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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 ...
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The aerospace sector, like most manufacturers, have traditionally used more, or larger bolts, than is strictly necessary in their assembly, in order to offset the effect of poor control of the bolted joint. With ever increasing pressure to design lighter structures to increase fuel efficiency, this practice is fast becoming obsolete.
Over-bolting is not confined to the aerospace sector. Even the humble car wheel uses four or five bolts to secure the wheel, when in reality the job could be done with fewer and smaller bolts if greater control was applied. This reduced un-sprung weight of the car would also, as a result, improve the acceleration, braking and ride quality of the car.
Similarly, in the aircraft industry, the cost of over-design, particularly its effect on fuel efficiency is even more noticeable. The rule of thumb is that a 1% weight reduction results in 0.75% reduction in fuel consumption; therefore a considerable saving potential.
Over 80% of the fully laden take-of ...
Norbar has devised easy-to-use online calculators that support the correct application of torque in three key areas:
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