Chapter Five Theory Of Operation; Ultrasonic Measurement Of Bolts; Features Of The Minimax - DAKOTA ULTRASONICS MiniMax v2.0 Operation Manual

Ultrasonic bolt tension monitor

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CHAPTER FIVE

THEORY OF OPERATION

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

very accurate results, the selection of the optimum bolt and transducer and their

coupling can be difficult. The MiniMax 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.

The Dakota Ultrasonics MiniMax, Ultrasonic Bolt Tension Monitor, defines the State

of the Art in the measurement of the actual clamp load produced by tightening a

fastener. The MiniMax 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 MiniMax minimizes operator training.

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