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Fasteners: Common
Myths, Misconceptions, and Errors. The technical library index is to the left in
the sidebar.
The heads are breaking: We hate this
phone call more than any other call. In every single case
over the past fifty plus years, it's never been the
screw. Ever. Not
once. Screws have recommended installation torques. Each
screw is different, but it's usually around 1000 to 2000 RPM. When
you go faster than that, the head will sometimes break. That is not
a fastener failure -- that is a user installation failure. "But
I've always done it that way," is a common response. For many
years asbestos was used in ceilings because they've "always done
it that way" --
that doesn't mean it was a good idea, and the many asbestos
lawsuits are a testament to that fact.
It's always worked that way/or/ I've always
done it this way: This one causes all sorts of difficulties.
Just because it's always worked, doesn't mean it always will. People
do things wrong for many years and get away with it. Then you get a
piece of metal that's on the high side of a tolerance and a screw that's
on the bottom side and it just doesn't work. That doesn't mean something's
wrong. And many times when something is wrong, it's an application
error. If a part is not designed to work in a particular way, one day
it might not work. We had a customer who was using a blind rivet to
fasten a grommet to a metal plate. The wall of the grommet was very
thin -- outside of specification the rivet was designed for. It worked
for many, many years without a hitch. One day the rivets stopped working.
Turns out the new batch of rivets were on the high side of the specification
and just wouldn't hold the grommet in place -- the difference was 3
microns (.003 of an inch). It happens regularly. Use the right fastener
for the application.
Self-Drilling versus Self-Tapping: These
terms are commonly confused. A self-drilling screw has a drill bit
style point. They are often called Tek® screws,
which is a brand name and not to be used to refer
to generic screws. A self-tapping screw has a sharp point normally
and is a type "A" or "AB" tapping
screw -- though there are other variations, such as "B" point
which are blunt. Confusing self-drilling and self-tapping screws is
the single most common terminology error we run into daily. Self-Drilling
screws are for metal only and do not work in wood (except for
specially designed parts with reaming wings). Self-tapping screws work
well in most materials, but are likely to need a pilot hole in many
applications.
Stainless Steel Doesn't
Rust: That's a commonly
believed statement, but it's wrong. Almost all alloys of stainless can eventually
rust because they contain traces of carbon. Any metal that has traces of
carbon can rust. 302, 304, 18-8, and 316 stainless do rust but it takes
a long time. 410 stainless rusts as quickly as steel, but since
it's generally surface rust, the integrity of the screw is
not usually compromised. Remember, it's called "stain less steel"
and not never stains....
Stainless Steel is not magnetic: In
stainless fasteners, 300 series stainless is non-magnetic in its raw
condition. Cold working it (making the parts) can sometimes cause
traces of magnetism in 300 series, depending on various factors, especially
in washers. An increase in magnetism is caused by the heat and
friction of cold forming and does not reduce corrosion resistance.
A higher portion of nickel can increase stability in stainless, thus decreasing
work hardening and any possibilities of magnetism. On the other hand, 400
series stainless is magnetic because of its high carbon content. This
article answers
the question very well. (For the record 18-8 is a family of stainless
steels and includes all alloys from 302 through 305 at the factory's option.
Different alloys are used for different types of screws. If you want to
know, just ask!)
316 stainless is stronger
than 304 stainless: That just isn't true. 316 stainless
may be slightly stronger in many applications using
larger diameter bolts, but 316 is not stronger in any statistically
significant way on most standard sized screws. 316 stainless
does, in most but not all applications, exhibit much
better anti-corrosive properties but that's not a strength issue;
see our Technical
Library for more on this.
