A piezoelectric load-sensing washer now under development by a professor and a recent graduate at the University of Alabama in Huntsville (UAH) provides what they say is a more accurate way to measure the clamping force exerted by the bolt.

Gang Wang and David Hissam show the prototype piezoelectric washer. Image credit: Michael Mercier/UAH. That may be a benefit for applications where the clamping force of bolts is critical, including construction, auto assembly and aerospace.

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 approximations of the true preload exerted because friction plays a role in the readings obtained, the researchers say.

"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% by using a torque wrench as an indicator because of the amount of friction involved," says Dr. David Hissam, a UAH mechanical and aerospace engineering doctoral graduate who works at NASA's Marshall Space Flight Center.

"That means to get 2,000 pounds of preload on each bolt, a different torque value would have to be applied to each one."

The washer Hissam is helping to develop 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 to 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," says Dr. Gang Wang, UAH assistant professor of mechanical and aerospace engineering. "Instead of a plus or minus 35% variance, we are shooting for a closer range, like 5% to 10%."

The two have filed a NASA Disclosure of Invention and New Technology and received a 2016 Marshall Space Flight Center Center Innovation Fund grant to further test and refine the washer.

Tests under way now will involve determining an appropriate low-cost material in which to embed the piezoelectric filaments, vibration and temperature testing and trials to accumulate the data necessary to provide accurate preload readings.

"Two things we are after are accuracy and to keep the costs down," says 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.