Methods for measuring bolt preload - a technical comparison for large-scale bolted joints
In safety-critical bolted connections such as tower flanges, process flanges, crane runway joints or large steel construction nodes, the structural integrity depends significantly on the preload present in the bolts. This preload determines stiffness, tightness, vibration behavior and fatigue resistance. However, it is not visible during operation and can change over time due to settling, temperature cycles or dynamic loads.
For operators who want to monitor flanges or large bolted connections, the key question is which measurement methods are suitable for determining bolt preload and which of these can also be used for continuous bolt monitoring.
This article compares the most important approaches and explains how different measurement methods behave during installation, operation and continuous monitoring.
Highlights
Torque-controlled tightening and tensioning methods are suitable for installation, but they do not measure the actual bolt force during operation. Friction variability and installation influences lead to significant uncertainties.
Ultrasonic measurement provides accurate single measurements, but it is temperature-dependent, slow and unsuitable for large groups of bolts, which makes it impractical for continuous monitoring.
Sensor-equipped bolts measure the actual bolt force and the dynamic load components directly in the shank. They are particularly well suited for continuous monitoring of highly loaded bolted connections.
Requirements for measurement methods in critical applications
In industrial applications with bolts from M24 to M80, measurement methods must meet several technical requirements.
First, the mechanical load-bearing capacity of the bolt must not be impaired by the measurement method. Bolts in tower flanges, crane runways or chimney flanges are highly loaded; any reduction of the core cross-section affects safety and design.
In addition, a measurement method must capture the actual bolt force and not only indirect parameters such as torque or rotational energy. Ideally, the force is measured through the elongation of the bolt shank, since the elastic elongation of the bolt is directly proportional to the acting force.
For operators, it is also important whether a method is suitable only for installation or whether it can also provide data during operation. For condition monitoring, measurement must be possible during ongoing operation, either continuously or automatically at defined intervals.
Furthermore, a robust mechanical design is required. Bolts should behave as closely as possible to standard fasteners, without special geometries or delicate additional components that complicate operation. From a measurement technology perspective, the integration should be open so that existing amplifiers, data loggers and control systems can be used without requiring proprietary hardware.
Classical methods for determining bolt preload
Torque and torque-angle methods
These methods are the most widely used techniques for installation. The preload is determined by the applied torque or by the combination of torque and angle of rotation. The force calculation is based on the assumption that a defined portion of the torque is converted into elastic elongation of the bolt.
In practice, this method is highly dependent on friction. Variations in lubrication, thread friction or bearing surface conditions lead to significant scatter in the achieved preload. The method is well suited for implementing a defined tightening specification, but it does not provide any information about how the bolt force changes during operation. Retesting is also difficult because reapplying torque alters the condition of the bolt itself.
How sensor-equipped bolts capture the actual bolt force directly and provide full transparency during operation can be found here.
Tensioning method
The tensioning method first elongates the bolt axially using a hydraulic tensile force before the nut is seated against the flange. This results in significantly more reproducible preload levels because variations in friction have almost no influence on the achieved elongation. This method is well established for large bolt sizes and waisted-shank bolts, for example in rotor blade bolts of wind turbines.
However, this method is also purely an installation procedure. It provides an accurate preload at the moment of tightening, but no information about how this force develops over the service life. The method is therefore unsuitable for monitoring during operation.
Ultrasonic measurement
In ultrasonic measurement, an acoustic wave is sent through the bolt. From the change in travel time, the change in length and therefore the preload -is derived. This method is very precise when the bolt is calibrated and is well suited for spot checks of individual bolts.
However, the measurement is limited to single points in time and depends on temperature, coupling conditions and surface condition. For flange connections with many bolts, the application is labor-intensive because each bolt must be checked individually. In most cases, the inspection takes place while the machine is stopped. The method does not provide an integrated solution for continuous bolt monitoring.
Sensor-equipped bolts - functional principle and design variants
Sensor-equipped bolts measure the elongation of the bolt shank directly and derive the acting bolt force from it. They differ primarily in how the sensor technology is integrated. How the measured bolt elongation relates to the resulting clamping force in the flange is explained in the knowledge article Preload vs. Clamping Force.
One variant uses bolts with longitudinal bores in which sensors and, if necessary, electronics are integrated. These solutions reliably measure axial elongation but reduce the load-bearing core cross-section of the bolt and require renewed mechanical qualification for safety-critical applications. Depending on the manufacturer, the measurement may be based on resistive, capacitive or inductive principles.
Another variant uses external sensors such as load cells, sensor-equipped washers or sensorized nuts. These measure the force indirectly via the pressure path under the bolt head or nut, but they change the installation conditions and local stiffness. They do not measure the actual shank elongation and provide only limited accuracy in dynamic or multi-axially loaded connections.
The third group consists of bore-free integrated systems in which the sensor is applied to the surface of the bolt shank and the lead routing is implemented within the existing geometric clearance of the thread root. These solutions measure the actual bolt elongation, preserve the full core cross-section and enable high-resolution measurement during operation.
Detailed information on integrated force measurement with S.Bolt XP can be found here.
The S.Bolt XP measurement concept
The S.Bolt XP belongs to the group of bore-free sensor-equipped bolt systems. The sensor consists of a full-bridge strain gauge applied directly to the bolt shank in the clamping area. The electrical connection is routed through the thread root, so no mechanical weakening occurs and the geometry of the bolt remains fully preserved.
Mechanically, the bolt behaves like a standard fastener; the full load-bearing capacity is maintained. At the same time, the full-bridge strain gauge enables precise measurement of the axial bolt force. Depending on the variant, bending moments or torsional components can also be measured, allowing detailed analysis of the load behavior in complex bolted joints such as blade bolts, crane runway bolts or long through-bolts.
From a measurement technology perspective, the S.Bolt XP is a standard strain-gauge sensor. It can be operated with commercially available strain-gauge amplifiers and industrial measurement modules, as it does not require proprietary evaluation electronics. Integration into PLC systems, data loggers or condition-monitoring platforms is possible without any additional special hardware.
FAQ
Comparison of measurement methods - technical classification
Torque and tensioning methods provide precise installation values, but they are not suitable for determining bolt force during operation. Ultrasonic testing is a very accurate inspection method, but it remains a single spot measurement and does not reveal the continuous force progression.
Sensor-equipped bolts are the only method that can record the actual bolt force continuously or at regularly automated intervals. By measuring the elongation in the bolt shank, the acting forces are represented directly. Variants that operate without bores also meet the high requirements for mechanical integrity in safety-critical applications.
For operators who want to monitor flange connections or introduce bolt monitoring, sensor-equipped bolts are therefore the only method that can capture both static preload and dynamic force components without mechanically altering the bolt. By analyzing the dynamic components of the signal, conclusions can also be drawn about the remaining clamping force in the flange.
Conclusion
The measurement of bolt preload can be carried out using different methods, each suited to specific applications. Torque and tensioning methods remain indispensable for installation, and ultrasonic testing is ideal for single-point inspections. For evaluating bolted connections during operation and for integrated bolt monitoring, sensor-equipped bolts with direct elongation measurement are the most precise and complete technical solution.
Bore-free systems such as the S.Bolt XP enable this measurement without changing the mechanical load-bearing capacity of the bolt. They can be integrated into existing industrial measurement technology and therefore provide a practical basis for monitoring large and safety-critical bolted connections.
What is your application?
Contact us
Sensorise GmbH
Fahrenheitstraße 1
28359 Bremen
Germany
+49 (0)421 220 834 0
