The correct bolt clamping force can be guaranteed by listening to the sounds produced as bolts are screwed in. And, in addition, eddy current sensors can be used to ensure that threads are present and of good quality in the first place.

Bolted joints remain the preferred method of joining for many applications, including those that are highly safety critical. Which is where the Sonofast system, developed by Weber Automatic Assembly Systems, comes into the picture. It listens to the faint sounds produced when screws stretch as they are tightened. These sounds are produced by twinning, when metal crystals flip over from one orientation to another, and by the movement of dislocations – the imperfections inevitably present in metal crystal structures.

A small proportion of sounds are produced by local yielding, well before the overall yield point for the screw when it starts to plastically deform generally. By integrating the amount of sound detected, it is possible to arrive at an accurate estimate of the clamping force, more accurate than that inferred from measured bolt head torque.

The sound is detected using Kistler quartz crystal transducers mounted on a special screw driving assembly. And the detected RMS signals are transmitted from an antenna, picked up by telemetry around the assembly and fed to a controller.

The system is set up by allowing the controller to detect automatically the correlation between clamping force and acoustic emission. The controller then calculates a reference curve, after which it can be programmed with the desired clamping force. In service, the controller monitors the acoustic emission during fastening, compares it to reference data, and stops fastening when the pre-defined, integrated signal level has been reached.

A typical application is in the insertion and tightening of engine cylinder head bolts, although many other potential uses must exist, especially with regard to chemical and process plant.

Such a system breaks down, however, if the hole in which the bolt is to be inserted either has no thread, or has one that is imperfect or damaged.

To overcome this problem, Kaman, represented in the UK by Ixthus Instrumentation, has developed the KDS 2440 eddy current sensing system. By inserting an appropriately sized sensor into a tapped hole, an analogue voltage is produced proportional to the radial distance between the sensor and the tapped hole. The output of the system corresponds with the average diameter as determined by the average of the peaks and valleys of the thread. If the hole is not tapped, the output of the system indicates the tap drill diameter and the minor diameter of the thread. If measured values are not what they should be, parts can be rejected and production stopped before any bolts become jammed in untapped holes.

The system can be used to detect the presence or absence of threads, and by using a ‘Side View’ sensor in conjunction with a waveform analysis instrument, it is possible to monitor the quality of threads.

Note: Ixthus Instrumentation will be exhibiting at the mtec exhibition at the National Exhibition Centre, Birmingham on 13-14 February 2002.

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