NUT DESIGN AND FAILURE MODES 4

[eli28]

Hello everyone,

I have to design a custom nut for my assembly for some reasons - I have a limited place and an exotic material.
As a result I have to make sure that the nut I design will not fail under the predicted loads.
Until now picking up a nut was quite a simple task (as long as you pick a standard hex nut) but now I have 2 simple questions that I couldn't find any data.
1. What possible failure modes a nut can undergo?
Everything I encountered on the web talked about the thread shear, but I couldn't find any reference to a failure due to tensile.
If we take the bolt as an example - there are 2 things that are checked:
Thread strength (due to shear strength) and shank strength (due to tensile strength).
Why there is no such treatment for the nut? I think that if its external diameter would be too small it may fail in tension as well...no?
2. How can I calculate the nut strength?
In shear - how many tooth are considered as participating in resisting the tensile force?
In tension - which area is taken? all the area over the tooth?

thank you all

 

[drawoh]

eli28,

Belleville washers will help distribute your load away from the clearance hole. You may want to stack your washers in parallel, to improve that distribution.

--
JHG

 

[SwinnyGG]

Am I the only one that seeing a contradiction here (I highlighted the contradicting sentences)?

That's not a contradiction, although it's worded a little weird.

It is accurate that base material strengths of nuts will be lower than bolts (typically, not always). Material strength is load per area.

That document is telling you that you need to select a nut that has a proof load higher than the ultimate strength of the bolt. Proof load of a nut is the load it can carry before failure- that's load, not stress- bad choice of words in that document but the intent is accurate. If the material used for the nut has half the yield strength of the material for the bolt, I need to make sure the stressed area of the nut is twice that of the bolt, etc. Not complicated at all to do so, since the thread area of the nut is by definition always larger than the thread area of the bolt.

 

[rb1957]

yes, we want the bolt to be critical. A example of this in practice is tapping Al ... now the fastener capacity is limited by the "nut".

another day in paradise, or is paradise one day closer ?