In certain critical bolting applications, factors such as friction or bolt/joint geometry are such that the measurement of applied torque alone will not provide the degree of accuracy necessary to control the preload to be monitored over the service life of the fastener. In these applications, ultrasonic measurement of fastener elongation and/or load has frequently proven to be the most accurate, reliable and cost effective solution.
Ultrasonic measurement of bolt preload is made possible by introducing a sonic pulse at one end of the fastener and accurately measuring the time of flight (TOF) required for the echo to return from the opposite end. Using material constants, the USM-3 converts this TOF into an "ultrasonic length" of the fastener, providing a baseline from which future measurements will be made. When the fastener is tightened, the TOF increases and the USM-3 will again utilise material constants to eliminate the effects of stress and temperature variations on sound velocity, providing an accurate elongation or load measurement.
The USM-3 uses state of the art hardware and software to achieve these measurements with maximum automation, and minimizes the need for operator interpretation. Offering digital recording and transmission of data, in addition to analog signal output the USM-3 provides a complete system for measurement, recording, and control of fastener tension in the most demanding applications.
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