A tightened bolt is a spring under tension. By measuring how much it has stretched you know the load it is exerting: a quantity that a torque wrench can only estimate.
- ✓ Measurement of the actual load, not of the applied torque
- ✓ Automatic temperature compensation
- ✓ Re-checking the same bolt after a period of time
- ✓ Continuous monitoring of joints under observation
The method is based on an elastic relationship. When the bolt is pulled it stretches, and as long as it remains in the elastic range the elongation is proportional to the load. A transducer placed on the head measures the time of flight of the pulse along the axis of the shank: as the length increases the time of flight increases, and the difference from the initial condition gives the elongation.
This explains why a reference reading on the unloaded bolt is always needed. Without that initial value there is no term of comparison, and the reading taken after tightening alone says nothing about the load. This is why the check must be set up at assembly, not when the problem arises.
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Temperature intervenes twice: it expands the material and changes its speed of propagation. On joints working at variable temperatures, temperature compensation is not a refinement but a condition for the validity of the measurement, and this is why dedicated instruments integrate it.
The in-depth topics of this section
| Topic | What it answers |
|---|---|
| Elastic elongation measurement | How elongation becomes applied load |
| Automatic temperature compensation | Why temperature distorts the reading and how it is corrected |
| Ultrasonic method and torque wrench | What each of the two really measures |
| Fastener preparation and calibration | Surface, geometry and initial reference |
Spot checks and continuous monitoring
Spot checking accompanies assembly: each bolt is measured as it is tightened and checked to be within the expected window. The portable instruments of the BT series are designed for this use, with a datalogger that keeps the readings for the assembly report.
Continuous monitoring is needed instead when the joint has to remain under observation over time. The Dakota MultiMax reads up to four bolts continuously with automatic temperature compensation, and the modules stack to follow more points. The range is in the Bolt tension monitors section.
Frequently asked questions
Yes. The elongation is obtained by difference from the initial length, so the reference reading must be taken and recorded before tightening.
Because it acts on two fronts: it expands the material and changes the speed of propagation of the ultrasound. Both effects alter the time of flight and must be compensated.
Access to one end, with a flat, clean surface on which to place the transducer, and knowledge of the geometry of the bolt to convert the elongation into load.
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Further information and contacts
Systems division and sensor integration partner : www.roder.it
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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