Designers and engineers who need to know precisely the clamp load on a bolted joint have traditionally been stuck with complex and expensive techniques such as ultrasonic extensometers or strain-gaged bolts with cumbersome cables and electronic readouts. Now, with the development of new high-resolution (HR) microindicators, the precision of built-in visual bolt tension indication has been increased dramatically to allow visual setting and monitoring of bolt clamp loads to an unprecedented 2-5% accuracy!

Precision HR SmartBolts® from Stress Indicators, Inc. have a yellow colored indicator in the bolt head, which doesn’t change its color until the bolt is tightened up to about 85-90% of its design tension, where it rapidly begins to turn green. The bolt design tension has been reached when the indicator color could be described as “grass green”. Over-tightening will cause the indicator color to darken to nearly black,. A bolt loosened by as little as 5% will similarly be obvious, because the indicator color will be chartreuse. If loosened by 10% or more, it will revert back again to yellow. This action is completely reversible, and the SmartBolt® may be tightened and loosened innumerable times without degradation or loss of accuracy.

SmartBolts® measure bolt tension by comparing the elongation of the stressed bolt relative to an unstressed gage pin. This is still the most reliable and most accurate known method of measuring bolt clamp load. The Precision HR SmartBolts® are individually calibrated at the design tension condition, while the standard DTI SmartBolts® are calibrated in the unloaded condition. The extraordinary achieved precision and accuracy of the new HR SmartBolt® is the result of the combination of the new high-resolution visual microindicators with precise calibration at the full design clamp load.

Precision HR SmartBolts® are currently undergoing certification for use in U.S. Army helicopters.