Understanding clamping force: how measured bolt elongation becomes the acting clamping force
In safety-critical bolted connections, preload is the decisive parameter. It acts inside the bolt and is responsible for the functionality of the joint. But in many applications, preload is only half the story. What ultimately matters is the clamping force, meaning the force that actually holds the two components together.
There is a clear relationship between these two quantities, but also important differences. This article uses the classical bolted-joint diagram to explain how preload and clamping force are related, why flange gaps can occur, and how sensor-equipped bolts enable continuous monitoring that allows the real acting clamping force to be derived from the measured elongation in the bolt.
Highlights
Preload and clamping force are identical only in an ideal, perfectly parallel flange. In real operation, they can diverge significantly due to settling, distortion and dynamic loads.
If the clamping force decreases, the load distribution shifts: the bolt increasingly takes over more of the dynamic loading until local flange opening suddenly changes the system behavior.
Monitoring detects these transitions at an early stage through changes in the force signal, especially in its dynamic components, long before damage or flange openings become visible.
Bolt preload and clamping force
Preload is the axial force acting inside the bolt. It is generated during tightening and produces an elastic elongation of the bolt shank.
Clamping force, on the other hand, is the force that actually acts between the connected components. It provides frictional locking and prevents the joint from opening.
How different methods determine preload is explained in the following article:
Methods for measuring bolt preload.
Bolted-joint diagram, spring stiffnesses and action = reaction
VDI 2230 describes the bolted-joint diagram as the fundamental model that shows how loads are distributed between the bolt and the clamped parts. In this model, the bolt and the clamped components are represented as linear springs:
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the bolt as a tension spring with stiffness c_s
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the clamped parts as a compression spring with stiffness c_p
During tightening, action equals reaction: the preload in the bolt is equal in magnitude to the force pressing the plates together. The bolt is elongated, and the plates are compressed. In an idealized, perfectly parallel flange, preload therefore equals clamping force.
The bolted-joint diagram shows how an external load is distributed between these two springs. When an external tensile load is applied to the joint, the bolt force increases and the clamping force in the plates decreases. The magnitude of this increase or decrease depends directly on the ratio of the stiffnesses of the bolt and the plates. Put simply: the stiffer the plates are compared to the bolt, the smaller the share of the external load that the bolt takes up.
Further details on the technical causes of preload loss can be found here.
In practice, it is often desirable to design the plates as stiff as possible and the bolt comparatively flexible.
Typical risks and misconceptions in practice
In many applications, it is assumed that a high preload automatically means a high clamping force - and that this clamping force remains largely constant over the lifetime of the joint. There are several typical situations in which these assumptions are not valid.
Flange opening
When the flange opens under load, an air gap forms. In the bolted‑joint diagram, this corresponds to the state in which the clamping force in the component has dropped to zero. The bolt still carries tensile force, but the plates at the considered location are no longer in contact. In this condition, the joint loses its functionality even though the bolt is still carrying a noticeable amount of force. This transition from “still closed” to “open” is critical in many applications.
Settling and relaxation
Settling processes in the contact surfaces -for example due to coatings, surface roughness, gaskets or micro‑plastic deformation -cause the compression of the clamped parts to decrease. In terms of the spring model, this means: the elongation of the bolt decreases, and both the preload in the bolt and the clamping force in the flange drop.
Without direct force measurement, this process remains invisible for a long time because the external behaviour of the joint changes very little at first. In safety-critical connections, it is therefore essential to quantify these preload losses through measurement data rather than relying on assumptions.
Dynamic loads
Changing external loads are distributed within the bolt/plate system according to the ratio of their spring stiffnesses. The bolt experiences only the portion of the dynamic load that corresponds to its stiffness in the system. At the same time, the clamping force decreases by a larger amount when the plates are pulled apart in the tensile direction.
This means that even comparatively moderate external load cycles can cause the clamping force to drop to very low values at critical load peaks, even though the increase in bolt force still appears uncritical. Without knowing the actual load paths and the stiffness ratio, these effects are often underestimated.
Non-parallel surfaces in the flange
In practice, flanges and plates are not always ideally parallel. Weld distortion, deformations from manufacturing or assembly, and local bending can cause the flange to be “warped” before the bolts are tightened. In such cases, the preload in the bolt initially works against the deformation of the flange: the bolt pulls the flange toward parallel alignment before any uniform clamping actually develops.
