Did I do that? Discover how a screw really works
Joe Stager,
Posted
05/01/2009
We've all seen it
before: a failed roof, a sagging wall or dipping trim caused by
poor installation or a component failure. Several factors can
contribute to such system failures, and care should be taken to
ensure each component is installed properly as recommended by the
suppliers.
When it comes to screws in particular, failures are often a result
of poor installation techniques or loosening due to cyclic impact
and service loads of the connection.
As a contractor, improper installation and screw failures fall on
your shoulders. By learning more about how screws work, reasons why
they loosen and more failsafe installation methods, you can
minimize screw back-out.
Sit Tight
A tapping screw stays tight for two basic reasons: clamping forces
and friction.
Clamping forces are generated by the thread pulling the washer face
against the work surface, keeping the connection tight. These
clamping forces are relatively low in thin metal connections.
Friction is generated by the threads and the head contacting the
steel. The more friction that can be generated, the more the screw
will be resistant to back-out.
A Loose Screw
Imagine for a minute that the angle of the screw's thread is a
ramp. As with any ramp, an object tends to slide down its surface.
This is the reason a screw loosens. If the forces exerted on the
thread are greater than the clamping force or friction, the screw
loosens.
In metal roofing, siding and trim applications, there are many
loads exerted on the joint. These loads can be caused by vibration,
prying, expansion and contraction, each of which causes a screw to
loosen. When screws are installed incorrectly, failure can occur at
a faster rate. This is why tooling and installation techniques play
a vital role in the performance of the connection.
Installation
Tips
Fastener installation is a critical part of ensuring that the roof
or wall panel performs properly. Too often, incorrect tooling and
poor installation techniques create an unstable or leaky
system.
Accessories It is not uncommon to hear about a "fastener problem"
that is actually caused by the use of worn-out or low-quality
accessories. Frequent problems include:
• Slow and inconsistent drill times
• Screws wobbling during installation
• Cam-out/stripping of the hex or recess
• Damage to painted heads
Most of these problems are easily and inexpensively solved by
switching to higher-quality accessories and utilizing the following
tips:
• Start every job with new drive sockets and recess bits.
• Replace worn-out accessories during installation.
• Be sure the hex of the screw is completely encapsulated by the
drive socket by "setting the magnet" depth before its use.
• Remove any metal chips that may have found their way into the
drive socket. An improper fitting hex can make the screw
wobble.
Power Tools
The correct selection and use of power tools is extremely important
to how well the fastener performs. If a tool is misused, it can
affect how well the fastener secures the components.
Take, for example, a screw-gun used to install self-drilling
screws. When selected and used correctly, self-drilling screws
install more consistently and the chance for the screw to loosen
due to improper installation is minimized.
The following are a few tips for selecting and using a screw-gun
for installing self-drilling screws:
• When installing screws in steel that is 0.125 inches (3 mm) or
less, use a screwgun with 2,500-rpm maximum speed. Utilizing a few
basic tooling techniques will greatly improve drilling consistency
and productivity while increasing the likelihood that the screw
will be tightened correctly. and some reasons why it loosens
• When installing screws into steel thicker than 0.125 inches, use
a screw-gun with 2,000-rpm maximum speed.
• Holding the screw-gun perpendicular to the work surface and
applying steady pressure will allow the drill point to cut faster
and more consistently.
• Using a depth-sensing or torque-control screw-gun will help you
install screws to the correct torque level, minimizing the chance
for leaks caused by screws that are too tight or loose.
Utilizing a few basic tooling techniques will greatly improve
drilling consistency and productivity while increasing the
likelihood that the screw will be tightened correctly.
New Thread and Testing
To provide extra insurance for protecting a screw from back-out, a
new patent-pending thread was developed. This thread, referred to
as Vibration Resistant Thread Technology, increases back-out torque
and reduces the potential for failed joints and leaks. The main
feature found on VRT is its proprietary shape, which is designed to
increase back-out resistance. As a screw tries to loosen and
rotate, VRT contacts the panel, thereby increasing friction and
resistance to back-out.
To determine the effects of VRT compared to a standard thread, an
independent laboratory was contracted to conduct torque angle
tests, which help quantify the back-out resistance. This type of
test is used extensively for analyzing bolted joints. The test was
conducted by RS Technologies Ltd., Farmington Hills, Mich.
The test setup consisted of two pieces of 24-gauge steel metal
roofing material with butyl tape between the laps. Sample screws of
VRT and standard threads were installed in typical lap conditions
and tightened to 40 inch-pounds (0.5 kg-m). The objective of the
test was to record the torque on a screw as it loosened. The
machine recorded the loosening torque for 360-degree rotation of
the screw. These tests confirmed that the new VRT thread produces
more than three-and a-half times additional resistance to back-out
than a standard thread.
Decrease Problems
Any actions that can be taken to help minimize back-out should be
evaluated and considered. To decrease potential problems, use
proper fasteners, evaluate new products and follow installation
procedures.
Joe Stager is director of corporate marketing and product
development for Triangle Fastener Corp., Pittsburgh. Details can be
found at www.trianglefastener.com.
www.trianglefastener.com