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Forrest Addy
05-24-2010, 05:15 PM
Never know what you find until it jumps up and bites you. I was looking for the FED-Spec formula for classes of fit for diameter, pitch and length of engagement components for Unified screw threads clearances and allowances and stumbled across this excellent tutorial:

http://www.ring-plug-thread-gages.co...y-Fastenal.pdf

Has a bit of everything including stress distribution on a loaded thread. Handy stuff. Print it and put it in the looseleaf of cool info you keep in your shop library.

DENedbalek
05-24-2010, 05:44 PM
http://www.ring-plug-thread-gages.com/ti-Article-Screw-Thread-Design-by-Fastenal.pdf

I think the above is the link Forrest was trying to post.

Dwayne

Mcgyver
05-24-2010, 08:04 PM
one item that caught my attention was the comment that coarse won't strip as easily as fine.

Is that correct? if draw a line as an imaginary shear plane though a cross section of either of the same length, whether nf of nc the same amount of material would have to shear, right?

Dr Stan
05-24-2010, 08:25 PM
one item that caught my attention was the comment that coarse won't strip as easily as fine.

Is that correct? if draw a line as an imaginary shear plane though a cross section of either of the same length, whether nf of nc the same amount of material would have to shear, right?

NF are stronger since there is more area of contact between the male and female threads. They are also stronger in shear perpendicular to the axis as the minor diameter is larger.

JMcTool
05-24-2010, 09:38 PM
Thanks, Forrest. Good info. I just ran across this site a few days ago and learned a thing or two. I've probably got a few more sites worth a look at if I can find them.

http://www.boltdepot.com/fastener-information/Default.aspx

Jim

Mcgyver
05-24-2010, 09:41 PM
They are also stronger in shear perpendicular to the axis as the minor diameter is larger

i know that it was the site's stripped reference that i think is wrong, or at least i didn't understand if it is correct


NF are stronger since there is more area of contact between the male and female threads. .

i dont understand why the area of contact a) matters or b) is different between nf or nc For threads to strip there has to be shear along the axis of the fastener...the shear plane is then a cylinder as i picture it there is no more less metal that has to shear (for a given length) regardless of the thread pitch

with the cylindrical area in shear the same area, why wouldn't they perform about the same so far as stripping is concerned ?

JMcTool
05-24-2010, 09:47 PM
Here's another "interesting" site, altho I have a hard time heartily recommending it. Too much distracting sidebar info, very little actual content per page. However, it covers some subjects I've never even heard of. The real question is, do any of them matter? At this point I'm not even advanced enough to know what I don't know, so I can't say.

http://www.gewinde-normen.de/en/index.html

Jim

cuemaker
05-24-2010, 10:27 PM
i know that it was the site's stripped reference that i think is wrong, or at least i didn't understand if it is correct



i dont understand why the area of contact a) matters or b) is different between nf or nc For threads to strip there has to be shear along the axis of the fastener...the shear plane is then a cylinder as i picture it there is no more less metal that has to shear (for a given length) regardless of the thread pitch

with the cylindrical area in shear the same area, why wouldn't they perform about the same so far as stripping is concerned ?


I am also under the impression that NF is stronger than NC..

Being that I work for Fastenal, I will call one of the important engineer types at corp to see if the document is wrong or if the info is correct. Let you know Wednesday AM

Here is a bunch of videos that are of our machine shop in MN

http://www.youtube.com/fastenalcompany

oops, you need to go on the right side of the page and click on the manufacturing videos...13 in all. Interesting stuff.....

Fasttrack
05-25-2010, 01:57 AM
Yes, NF is stronger but that is not the issue that Mcgyver has. He is talking about which type of screw thread is less likely to strip.

The mechanism for stripping threads is actually more complex than it may seem.

The shear area (in regards to the material sheared during stripping) is given by the following equation:

A = 0.5*pi*L*(D - 0.64952(1/n))

Where L is the thread engagement length, D is the major diameter and n is the number of threads per inch. But that is not the whole story...



