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Distributor's Link Magazine Winter Issue 2016 / Vol 39 No1

30 THE DISTRIBUTOR’S

30 THE DISTRIBUTOR’S LINK UNIVERSITY OF ALABAMA IN HUNTSVILLE 301 Sparkman Dr NW, Huntsville, AL 35899 TEL 610-832-9500 NEW WASHER CAN ACCURATELY MEASURE A BOLT’S CLAMPING FORCE by Jim Steele, Research Writer/Editor A piezoelectric load-sensing washer being developed by a professor and a recent graduate at The University of Alabama in Huntsville (UAH) provides a more accurate way to measure the clamping force exerted by the bolt it is on. That’s a benefit for numerous industries where the clamping force of bolts is critical, including construction, auto assembly, nuclear power, racing, aviation and space. “Any performance application will have a need for this,” says Dr. Gang Wang, assistant professor of mechanical and aerospace engineering who is testing the new washer system with Dr. David (Andy) Hissam, a UAH mechanical and aerospace engineering doctoral graduate who works at NASA’s Marshall Space Flight Center (MSFC) and is the team lead for the Propulsion Detailed Design Branch (ER34). A bolt’s clamping force, called preload, is generated as the bolt stretches during tightening. The two most common methods of applying preload are torque control and turn control. In torque control, a specified torque is placed on the fastener, typically with a torque wrench. In turn control, the nut is turned through a specified angle that stretches the bolt the desired amount. Both methods are only approximations of the true preload exerted because friction plays a major role in the readings obtained. “When you use a torque wrench, you are measuring torque, not the preload,” says Dr. Hissam. “You can be off the bolt’s specified preload by plus or minus 35 percent by using a torque wrench as an indicator because of the amount of friction involved, which is very difficult to quantify,” says Dr. Hissam. DR. GANG WANG, LEFT, AND DR. DAVID (ANDY) HISSAM WITH A PROTOTYPE PIEZOELECTRIC WASHER IN DR. WANG’S HUNTSVILLE ADAPTIVE STRUCTURES LABORATORY. PHOTO COURTESY OF MICHAEL MERCIER/UAH “That means,” says Dr. Hissam, indicating a drawing of a flange, “that to get 2,000 pounds of preload on each bolt, a different torque value would have to be applied to each one.” The washer the pair is developing uses piezoelectric filaments connected to a handheld device that registers the preload value by reading the electrical output of the filaments. Piezoelectric materials have the capacity generate a voltage when they are subjected to mechanical stress. “When you tighten it up, this directly indicates how much force this washer is experiencing, which is directly related to the bolt’s preload,” say Dr. Wang. “Instead of a plus or minus 35 percent variance, we are shooting for a closer range, like 5 percent to 10 percent.” The two have filed a NASA Disclosure of Invention and New Technology and have received a 2016 MSFC Center Innovation Fund grant to further test and refine the washer. “We are continuing to look for additional sources of funding in order to further develop it,” Dr. Wang says. TECHNICAL ARTICLE CONTINUED ON PAGE 124

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