Imagine this: a torque wrench shows exactly the specified tightening torque - yet the joint still fails in service. No measurement error, no material defect. The root cause is a parameter you cannot read directly, but which determines the safety and service life of every bolted joint: preload force.
In industrial bolting technology, the preload force is the primary target variable - and at the same time the most difficult one to control. This practical guide explains what preload force is, how to calculate preload, which factors influence it, and how to control it reliably in production.
Definition: What is preload force?
Definition of preload force (F_V): The preload force is the force that, after tightening a screw in the screw connection, acts as clamping force. It holds the components to be joined together and largely determines the reliability, tightness, and load-bearing capacity of the connection. Unit: Newton (N) or kilonewton (kN).
In technical literature, preload force - denoted as F_V - is generated by the elastic elongation of the bolt during tightening. The bolt behaves like a stretched spring: it clamps the components together and creates the clamping force in the joint interface. As long as external service loads do not exceed this clamping force, the joint remains leak-tight, slip-resistant, and load-bearing.
In everyday language, the terms preload force and clamping force are often used synonymously. Technically precise, however, the following applies: the preload force F_V acts in the bolt as a tensile force, while the clamping force F_K is the resulting compressive force in the joint interface - after considering embedment losses and service loads in accordance with VDI 2230. In engineering English this is often referred to as bolt preload or torque preload.
Why is preload force so important?
The operational safety of highly loaded bolted joints depends critically on the level of preload force. Sophisticated design calculations and advanced manufacturing methods are ineffective if a joint is tightened too high or too low during assembly.
Concretely: if the preload force is too low, you risk self-loosening of the bolt, leakage at flanged joints, and fatigue failure due to fluctuating loads in the components. If it is too high, you risk bolt fracture, thread damage, or plastic deformation of the clamped parts.
VDI 2230 as the basis for calculation
VDI 2230 "Systematic calculation of high duty bolted joints" is regarded worldwide as the standard reference for the design and bolt joint analysis of safety-critical joints. It provides engineers with a systematic method to determine the required minimum clamping force, the permissible assembly preload force, and the necessary tightening torque - taking into account all relevant influence factors.
In safety-critical industries such as automotive and aerospace, compliance with VDI 2230 is not optional - it is mandatory, and the basis of every standards-compliant A-class bolted joint.
Embedment losses: the invisible enemy of preload force
A frequently underestimated phenomenon: shortly after tightening, every bolted joint loses part of its preload force due to embedment. Micro-roughness on the bearing surfaces, threads, and joint interface flattens plastically under load - the joint "settles". Typical embedment losses range, depending on joint type, from 5 to 25% of the assembly preload force. VDI 2230 accounts for this effect via the embedment value f_Z.
Especially with multiple joint interfaces and soft materials, embedment losses can significantly reduce the clamping force.
Factors influencing preload force
Preload force results from a complex interaction of several parameters. Only those who know and control these parameters can achieve reliable repeatability in torque control and bolt preload:
| Influence factor | Impact on F_V | Practical note |
|---|---|---|
| Friction coefficient μ (thread and head contact) | Very high - determines how much of the torque is converted into clamping force | Document lubricant, coating, and surface condition precisely |
| Surface roughness (R_z) | Rougher surfaces increase friction -> lower F_V at the same torque | Check roughness classes according to the drawing and clean contact surfaces |
| Tightening torque M_A | Direct proportionality - higher M_A -> tend to higher F_V | Use calibrated tools; uncalibrated wrenches can vary by ±30% |
| Screw material / strength class | Determines the allowable preload (R_p0.2 yield strength) | Strength classes 8.8, 10.9, 12.9, each with different F_V limits |
| Temperature | Thermal expansion can decrease or increase F_V (material-dependent) | Especially relevant in automotive and aerospace: account for temperature fluctuations |
| Settling behavior (f_Z) | Plastic deformation of micro-roughness -> preload loss after assembly | Settling losses can be 5-25%; account for the preload according to VDI 2230 |
Calculating preload force: formulas and relationships
The full formula according to VDI 2230
The physical relationship between tightening torque M_A and preload force F_V is described in the full torque formula according to VDI 2230:
M_A = F_V × [d₂/2 × tan(φ + ρ') + μ_K × D_Km/2]
Where:
- d₂ = flank diameter of the thread (mm)
- φ = thread pitch angle
- ρ' = thread friction angle (depending on μ_G and flank angle)
- μ_K = friction coefficient under the bolt head or nut face
- D_Km = effective friction diameter under the head or nut (mm)
This formula makes it very clear: the assembly torque required depends on the resulting preload force, thread pitch, flank diameter, and the friction conditions in the thread and under the head or nut.
