Why preload is critical

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A bolted joint functions through preload, not through the external operational load. Preload ensures frictional locking, tightness and resistance to vibration. As soon as preload decreases - due to settling, relaxation, temperature changes or dynamic loads - the risk of failure increases significantly. A large share of damage to flange connections, steel construction nodes and machine components can be traced back to undetected preload losses. This creates a central problem for operators: the most safety‑relevant parameter of the connection has traditionally been invisible.

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The traditional problem: installation without direct measurement

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In practice, preload is usually generated indirectly, for example through torque control, torque‑angle methods or hydraulic tensioning. These methods are strongly influenced by friction at the bearing surface and in the thread. Even small variations in friction can change the achieved preload by 20 to 40 percent. After installation, no one knows what force is actually acting in the flange. VDI 2230 describes these variations in detail and shows how large the uncertainties can be, even with installation that follows all standards.

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Normative context: the central message of VDI 2230

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VDI 2230 is considered the most important standard for the design of highly stressed bolted joints. It describes calculation approaches, friction dependencies and settling processes. Its key message is decisive: all uncertainties of conventional tightening methods can be eliminated when the elongation of the bolt shank - and thus the preload - is measured directly. This provides the normative justification for the technical superiority of sensor-equipped bolts. Additional relevant standards include ISO 4014/4017 (geometry), ISO 898‑1 (strength), DIN EN ISO 16047 (friction testing), DIN 25201 (vibration‑loaded bolts) and DaSt 021 or IEC 61400‑1 for wind energy.

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Typical risks and misconceptions in practice

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In many projects, the same pattern appears:

1. Installation follows standards, yet the actual preload remains unknown.

2. Operators rely on statistical assumptions instead of real measurements.

3. Preload losses are detected only when damage occurs or when noise, leakage or flange openings appear.

4. Retightening is seen as a solution, although the underlying issue is usually the lack of force information.

5. Dynamic loads, vibrations and temperature changes are underestimated, even though they significantly affect preload.

 

What measurements reveal - and what remains invisible without sensors

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Measurements using sensor-equipped bolts reveal what conventional methods cannot detect:

• the actual preload achieved immediately after installation

• settling processes during the first hours and days

• changes under dynamic load, bending or vibration

• gradual preload losses caused by relaxation or temperature changes

• load peaks that are frequently underestimated in steel structures and wind turbines

 

This information enables a completely new level of analysis and operational management.

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How sensor-equipped bolts work

 

Sensor-equipped bolts integrate a measurement element in the bolt head or shank. It records the elastic elongation of the bolt, which is a physically clear indicator of bolt force. The bolt is installed like any standard bolt and retains its mechanical properties. The electronics transmit the measurements to an evaluation system. Installation methods, friction, material tolerances and tool accuracy no longer influence the measurement. Operators receive real values instead of statistical assumptions.

 

Benefits for operators, planners and infrastructure owners

 

Sensor-equipped bolts offer significant advantages:

• direct measurement of the actual preload

• traceable documentation of each connection

• early detection of preload loss

• well‑founded maintenance decisions in the sense of predictive maintenance

• reduction of service operations, retightening cycles and downtime

• increased safety in critical applications such as wind turbines, steel construction nodes and machine components

 

In dynamically loaded structures - offshore and onshore - the benefits are particularly high. For the first time, operators

can understand and control the actual mechanical state of their connections.

 

Outlook and contact

 

VDI 2230 provides the normative foundation, and practical experience confirms the technical necessity: direct measurement of preload is the key to safe, traceable and durable bolted joints. Sensor-equipped bolts make forces visible and create transparency regarding real mechanical safety - an essential step for modern maintenance strategies, digital monitoring and the future of critical structures.

 

For technical questions or project inquiries, we are happy to assist.