Loctite Retaining Compound effect on bending fatigue behavior

[JosephKohrman-Glaser]

Hi everyone,

I've got a 9mm 4130 steel stud slip-fit into anodized 6061. I could only fit around 8mm of thread engagement, so my pre-load torque is limited to 10-12 N-m, around half of typical. I'm using 638 retaining compound on the shaft and threads to lock the stud in, but I want to know how the retaining compound will effect bending performance. The external side of the stud will be seeing pretty significant vibration and bending loads.

The stud has a flange at the surface of the aluminum that will be bonded with 638 to the face of the aluminum as well. Can anyone with similar experience give me some advice here? Don't have time to make big changes but I could switch to a different Loctite and just lock the threads instead of the shaft if that would be stronger.

Thanks!
JGK

 

[drawoh]

JosephKohrman-Glaser,

Are you bending the stud, or is it loaded entirely in tension? If in tension, is your tension force well in excess of your external load?

--
JHG

 

[JosephKohrman-Glaser]

Hi JHG, thanks for responding so fast!

The preload on the stud is entirely tension. External loads will be exclusively bending. I expect low-cycle external loads to cause stress in the stud that well exceed the pre-load. More typical cyclic loading will cause stress that is significantly less than preload.

 

[Tmoose]

"The stud has a flange at the surface of the aluminum"
Does the stud look Like this, but with unfortunate lack of thread engagement ? -

https://smhttp-ssl-39784.nexcesscdn...525d08d6e5fb8d27136e95/a/r/arp-134-7104_4.jpg

what is the material with 8mm of internal threads, and what is the aluminum slip fit part? Is the tapped hole in 8mm thick material, or is 8 mm the deepest blind thread you can make?

 

[JosephKohrman-Glaser]

Hi Tmoose,

My stud looks fairly similar to that, but with a much longer shoulder.
The material with 8mm of internal threads is 6061. The stud is 4130. This is why my preload is limited. The blind hole is longer than 8mm to allow tapping clearance, but the stud is constrained geometrically by other factors, and the design is already locked.

What i'm really trying to figure out is how the retaining compound will affect fatigue performance in bending, and if significant bending fatigue could cause the bond to break. As far as I know Solidworks FEA isn't capable of modeling such a complicated contact set. I can choose how I want to lock the stud in (threads only, retaining compound on the shaft, combination of the two) but I'd like to understand both how that bond will hold up and how it will affect strength.

Appreciate the interest here, thanks for helping me out.

 

[Tmoose]

If the geometry of the flange is good, it absorbs most of the bending leaving most of the thread forces as tensile and maybe some shear. There are plenty of posts and on-line info about how sufficient fastener preload, like love, is all you need.

Depending how thick the aluminum is, and how it is supported, there may be a bunch of oil-canning and potato-chipping in response to the loading.

I know you say the tapped hole design is fixed, but I'd still be looking at a thru tapped hole, and relying on the loctite for retaining AND sealing. Maybe add an o-ring groove to the plate without reducing the flange footprint, which I think is your best friend today.

 

[MikeHalloran]

I'd take a serious look at the slight extra expense of using a bottoming tap to get a little more female thread, and add a corresponding amount to the male thread.

Like Tmoose, I'd also maximize the diameter of the flange, and stiffen it, so as to convert more of the moment to tension in the stud.

If, despite all that, that short stud still pulls out or fractures, I would not blame the Loctite.


Given that the problem is important enough to ask for help, I'd also look at unfreezing the design and finding a more conservative way to meet your goals.

Further, it smells like there is some economic impact to someone, so you'd need to analyze the cost balance, including the downside risk of failure(s).





Mike Halloran
Pembroke Pines, FL, USA

 

[JosephKohrman-Glaser]

Hi all,

thanks for your suggestions. The flange diameter is maximized, and I'm looking at getting more thread engagement.

JKG

 

[Tmoose]

Hi JosephKohrman-Glaser ,

After re-reading your OP, there is mention of "slip fit" shaft and threads, and applying Loctite " to lock the threads instead of the shaft."
Is the "shaft" of which you speak the portion of the stud below the flange, that engages the aluminum, making it resemble a shoulder bolt?

https://ampg.com/catalog/media/SHOULDERSCREW-DOUBLEDRIVE.jpg

If so, I'd say lose the shoulder/shaft. Its ability to reliably resist bending loads is very pale indeed compared to the flange, well tightened.
If the "shaft" is there to provide radial centering/location, or resist "shear", then a short pilot diameter on the flange to engage a shallow snug fitting spot-face on the aluminum might use less of the real estate that would be much better used for increased thread engagement.

Some realistic detailed drawings and pictures are a week overdue.