Assembly - January 2009 - (Page 58) ASSEMBLY Screwdriving Keeping Screws Tight Preventing screws from loosening requires the right combination of thread design, torque specification and fastening tool. T When fastening plastic, friction between the threads and the material is much more important to maintaining clamp load than the spring-like action of a stretched shaft. Photo courtesy Chicago Pneumatic By John Sprovieri Editor he worst Thanksgiving in John Souza’s memory happened more than 20 years ago, back when he was an engineer for the now-defunct Digital Equipment Corp. in Maynard, MA. Days earlier, the company completed assembly of a multimilliondollar computer system, loaded it into a semitrailer, and sent it out to a Fortune 100 customer in Southern California. The truck arrived the day before Thanksgiving, but when the trailer doors were opened, the customer received an unwelcome sight: The plastic panels that enclosed the electronics had fallen off in transit! It was not exactly what they wanted to see after spending millions on state-of-the-art technology. Holiday or no holiday, Digital quickly dispatched Souza to California to reassure the customer and learn what went wrong. “I couldn’t understand it,” recalls Souza, now a manufacturing consultant for Quality Screw & Nut (Wood Dale, IL). “The panels were fastened with #10 screws. The standard formula called for 27 in-lb of torque, and that’s what they were tightened to. “However, I didn’t consider how hot it would get in that trailer. To get from Massachusetts to Southern California, the truck had to pass through Texas, New Mexico and Arizona. It was 120 F inside that trailer! It was so hot that the plastic migrated away from the fasteners. All those screws that were nice and tight became loose. Once we tightened them back up, there was never a problem, but then the system was in a climate-controlled environment.” Souza’s story is not uncommon. You develop the perfect torque specification. You invest in high-tech tools to verify every fastener is tightened to that specification. You monitor statistics to ensure your process stays on track. And yet, sometimes, assemblies still come back with loose fasteners. Often, the culprit is some environmental factor, such as thermal cycling or vibration, that engineers simply didn’t account for when designing the product or the process. To keep fasteners from loosening, engineers need the right screw, the right torque specification and the right tool. Lock washers, threadlockers and other additions to the joint are effective in preventing loosening, experts say. However, they also add cost and time to the assembly process, and most can only be used once. Often, such extras can be avoided by choosing a more appropriate screw. “There’s no simple answer to the problem of loosening,” notes Ken LeVey, director of global product development for ITW Shakeproof (Broadview, IL). “You have to consider the joint, the internal thread, external thread, grip length, coatings, fastener spacing, serviceability and the environmental conditions.” 58 ASSEMBLY / January 2009 www.assemblymag.com http://www.assemblymag.com
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