Assembly - January 2009 - (Page 63) engagement with the plastic. Fasteners with a flat root can cause material to jam, which can concentrate stress in the plastic and lead to cracking. The cored root eliminates stress concentrations and provides nearly 100-percent flank engagement. The head of the Delta PT has a large bearing surface, which distributes pressure over a wide area of the boss. It also enhances joint stability by reducing creep and increasing break-loose torque. The increased minor diameter and large cross sectional area of the Delta PT boost torsional and tensile strength, so the fastener can be used in thermoset and glass filled thermoplastic materials. “You want the fastener to engage as much surface area as possible to hold the load,” says Simpson. Another option is the BosScrew from ITW Shakeproof. The threads of this screw have tiny notches on the upper surfaces. The plastic flows into these notches, mechanically locking the thread with the material and greatly increasing resistance to loosening. The notches are inactive during installation, and the screw can be removed and reinstalled. “The BosScrew is like a car parked between speed bumps,” says LeVey. “You can drive over them, but if you’re parked between them, you’re not going to move if you rock the car back and forth.” Even with a screw for plastics, engineers still need to set the right torque specification. Looking back on the problem of fastening the panels for the computer system, Souza later realized that the standard formula he used to determine the torque was more appropriate for steel than for plastic. “The formula is based on the root diameter of the screw and the point at which the shaft will elongate,” he explains. “That won’t work in plastic, because the screw never gets into the elastic range. After all, plastic is only one-tenth as strong as steel.” When fastening plastic, friction between the threads and the material is more important to maintaining clamp load than the spring-like action of a stretched shaft. “If you overtorque the The multi-angled thread profile and cored recess of the Delta PT fastener maximize engagement with the plastic. Photo courtesy Semblex Corp. www.assemblymag.com January 2009 / ASSE M B LY 63 http://www.tridak.com http://www.tridak.com/offer http://www.assemblymag.com
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