Monitoring of bolted connections - causes of preload loss and why bolt monitoring during operation is important
In many installations, the reliable performance of flange and bolted connections is critical: tower flanges and rotor blade bolts in wind turbines, crane runway connections, steel construction nodes, chimneys and process flanges in the chemical and petrochemical industries. In all these cases, structural safety depends almost entirely on the acting preload in the bolts.
During design, preload is defined, calculated and introduced through installation methods such as torque control, torque-angle procedures or tensioning methods. In operation, however, the actual preload often deviates significantly from these target values. Operators who want to monitor flange connections or monitor preload in bolted joints frequently observe preload losses, loose bolts, increased inspection efforts and sometimes unexpected leakages. How preload can be measured reliably is explained in the knowledge article: Methods for measuring bolt preload.
Monitoring of bolted connections - meaning continuous bolt monitoring or condition monitoring of bolted joints - aims to record the actual bolt force and its changes during operation. This article describes the most important technical causes of preload loss and the typical problems that are difficult to detect without monitoring.
Highlights
Preload losses occur invisibly and often shortly after installation.
Settling, relaxation, temperature effects and dynamic loads lead to significant force losses that remain undetected without measurement.
Typical issues such as leakages, loosening and high inspection efforts are a direct consequence.
Flange openings, the need for retightening and unclear safety margins occur because the actual bolt force during operation remains unknown.
Continuous bolt monitoring makes force progressions visible and creates real operational safety.
Monitoring reveals settling, temperature cycles, load redistribution and dynamic forces, and it enables condition-based maintenance instead of fixed intervals.
Mechanisms that lead to preload losses
Settling and relaxation after installation
Immediately after installation, settling processes begin: gaskets, coatings and contact surfaces adapt, and surface roughness is compressed. As a result, the bolt loses part of the introduced preload without any visible external signs. In many cases, preload loss during the first hours and days can amount to several tens of percent. Operators who need to retighten bolts shortly after installation are experiencing exactly this effect.
Temperature effects in flange connections
High temperature cycles affect chimney flanges, exhaust piping of combined heat and power units, process flanges and hot gas lines. Bolts, flanges and gaskets expand differently. With each heating and cooling cycle, the bolt force and the clamping force in the flange connection change. Over many cycles, the average preload can gradually shift downward.
Dynamic loads
In wind turbines, changing wind loads, bending moments and torsional loads act on bolts in tower flanges and rotor blade connections. In crane runways and steel construction nodes, dynamic load patterns arise from driving and braking operations. Large machines experience vibrations and variable process loads. These dynamic forces cause micro-movements in the bolted joint, which over time lead to frictional wear and additional preload loss.
How sensor-equipped bolts make preload losses visible during operation is described here.
Load redistribution in the flange
In real flanges, load distribution is rarely ideal. Manufacturing tolerances, differing stiffnesses, uneven contact surfaces or local deformations can cause some bolts to be loaded significantly higher than others. The affected bolts operate at higher bolt force, while others work at lower levels. Without monitoring, this uneven force distribution remains invisible.
Friction variability during installation
Many operators rely on a defined installation method, for example a specified tightening torque. However, the relationship between torque and preload is strongly dependent on friction. Differences in lubrication, coating condition, corrosion or thread quality lead to considerable scatter in the achieved preload. As a result, a group of bolts enters operation with different force levels from the start.
Changes in clamping force
In addition to bolt force alone, the clamping force that keeps the flange faces together is crucial. Due to settling, gasket flow, coating degradation or corrosion, the clamping force can decrease faster than the bolt force. A detailed examination of the relationship between preload and clamping force is provided in the separate article Preload vs. Clamping Force.
Typical operational problems in bolted connections
Leaking or critical flange connections
In thermally and mechanically highly loaded flange connections, such as process flanges, chimney flanges or exhaust lines, decreasing clamping force leads to local flange openings. Operators perceive this as leakages, increased leak rates or irregularities during tightness tests. Monitoring of the flange connection can show how bolt force decreases during operation and at which point critical conditions appear.
The relationship between preload and clamping force is explained in detail in this article:
Preload vs. Clamping Force
Loosening bolts and the need for retightening
At crane runway connections, steel construction nodes or rotor blade bolts in wind turbines, there are cases where bolts must be significantly retightened during inspections or show visible movement. The causes are usually combined effects of dynamic loading, settling and uneven load distribution. Without bolt monitoring, it remains unclear when this condition developed and how quickly it progressed.
