Bolt Thread compatibility 1

[Nivrah]

Hi All,
Does anyone know if a BSW (british standard whitworth) bolt can mate with an UNC nut or vice versa?
I looked up charts and found that the threads per inch (TPI) for BSW bolts and UNC bolts are the same for all sizes except for 1/2" size.

 

[GregLocock]

BSW has a different threadform to UNC. If you apply enough torque they'll probably mate.



Cheers

Greg Locock


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

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[unclesyd]

Your combination cost us a ton of money several years back with some 1" studs. The threads will start and take up 4 or 5 threads and stop. If you back off at this point you can still use the proper thread form nut to make the joint.
Our problems were we tried to mix and match instead of dumping the Whitworth studs. This episode did one thing as it convinced the chemical side of the road to spin on the nuts and supply a stud and nuts togather like the fiber side had been doing for years.

Even to try this with materials that will gall will cost you money.

 

[MiketheEngineer]

If you get them together tight - I doubt you will ever get them apart...

 

[Tmoose]

55 degree (whitworth) vs 60 (unified) and whitworth "shortening" 1/6 H vs unified 1/8 H looks like potential for some hard interferences.

It makes me wonder similar things as to what happens with this 'locking" thread

http://www.spiralock.com/technology

Dan T

 

[drawoh]

Nivrah,

I worked this out. The to-specification thread profiles interfere.

I was very surprised to find out that camera tripod attachments are 1/4-20BSW, not 1/4-20UNC. This actually makes sense when you remember that tripods predate the Unified National standard. We stick 1/4-20UNC screws into these things all the time. I do not really know if modern cameras and tripod screws still are BSW, or if they are extra sloppy to allow more clearance.

Camera tripod do not engage very many threads. I do not think I have seen a tripod mount that violates unclesyd's conditions, above.

JHG

 

[unclesyd]

The number of turns to lock will depend on the size and threads in the mating part. My reference in particular was to 1" stud bolts. All our problems were with 430 SS fasteners used on a Nitric Acid Plant.


tmoose,
Here are two of several papers from the old NASA.
The most impressive use for the thread form is used on fasteners that hold the Shuttle's main engines on and can be reused. I can see no worse case. We use them on some high speed equipment due periodic periods of instability excursions.

http://www.google.com/url?sa=t&source=web&cd=6&sqi=2&ved=0CDgQFjAF&url=http://

http://www.eaton.com/ecm/idcplg?Idc...0IHaAw&usg=AFQjCNGrZz0DGxUjIb9DF0-mES7InH_zQQ

http://www.techbriefs.com/component/content/article/9590

 

[racookpe1978]

A camera tripod will be very short (from around 3/16 up to maybe - at most - 3/8 inch long) so the number of threads engaged and binding is relatively few.

Also, tripods will be attached to "something precious" only "finger-tight" so as soon as binding occurs, the camera is considered tight and the motion stopped by the user. Who is usually pretty cautious anyway about his camera or laser.

 

[nenesteamer]

I'm surprised at the comments about the threads locking up.

A lot depends on the class of fit, of course, but in experimental tests I've found that Whit and UNC will run right up in all sizes from about 1/4" to an inch or larger, other than the 1/2" with its different TPI. But it should never be done for real!

The difference in thread angles, mentioned above, is the problem. Putting 55 and 60 degree threaded components together with cause the load to be taken on the crest of the thread rather than fully on its flank, and there is a very good chance of fretting and loosening to follow.

My regular Whitworth experience is on historic steam railway locos and I never mix the threads (though I've had a few near misses!) but I have heard of problems with mixed Whit/UNC fasteners coming loose on motorbikes.

David W.
Mechanical Engineering Group
Nene Valley Railway

 

[nenesteamer]

I should have mentioned that my 'experiments' were with UNC nuts on Whitworth bolts, not vice versa. In theory there should still be galling, but I didn't find that in practice.

David W.
Mechanical Engineering Group
Nene Valley Railway