⚙️ The Hidden Complexity of Bolt Tightening
Screwing is often perceived as a simple and easy-to-implement assembly technique. Yet behind this apparent simplicity lies a technical complexity that is often underestimated. From machine construction to large-scale infrastructures such as airplanes or bridges, bolts are everywhere. Unfortunately, this perceived ease of use sometimes leads to negligence in the design, implementation, and monitoring of bolted assemblies.
Guilhem de Nucé, CEO of ATEC, believes that this type of assembly is often the “poor relation” of engineering. Meanwhile, Romain Barrellon from the Technical Center for Mechanical Industries (Cetim) observes a loss of know-how in tightening practices, resulting in lower-quality designs and implementations and ultimately an increase in failures. For Michel Berbesson, manager of Traxx, there is a significant “cultural lag” surrounding this technique.
The Limitations of Torque Measurement
The primary goal of bolting is to generate a specific tensile force between two parts, as defined by the designer. To achieve this force, the most common method is to measure the torque applied to the nut or bolt head using tools such as controlled screwdrivers or torque wrenches. This technique is the most widely used in industry.
However, torque tightening exhibits significant variability. Even when applying the same torque to identical joints, it is impossible to achieve identical tension levels. Several factors contribute to this inaccuracy, including variations in steel grades, surface treatments, lubrication, the presence of rust, or thread quality. All of these variables affect the friction coefficient, meaning that a portion of the rotational force is not efficiently transmitted to the bolt. This imprecision highlights the importance of using a more reliable system to monitor the actual tension in the bolt.
Another method, tension-based tightening, can be used to achieve more precise results, particularly for large-diameter assemblies. This technique, which involves using hydraulic jacks to stretch the fastener, also results in variability, which can reach 20–25%. Material relaxation, due to its elasticity, sometimes requires repeating the operation to achieve accurate tightening.
Ultrasonic Measurement: A Precision Solution
To overcome the inaccuracies of traditional methods, another technique is gaining popularity: ultrasonic tension measurement. Developed over the past 30 years, this method is similar to those used for crack detection or thickness measurement in non-destructive testing (NDT). It has the added advantage of being a non-destructive measurement system itself.
The principle is simple: the time it takes for an ultrasonic wave to travel back and forth within the bolt is measured. The wave’s time of flight, measured in nanoseconds, is directly related to the bolt’s tension. It is influenced by the bolt’s elongation and axial stress, which alter the material structure and, consequently, the wave propagation speed.
Using this method requires prior calibration. The bolt is tensioned to establish a curve that correlates time of flight with tension. The measurement is also sensitive to temperature, making the use of a temperature probe essential for reliable results.
Advantages and Limitations of Ultrasonic Technology
Ultrasonic measurement offers numerous advantages. It is effective regardless of the tightening method used (manual, torque-based, or tension-based). It can be employed both during tightening, to verify that the desired tension is reached, and over the long term, to monitor the evolution of the assembly.
However, the method also has its limitations. Bolt geometry is a crucial factor, as the surface must be sufficiently flat to avoid unwanted reflections and large enough for the sensor to be correctly positioned at the center. Additionally, the bolt should not be too short, to ensure an adequate time of flight, nor too long, to prevent excessive wave attenuation.
To facilitate use, some manufacturers have developed innovative solutions. Traxx, for example, has designed a device capable of taking measurements on any bolt surface, avoiding additional costs and modifications (such as abrading the anti-corrosion coating). Others, like Intellifast, offer the integration of permanent sensors directly on the bolts, ensuring that each measurement is performed under the same conditions and eliminating variations caused by sensor positioning or gel thickness.
The Importance of Training and Monitoring
Despite the effectiveness of ultrasonic measurement, it cannot, by itself, compensate for poor design or low-quality materials. The robustness of a bolted assembly primarily depends on its design and execution. Tightening is often overlooked, and the loss of expertise due to the retirement of skilled personnel is an increasingly common problem. Furthermore, the drive to reduce costs can lead companies to use lower-quality bolts, compromising the strength of assemblies.
To address these challenges, training programs are offered—particularly by Cetim—covering design, calculation, sizing, failure analysis, and tightening techniques. Companies like TRAXX also provide training to familiarize engineers, technicians, and operators with their devices and measurement methods.
Purchasing an ultrasonic measurement device is a solution for companies with recurring needs. However, one-off intervention services are useful for addressing urgent issues and can often reveal flaws in tightening processes that justify investing in equipment. For long-term monitoring, traceability is essential. Devices like the TRAXX-M2 can store millions of measurements, along with additional information such as operator names and intervention times. Bolt identification can be managed through numbering, barcodes, or RFID tags.
In short, although bolting may seem simple, its correct execution and the precise control of tension are crucial for the safety and reliability of assemblies. The use of advanced technologies such as ultrasonic measurement, combined with a better understanding of tightening processes and proper training, helps overcome the limitations of traditional methods and ensures the robustness of structures.