Assembly - January 2009 - (Page 60) Screwdriving The threads of the BosScrew have tiny notches on the upper surfaces. These notches grab the plastic before it can slide down the helix. Photo courtesy ITW Shakeproof Ideally, engineers would ferret out loosening problems at the design stage through torque testing and environmental stress testing. But, few manufacturers devote the resources to such work, unless the assembly is safety-critical. “Generally, engineers don’t worry about the fasteners until the very end,” admits LeVey. “Sure, testing will identify potential problems, but it’s expensive and it takes time. The shortest path is to consult people who have years of experience with fasteners and to do it up-front—use your suppliers.” Fastening in Metal Vibration loosens fasteners by creating relative motion between the threads. “Imagine you’re sitting on a sandpaper slide with a weight on your shoulders,” explains LeVey. “Friction and the weight will keep you from moving. But, if you bounce, you will slowly move down the slide.” A screw is little more than a sandpaper slide wrapped around a metal shaft. Thermal cycling poses two additional mechanisms for loosening. One is through annealing—heating and cooling the fastener to the extent that the strength it received from heat-treatment diminishes. The other is through expansion and contraction, which, like vibration, causes relative motion between the threads. Loosening due to annealing can be prevented by choosing the right grade of fastener to match the expected temperatures. Loosening from vibration, expansion and contraction can often be prevented just by ensuring the right amount of clamp load is applied to the joint. When tightening a screw, the goal is to stretch the fastener to its yield point, without going over, says LeVey. In other cases, engineers need a fastener that will maintain clamp load in spite of vibration and thermal cycling. One such fastener is the Sinuloc from ITW Shakeproof. In most threaded fastener assemblies, space exists between the screw threads and the mating threads. When subjected 60 ASSEMBLY / January 2009 www.assemblymag.com http://www.ODU-USA.com http://www.assemblymag.com
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.