In an ideal world, each bolted joint connection in an aircraft or spacecraft would be fastened to achieve precisely the desired load and all bolts belonging to a specific application area would get a consistent load to ensure even force distribution. ö
“Reality is quite different, though,” said Robert Molsbergen, President of the King of Prussia, PA-based company Load Control Technologies (LCT). “Tests have shown that bolt clamp loads scatter by up to ±30% when the tightening is merely torque-controlled and by up to ±15% if torque and angle control are applied.”
The conclusion to these findings is to specify the tightening load much higher than necessary to make sure that the designed load is achieved even under worst conditions. “In other words, clamp loads are indirectly estimated,” said Molsbergen.
Frank Scheuch, Managing Director of German Intellifast says one primary goal of load-monitored fasteners is “to eliminate exceeded loading and thus to minimize strain and stress during assembly.”
The scatter between minimum and maximum bolt loading has a number of unwanted consequences. For one, high bolt loads limit the lightweight design freedom as the load scatter necessitates more and stronger material to cope with the high tightening forces. Lightweight composite structures for instance are particularly sensitive to overloading.
Uneven pre-loads caused by conventional bolt tightening due to the influence of under-head friction and thread friction make things worse. According to Scheuch, load-monitored fasteners facilitate even bolt loads that safely achieve locking torque with minimum nominal load. This feature will help to avoid damages at the ply edges of composite materials by minimizing interface compression.
Another downside of the load scatter is that there will be an element of uncertainty about the remaining pre-load of a joint after a number of flight cycles. In practice, the bolts have to be checked with a tool one by one to make sure that potentially problematic joints are brought back to normal. This process is slow and labor intensive, which also means it is costly. Maintenance intervals are a balance between cost and possible bolt failure or vibration loosening.
An answer is found in electronically controlling the preload. Currently there are two variants of the same technology. Both rely on fixing a transducer to the head of the end user’s bolt.
For precise tightening, the fastening tool is equipped with a probe and additional electronics that adjusts the tool’s torque to the actual preload of the bolt. To measure this load, an ultrasonic signal is sent down the bolt. The time of flight until the echo arrives back at the transducer directly correlates to the stress and bolt elongation, which in turn is precisely defined for each bolt material and design.
“It is the principle of a bat shouting down a bolt,” said Scheuch. Intellifast calls its assembly load control solution Permanent Mounted Transducer System (PMTS) and has recently extended the product portfolio by a high-temperature version of the PMTS that can withstand up to 320°C and also by a version that is highly corrosion resistant. Bolts with PMTS are available down to the metric size M4. Currently, PMTS technology is in the verification phase for various Airbus aircraft and is part of the Rolls-Royce lift system for the Lockheed Martin F-35B.
By measuring the signal time of flight, both the LCT and Intellifast technology versions of the transducerized bolts allow a measurement of the installed load to 3% accuracy. Inspection during maintenance is done with a simple probe instead of a tool. In under 2 s the bolt load is measured in the actual joint without disturbance and to 5% accuracy through the life cycle of the bolt, claims LCT. In addition, each single bolt can be identified via a 2-D barcode during fastening and inspection and the load control data can be stored.
The difference between the LCT fastener system and Intellifast technology is the process by which the transducer is fixed to the bolt. Intellifast uses a high vacuum process for bonding the transducer to the substrate. LCT on the other hand has completely overhauled its originally commercialized Ultrafast Intelligent Fastener transducer and electronics. For its current i-Bolts the company uses a cold fusion manufacturing process that can also be licensed. After upgrading the piezo ultrasonic transducer’s capabilities, the first big validation project for the i-Bolt is the Boeing 787 Dreamliner.
“Going into production for this airplane after a four-year certification process is probably the best validation one can get,” said Molsbergen, adding that NASA has also certified i-Bolts for the booster tanks of the space shuttle.
The sheer speed by which bolts can be checked may soon lead to other applications, Molsbergen is confident. “Helicopter rotor bolts for instance can be checked within 15 min if the bolts are transducerized, so the aircraft is quickly available again for military or combat service.”