Bolt Coating Thickness _amp; Nut Overtapping- A Lesson in Practical

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					About the Author/WAYNE WALLACE


   Wayne Wallace is the president of Applied Bolting Technology Products.
The company provides bolting consulting services and manufactures direct
tension indicating washers from its base in Rockingham, Vermont. Wallace
is a member of the Research Council on Structural Connections, owner of
several patents, and author of numerous papers on the practical aspects of
quality assurance in structural bolting. He can be reached by phone at 800-
552-1999, by fax at 802-460-3104, or e-mail at wwllace@sover.net.




 Bolt Coating Thickness & Nut Overtapping-
       A Lesson in Practical Necessity
W          e recently were faced with a problem. A client
           wanted us to have about a million bolts, nuts,
flat washers and their associated Squirter DTIs coated
                                                                   ure 1) told us that for ISO (read 60 degree included
                                                                   angle) threads, you overtap eight times the coating thick-
                                                                   ness! That was a bit of a surprise, but when you look at
with a fluoropolymer called Xylan® applied over a zinc             the geometry of the thread interface, it’s correct, of
phosphate treatment. We knew it’s done all the time for            course.
small quantities of studs for the offshore oil industry,               Eight times 20 microns is 160 microns. Practically
pipe flange bolting, that kind of thing — but a million            speaking, overtap thread dies come in 200 micron in-
bolts!                                                             crements (0.2mm and 0.4mm), so we opted for the
    How to get it done, what would be the cost, how                former. Then the question arose, how are we going to
long would it take, what overtap to specify if any, and,           be sure the bolt/nut strength is not impaired? After all,
above all, what happens to the strength of the bolt/nut            we didn’t want to measure coating thickness, and we
assembly when the nuts are overtapped to suit the                  are simply not capable of measuring actual overtap
coating thickness?                                                 by go/no-go or indicating gages. The bolts and nuts
    First, we started with Whitford Corporation, the U.S.          had been coated by a dip/spin process, for reasons of
maker of Xylan, and they told us how to do it, where to            economy, but the finished product looked good (See
get it done, and whether an overtap would be required for          Figure 2). We thought of the “rotational capacity” test
a 16 to 20 micron (a micron is a millionth of a meter) thick       in A325, which doesn’t measure bolt tension, and we
coating. Whitford indicated an overtap would probably NOT          looked at the similar test in AASHTO but it also does
be needed. This was a key recommendation, because at               not mandate a minimum achieved tension. Conse-
the time we had already procured 200,000 A563M nuts,               quently, we were surprised to learn that there is no
without overtap, assuming the bolting on this project would        accepted test for the minimum strength of overtapped
be uncoated. And, we knew if nuts are not overtapped,              assemblies in place in North America.
there cannot be any degradation of bolt strength due to                So, we invented one. It seemed to us that if you put
poor nut fit, stripping, galling, all the really bad things that   the bolt, the nut, the flat washer, and the Squirter DTI
can happen with overtapped nuts get used.                          on a Skidmore bolt tension calibrator, and tighten it
    Maybe their recommendation was based on the kind               until the strength will not increase any more, the as-
of individual cleaning, pre-treatment and spray coating            sembly should demonstrate at least 90% of minimum
typically done on oil platform studs, but the first thing          ultimate strength of the bolt. Why 90%? Why not 100%?
we found, after (for cost reasons) dip/spinning the coat-          Why not 80%? We theorized that 90% was sufficiently
ing on, the nuts would not assemble. Some would, most              above the intended installation tension (70-80%) to
wouldn’t, and it’s true, if you could get them started, a          assure us that, with correctly manufactured bolt and
wrench could make them run up the bolt threads. But                nut components, the overtap was not causing a sig-
we could not ask a steel erector driving a million bolts           nificant reduction in bolt strength.
to do this. We’d be drummed off the site.                              And, in short, after testing a few hundred lots of
    So, back to the drawing board. Scramble for                    bolts and nuts with the 0.2mm overtap on the nuts,
overtapped nuts, but how much overtap?                             and coated with what we were told was 16-20 microns
    We learned that bolt threads are bolt threads, and             of Xylan, only two lots failed to deliver 90% of ulti-
are never (on this side of the Atlantic) made under-               mate, and in fact most demonstrated a tension greater
sized. All coating space is gathered by making the                 than 100% of specified minimum. The two lots that did
internal thread on the nut a bit bigger, called “diametral         fail were caught in time and the nuts segregated, and
overtap.” Quick reference to a British publication (Fig-           subsequent investigation showed they were probably not
overtapped at all, so the reduction in assembly strength            strength structural bolting assemblies for preloading
likely arose from extremely high torque build-up.                   — Part 2: Suitability test for preloading,” and guess
    Now, after doing this work, and after beginning to              what? It includes, among other criteria, the 90% test
believe that the 90% level was really an important as-              of the assembly (see Figure 3).
sembly criterion, we find that there is a new British                  And here we knew it all the time. Go figure.
Standard specification BS/EN 14399-2:2005 “High-



            Threaded Work
            Although Sheradizing gives a uniform coating without any significant changes in the profile of threads, there
            must be adequate clearance between external and internal threads before Sherardizing to allow for the coating
            and to avoid interference. The method of calculating the theoretical clearance is shown below. A table of
            clearances required for several coating thicknesses on different thread forms is also given.


                            PROTECTIVE COATING t




            The relationship between coating thickness and increase in effective diameter of an external thread is
            shown by the triangle ABC where AB is the coating thickness ‘t’ and BC is half the increase in effective
            diameter.

            Increase in effective diameter = 2BC = 2t
                                                 sin x
                                                     2

            Increase on I.S.O. Metric, UNF and UNC threads = 2t = 4t
                                                         sin 60°
                                                              2

            Increase on BSW and BSF threads = 2t = 4.33t
            (angle x = 55°)               sin 55°
                                               2                                                Figure 1




                          Figure 2                                                              Figure 3

                                     Reprinted from Distributor’s Link Magazine, Spring 2006

				
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