Over torquing a bolt above its yield stress will lead to premature failure of the bolt whereas not applying enough torque or joint relaxation will allow the fastener to loosen, and resulting in insufficient working clamp load. Both of these scenarios can be considered critical failures in applications (such as the wheel nuts on your car) potentially having catastrophic and costly results.
For companies assembling bolted joints there are two main ways of ensuring quality assembly, adequate personnel training and the periodic calibration of torque tools. Calibration of manually operated torque wrenches uses the known standard (BS EN ISO 6789:2017 in this case) and the measurements recorded for a specific torque tool. A certificate of calibration is proof that the tool is operating to the required accuracy and establishes a traceable paper chain for quality assurance. Time interval between calibrations is not specified but once annually is common practise unless a critical application requires more regular calibration. Calibrations should be performed to ISO 6789 either in house or by dedicated calibration laboratories using proven equipment.
Advanced Witness Systems Ltd specializes in design and manufacture of torque calibration, control equipment and certification software. More recently the company has installed and had accredited several of their Universal Torque Wrench Calibration Machines (UTWCM) in calibration laboratories and will soon have the Universal Torque Screwdriver Calibration Machine (UTSCM) (Pictured) on sale. These are both all in one calibration solutions designed for ISO 6789:2017 or in house standards, and to cater for the widest range of torque wrenches and screwdrivers on the market. This includes a wide range of annular, inline and rotary torque transducers, precision adaptors and displays. More information can be found on their website www.awstorque.co.uk
