Tightening against incomplete threads 2

[shingouz]

Hello guys,
Is there any effect on static of fatigue behavior of fasteners if the nut is tightened against the incomplete threads of the screw? Is there any studies that was performed to study this configuration?
Perhaps I am not clear..., please see attached the definition of incomplete threads .
Thanks

 

[rb1957]

there's a reason why we have design standards against doing this kind of thing ...

Quando Omni Flunkus Moritati

 

[MintJulep]

Doing so will most probably prevent the development of the intended design preload.

The effects of insufficient preload are well documented.

 

[Screwman1]

I agree fully with rb. You do not ever want to allow this condition to occur; we control run-out geometry carefully to reduce the kind of stress concentration that you would be inducing with this design. Get a shorter grip fastener.

 

[tbuelna]

As noted by others, in the aerospace world it is never acceptable to allow this condition to occur. During the design process a tolerance stack-up analysis is usually conducted for each fastener installation. And there are QA steps used during manufacturing that check for these types of interference conditions.

Tension bolts usually do not control the thread length/runout as tightly as shear bolts do. With tension fasteners, it is common practice to allow the addition of washers under the nut to prevent the nut from "bottoming out" on the bolt threads. With shear fasteners, it is common practice to allow the substitution of a bolt with the next higher/lower grip dash number to ensure that there are no threads in shear and that the nut/bolt threads do not bottom out.

In terms of fatigue, bottoming out the threads of a tension bolt will likely produce a significant local stress concentration, and this would have an adverse effect on fatigue life.

Hope that helps.
Terry

 

[shingouz]

Hello guys,
Thanks for your explanations, I fully agree with you on the effect of this tightening on preload. But Can you explain to me what can happen in theroritical conditions : assuming a non preloaded joint? I know that this application may not exist in industrial conditions, but I need to have more details about the creation of "stress concentration" as you said.
Shingouz

 

[rb1957]

why ? ... don't do it, it's Bad; it shows sloppy assembly practices. next you'll ask "why have 2 threads protruding from a nut?"

Quando Omni Flunkus Moritati

 

[shingouz]

Can we just discuss why it's bad? I mean physical explanation that shows "it's bad".

 

[MintJulep]

It's bad because it violates many of the assumptions that are generally used in the calculations associated with bolted joints.

The geometry is uncontrolled, and you don't know what you will get.

The primary purpose of bolts is to clamp things together. If you are bottoming out before the required preload is developed then the joint is failed before it's even assembled.

However now you have introduced the term "non preloaded joint". What does that mean to you?

 

[berkshire]

Does the idea that a correctly torqued nut and bolt pair installed to your criteria, can be spun with your fingers, not scare the hell out of you?
B.E.

You are judged not by what you know, but by what you can do.