Smart bolt preventive and anticipatory role aims to avoid loosening or the risk of failure, especially under conditions exposed to dynamic forces. Currently, smart bolts are designed to remotely measure length variations, thus enabling early detection of any potential loosening that could jeopardize the integrity of the installation. This remote measurement function significantly contributes to reducing costs related to on-site inspections while ensuring continuous monitoring of installations in hazardous, contaminated, or hard-to-reach areas.
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How does smart bolt work ?
The fundamental principle of the smart bolt relies on the insertion of an optical fiber into the bolt, operating based on the well-established concept of white light polarimetric interferometry, which has been widely used in the nuclear industry for many years.
Specifically, this technology involves integrating a 0.25 mm diameter optical fiber into a 0.5 mm axial hole, located at the center of bolts, studs, and other threaded rods. The fiber is bonded to the fastener using a polymerized adhesive, which has been certified for over 40 years in aerospace and nuclear industries, ensuring resistance to temperatures up to 320°C. The optical fiber is based on a patented white light polarimetric interferometry process, allowing precise calculation of length variations through light emission.
The fiber is connected before or after tightening using a connector, linking the sensor to a device called an “interrogator,” which can be located up to 7 kilometers away. This interrogator can manage up to 1000 sensors simultaneously, providing considerable flexibility. Other sensors based on this technology can detect temperature, pressure, and displacements with high precision, centralizing all information on a single interrogator.
Why use a smart bolt ?
The smart bolt allows for precise monitoring of the elongation of assembly components, with a range of up to 7 km from the measurement sensor. The optical fiber used is resistant to beta and gamma radiation and operates within an exceptional temperature range from -290 to +320°C. Designed for critical assemblies requiring constant vigilance, the smart bolt can be installed in various industrial areas and intervention sectors. It is not only non-emitting but also insensitive to gamma/beta rays, magnetic fields, and frequencies. Energy consumption is moderate, limited to the interrogator (2 to 5W). Additionally, a variety of protective sheaths are available, which can withstand extreme conditions such as immersion, shocks, chemical aggression, radiation, and significant curvature radii.
The optical fiber used in ionizing radiation zones is made of pure silica, with tests showing favorable reports regarding its use in irradiated environments. Notably, the optical fiber remains insensitive to temperature changes, with no thermal expansion, ensuring accuracy in elongation measurements. Furthermore, the gauge’s placement on the neutral axis makes it insensitive to torque phenomena, allowing exclusive visualization of elongation during tightening.
The benefits of the smart bolt
The benefits of the smart bolt are numerous, particularly in terms of safety. During tightening operations, its reactive signal, ranging from 1000 measurements per second to 5 measurements per second depending on the device used, allows for precise management of elongation with exceptional accuracy of 0.5 µm. However, practical considerations arise, such as the need to have at least one end available to connect the sensor to the interrogator. This requirement can become more complex in the case of an installed CHC screw or a torque-tightened TH screw on a threaded frame, sometimes necessitating the setup of a specific system.
Real-time monitoring of assemblies ensures the integrity of installations during operation, triggering alerts via email or SMS in the event of potential failures. It is crucial to note that this technology remains neutral from the sensor’s side, with the optical fiber being entirely passive and emitting only light, making it suitable for hazardous environments without any harmful impact.
From an economic standpoint, the smart bolt offers instant detection of emerging failures, enabling conditional preventive maintenance. This approach, also known as predictive maintenance, optimizes the timing and location of interventions, thereby significantly reducing production downtime and precisely targeting necessary work. This feature is particularly advantageous for assemblies located in hard-to-reach areas, such as hazardous locations or underwater installations.
Examples of smart bolt applications
A concrete example of the smart bolt application can be observed in the rail industry, where rail maintenance is a major concern for SNCF. The smart bolt, acting as a sensor measuring the tightening force, transmits real-time critical data on rail tightening. This allows for anticipating future maintenance work, thus improving the safety of rail journeys.
Disadvantages of the smart bolt
Beyond its many advantages, the smart bolt also presents some disadvantages that should be considered. Three critical aspects to take into account are:
Potential alteration of mechanical properties: The insertion of the sensor into the bolt can compromise the assembly calculations. A 0.5 mm diameter hole along the axial direction, equivalent to 0.2 mm² in the bolt, can affect the mechanical strength of the bolt and its nature, introducing potential risks into the structure.
High installation cost: The installation of the smart bolt can be expensive, requiring complex machining and preparation for each bolt. These steps increase the initial installation costs, which may be a barrier in projects with strict budgets.
Energy consumption and Wi-Fi connectivity issues: The connectivity of each bolt represents a notable energy consumption, which can pose challenges in terms of energy management. Additionally, dependence on Wi-Fi transmission systems exposes the device to potential issues such as interference or network outages.
