Stamp it out: Textron Fastening Systems
already performs its sheet-metal-extrusion process on specialized presses
shown here in Germany, producint structural stamping with built-in
fastening locations for the likes of Mercedes-Benz. Textron plans to make
a similar system available in North America too.
Rockford, IL—If the thought of joint failure drives you crazy, try getting rid of the nuts. That's the approach taken by Textron Fastening Systems (www.textronfasteningsystems.com). The company has developed a metal forming method that does away with those failures related to weld nuts and similar hardware by integrating fastener attachments directly into structural stampings. Used by European automakers for the past seven years and soon to be made available here, the technology targets high-stress, safety-critical joints. "It's for the kind of joints that engineers tend to describe with the words 'catastrophic' and 'failure,'" says Tim McGuire, Textron's applications engineering director.
Based on a proprietary process that combines stamping, deep drawing, and cold forming in a custom 2,000-ton mechanical press, Textron's metal working technology at first glance seems to involve simply adding bolt holes into stampings. But the technology features an important design twist: It can selectively thicken the stamping's base material, a capability Textron uses to beef up the sidewalls of the bolt locations. According to McGuire, these sidewalls can be made twice as thick as the nominal thickness of the base material. A traditional punching operation, by contrast, would stretch the sidewalls, thinning them out and weakening them. "For light load applications this thinning isn't necessarily bad, but it causes problems in high-load joints," he says.
By beefing up the wall sections surrounding the bolts, Textron's system promotes robustness. "The bolt will fail before the parent material does," McGuire notes. Equally important, though, the system makes the joints more amenable to failure analysis during the design stage. McGuire points out that the number of failure modes decreases dramatically as you take fastening components out of the equation. "We've eliminated a host of failure modes associated with the parent material, nuts, and brackets," he says. "Assuming a proper pre-load, the bolt's tensile strength becomes the sole determination of joint strength."
Joints in motion
Over the past five years, this ability to create strong, predictable joints has pushed the technology into automotive applications in Europe. "Mercedes Benz has taken the lead, but BMW, Volkswagen, and Porsche have used it too," reports Scott Nelson, Textron Fastening's director of product management. These automakers have used it for a variety of joints in which safety concerns—or warrantee costs—loom large. These joints include mountings for drive train and suspension components as well as seatbelt anchors.
Safety aside, Textron's metal-forming system offers some secondary benefits too. It avoids heat-treating and the potential embrittlement problems it entails. With its extra wall stock at bolting locations, it also produces parts that can be re-worked in the event of a tapping or bolting mishap. "If you were to cross thread a bolt, you could just tap in the next size," notes Tim Thompson, product manager for the extrusion system.
For all its potential benefits, Textron's metal forming technology doesn't easily drop into existing applications. Because it requires that stampings and fasteners be designed in tandem, as well as requiring an investment in new tooling, the method works best for new projects and ones where production volumes can justify the capital investment. "It's not really a retro-fit technology," says Thompson.