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Ultrasonic clamping tension measurement: a new industry standard

Ultrasonic clamping tension measurement: a new industry standard

Tension tightening

The need to measure bolt tension has become essential in many industrial sectors, such as automotive, energy, rail, aerospace and heavy industry. These areas, which are particularly sensitive, require precise tightening and measuring tools. Indeed, the reliability of these tools is crucial to guarantee the optimal maintenance of mechanical assemblies, ensure the safety of operations and prevent possible failures or ruptures.

In a context where quality and performance are top priorities, the ability to accurately measure bolt tension is of strategic importance, directly contributing to the reliability and durability of equipment and structures. This article will look at the use of ultrasound to measure clamping tension.

Thinking in tension: from the design office to the tightening operation

During the design stage, consider that the design team determines the dimensioning of its assembly as well as the materials that compose it in order to meet, in particular, mechanical strength objectives. This approach is based on the concept of tightening tension. When defining assembly methods, in 90% of cases the tightening process is implemented by applying tightening torque. Despite the existence of charts and calculation formulas, it is impossible to establish a direct link between torque and tension rigorously because many parameters can lead to significant dispersions in the applied voltage (range and dispersion of the coefficient of friction mainly and dispersion linked to the precision of the tool). This conceptual and operational gap causes a break in the product development and industrialization process. To compensate for this discontinuity, designers double, as a precaution, their safety margins on all the parts they design and the screws they prescribe. The consequences are significant: the assemblies are oversized, assembly is therefore more difficult and longer and maintenance and operating costs are significant. Controlling the tightening tension makes it possible to reduce these factors, because it restores continuity between the different services: all teams work in tension.

What does ultrasonic tightening measurement consist of?

The measurement principle is simple. It involves measuring the elongation of the screw or stud under the influence of the tension generated by tightening. Since this elongation is proportional to the tension (assuming, for the moment, that we limit ourselves to the elastic domain), it becomes possible to deduce the tension by simply applying a coefficient.

The ultrasonic translator placed on the head of the screw or stud acts as a transmitter which transforms the electrical impulse supplied by the measuring system into an ultrasonic wave which will propagate longitudinally in the material of the screw. Then, as a receiver which will collect the very weakened ultrasonic wave which is reflected on the bottom of the screw, to transform it into an electrical signal which will be processed by the measuring system.

The ultrasonic translator, placed on the head of the screw or stud, performs this operation, which is repeated at a frequency of several hundred or thousands of times per second.

With each "shot", the measurement system times the time elapsed between the emission of the pulse and the return of its echo. Although the principle is similar to that of sonar, the high speed of ultrasound propagation (around 5800 ms-1 in steel) and the resolution required for elongation (a few micrometers) require a time measurement resolution of order of nanosecond (1e-9 s).

To find out more, read the article on how to measure tightness .

Ultrasonic length: the central concept

The time measured by the system is called “ultrasonic length”. It is not imperative to know precisely the speed of propagation of ultrasound in the material. In the initial state, at rest, an ultrasonic length Li is measured. After tightening, in the final state, an ultrasonic length Lf is measured. This ultrasonic elongation (Lf - Li) results from two factors: the mechanical elongation of the screw and the reduction in the speed of propagation of the ultrasonic wave due to the tension introduced into the material.

In general, the effect of speed largely predominates and varies depending on the material. Although the system cannot distinguish these two causes, research, including that of CETIM in France over a period of forty years, has demonstrated that their combination is proportional to the voltage, thus making the method applicable.

The proportionality ratio between the ultrasonic elongation (the measured quantity) and the induced voltage (the desired quantity) is established by prior calibration of the assembly. A model representative of the assembly (with the same type of screw and over the same tightened length), is subjected to calibrated forces, for example on a traction machine. For each tension value measured by the traction machine, the measuring system records the corresponding ultrasonic elongation. The system then calculates and records the proportionality coefficient K, expressed in daN.ns-1 or kN.ns-1.

Voltage (daN) = K x Ultrasonic elongation (ns)

The TRAXX-M2 measuring system

The TRAXX-M2 device is one of the systems that allows these measurements to be carried out. The TRAXX-M2 is an ultrasonic tightening tension measurement system which allows the tightening of screwed assemblies to be controlled with very high precision. It makes it possible to control the quality of assemblies on site and to control the tightening of screwed connections.

Le 11 juin 2024 par TRAXX

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