Increased maintenance complexity and cost: Integrating electronics into bolts adds complexity to maintenance operations, increasing the associated costs for managing and repairing the equipment.
Alternative methods to the smart bolt : the connected tensioner nut
The connected tensioner nut represents a significant advancement in preload monitoring.
Equipped with a stress measurement device, this patented system analyzes the variation in the circumference of the tensioner relative to changes in the preload within the bolt. Additionally, the system offers remote monitoring and direct data reading, with a precision superior to +/-5%.
One example of the connected tensioner nut application is the critical joint on wind turbines. As with all critical joints, wind turbine connections require extremely precise and regular preload checks. This is necessary both during installation to ensure the achieved preload meets design requirements and throughout the turbine’s lifespan. Before the introduction of the connected tensioner nut, these preload checks required considerable effort and some modification to the bolted joint. Today, preload can be regularly monitored remotely and without altering the joint.
The theory behind the system
Under the effect of an axial load on the bolt, the body of the nut undergoes circular stress, leading to radial expansion (bending up and down). Measuring this expansion relative to the circumference of the nut body allows for a broader and more precise response. By controlling the relationship between the circumferential expansion and the axial preload, it becomes possible to monitor the preload through this unique characteristic.
Advantages of the connected tensioner nut
Remote monitoring of preload and direct data reading: It allows real-time monitoring of preload, providing immediate responsiveness to changes in the assembly. Direct data reading during tightening simplifies result interpretation and facilitates quick decision-making.
Increased availability during operation: It ensures continuous equipment availability without major interruptions for periodic checks, thus improving operational efficiency.
Accurate and reproducible preload reading: It provides precise and reproducible measurements, ensuring a consistent evaluation of the assembly’s condition over time.
Eliminates the need to modify the bolt: It avoids costly or complex modifications to the existing structure, simplifying the installation process.
Robust and reliable, rated IP67: It withstands harsh environmental conditions, ensuring long-term reliability even in humid or dusty environments.
Precise preload verification without adjusting tightened parts: It removes the need to reconfigure or adjust tightened parts for accurate measurements, simplifying maintenance.
No modifications needed for the assembly (including design or geometry of the stud or joint): It preserves the structural integrity of the assembly without requiring major modifications, which is crucial for specific designs.
Eliminates the risk of operator injuries in hazardous areas: By avoiding risky manual procedures, it contributes to operator safety, especially in hazardous environments.
Overview of the disadvantages of the connected tensioner nut
Costly, making it a non-economic solution: The initial investment can be high, which may be a limitation for some companies.
Tightening requires time and specialized skills: Requires specific expertise to ensure precise tightening, potentially increasing labor costs.
Not compatible with all bolts and rods, only for M36 to M100: Limited compatibility may restrict the application of this technology to certain types of assemblies.
Requires access to the nut for measurement, which may not always be possible during assembly: Access constraints can make it difficult to perform measurements in certain assembly configurations.
Troubleshooting can be painful and difficult: Problem resolution may require considerable effort, leading to extended downtime.
Complex and expensive maintenance: Maintenance costs, particularly for repairing potential electronic faults, can be high.
An alternative to connected assembly solutions: TRAXX Wrench
The TRAXX torque wrench offers substantial advantages for the assembly process. By increasing tightening accuracy by 30%, it also simplifies operations by allowing both the tightening of bolts and verification of tightening force with a single tool.
This wrench provides real-time consultation of tightening force during assembly while preserving existing operational habits. Functioning as a torque/angle wrench, the TRAXX wrench is distinguished by its enhanced precision, measuring tightening force directly in tension (kN). Accessories tailored to individual needs are available, offering customization of the assembly process.
Real-time monitoring of tightening tension
Real-time monitoring of tightening tension is crucial for several reasons:
Ensures correct component fixing: It guarantees that each component is properly secured and meets technical specifications. Incorrect tension can lead to issues such as leaks or premature failures.
Immediate detection of deviations: Immediate detection of deviations from recommended parameters allows for corrective actions before serious damage occurs. Real-time monitoring also aids in adjusting and optimizing the assembly process, avoiding over-torquing or under-torquing that could compromise safety and performance.
Traceability and documentation: It contributes to traceability and documentation of the assembly process, which is critical in sectors such as aerospace, automotive, and industrial equipment. Real-time consultation of tightening tension ensures reliability, safety, and performance of the equipment while optimizing assembly processes.
In the long term, the TRAXX-M2 wrench preserves and maintains data on each bolt’s tightening. It records the bolt’s identifier, characteristics, date and time of tightening, and the applied tension. This information is valuable for tracking bolt performance over time, allowing precise control of the work performed. This functionality facilitates traceability of operations, analysis of results, and proactive planning of tightening maintenance schedules.
