SOUTHFIELD, Mich., April 29, 2011 – A manufacturing research and development group of the United States Council for Automotive Research LLC (USCAR), the collaborative automotive technology research organization of Chrysler Group LLC, Ford Motor Company and General Motors, recently made breakthroughs to adapt highly repeatable aerospace bolt-tightening technology for use in automotive manufacturing.
The process, known as real-time ultrasonic control of bolt tightening, uses high frequency sound waves to estimate the amount of force a bolt provides to clamp two components together. While tightening, bolt stretch and increased stress increase the time of an acoustic wave to travel the length of the bolt and return by reflection from the bolt end. Several hundred measurements per second permit feedback control to achieve consistent clamp force.
The process has the potential of reducing the variations in clamping force from plus or minus 30 percent, through using torque as a measurement of clamp load, to as precise as one percent using the ultrasonic technique.
“We found out from field studies that even though we can apply torque consistently, it still doesn’t consistently indicate clamp load,” said 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, Mich., determined the benefits of real-time ultrasonic clamp-load measurement could result in improved product quality at reduced manufacturing costs, while also reducing warranty costs and improving customer satisfaction.
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 said. “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 – which was key, if ultrasonic technology was to be affordable for mass production.
The group brought in experts from Battelle in Richland, Wash., to help develop a couplant 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 said.
Of the 1,000 bolts, 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. Cockman also tested it at speeds up to 550 revolutions per minute, exceeding the original 200 rpm goal.
“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 said. “That level of control just wouldn’t be found in conventional torque control systems.”
The group plans on working with tool and fastener manufacturers to generate a demonstration-ready prototype system and to validate it prior to beginning a pilot production test phase.
“We hope to continue collaborating to get this closer to reality,” Cockman added. “There are a number of safeguards and development to do before we get this on a factory floor, however, we will continue to work collaboratively to see that happen and to save costs on development.”
Founded in 1992, USCAR is the collaborative automotive technology organization for Chrysler Group LLC, Ford Motor Company and General Motors. The goal of USCAR is to further strengthen the technology base of the domestic auto industry through cooperative research and development. For more information, visit USCAR’s wesite at www.uscar.org.