The simplified approximation formula
For day-to-day engineering practice and quick estimation, a simplified form is often used to calculate preload:
M_A ≈ F_V × k × d
Where:
- M_A = tightening torque (Nm)
- F_V = preload force (N)
- k = overall tightening factor (typically 0.16-0.20, depending on friction and geometry)
- d = nominal / outside thread diameter (m)
Rearranged for F_V:
F_V = M_A / (k × d)
This approximation is suitable for initial sizing of clamping force in a bolt. However, it does not replace the full bolt joint analysis and calculation according to VDI 2230 for safety-critical connections.
Interactive preload force calculator
Try it out directly: our calculator illustrates how torque, friction, and bolt diameter influence the achievable preload force - and whether your bolt design is still within the permissible range.
Torque vs. angle: why tightening torque alone does not guarantee preload force
This is the most critical point in industrial bolting technology - and one of the most frequently underestimated in torque analysis and assembly testing:
In practice, depending on friction conditions, up to 50% of the applied torque is consumed solely in overcoming friction under the bolt head or nut. Only a small fraction of the input torque is actually converted into bolt preload.
The consequence: even small changes in the friction coefficient μ - for example due to a drop of oil, varying coating quality, or temperature effects - can cause large changes in the resulting preload force. With purely torque-controlled tightening, the scatter of preload force can easily reach ±30% or more. In terms of repeatability, torque control alone is not sufficient for many safety-critical joints.
Angle-controlled tightening as the superior alternative
The torque-and-angle method (combined torque/angle method) addresses exactly this issue: first, a defined snug torque is applied to seat the contact surfaces and overcome initial friction. Then, a precisely specified tightening angle is applied. Since this tightening angle correlates directly with the elastic elongation of the bolt - and is less sensitive to varying friction - the preload force can be set with significantly better repeatability.
This is where precise angle analysis and a robust angle gauge or torque/angle tool become crucial.
| Criterion | Torque control | Torque-rotation-angle method |
|---|---|---|
| Control principle | Tightening until target torque M_A is reached | Preload torque + defined rotation angle φ |
| Effect of friction | Very high - friction directly affects the result | Low - angle correlates with strain |
| Variability of preload | Up to ±30 % (with varying friction) | Significantly reduced |
| Measurement effort | Low - only torque sensor needed | Higher - torque and angle sensor needed |
| Standards-compliant testing | Basic requirement according to VDI/VDE 2862 | Recommended for A-class bolted connections |
| Typical application | Standard fastenings, Category B/C | Safety-critical connections, Category A |
| GWK tool | OPERATOR®, Q-CHECK® | QUANTEC MCS®, OPERATOR® with angle measurement |
GWK QUANTEC MCS®: precise torque and angle analysis for preload force control
Direct measurement of preload force in running series production is complex - it requires strain gauges or piezo sensors directly on the bolt. The practical solution is highly precise indirect control via combined torque and angle analysis.
This is exactly where GWK QUANTEC MCS® comes in - the compact "screw lab" for development, torque analysis, and quality assurance.
What QUANTEC MCS® delivers:
- Simultaneous measurement of torque and angle in a single tool
- Measurement accuracy of ±1% between 10 and 100% of the nominal range - for reproducible, meaningful test and assembly testing results
- Fixed-point-free angle measurement using patented angle sensor technology with 0.1° resolution - no reference point required, no mounting-related errors
- External measurement channel for preload force measurement (16-bit resolution, connection for external piezo sensors) - enables direct F_V acquisition in bolt preload analysis
- Wireless WLAN data transmission and 2 GB internal memory for complete, gap-free documentation
- Measurement range from 3 to 1000 Nm - from fine threads to high-load bolted joints
Fixed-point-free angle measurement is a decisive advantage over conventional systems: it eliminates measurement uncertainties caused by incorrect reference setting and delivers consistent, auditable torque and angle analysis results - especially for safety-critical A-class bolted joints. For engineers focusing on precision measurement technology and repeatability, this is a key enabler.
Testing and controlling preload force in practice
Residual torque measurement with Q-CHECK®
The GWK Q-CHECK® is a specialized quality assurance and audit tool for residual torque measurements in accordance with VDI/VDE 2645-3. It measures the torque required to further turn an already tightened bolt - a direct indicator of the actual clamping force bolt condition and the embedment behavior of the joint.
Important: Q-CHECK® is not a calibration device, but a torque tester and inspection tool for process capability studies (PFU) in quality assurance. You can find more details in our article on process capability testing according to VDI/VDE 2645-3.
DAkkS-accredited calibration with DWPM-1000®
To ensure full metrological traceability of your torque tools and torque tester systems, GWK operates its own DAkkS-accredited calibration laboratory with the fully automatic DWPM-1000® test machine (accuracy class 0.2). This guarantees that all torque and torque/angle tools in use are properly calibrated and traceable - an essential prerequisite for compliant processes according to IATF 16949, VDI/VDE 2862, and ISO 9001.
The fully automatic DWPM-1000® operates in accuracy class 0.2 and enables DAkkS-accredited calibration of torque wrenches and torque-angle wrenches.
