U.S. auto manufacturers are hoping to apply a high-tech
fastening technology adapted from the aerospace industry.
In the process, real-time ultrasonic control of bolt tightening uses high-frequency sound waves to determine the force needed to clamp two components together.
"We found out from field studies that even though we can apply torque consistently, it still doesn't consistently indicate clamp load," says Adrian Cockman, chair of the USCAR fastener committee and supervisor of the Ford Fastener and Joint Test Laboratory.
Material variations, thread cutting debris, oils, tool misalignment, defective parts and temperature variations can all vary torque readings and skew actual clamping loads.
Initial studies and research in ultrasonic bolt tightening at Oakland University in Rochester, MI, determined that real-time ultrasonic clamp-load measurement could cut manufacturing costs and improve vehicle assembly quality.
In 2006, the USCAR's Real-Time Ultrasonic Control of Bolt Tightening Task Force began work to leverage existing ultrasonic sensing technologies for automotive applications. "We'd heard of ultrasonic systems being used in aerospace and windmill generation, but their products are very low volume and quite a bit more expensive than our products are," Cockman says. "We needed to figure out a way to make this work for our high-volume, lower-cost applications."
The group first needed to establish accurate correlations between acoustic bolt stretch and clamp load. Once they developed and verified that data, the next phase was to determine if there was a way to mount the ultrasonic bolt sensor into a tool socket instead of having to use a custom-made bolt fitted with an ultrasonic sensor.
The group brought in experts from Battelle in Richland, WA, to help develop a device that could fit inside a typical tool socket and run at speeds up to 200 revolutions per minute. By fall 2009, a prototype sensor had been designed and delivered to Ford's fastener labs for evaluation and a prototype sensor-socket assembly was designed and delivered by summer 2010 for testing.
"Considering the complexity of the process, I was a bit skeptical if it was going to work at first, but we eventually ran 1,000 'off-the-shelf bolts' and it exceeded all expectations," Cockman says.
Of 1,000 bolts tested, 994 tightened to tolerances within four percent and once the specified clamp load was achieved, the system stopped the nut runner in less than one-quarter turn. The remaining six bolts were determined to be defective based on the ultrasonic readings.
"What we learned was there are a host of other aspects of fastening which we can now control, such as identifying defective parts and improving product designs because we can reduce the number of fasteners required to achieve consistent clamp load," he says. "That level of control just wouldn't be found in conventional torque control systems."
The group will now work with tool and fastener manufacturers to generate a demonstration-ready prototype system prior to beginning a pilot production test phase.
Founded in 1992, USCAR is the collaborative automotive technology organization for Chrysler, Ford Motor and General Motors.