I have always heard that coarse threads are best suited for soft materials and fine for harder, more dimensionally stable materials. Softer materials have a tendency to "open up". This is because the threads can act like "cams" to push a nut or threaded hole open as it is tightened. This action is more pronounced is softer materials, so if I was threading a hole for aluminum, I would want coarse threads over fine.

There is a technical name for the expansion of the nut as it is tightened but I can't recall what it was...


EDIT: Ahh - found it. "Nut dilation". Sounds painful.

Paul Alciatore
05-25-2010, 02:14 AM
Here's a thought on stripping. NOT overall strength, just stripping.

A threaded screw that is under load, under stress will distort. It stretches in a non-uniform manner such that the first thread is under the largest load, the second thread has a little less, the third less still, etc., until at about five threads or so, any further engaged threads add only a very small percentage to the overall strength.

So look at that first thread where the stress is the greatest. It will fail first if the screw is going to strip. A fine thread will have a smaller area in the thread form just above the minor diameter than a coarse thread would. So, it would seem that, ALL ELSE BEING EQUAL, the fine thread would strip first. Now, if the first thread fails (strips) then that greatest load will be transfered to the second thread and it will now fail. And then the third. ETC. It would be a sequencial failure, one thread at a time.

From this arguement, it would appear that the coarse thread would resist stripping more than a fine thread would.

Remember I said above that this only applies to stripping, not any other failure mechanism. In my experience, a new, properly fitted screw, with enough threads engaged, will fail in some other manner, not stripping. Stripping usually only occurs if the thread is damaged or if the fit is too loose or too few threads are engaged.

oldtiffie
05-25-2010, 06:20 AM
Never know what you find until it jumps up and bites you. I was looking for the FED-Spec formula for classes of fit for diameter, pitch and length of engagement components for Unified screw threads clearances and allowances and stumbled across this excellent tutorial:

http://www.ring-plug-thread-gages.co...y-Fastenal.pdf

Has a bit of everything including stress distribution on a loaded thread. Handy stuff. Print it and put it in the looseleaf of cool info you keep in your shop library.

Forrest.

That was excellent info.

The part that I thought was potentially best was pages "Thread Fits" on pages 2 and 3 of 7 pages.

It is a simple table for tolerances, allowances and limits - just the same as those for "Limits and Fits" and "Surface Finish" - and apparently not used or just ignored or not understood.

They are a gold-mine of information and should be regarded and used as such.

There is far too much guess-work and "rules of thumb" none of which is very productive and very rarely needed - if the "Fits" tables are used - "Thread Fits" very much included.

Most - or many - taps have the "Class of Fit" (ie 1B, 2B, or 3B) imprinted on them and most cut pretty well to size and in many or most normal work the tapped hole is very close to the size and form of the relevant "B" tap which can be used as a guage to make the 1A, 2A or 3A male screw.

The limits for the screw can be obtained directly from the "Fit" tolerance tables.

Translating the sizes/figures to the job is a problem for many with any screw - male or female - but especially for the female (nut) as some will use the "3-wires" or a thread micrometer for the male (screw).

It is not difficult at all to get very close to the clearances required within the B tolerances and limits in an internal thread (nut) if/when the A (bolt) exists - similarly for the converse.

I will pursue this further if needs be.

Mcgyver
05-25-2010, 08:20 AM
From this arguement, it would appear that the coarse thread would resist stripping more than a fine thread would.

.

that's a good point, I'd been thinking of the shear as a single event rather than a sequence, probably why they say coarse is stronger in so far as stripping is concerned

demerrill
05-25-2010, 01:08 PM
that's a good point, I'd been thinking of the shear as a single event rather than a sequence, probably why they say coarse is stronger in so far as stripping is concerned

Paul's explanation agrees with the "Cautiion" paragraph on page 7 of the Fastenal *.pdf document.

David Merrill

jb-mck
05-25-2010, 03:42 PM
Yet another link..........http://www.americanfastener.com/techcharts.htm

I was doing some reading and happened across this reference.

Thomas Staubo
05-25-2010, 04:05 PM
I think I'm going to have a thread fit soon...






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