In an extreme scenario, the bolt may be correctly preloaded according to specification while the flange at that location is still not fully closed. This means that the local clamping force in that area is significantly lower than the bolt force would suggest. Such situations can only be represented to a limited extent in the classical bolted-joint diagram, yet they are highly relevant in practice, especially for large flange diameters, welded structures and uneven support conditions.
Temperature changes
Temperature changes can influence preload when the bolt and the connected components have different coefficients of thermal expansion or significantly different temperature profiles. In pure steel-to-steel systems, this effect is usually smaller than in mixed-material joints, for example steel and aluminium. Nevertheless, local temperature gradients or uneven heating can cause the load distribution between the bolt and the clamped parts to shift. Whether temperature changes play a dominant role depends strongly on the specific application.
What measurements reveal - from bolt elongation to clamping force
Sensor-equipped bolts measure preload directly through the elongation of the bolt shank. This measurement is physically unambiguous: the elastic elongation of the bolt is directly proportional to the bolt force as long as the yield point is not exceeded.
From the measured bolt force, the clamping force can be derived using the bolted-joint diagram once the spring stiffnesses of the bolt and the clamped parts are known. These stiffnesses can be:
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calculated from geometry and material properties,
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determined in a one-time test (for example on a test joint), or
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derived from normative assumptions.
Within the linear range, the following applies:
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The change in bolt force is the share of the external load taken up by the bolt.
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The change in clamping force is the corresponding negative change in the clamped parts, based on their individual spring stiffnesses.
As long as the clamping force remains greater than zero, the flange stays closed and frictional locking is maintained. When the clamping force approaches zero, the flange begins to open - the point at which the joint loses its intended function. Knowing the actual bolt force progression therefore makes it possible to assess the condition of the flange indirectly but with clearly definable uncertainty.
Why continuous monitoring is essential
In dynamic applications, the behaviour of flanges and bolts is often not strictly linear. Settling, locally changing contact conditions, temperature variations, bending effects and fluctuating loads lead to time‑dependent changes in the effective spring stiffnesses and in the preload. These effects can only be estimated to a limited extent at the time of installation.
Real-time monitoring of preload
The actual bolt force is recorded continuously. This makes load peaks, preload losses caused by settling and relaxation, and changes due to dynamic loading directly visible. Instead of documenting only the installation result, the operational behaviour of the joint becomes transparent.
Deriving clamping force from the measured values
Based on the bolted‑joint diagram, Sensorise can derive the clamping force from the measured bolt elongation, taking into account the known or calibrated spring stiffnesses. This makes it possible to identify when the clamping force is approaching a critical level, even if the absolute bolt force still appears to be within the permissible range.
Early warning system for flange opening and uneven clamping
If the clamping force approaches zero in certain load situations, or if the development of the bolt force indicates uneven clamping in the flange (for example due to distortion or non-parallel contact surfaces), the operator can react at an early stage. Maintenance actions can be initiated before visible damage, leakages or flange openings occur.
Importance for operators
The ability to know not only the bolt force but also the resulting clamping force and its development during operation fundamentally changes how operators assess bolted connections. Instead of relying on static assumptions, they can base decisions on reliable measurement data and model-based evaluations.
Operators can:
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plan maintenance in a targeted way, based on real force histories rather than fixed intervals
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quantitatively evaluate critical load situations
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operate flanges and other highly loaded joints more safely
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reduce inspection and testing costs by complementing or replacing traditional torque checks and purely visual inspections with data-based assessments
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document mechanical safety and compliance with internal or regulatory requirements
Outlook and contact
Clamping force determines whether a flange remains closed and fulfills its intended function. Preload alone does not provide complete information - especially not over the entire service life of a joint. Through continuous measurement of bolt force and the derived evaluation of clamping force, the actual condition of a bolted connection can be assessed far more accurately than with classical methods.
If you would like to know how this concept can be applied to your specific use case - especially for highly loaded flange connections, large-diameter bolts and dynamically stressed systems - feel free to contact us.
Further information on integrated force measurement in highly loaded bolted connections can be found here.
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Sensorise GmbH
Fahrenheitstraße 1
28359 Bremen
Germany
+49 (0)421 220 834 0
