An end to over-bolting? [Part 2]
The aerospace sector has been very forward thinking in its use of materials to reduce aircraft weight. When it comes to bolting design and practise however, the industry has not experienced the same level of advancement.
Torque is an indicator to bolt tension (sometimes called pre-load) and it is the bolt tension that we really need to control. The final stage of the bolting evolutionary ladder is therefore direct control of the tension induced in the bolt. There are various ways of doing this, including strain gauging the bolt or adding a load cell, load indicating bolt or washer into the assembly. However, such methods can be costly or impractical to implement. A more efficient method which requires minimal modification to the bolt is to ultrasonically measure the extension of the bolt due to the tightening process. For every bolt type there will be a relationship between the extension and the induced load so, measuring the extension allows accurate calculation of load. This is the ultimate level of bolting control that the aerospace sector and other safety critical bolted assembly operations need to work towards.
Methods that directly measure load such as strain gauging or that measure the extension in the bolt and calculate the load from a known material constant, bypass the effect of friction, the biggest variable in the bolted assembly. Norbar’s USM-3 for example, provides a very precise method of determining the elongation and load in the fastener due to tightening. USM has been used for many highly critical bolts including at NASA’s Stennis Space Center. The potential for composites and other lightweight materials to reduce aircraft weight may not yet have been reached but it is finite. However, it is clear that finding a ‘cure’ for over-bolting offers potential in the race for ever-lighter aircraft which can benefit from increased fuel efficiency. That could be direct control of the bolted joint which can offer the aerospace sector a means of reducing the number of bolts, and in turn aircraft weight, whilst maximising efficiency.
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