Ultrasonic measurement (supplementary)
For selected, highly critical applications - for example in aerospace or bridge construction - preload force can also be measured directly via ultrasonic determination of the elastic elongation of the bolt. This method is independent of friction, but requires significant effort and specially prepared bolts.
Typical errors and their consequences
Preload force too low
- Causes: tightening torque too low, high friction (uncalibrated tool, dry/corroded threads), excessive embedment losses
- Consequences: self-loosening under vibration, leakage in flange joints, fatigue failure due to increased load cycles in the bolt, safety failures in A-class joints
Preload force too high
- Causes: tightening torque too high, very low friction (over-lubricated joint), wrong strength class
- Consequences: plastic deformation of the bolt beyond yield strength, thread damage, bolt breakage during tightening, overstress damage in clamped components
The golden rule: neither too tight nor too loose - and the target preload must be verified by measurement.
How to control preload force in your production: step by step
To keep preload force reliably under control and to optimize repeatability in torque control, we recommend the following approach:
Design according to VDI 2230: Calculate the required minimum clamping force, assembly preload force, and tightening torque, considering all influence factors - including friction coefficient, embedment, and service loads.
Select the tightening method: For safety-critical A-class joints, use the combined torque-and-angle method to minimize scatter in preload force.
Use calibrated tools: Only tools calibrated and traceable to DAkkS ensure that the specified tightening torque is actually achieved.
Analyze the tightening process: Use QUANTEC MCS® to record and evaluate the torque-angle curve for each bolt case - any irregularities in the curve provide direct indications of friction problems or embedment effects.
Prove process capability: Use Q-CHECK® for regular residual torque measurements and process capability studies (PFU) according to VDI/VDE 2645-3 - as a basis for the audit checklist of your bolting processes.
Document everything: Archive all measurement data electronically - using software such as QuanLab Pro® or EasyWin® - to ensure continuous, gap-free traceability.
FAQ: preload force in practice
What is the difference between preload force and clamping force?
The terms are often used synonymously, but technically describe different quantities: The Preload force F_V is the tensile force acting in the screw, generated by tightening. The Clamping force F_K is the resulting compressive force acting in the joint gap between the clamped components. According to VDI 2230, the clamping force may be smaller than the mounting preload force due to settling losses and operating loads.
Why does the preload vary so much in torque-controlled assembly?
Because the tightening torque M_A is largely — up to 90% — consumed to overcome friction in the thread and under the screw head. Even small changes in the coefficient of friction μ (e.g., due to lubricant residues, surface coatings, or temperature) lead to substantial scatter of the actually achieved preload, even when the target torque is applied precisely.
What are settling losses and how large are they typically?
Settling losses arise from the plastic deformation of micro-roughness at the contact surfaces (thread, head bearing, the joint gap) shortly after tightening. As a result, the clamping force decreases without the screw turning. Typical settling losses are, depending on the connection type, number of separation gaps and surface finish, around 5 to 25 % of the mounting preload and must be accounted for in the design via the settling amount f_Z according to VDI 2230.
When is ultrasonic preload measurement useful?
Ultrasonic preload measurement (Bolt Elongation Measurement) measures the elastic elongation of the bolt directly and independently of friction. It is especially useful for safety-critical connections (e.g., aerospace, wind energy, bridge construction), where the indirect control over torque and angle is not sufficient. However, the effort is considerably higher than the torque/angle analysis.
Which standard governs the calculation of preload forces in highly stressed bolt connections?
VDI 2230 ("Systematic calculation of highly stressed bolt connections") is the globally recognized standard for the design of highly stressed bolt connections. Part 1 covers single-bolted connections, Part 2 multi-bolt connections. For the process safety of fastening tools, the additional standard VDI/VDE 2862 is relevant as well, which defines bolt-case classes A, B and C.
How can I monitor the preload in ongoing production?
In serial production, preload is controlled indirectly through torque and angle analysis. Analysis tools such as the GWK QUANTEC MCS® enable simultaneous capture of torque, angle, and — via an external measurement channel — also directly of the preload force. In addition, the Q-CHECK® as a QA and audit tool provides further torque measurements according to VDI/VDE 2645-3 to perform settling losses and process capability studies (PFU).
Conclusion: preload force is more than just a calculated value
Preload force is the invisible foundation of every reliable bolted joint. It can be calculated precisely - but it can only be maintained reliably if the tightening method, friction conditions, tool accuracy, and process monitoring are consistently aligned.
Practical experience shows: relying on tightening torque alone is not enough in safety-critical applications. The combination of fixed-point-free angle measurement, a combined torque-and-angle tightening method, and systematic process capability testing is the way to reproducible, auditable preload forces.
GWK supports you along this path - from bolt joint analysis with QUANTEC MCS®, through PFU testing with Q-CHECK®, to DAkkS-accredited calibration in our in-house laboratory. Accuracy by GWK - backed by 30 years of precision measurement technology Made in Germany.
