An end to over-bolting? [Part 1]
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-off weight of a modern aircraft is the aircraft itself with fuel accounting for a large part. It follows that a reduction in weight of an aircraft requires a reduced fuel load, which in itself allows further weight reduction. This effect is even more pronounced with space travel. According to NASA the current cost of putting one lb in weight into space is $10,000. The target is to generate a 100 fold reduction by 2025 and high on NASA’s priority list is more efficient design, which will include bolting. Whilst the aerospace sector has been very forward thinking in its use of composites and materials such as carbon fibre to reduce weight, the way that bolted joints are designed and subsequently tightened has not experienced the same level of advancement. With fully evolved bolt control, where bolt tension is controlled and measured, we can think ahead to reducing over-bolting. Check out part 2