UNRC Threads? 5

[tc7]

Has anyone ever heard of UNRC threads? I have been told the "R" means rolled threads and I have also been told the "R" designates a special root radius thread form. What I really need to know is if a UNRC Class 2 or 3 bolt will fit without interference into UNC class 2 or 3 threaded holes.
Thanks for helping.

 

[swertel]

Yes, it will fit fine.

The R stands for "rolled" which gives you a special root radius. Basically, because these threads are rolled and not cut, the grain structure of the metal changes making these threads slightly stronger than regular UNC threads.

If Corypad is around, hopefully he can elaborate more.

--Scott

For some pleasure reading, try FAQ731-376

 

[israelkk]

Machinery's handbooh 26th edition page 1713. There is no UNR thread and UNRC bolt will fit into UNC internal thread.

 

[Screwman]

The "R" stands for rounded root contour (see ASME B1.1 sec 1.3). It has a radiused root contour which greatly helps the fatigue properties compared to a flat bottom root. One interesting thing to know is that the vast majority of external thread are rolled not cut and if they are rolled, most of them will be rolled to the UNR thread form because it provides much better tool life. This is because the UNR form falls withing the allowable envelope for UN.
Even at maximum material condition you will have no problems with fit and interchangability for the two systems. They are designed to be fully compatible.

Dick

 

[CoryPad]

Agreed that UNRC screw will mate with UNC tapped hole. Just to clarify, UN means unified thread, R means controlled-root radius while C means Coarse pitch. Personally, this makes me yearn for M and MJ threads...

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[tc7]

Thanks all very much with rapid answers.
V/R
T

 

[aviat]

Originally the "R" thread was made to MIL-B-7838 specs and under that spec it may or may not be rolled. That spec was on the internet last year but isn't anymore. It was changed to ASME B1.1,as said above,in 1992. A male thread made to UNR/MIL-B-7838 has a radius at the root of the thread that is equal to 0.144 times the pitch of the thread (0.144p). This is the largest radius that can exist there and still guaranteeing that the male fastener will engage with a standard nut or other female thread, without interference.

 

[tc7]

Aviat-
Thanks for the history on the "R" designation.
It seems like common knowledge to you, swertel,israelkk, Screwman and Cory that the UNRC thread will mate with the UNC threads, yet without going through the geometry which you all have now made me aware of, I would never have known this. The "average" designer, in making bolt selections, does not need to delve into thread form geometry, so how would the average designer know that UNRC will mate with UNC? What text, reference or footnote did I miss along the way?????
T

 

[CoryPad]

I can't provide a single piece of information to answer your last question - it is from accumulated knowledge. However, I can elaborate on my "yearning" stated above: metric threads don't have the variations in terminology that inch-based threads do, and therefore are identifiable as interchangeable to "average" designers.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[arto]

Look in ASME B1.1:
"UNR applies only to external threads; the difference
between UN and UNR threads, in addition to
designation, is that a flat or optional rounded root
contour is specified for UN threads, while only a
rounded root contour is specified for UNR threads."