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 firstname.lastname@example.org.
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-
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
Increase in effective diameter = 2BC = 2t
Increase on I.S.O. Metric, UNF and UNC threads = 2t = 4t
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