A tensioned bolt behaves like a spring: it stretches in proportion to the load. Measuring that elongation means measuring the load, without having to infer it from torque.
- ✓ Elastic relationship between elongation and applied load
- ✓ Time of flight increases as the bolt length increases
- ✓ Reference reading on the unloaded bolt is indispensable
- ✓ Validity limited to the elastic range of the material
The principle is that of elastic behaviour. As long as the material stays within its elastic range, the elongation it undergoes is proportional to the force tensioning it: doubling the load doubles the elongation, and removing the load returns the bolt to its initial length.
The ultrasonic method measures that elongation without touching it. A transducer resting on the head sends a pulse along the axis of the shank, which reflects off the opposite end and returns: the time of flight depends on the length travelled, so it increases when the bolt stretches.
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Comparing the time of flight on the unloaded bolt with that after tightening yields the elongation. From this, knowing the geometry of the bolt and the characteristics of the material, the applied load is derived. This is why the initial reading, the one on the untightened bolt, is the condition without which the method does not work.
What needs to be known
| Data | What it is for |
|---|---|
| Reading on the unloaded bolt | It is the reference from which the elongation is derived |
| Clamping length | The portion of the bolt that actually stretches |
| Stress area | Converts elongation into applied load |
| Bolt material | Determines the elastic behaviour and the acoustic velocity |
| Temperature | Must be compensated because it alters the time of flight |
The limitations of the method
The first limitation is proportionality: it holds within the elastic range. A bolt taken beyond yield does not return to its initial length, and the relationship between elongation and load is no longer the one on which the calculation is based.
The second is access: a reachable end is needed, with a flat, clean surface on which to rest the transducer. The third is temperature, which acts both by expanding the material and by changing its acoustic velocity: this is the reason for temperature compensation.
Measuring instruments
| Instrument | Typical use |
|---|---|
| Dakota BT1-DL MiniMax | Tightening check in the workshop, with datalogger |
| Dakota BT2-DL Max II | Multi-channel benchtop version |
| Dakota MultiMax | Four-channel continuous monitoring |
| DakMaster™ | Analysis of readings and assembly report |
Frequently asked questions
Because the bolt stretches: the pulse has to travel a greater distance to reach the opposite end and come back, so it takes longer.
No. The proportionality between elongation and load holds within the elastic range of the material: beyond yield the relationship on which the calculation is based is no longer valid.
The elongation cannot be derived, because the term of comparison is missing. The reading after tightening alone provides no information about the applied load.
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Further information and contacts
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More information about RODER SRL : about us
Contact for general information : info@roder.it
Contact for information about these products : info@roder.it
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