High effort for regular torque inspections
Where inspection regulations require annual or more frequent torque checks, bolts must be inspected with considerable effort. This applies for example to chimney flanges on CHP units, critical steel construction connections or wind turbines. However, torque inspection provides only a snapshot and influences the preload itself. Operators who are looking for an alternative to torque-only inspections increasingly turn to monitoring solutions that allow bolt force to be tracked over time.
Unclear safety margins
In many existing installations, it is unclear what safety margins the current bolted connections actually have. The original calculations are often many years old, the installations have been modified several times, loads have increased and operating conditions have changed. Without knowing the actual bolt force during operation, any assessment of the available safety margins remains speculative.
Reasons for monitoring bolted connections
Monitoring of bolted connections, often referred to as bolt monitoring or flange connection monitoring, provides operators with several technical and organizational advantages:
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transparent representation of bolt force development after installation
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detection of preload losses caused by settling and temperature cycles
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identification of dynamic loads in bolts and flanges
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assessment of load distribution within bolt groups (for example tower flanges or crane runways)
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basis for condition-based inspections instead of purely interval-based checks
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reduction of dependence on recurring torque inspections
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support in evaluating safety margins and remaining service life
Monitoring therefore complements traditional inspection methods and forms the basis for condition monitoring of bolted connections, especially for bolts of size M36 and larger and for safety-critical joints.
Relevant measurement parameters in bolt monitoring
Bolt force
The central measurement parameter in monitoring is the acting bolt force. It indicates the actual tensile force present in the bolt shank. By analyzing the progression of bolt force over time, settling effects, thermal influences and changes caused by dynamic loading can be identified.
Dynamic force components
In addition to the static force level, dynamic components such as vibrations, load peaks and load spectra are relevant. They provide insight into how strongly a bolted connection is exposed to varying loads during operation and whether this behavior changes over time.
Relationship to clamping force and flange behavior
From the bolt force and the stiffness of the bolt and the flange, the clamping force behavior can be derived. Changes in the force signals may indicate that flanges are opening locally or that load paths are shifting. The theoretical foundations of this relationship are explained in the article Preload vs. Clamping Force.
Limitations of traditional inspection methods compared to monitoring
Traditional installation and inspection methods are not suitable for continuous monitoring of bolted connections. A detailed description can be found in the article Methods for Measuring Bolt Preload.
Torque inspection
Primarily checks the torque, not the actual bolt force. It is strongly dependent on friction, provides only single measurements and may even alter the preload.
Ultrasonic bolt inspections
Can determine preload at individual points in time, but they are labor-intensive, dependent on temperature and coupling conditions, and usually not designed for continuous monitoring.
Visual inspections and simple functional checks
Do not detect internal force conditions. Many preload losses remain unnoticed as long as no obvious damage or leakages occur.
FAQ
Application areas with high benefit potential
Monitoring of bolted connections is particularly useful where:
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large bolts (typically M36, M42, M48, M64 and larger) are used
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safety-critical flange connections are present
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dynamic loads and temperature cycles occur
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bolts are difficult to access or cause high downtime costs
Typical application examples:
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tower flanges and transition pieces of wind turbines
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rotor blade connections (blade bolts) with high dynamic loading
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crane runway connections and highly loaded steel construction nodes
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chimney flanges and exhaust flanges in CHP units and power plants
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process flanges in chemical plants, petrochemical facilities and refineries
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heavy machinery (crushers, mills, rolling mills)
Conclusion
Preload losses and changing load distributions in bolted connections are a central cause of leakages, loose bolts, increased inspection effort and safety-critical conditions. The underlying mechanisms - settling, temperature cycles, dynamic loading and load redistribution - act continuously and are difficult to detect without measurement. For real-time monitoring of preload and dynamic loads, the integrated sensor technology of the S.Bolt XP is suitable.
Monitoring of bolted connections provides operators with direct insight into the acting bolt force and its development over time. It creates the basis for evaluating flange connections, tower flanges, blade bolts or chimney flanges not only through torque inspections and visual checks, but through actual measurement data. For critical bolted joints and large-diameter bolts, bolt monitoring is therefore an essential component of safe and economically efficient plant operation.
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