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Did I do that? Discover how a screw really works

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.