Questions about helicoil screw assembly strength

[Mirui]

Attached file is an excerpts from HELI-COIL tensile strength of threaded insert assembly.
The parent material is Al6061, and bolt strength is 420MPa, in this case, its intersection is below the 1 dia insert length.
• If I use 1 Dia insert, what’s the strength of assembly?
• If I use 1.5 Dia insert, what’s the strength of assembly?

 

[SwinnyGG]

That chart is not designed to tell you the ultimate strength of the assembly- its a guide to selecting which insert to use, so that your assembly strength is controlled by the strength of the fastener and not by pullout of the insert.

 

[Mirui]

I think what I really want to know is, what if the intersection does fall onto the insert length, then what's the assembly strength?

 

[SwinnyGG]

The assembly strength is whatever you calculate it to be.

The point of that chart is not to tell you what the assembly strength is.

 

[Mirui]

Thanks.
Do you have reference book recommend, or formula I can learn?

 

[IRstuff]

Seems to me that on the face of it, the graph is telling you that you are limited by the fastener that you've chosen.

TTFN (ta ta for now)
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[Mirui]

Hi IRstuff, can you explain in more detail?

 

[IRstuff]

The diagonal lines are the pullout strengths of the inserts, assuming infinite strength fasteners, so ~2.6 kN is the pullout strength of the 1 dia. insert and ~4.4 kN for the 1.5 dia insert, in 6061T. The 420 MPa fastener line is below the intersection, so the graph is saying that ANY insert with that fastener will be limited to ~1.4 kN

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[Mirui]

Thanks IRstuff, I am getting there.
How about the case that intersection is above the dia. line?

 

[SwinnyGG]

If the intersection is above a diagonal, then the chart is telling you to use a different diagonal so that your assembly strength is controlled by the fastener.

The fastener should always fail first.

The strength of the joint is the strength of the fastener.

 

[Mirui]

Thanks JgKRI