The use of a threaded bolt or stud is one of the oldest ways of fastening together two pieces. This method is accepted in the mechanical, marine and civil engineering industries as a reliable fixing system. Examples are slewing bearings, pipe flanges, marine engines, bridges etc. This fastening method is dependent on the pieces being assembled and held together reliably by the nut and bolt assembly. It can only do this 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.

The problem

How can it be determined that threaded assemblies have achieved sufficient preload and cannot come loose due to either under- or over- tightening? The general rule is that bolts are preloaded between 70% and 90% of the yield of the bolt material. This is actually a large spread and the means of measuring this preload is generally a theoretical calculation which involves determining the preload and converting it into an equivalent hydraulic pressure to be used by the hydraulic tensioning unit (the means of stretching the bolt before tightening the nut).

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%.

As a way of explaining this, imagine a 14 inch diameter high-pressure gas pipe which is jointed at intervals by flanges. It is obviously important to maintain sufficient pressure on these flanges and the gaskets between them to prevent dangerous leaks; this pressure will be achieved by the flange bolting. The pressure in the pipe when it is in use will be trying to force the flanges into one another, compressing the joints. In addition to this force there will be a thermal influence, with the temperature in the gas pipe changing. This thermal force will introduce a cyclic effect onto the flanges, hot for a while then cooler. During the heat cycle the material of the flange and the pipe will expand, thus exerting more compression on the flange faces. During the cooling down cycle everything will compress, thus reducing the flange face pressure.

These cycles will not only introduce different forces on the bolts which in turn will affect the preloads of the bolts, but the cycles will also cause fatigue in the bolt material which will weaken the bolts and change their elastic properties.

Other applications that are susceptible to bolting preload are marine engine bolts, drive shaft bearing bolts, rudder stock bolts and any bolting cases that are subject to vibration cycles or heat cycles that are load critical and require a graded bolt. All these examples are likely to cause injury to personnel should a bolt fail.

A solution?

A Norwegian company called 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 same 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 as any changes occur. 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.

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