Custom screw threads strengthen sheet metal joints

Bensenville, IL-What do expensive automobiles and prison cells have in common? No, this isn't a lawyer joke. Both of these applications, and others, have recently made use of a patented sheet-metal fastening system designed to boost joint strength and slash assembly costs.

The High Torque Fastener system from Quality Screw and Nut Co. (QSN) consists of a custom-designed screw whose threads correspond to a helix stamped into the base panel. According to John Souza, the company's applications engineering manager, the resulting joint is typically between two and three times as strong as joints fastened with similarly sized sheet metal screws. In one torque-out test, for example, Souza found that a #8 sheet metal screw failed at an average of 12 in-lbs, while the High Torque system held out to 32 in-lb in 0.036-inch panels with a drilled 0.134-inch hole.

The reason for the added strength boils down to the interface of the base-panel material and the screw. As Souza explains, the High Torque's buttress-type threads match the angle of the stamped helix in a way that allows the threads to capture the outside edge of the base panel material. That is, the panel's edge ends up normal to a flat, angled surface at the base of the screw threads. "This allows the joint to use 100% of the panel material's total strength," Souza says. What's more, it directs compressive loads toward the center of the screw. "As the screw is tightened, the formed sheet metal is compressed toward the center of the screw," he says. "You have to overcome that load before the screw can strip out," he adds.

A few other fastening methods also create a helical receptacle for the screw-whether in the base panel or on a clip. "The problem with these is that they don't change the thread form," Souza says, pointing out that the square internal edge of the panel inherently has a reduced contact with v-shaped threads of an ordinary machine screw. As for self-tapping screws, these tend to pull material up into the clearance hole. "So again, you have less material available to form a strong joint," Souza says.

Aside from corresponding to the screw threads to increase contact area, the helix performs other roles, too. For one, it moves the thread-contact area out approximately 0.090-inch. "That eliminates the problem of thread run out, allowing for the assembly of very thin materials," Souza says, citing 0.010 inch as the thinnest tried so far. The helix also serves as a lock washer.

If the High-Torque System has a downside, it can be found in the manufacturing process. Creating the helix requires a two-step process. A piercing operation creates a hole and dimple, while a second stamp adds the helix. But in return for the extra stamping step, the system can do away with all sorts of parts. "It completely eliminates the need for weld nuts, inserts, clips, and tapping operations," says Souza. Installable by hand, the High Torque screws also do away with the comparatively high drive torque needed by thread-forming screws, he adds. Tallying up all savings from component elimination and easy installation, Souza estimates that the system can cut fastener-related assembly costs by 30 - 50%.

And it looks like those savings have started to attract some users. Since it was developed in the United Kingdom more than five years ago, the High Torque system has been used by Jaguar and Land Rover. And since QSM began selling the system in North America, it has chalked up applications in Blodgett commercial ovens and in the rugged prison-cell fixtures made by Acorn Engineering. The latter company has even started investigating a way to laser cut the keyhole profile to further drive down costs, Souza says.

The system targets sheet metal thicknesses from 0.020 to 0.105 inches and covers thread sizes from 2.5 mm (#4) to 16 mm (5/16).

For more information about fasteners from Quality Screw & Nut: Enter 533