Dakota MAX II, Operation Manual

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CHAPTER FIVE
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
MAX II 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.
5.2 Features of the MAX II
The Dakota Ultrasonics MAX II , Ultrasonic Bolt Tension Monitor, defines the State of
the Art in the measurement of the actual clamp load produced by tightening a
fastener. The
MAX II can measure time, elongation, load, stress, or %strain in bolts
of virtually any material from 1 inch to 100 feet in length. By storing the reference
waveform and displaying it for comparison while the elongation is being measured,
the
MAX II minimizes operator training.