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Dakota Ultrasonics
32
Chapter 5 Theory of Operation
5.1 Ultrasonic Measurement of Bolts
Note: The terms bolt, fastener, and threaded fastener are used interchangeably.
Ultrasonic measurement has proven to be the most reliable and cost effective
solution when:
•
Variations in friction or joint geometry prevent applied torque from controlling
the actual clamping force produced by the fastener with the required
accuracy.
•
The clamping force must be monitored over the service life of the bolt.
Ultrasonic measurement of clamping load is obtained through a predictable
decrease in the sound velocity within the body of the bolt as the tensile load is
increased. By introducing a sonic pulse at one end of the bolt and accurately
measuring the time required for the echo to return from the opposite end, the
ultrasonic length is determined. As the fastener is tightened, the change in this
ultrasonic length is used to calculate and display the actual clamping force
produced.
The physics governing this process are clearly understood, and have been
employed for many years in the fields of active sonar, or radar. Send a pulse of
energy toward an object (in this case the opposite or reflecting end of the
fastener), and then measure the time between the initial pulse and the returning
echo.
While the concept is comparatively simple and ultrasonic measurement can
produce astoundingly accurate results, the selection of the optimum bolt and
transducer, and their coupling can be difficult. The MINI-MAX minimizes these
difficulties to the greatest extent possible:
•
The variable width pulser system can send the maximum amount of energy to
the ultrasonic transducer, allowing the broadest possible range of transducers
for a given application.
•
The low noise and gain features of the receiver system allow signal detection
and measurement in the most difficult applications.
•
The digital signal processor optimizes the measurement process.