An end to over-bolting?

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.  Philip Brodey, Director of bolting specialist Norbar Torque Tools examines why manufacturers over-bolt and offers an alternative.

Most bolted assemblies are overdesigned to allow for poor control of the bolting process and the aerospace industry is no exception.

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 will 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 profound. 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 bolt control, four stages of evolution can be identified. The first stage is having no control over the bolting process and the result of this is that massive over-design is required to ensure the security of the joint.  The joint will be designed with extra bolts and/or larger bolts than are necessary.

The second stage involves the use of torque control methods, but the process itself is out of control. Often with this stage there is limited operator understanding due to poor training which results in inadequate control of induced tension in the bolt. This situation is very common: torque tools are in use and may even be calibrated but then other changes are made which totally upset the torque versus load relationship.  For example, the bolt lubrication regime might be changed, the bolt type might be changed for one with a different finish and therefore a different friction factor and washers might be added or removed from the assembly.

The third stage of evolution is where torque control methods are used along with a process that is under control. In other words, trained operators controlling physical factors such as thread and under bolt-head lubrication. This is the situation that you would expect to find in a modern car plant and aircraft assembly plant.  This will often be coupled with direct torque measurement from the tools rather than relying on indicating torque wrenches which simply click at the set torque but give you no record of the tightening event.

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 this 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.  Even if this is not used as a production process, the technique can provide invaluable data for engineers to use when designing bolted assemblies and then creating the production process.

Provided that the bolting process is under control, torque provides an acceptable method of tension control under the majority of circumstances. However, friction under the bolt head and in the threads absorbs the majority of the applied force. Thus, a small change in the friction causes a large change in the applied load.

By way of contrast, 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 Torque’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 ‘cure’ 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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