`Smart` washer monitors bolt tension

The tightness and pre-tension of critical bolted joints can now be monitored continuously using an "intelligent" washer developed by a Norwegian company. The device, called BoltSafe, is used like a standard washer but measures the compressive forces between a nut and its interface surface to determine the pre-tension in the bolt joint.

The BoltSafe uses a sensor formed from an amorphous ribbon alloy to monitor the changes in magnetic field that occur when pressure is applied to it. This sensor is linked to a custom chip which processes the signal and provides a digital connection to the outside world.

As well as gauging the bolt tension, the device provides temperature readings and timing data and a 64-bit number identifying the washer.

There are two version of the BoltSafe. One uses a permanent cable connection to provide continuous readings throughout the life of an installation, and identifying any changes in the set pre-loading. Up to 32 of the washers can be linked to form a network.

The second version allows manual readings of pre-tension to be taken during routine maintenance, by docking a manual probe with a port built into the washer.

The BoltSafe, developed by Scan-Sense and available in the UK from SKF Engineering Products, can be used on bolt sizes from M12 to M64 and with clamping loads up to 2,100kN. A companion software package can flag alarms and produce graphs showing how the pre-tension varies with time.