THE Smartbolt system was developed by Ajax Fasteners to accurately, reliably and economically measure and monitor bolt tension directly from the fastener.
The operating principle of the Smartbolt is very simple. A precision sensor element is positively attached to the far end of a centre-drilled hole at a known distance below the under head bearing surface of the bolt so that the other end is flush with the head of the bolt.
Once the bolt is loaded the shank elongates in proportion to the tension load in accordance with Hooke's Law stress strain relationship. As the element is not loaded, the movement at the bottom of the hole is directly transmitted to the top surface.
By accurately measuring the travel of the polished top surface of the element with respect to the polished top surface of the bolt, tension can be determined.
The sensor is capable of reliably measuring the movement to ± 0.5 mm accuracy over a travel of 100 mm. The solidity of the attachment at the bottom of the hole is crucial to the reliability of the operation. The dimension of the hole is precisely designed by Ajax engineers through extensive finite element analysis and experimentation in order to maintain the original strength of the bolt.
The most critical parameter in a bolted joint is the tension of the bolt. This tension in turn will provide the various clamping loads through friction forces. The pre-load (tension) in a bolted joint is extremely important when the joint is subject to dynamic loading.
All structural bolts must retain a preload, which exceeds the applied load.
Standard practice requires that the bolt be preloaded to 65% of the yield load of the bolt. Provided that the preload exceeds the working load then dynamic applied loads are not directly transmitted to the bolt and hence bolt fatigue is controlled.
Statistics show that most structural failures are due to incorrect tightening of the bolts. It is well understood in the engineering community, that the tightening of a bolt to the correct tension is as important as selecting the correct bolt. It is also understood that the existing methods of tightening are either very unreliable or extremely expensive.
Innovation in strain measurement technology using multiple inductors in mutually integrating and error cancelling configuration enabled Ajax to achieve extremely high accuracy with reliability and speed.
With engineers focusing on lean designs, Smartbolt provides a means to comfortably use minimum size fasteners and assure their adequacy. Higher proof loads can be used to achieve joint integrity when being monitored by Smartbolt.