Stress-indicating bolts color the plant floor
by Kathryn Tomiello, Staff Editor
Saturday, September 01, 2007
Stress Indicators, Inc., has been manufacturing bolts that verify clamp load condition for the Spectra Series Busway systems from the GE Busway, Electrical Distribution division of General Electric, since 1997. The fasteners join modular electrical con-ductors that supply power throughout industrial plants.
Stress indicators’ DTI (Direct Tension Indictor) SmartBolts eliminate the use of torque wrenches, strain gages, wires, and electronic or ultrasonic equipment in setting up proper bolt tension. They turn bright red when loose and gradually darken to a deep black when tightened. Unlike traditional torque wrenching methods whose accuracy varies by as much as 5 to 1, they have a standard tension accuracy of ±10%. The DTI series has been the company’s flagship, until recently, when Stress Indicators added a bolt to its line-up that could indicate an over-tension condition.
The HR (High Resolution) SmartBolt’s indicator, located in the center of the bolt head, remains yellow as the fastener elongates. When the design tension reaches approximately 85%, the indicator rapidly turns bright green. If the bolt is tightened beyond specifications, the indicator darkens until it is almost black. Should the bolt loosen more than 10 to 15%, the indicator returns to bright yellow, warning observers of a problem.
The HR SmartBolt indicates bolt tension through direct measurement of bolt elongation. Accuracy is improved over the DTI SmartBolt because each bolt is calibrated at design tension during assembly. The HR indicator is sensitive to an elongation of 60 millionths of an in., which corresponds to a tension precision of approximately 2% when installed in high-strength bolts.
Torque wrenches, however, respond to applied torque, which is only weakly related to tension. As much as 80 to 90% of this torque is required to overcome friction, which can vary significantly in different situations. An accurate torque wrench may produce errors in bolt tension from 25 to 50%.
Stress Indicators, Inc.
www.smartbolt.com