 View the latest power industries news [June 2007] Whether it is a rotating wind turbine blade, an F1 racing car or a complex military fighter the loads applied at critical joints require bolts to be tightened to the design load and subsequently inspected and recorded as part of the condition monitoring process. Modern ultrasonic techniques have made significant improvements which are attracting close attention from engineers in a wide range of engineering applications. When a wing flap moves at 35,000 feet or a tail plane rudder starts to move the pilot and designer benefit from the knowledge that the clamp loads are within the design parameters. One of the issues designers face is that as equipment meets the need for less weight the use of lighter components introduces new alloys.
Some of these alloys are very tough but can have unusually high friction factors which make it far more difficult for the fitter using his skills depending on the measurement of torque and angle to determine the clamp load exerted by the bolt.
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Torque control does not allow design optimisation | The widely recognised German VDI Guidance No 2230 for the design of bolted connections uses a tightening factor of 1.4 –1.6 where the tightening is based on the measurement of torque. If however ultrasonic technology is being used the tightening factor is reduced to 1.05 – 1.2. Applying these factors can significantly reduce the weight of the joint or enable a lower cost material to be used and thus the cost. Far more important is the reduction in the cost of labour as the joint can be tightened faster and more accurately. Engineers used to using ultrasonics are very reluctant to go back to strain gauges as probably the only accurate alternative.
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Ultrasonic clamp load measuring method |
A major step forward has been the success of the transducer fitted to the head of the bolt which is permanent resisting temperatures up to 320 °C, salt spray, mud splattering and the usual oily crud associated with aircraft on runways. The transducers are being fitted to the working components of high speed diesels, to cylinder head bolts, con-rods, and transmission systems. They are also being used extensively in space projects where the weight saving is critical.
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High vacuum chamber for transducer application |
A typical transducer is about 5mm in diameter and has a thickness of up to 30 microns. It is applied to the bolt head usually but it can be the other end. It is protected and incorporates an electrode. When energised the piezo – electric effect produces a pulse of energy which is reflected at the end of the bolt. The returning echo is analysed over a broad spectrum and a signal pulse used to determine the time to pass out and back. This time in nanoseconds is factorised by an earlier accurate calibration of a similar bolt in a load cell under strict laboratory conditions.
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M42 with Data Matrix Code |
The factor once established can be bar-coded onto the bolt head enabling similar bolts to have their clamp loads measured in – situ. This can be invaluable on complex joints where time is of the essence and incorrect bolt recognition might be disastrous.
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Data Matrix Reader for Bolt Identification |
Ease of inspection is another critical advantage. The tightened bolts need only to be touched with a small hand held probe to display to the inspector the clamp load. Using the portable meter the full history of the joint can be reviewed immediately. The system records who tightened, when and to what load.
In the composite structures field the achieved clamp load can be displayed at any desired time in the assembly process. It has also been possible to display what is happening remotely to a team in a safe area.
The accuracy of the system is usually better than 3%.
The cost of the system is a function of the number of bolts supplied to be fitted but for the larger users the costs can be a few pounds per bolt with the meter / laptop running at about 800 euros rental per week or outright for 8,800 euros. A calibration at the laboratory costs 500 euros. This is only required for the establishment of the factor used to convert time to load. It is applicable to the batch of bolts received at one time.
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Load Probe 3000B |
The meter has several modes of operation. It can provide a simple green OK to proceed to the next bolt if the bolt meets all the design criteria. It can give a red STOP mode if the design criteria are not met.
The meter can work with electric powered tightening tools where a large number of operations make it economic.
The meter uses a Windows Excel system for data storage and retrieval. This is simple and easy to use. An expert mode enables the data to be interrogated in greater depth.
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Coating control |
The system has been accepted and certified for use in the aerospace industry. It meets DIN EN 9100:2003 and DIN EN ISO 9001:2000. The measurement devices have been tested and certified by TUV SUD for CE conformity.
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Certificates |
All transducers that are sputtered on to the bolts supplied by clients are tested 100% before dispatch.
Please contact Intellifast GmbH in Germany on 0049 6232 91950 or by internet info@intellifast.de and the website address is www.intellifast.de
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