Trademark Misuse: This happens
often. People ask for a Tek® when they really want a self-drilling
screw. Or they ask for a Tapcon® when they want a Masonry Concrete
Screw. This happens with many brand names (Tampin®, Red Head®,
etc) but it can be dangerous. Sometimes these brands have significant
differences. It may be hard to believe, but sometimes a fastener costs
more for a good reason. When you ask for a particular brand you are implying
that you want anchors that meet specific technical specifications and
have the proper approvals of the brand name products. More importantly
it's illegal to use a trademark to refer to
a generic item. It's no different than asking for a Coke® and
getting a Pepsi® -- any company that provides
you with a brand other than what you ask for is legally required to tell
you they are doing so: otherwise it's a simple bait-and-switch.
Maybe the substitute you get is as good or better, but maybe it's not:
you just don't know. If you want a Tampin® brand machine
screw anchor, insist upon it. If you think you've been hoodwinked,
asked your vendor for proof or contact the owner of the trademark. (A
good sign of trouble is when the quote, invoice, packing slip, and/or packaging
don't match what you asked for.)
Sheet Metal Screws / Wood Screws / Drywall
Screws / Particle Board Screws / Deck Screws: Few
people know the difference. Here's a short summary. As with all rules,
there may be exceptions.
A sheet metal screw is hardened, fully threaded to at least
3 inches, and has a straight shank. They usually have single lead symmetrical
threads, though double lead threads are common in some industries. The
thread pitch is finer than a wood screw. Most people now use sheet metal
screws instead of wood screws -- this is the most common kind of screw
used in wood, despite the misleading name.
A wood screw is soft, fully threaded when under one inch in length
and threaded approximately 2/3 of
the screw length when over one inch in length, and has a tapered shank.
They always have single-lead coarse symmetrical threads.
A particle board screw is hard, fully threaded when under one inch
in length and threaded approximately 2/3 of
the screw length when over one inch in length, and
has a tapered shank. They have asymmetrical threads and are often waxed
or have some form of lubricating agent applied.
A drywall screw is hard, fully threaded when under one inch in length
and threaded approximately 2/3 of
the screw length when over one inch in length, and
has a almost straight shank. The threads rise well up above the shank
of the screw and they all have a bugle or flat head. If doesn't have
a bugle or flat head, it's not a drywall screw no matter what anyone
tells you. It may be used in drywall but that doesn't make it a drywall
screw. Saying something over and over doesn't make it true.
A decking screw is similar to a particle board screw but is generally
prepared with superior corrosion resistance.
Coated versus Uncoated Screws:A
lot of people love to say "oh, the grey screws don't work" or some
variant thereof filling in whatever color may be appropriate for their complaint.
Generally the color of a screw cannot affect its performance in a negative
way. Sometimes the color coatings add a bit of lubricity but they don't cause
fastener failure.
Nylon Insert Lock Nuts Making Bolts Break: We
often get complaints that the nylon-insert lock nuts don't go all the way on.
They're not supposed to. They're supposed to lock after one or two threads
clear the locking ring. That's how they work. This is not a defect in the product.
You can force them but they'll break or cause the screw to break. If you insist
they go all the way, applying dry wax to the bolt will work. Or they make special
waxed nylon insert lock nuts which are substantially more money. You may
wish to consider a k-lock nut or serrated flange lock nut instead which will
go all the way on.
What screws are RoHS compliant? See our news
page for
full details, but a quick summary is: 300 series and 400 series stainless are
RoHS compliant if uncoated and unplated (our part numbers ending in S, 3, and
4) because in most alloys of stainless steel, chromium is in the metallic state,
which is not hazardous. The same goes for other alloys (parts ending in B,
R, and A) The chromium in the oxide layer of passivated stainless steel,
is dichromium trioxide, which is a trivalent compound. The chromium banned
by RoHS is hexavalent chromium. Zinc and HDG parts are generally not RoHS
compliant unless the label indicates "RoHS compliant" or "Trivalent" or "Cr3"
on it (our part numbers ending in Z, Y, G, Q, H, T, or X.) The actual regulation
reads "..if
stainless steel bolts and nuts are used as parts, they are not considered
to meet the requirement of products specified by Article 5 of the enforcement
regulations (ppIII-268). SO, establishments that purchase such parts and
handle them as components of manufactured products do not have to make a notification.
