The use of a threaded bolt or stud is one of the oldest ways of fastening together two pieces. A crane’s integrity and safety is dependent on the parts of a slewing ring for example being held together reliably by nut and bolt assemblies. This is achieved by using the elastic properties of the materials of the nut and bolt pulling against each other, ie. under preload.
To further explain this elastic property it is often referred to as the yield of the steel. When assembling the nut and bolt the trick is not to push this elastic property too far as the steel will not recover if over stretched, ie. if permanent plastic deformation has taken place. Instead one should stretch the steel up to that point of permanent plastic deformation. If the steel is still stretchy the threads of the nut and bolt will grip each other, thus holding the assembly together.
Determining that threaded assemblies have achieved sufficient preload and therefore will not come loose due to either under- or over- tightening is generally a theoretical calculation. The existing methods, either a torque wrench or a hydraulic bolt tensioning system both suffer from not knowing what the friction losses are, for the torque wrench friction loss can be as much as 30% and the tensioning method 15% to 20%.
The crane’s operating cycle introduces different forces on the bolts which in turn affects the preload, but the cycles will also cause fatigue in the bolt material which will weaken the bolts and change their elastic properties. Bolt failure is likely to cause injury to personnel and damage to machinery.
Norwegian company, ScanSense, reckons that one answer is to have a system that measures the preload of the bolt at the assembly stage in units of force and eliminates the guesswork and theory from bolting. This system should also be able to monitor any changes in preload that occur during the operating life of the installation. The preload information should be capable of being downloaded onto a computer and plotted. Corrective action can then be taken prior to any failure.
ScanSense claims that its BoltSafe sensor system can do all these things in a simple and reliable way. Development of the BoltSafe sensor washer was made possible by a new semiconductor amorphous material which ScanSense produced. The washer can detect any change to the magnetic flux in its material as pressure is applied to it and thus measure the force applied to the bolt.
