?"

 

[aggieengineerrmb]

Although I know what a helicoil is, I don't understand what "helicoil by design" means. Can you elaborate?

 

[barbless]

aggieengineermb,

According to the textbook "Mechanical Engineering Design" (McGraw Hill) by Shigley and Mitchell (Pi times mean diameter times Length of engagement)/2 equals the shear area of thread required to cause the internal thread and the external thread to shear simultaneously. This is when the bolt and nut are of the same material and is highly approximate. By using the data on a 1/2-13 UNC thread and transforming the formula, the length of engagement equals 0.2". Because the materials are different one must multiply this value by the ratio of the tensile strengths. That would then equal 0.525". Because the length of engagement is highly approximate, a safety factor of 2 should be used.
The calculated length is then 1.05". Round off to 1 inch.

 

[arto]

Usually Helicoils are used to correct mistakes.[bad threads etc.]
It blows some people's minds when you call for "Helicoils by design" to get the high pullout strength for the assembly. Look in their catalog for pullout strengths.

 

[Double-Edged]

For thread stress see

http://www.engineersedge.com/thread_strength/thread_bolt_stress.htm

which has all of the equations for thread engagement per ISO 898-1, 1999 and FEB-STD-H28/2B, 1991.

Hope this helps

 

[CoryPad]

Edged's link should not reference ISO 898-1. That ISO standard does not have thread engagement calculations, especially not ones that use threads per inch.

Regards,

Cory

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