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.
The process has the potential of reducing the variations in
clamping force as precise as one percent. The variation in the current system,
using torque as a measurement of clamp load, is ±30 percent.
"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
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
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
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
"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.