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

 

[HTURKAK]

1- A nut can experience stripping failure , that is , shear failure of nut thread(s)..

2- The nut material can be weaker than bolt material (as is the case in general ) the stripping of nut material will be the failure mode and strip of its threads at its major diameter .
If the nut length is at least 0.5 d for small diameter bolts and 0.6 d for bolts with dia greater than 1.0 in, the load required to strip the threads will be greater than the tension strength of the bolt .

P.S. The nut can only experience tension or compression stresses when the nut is used to connect two different bolts like a coupling sleeve. If you connect two threaded rods with a nut long enough to connect the two rods, then apply tension or compression forces to the rods, the nut will experience similar stresses tension or compression with the rods....

 

[hydtools]

The loading of a nut will be compression through its body. Therefore no tensile failure.
Another failure mode may be bursting due to radial force of the thread if the nut wall is too thin.

Ted

 

[goutam_freelance]

Another possible failure is bearing failure at hexagon corners during tightening.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[XL83NL]

Could galling or cold welding be considered a failure mode?

Huub
- You never get what you expect, you only get what you inspect.

 

[saplanti]

I suggest you to have a look at the shortcut document page 10.4.5;

http://users.encs.concordia.ca/~nrskumar/Index_files/Mech344/Lectures/Lecture_6.pdf

Machinery’s Handbook gives you the thread tolerances with applicable standards. You need to assign the tolerance to be used in the bolt/nut threads. Additionally there are useful information about the shapes, thread formulas and materials.

The document above gives you used nut heights as well. Some cases height might be 1d even 1.5 d depending on the application.

 

[saplanti]

The following seems that answers your questions in detail;

http://www.hexagon.de/dose/dose-1e.pdf

However loading type, clamping, preloading etc... might need to be reviewed before the final decision for the connection and components of it.

 

[saplanti]

Some more useful information;

https://stuff.mit.edu/afs/athena.mi...UNdaMENTALs Book pdf/FUNdaMENTALs Topic 6.PDF

 

[saplanti]

Another useful information;

http://escies.org/ftp/ecss.nl/Handbooks/ECSS-E-HB-32-23A(16April2010).pdf

 

[CWB1]

If we take the bolt as an example - there are 2 things that are checked...

There's quite a bit more than that. Please pick up a few books on bolted joint design and immediately request a senior engineer review any/all critical bolted joints you have evaluated in the past before they fail. Bickford's "Introduction to the Design & Behavior of Bolted Joints" is the usual first choice.

Evaluating the nut or female threaded part is a standard part of a BJA and essentially the same as evaluating the bolt/male side. Thread shear, shank stresses, head shear, and underhead contact/bearing stresses are the common considerations for standard single-piece fasteners and there's a few more for multi-piece, multi-material fasteners. A quick BJA analyzes the bolt, the nut, and everything in between. Obviously a nut doesnt have a shank so you're evaluating the other three for nuts.

 

[eli28]

Thank you all for replying and adding useful attachements that I am going to read now.
I made a mistake by saying that there is a normal tension, and you are right there is compression.
This case (compression) shouldn't be considered carefully as well? I know that a more dangerous load is tension, but still the material have a yield strength.

saplanti - the link below doesn't lead to any page:

https://stuff.mit.edu/afs/athena.mi...UNdaMENTALs Book pdf/FUNdaMENTALs Topic 6.PDF

CWB1- Thank you for the useful book, I will search for it.
In the university we studied about bolted joint from "Shigley's Mechanical Engineering Design" book.
I couldn't find there any reference to head shear and underhead contact failures... Can you please tell me how you treat these?

 

[robyengIT]

few more info

 

[robyengIT]

and here too

 

[saplanti]

The link in my post does work, however you eed to wait a bit longer to open. Just in case I attached it again;

https://stuff.mit.edu/afs/athena.mi...UNdaMENTALs Book pdf/FUNdaMENTALs Topic 6.PDF

I guess you need a book that gives all the fundamentals about the thread calculation. All these attachments are web information that can be used as reminder.

 

[eli28]

saplanti - I still can't open the link. it's written that I am blocked (even when I tried another computer and browser).
Can you attach the file here?

 

[eli28]

hello guys,

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

here is the kink to the original document:

https://www.fastenal.com/content/feds/pdf/Article - Screw Threads Design.pdf

 

[CWB1]

In the university we studied about bolted joint from "Shigley's Mechanical Engineering Design" book.
I couldn't find there any reference to head shear and underhead contact failures... Can you please tell me how you treat these?

Underhead contact stress causes yielding of the part material directly underneath the fastener's bearing/washer area - a major concern with soft materials, small diameter washers, and/or large clearance holes. Head shear causes a cylindrical deformation and failure up through the head - prevalent with thin or large diameter heads and/or large clearance holes. Both of these are highly dependent on bearing/washer area, head geometry, and hole size. Hand calcs can get you in the ballpark but in product development acceptable values are typically based on experimental data correlated back to FEA for accuracy. To be fair, in the civil world they commonly just use heavy hex fasteners, big washers, and ungodly safety factors so to each their own methods.

Shigley's is like most engineering texts - a good reference but far from a "how-to" manual. Usually the "how-to" level of complete simplicity is only found in a well-run mega-corp's design guides, beyond that you start by reading a few commercial texts then find a SME to teach the rest.

 

[GregLocock]

eli28 that's a head scratcher. I think the point they are making is that the nut needs to be strong (high UTS) but soft (low yield stress). But are they talking about stresses or strengths (ie stress*area)?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[eli28]

I think they speak about material stresses.
What you said about High UTS Stress and low Yield stress sounds reasonable as long as we take in account that the nut thread will undergo yield.

 

[SWComposites]

If you are going to design a custom nut, then you need to plan to test it. Multiple replicates to determine allowable strength.