New Washer can Accurately Measure a Bolt\'s Clamping Force

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, works at NASA's Marshall Space Flight Center (MSFC).

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. 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.

"Two things we are after are accuracy and to keep the costs down," says Dr. Wang. "We want to test so that we can be sure that when a technician tightens a bolt he gets that specific value he is looking for every time."

Because a piezoelectric load cell is ceramic and fragile, the tests will also provide information about optimizing washer design to avoid breaking the sensing elements.

"We are also looking at what type of material we should use to surround the piezo material that will protect it best," he says. "That way we can be sure the piezo element will provide an accurate reading and not break."

If the elements can be preserved, the washer could be used many times.