rolling thread 1

[simthehunter]

Hi,

I read that rolling thread increase ultimate strength by 30% but I don't find more documentation on the subject. Can someone help me on this subject ?

THX
Simon P.

 

[btrueblood]

Yes, depending on the fastener size and material. See ASTM A193, F593, and/or F837, and look at alloy group 1 (austenitic stainless steels), comparing annealed vs. condition "CW" or "SH".

 

[hydtools]

Here is an extract from Metals Handbook:

http://books.google.com/books?id=5h...X&oi=book_result&resnum=3&ct=result#PPA281,M1

Ted

 

[dbooker630]

I have heard reports of fatigue life increases with rolled threads. We use some class 10.9 capscrews in our applications. The only problem I have seen is that some manufacturers will quench and temper the fasteners to the low side of the hardness spec to facilitate rolling, 33HRC is easier than 39 HRC.

 

[TVP]

There is a lot of information available on this subject, including detailed technical articles that can be freely downloaded:

http://www.threading101.com/resources.html

scroll through the various articles, especially those by Professor Wilhelm Friedrich Bethlehem

Fatigue acceptance test limit criterion for larger diameter rolled thread fasteners

http://www.osti.gov/bridge/servlets/purl/350965-cTcd3z/webviewable/350965.pdf

Factors Which Affect Fatigue Strength of Fasteners

http://www.osti.gov/bridge/servlets/purl/10173808-qsEerw/10173808.pdf

Fatigue Crack Growth Analyses and Experimental Verification of Aerospace Threaded Fasteners

http://www.ohiolink.edu/etd/view.cgi?acc_num=case1085750243

The effect of manufacturing processes on the fatigue lifetime of aeronautical bolts

http://www.eng.tau.ac.il/~neliaz/Papers_Files/C16.pdf

Possibilities of increasing the fatigue strength of threaded components

http://www.springerlink.com/content/kg6l302m1645124v/

Influence of Cold Rolling Threads Before or After Heat Treatment on the Fatigue Resistance of High Strength

http://www.astm.org/JOURNALS/JAI/PAGES/JAI13075.htm

 

[swall]

I must take exception to the idea stated by the original poster that thread rolling increases strength of fasteners.
As per the articles TVP has linked to, thread rolling increases FATIGUE strength in fasteners. That is the main engineering benefit.

 

[BillPSU]

The original poster was talking about all threads including internal threads which can also be rolled using a special tap. I would expect to see an increase of strength from the cold working of the material by the tap.

 

[btrueblood]

It may be the main benefit, but the yield and ultimate strength typically rise also, thus the benefit to fatigue strength is at least partially due to the work hardening. A secondary benefit for fatigue is the smoother profile generated by rolling vs. cutting.

 

[hydtools]

That the surface retains residual compressive stress contributes to increased fatigue life. The rounded root helps, too.

Ted

 

[btrueblood]

Sorry, right, should have said that _cold-rolling_ improves yield and tensile ultimate.

 

[swall]

No argument from me about the yield strength increase due to cold rolling. However, one needs to keep in mind that with fastener specs, you are always working with specified min tensile strength values--how you get there is immaterial. Also, comparing yield or tensiles on a fastener with rolled threads vs cut can be misleading unless you take into account the different thread root geometries (and stress concentration factor) of each. On internal threads--one advantage of the process on aluminum die castings is that the cold work will smooth over internal casting porosity, resulting in an improved surface finish, which is beneficial when subsequent disassembly/reassembly is required. Bottom line--I would hope that those reading this thread would not come away with the idea that thread rolling makes your fasteners stronger and that is why it is used. As I said before, the main benefit is the enhanced fatigue life. It is also a very practical process for high volume production.

 

[simthehunter]

Thank you for all your posts !

I forgot to say in my question that I was also talking about formed thread.

Swall you said : "However, one needs to keep in mind that with fastener specs, you are always working with specified min tensile strength values--how you get there is immaterial."

With that, want you to say that the increase of tensile/yield strength is not enough significaly to importing these values in our resistance calcul ?

Sorry for my english...I'm a French-Canadian

THX

 

[swall]

What I am saying is that you first determine what strength level you need for the application. Then you pick the material that will give you the needed strength. Finally, you look at method of manufacturing. If you go with rolled threads, any incremental strength increase arising from the process is a bonus, above and beyond the basic strength level of the chosen material. Consider this as adding to your margin of safety, if you will. And adding to fatigue strength, if your application is fatigue critical.

 

[EdStainless]

For some heat treatable materials you can use cold finished bar and then roll threads, or you can thread and then heat treat.
With alloys that only work harden (austenitic stainless) then you need to be more careful since you have to rely on the cold work. The selection of the starting strength is a real art. Often you can't use annealed material since it is too soft, but if you use bar that is too hard you risk cracking or destruction of tooling.

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