⚙️The bolt tension tightening method: the true measure of safety
In the world of engineering and industrial assembly, the reliability of a structure often depends on the strength of its connections. This is where the bolt tightening method becomes a precision operation, far from being a simple mechanical action. The ultimate goal is not merely to apply torque, but to induce in the bolt an axial tension—or “preload”—sufficient for the assembly to withstand dynamic stresses such as vibrations, temperature changes, or shocks, without loosening or deforming.
For a long time, industry has relied on torque-based tightening methods, an indirect force that, even with the most advanced tools, cannot reliably guarantee preload. As many industries now embrace connected and automated technologies, it is crucial not to be misled by appearances and to analyze the intrinsic weaknesses of these approaches. This article explores the limits of modern tightening methods, demonstrating why only an approach that directly measures bolt tension, like the one offered by TRAXX, can truly meet the safety and quality requirements of Industry 4.0.
The fundamentals of bolt tightening: torque and angle
The popularity of traditional tightening methods is explained by their simplicity, but using them in critical applications can have disastrous consequences. Studying their limitations highlights the imperative need for a more rigorous approach to ensure reliable bolt tension.
Torque-based tightening method: inaccuracy at the heart of the bolt tension process
Torque-based tightening is the most widespread bolt tightening method due to its ease of implementation. It involves applying a force at a given distance, measured with a torque wrench. However, the major weakness of this method lies in its variability. Only a small fraction of the applied torque is converted into the desired bolt tension, while the majority of the effort—up to 50% or 60%—is lost due to friction between the threads and under the bolt head.
Even when the measured torque is constant, the actual preload can vary unpredictably, making this approach highly unsuitable for assemblies where safety is critical.
Angle-based tightening method: a step forward, but still in the dark
To compensate for the weaknesses of torque-based tightening, the angle-based bolt tightening method, or “turn-of-nut” method, has gained popularity. It is performed in two steps: an initial snugging phase with low torque, followed by rotating the bolt through a precise angle. This method is less sensitive to friction variations than the torque method.
However, it is not foolproof. It requires perfect knowledge of the stiffness of both the assembly and the bolt. Uncertainties such as inaccuracies in friction coefficients, although reduced, still persist and add to system variability. As a result, even with angle-based tightening, the actual preload—and therefore the bolt tension—cannot be guaranteed if assembly conditions vary, such as the presence of foreign material or insufficient assembly rigidity.
Promises and limits of modern tools and automation
In their pursuit of performance, manufacturers of tightening tools have integrated many Industry 4.0 technologies. Connected tools that measure torque and angle, and even robotic tightening systems, are presented as cutting-edge solutions. However, it is essential to emphasize that these innovations, while effective for productivity and traceability, do not solve the fundamental problem of tightening inaccuracy.
Digitization: gain in traceability, not in precision
Modern torque wrenches and electric screwdrivers from manufacturers like Bosch Rexroth or HS Technik incorporate sensors and Wi-Fi connectivity, allowing them to measure torque and angle simultaneously and store the data for full traceability. Tools like the Opex digital torque wrench can archive thousands of data points, such as final torque, rotation angle, and tightening time, for rigorous quality control.
However, these technologies focus on measuring indirect variables. They tell you what has been done, but not the actual bolt tension achieved. The weakness of this approach is that even the most advanced tools cannot compensate for the fundamental inaccuracies of torque- and angle-based tightening methods. Angle-based tightening, even with electronic monitoring, cannot guarantee consistent tension if assembly conditions vary. Moreover, these sophisticated tools require significant investment and operator training.
Automation and robotics: efficiency without total control
Robotic tightening, using solutions such as IMAO Nutrunner systems, is undeniably a productivity breakthrough. Robots can perform repetitive tasks with high fidelity, handle heavy components, and apply large tightening forces without deforming critical parts. This approach is a major asset for large-scale production efficiency.
Nevertheless, automation alone does not guarantee assembly quality. The effectiveness of robots depends entirely on the tightening method programmed into them. If robotic tightening is paired with torque- or angle-based methods, the same inaccuracies are multiplied across thousands or even millions of assemblies. Robotics improves repeatability, but it does not resolve the variability in friction that skews results. To truly ensure safety, it is necessary not only to have flawless automation but also a bolt tightening method that measures the bolt tension directly—the actual result of the operation.
The technological leap of traxx: direct measurement of bolt tension
Faced with the limitations of torque- and angle-based methods, a different approach is required: the direct measurement of axial tension. This is the very essence of the innovation offered by the TRAXX-M2 measurement device.
The traxx-m2 revolution: an ultrasonic-based approach
The TRAXX-M2 represents a technological breakthrough by abandoning indirect measurements of torque or angle in favor of precise, direct measurement of bolt tension. This device uses ultrasonic technology to directly measure the deformation (elongation) of the bolt during tightening. By measuring the travel time of an ultrasonic wave along the bolt axis, the device determines the installed preload with minimal variability.
This approach is presented as a solution for controlling tension in critical assemblies. The TRAXX-M2 is robust and can operate in harsh industrial environments, such as dusty, humid, or shock-prone conditions. Its onboard system, the TMS (Tightening Management Software), ensures complete traceability: each measurement can be saved, annotated, timestamped, and exported, enabling rigorous industrial control that complies with the highest standards.
Conclusion: safety is measured, not estimated
The evolution of bolt tightening methods in industry highlights a crucial realization: indirect measurement, even when digitized and automated, is no longer sufficient for applications where safety and quality are non-negotiable. Technologies that merely monitor torque or tightening angle fail to guarantee the most important element: the actual bolt tension.
In this context, the TRAXX method and its TRAXX-M2 measurement device set a new standard of excellence. By focusing on direct tension measurement using ultrasonics, they offer a solution that is not only more precise but also more reliable and transparent. Investing in this type of technology is not merely a productivity gain—it is a guarantee of safety, allowing companies to ensure the compliance of every assembly and optimize processes based on real data, not mere estimates. The future of bolt tightening belongs to those who understand that true control lies in measuring bolt tension, not in applying force.