Regarding the MSDS, if bolts and nuts are used as components by an establishment
and they are not processed through fusion or the like (ie welding), they
do not meet the requirements of the products, thus MSDS need not be submitted."
There are other topics of interest discussed on our news
page. Look for items
marked Informational Bulletin.
What is the difference between a bolt and a screw?
What is the difference between a bolt and a screw? This is a surprisingly common
question with a complex answer. There are two three-step critera to go through.
The first will tell you if it qualifies as a bolt or screw and the second set
tells you which one it is.
First and foremost, you cannot tell by the
thread length*. The many websites and reference books that make this claim
are all wrong
and they shouldn't be trusted if they can't get something this basic correct.
Secondly, it must be externally threaded and it must have a drive
mechanism -- either a head or a drive. So a fully threaded rod (or stud) is
neither a bolt nor a screw because it has no head and no drive mechanism. The
commonly used term "stud bolt" is erroneous. A stud is not a bolt.
Thirdly, it must have continuous threads that circumnavigate the shaft of
the part at an angle to the appropriate length* -- the tips of some screws
have notches and cuts and special points but that doesn't affect the determination.
If it doesn't have external threads, it's neither a bolt nor a screw. Some
items that are threaded both inside and outside, such as bushings, are not
screws or bolts. If it's got internal threads, it's not a bolt or screw.
Once you've gone through the first three steps, then you know it's a bolt or
a screw and not some other fastener.
If you need to put it in with a wrench or some other non-drive tool, it's
probably, but not always a bolt.
If it can't take a nut -- or some other matching thread or tapped mating
hole -- then it's a screw. Period. Without exception. (And, before you suggest
Lag Bolt, those are properly called Lag Screws though nobody actually
calls them that.)
If it has a drive (slot, Phillips®, square, Torx®, Allen/Socket/Hex, etc)
then it's a usually screw. (Hex/Square Cap Screws are really Hex/Square
Machine Bolts with a chamfered washer face, though almost nobody knows the
difference, even those in the industry. But a knowledgeable person can tell
by looking. Sidewalk Bolts are really screws, but like Lag Bolts, the proper
name is not commonly known.)
Those two three-step criteria are all you really need to make the determination.
Are there exceptions? Not many, but some parts can be very confusing. For
example, the parts listed below are the only parts that might cause confusion.
Sometimes you have to accept what the market calls a product even if it's completely
wrong.
Anchor Bolts (J Bolts, L Bolts, U Bolts, Stud Bolts) are not bolts.
These are really just bent and/or cut threaded rods. Remember, for something
to be a bolt or screw it must have a drive mechanism. (Incorrect Terminology)
Hanger Bolts are dual use
parts, but they are classified as bolts because it takes a nut and has no drive.
(Correct Terminology)
Toggle
Bolts look like screws, but really are anchors and don't qualify as a
true bolt or screw. Wedge Bolts® which is a
trademark of Powers Fasteners is also an anchor and is not a bolt. (Incorrect
Terminology)
Shoulder Bolts are really screws, but they are not called screws and
they are part of the socket family. (Incorrect Terminology)
Square Head Set Screws are really
bolts, but are not called bolts because they are part of the socket family
but since they have no drive, they are bolts. (Incorrect Terminology)
U-Drive screws qualify as screws on
a
technicality, but they're really friction fasteners -- closer to a rivet or
nail -- and are not a true screw as they are installed with a hammer. (Incorrect
Terminology)
*
Current IFI standards dictate a standard bolt is threaded at minimum twice
the diameter plus a 1/4" for all bolts 1" in diameter and smaller and
6" long
and shorter. So, a 1" diameter machine bolt that is 2" long is fully
threaded and 5"
one is not. It's still a bolt. A screw for wood is threaded 2/3" of the
length with a 1" minimum. A screw for metal is fully threaded up to 3" minimum.
Bolts designated A325, A490, and such follow ASTM specifications for thread
length and not IFI. Metric standards follow the DIN book for thread length.