Test yourself: How many of these five statements do you believe are true?
"High torque = secure joint." - "Retightening makes it safe." - "The friction coefficient is constant." - "Calibrated tools guarantee quality." - "Category C doesn't need testing."
If you hesitated or agreed with even one of these, keep reading. These misconceptions are widespread in day-to-day production, technically incorrect - and in safety-critical applications they cost far more than a simple rework.
First, check your baseline knowledge with the interactive myth check:
Myth 1: "High torque = secure joint"
Why this myth is so persistent
Torque is measurable, tangible and can be checked directly with any torque wrench or torque tester. It is tempting to treat it as a direct measure of bolt joint quality. Many assembly instructions reinforce this impression by specifying only torque values.
Why it is wrong
Torque is an auxiliary quantity - not a direct measure of preload force. From the applied tightening torque, roughly 50% is physically lost to thread friction and about 40% to under-head friction. Only the remaining ~10% actually generates the desired preload force (clamping force) in the bolt joint.
The key consequence for torque monitoring: with torque-controlled tightening, friction variability creates a large scatter in clamping force. To achieve the same preload force with a constant torque, the friction coefficient would also have to remain constant - and that is exactly what you cannot guarantee in series production.
| Friction coefficient μ | Tightening torque | Resulting preload force | Deviation from target value |
|---|---|---|---|
| μ = 0.08 (very smooth / lubricated) | 50 Nm | ~24 kN | +33% |
| μ = 0.12 (target / defined) | 50 Nm | ~18 kN | 0% (Reference) |
| μ = 0.16 (dry / slightly corroded) | 50 Nm | ~13 kN | -28% |
| μ = 0.20 (strongly increased / uncoated) | 50 Nm | ~10 kN | -44% |
As the table shows: if the friction coefficient varies between µ = 0.08 and µ = 0.20 - which easily occurs in practice due to batch differences, coating variance or temperature effects - the resulting preload force changes by up to 44% at identical tightening torque. The torque value is "correct". The bolt joint is not.
What is correct
The combination of torque AND angle provides a reliable picture of joint quality and is the basis for robust torque analysis and bolt joint analysis.
A common approach: a defined snug torque, followed by an additional specified angle. The basic idea: once the contact surfaces are fully seated and the bolt is working in a stable elastic region, an additional rotation angle correlates far better with actual elongation and preload force than torque alone.
The QUANTEC MCS® analysis tool from GWK captures exactly this combined torque-angle curve with reference-free angle measurement and a measurement accuracy of ±1% between 10 and 100% of the nominal range - making visible what is really happening inside the bolt joint. Used as an angle torque meter in development and assembly testing, Quantec MCS gives you a precise "fingerprint" of each joint.
Practical consequence: Check whether your tightening specifications contain only torque values. Add angle monitoring - especially for safety-critical bolt joints and wherever reaction torque and friction can vary. The article on torque-angle analysis and its interpretation explains step by step how to use the tightening curve as the fingerprint of your joint.
Myth 2: "Retightening makes the joint safe"
Why this myth is so persistent
The logic sounds intuitive: a bolt that may have settled is brought back to the required preload force by retightening. In some applications, retightening is even explicitly specified - which reinforces the impression that it is generally beneficial.
Why it is wrong
When you tighten a bolt a second time, local surface pressures increase significantly. The friction conditions under the head change because the contact geometry is no longer cleanly defined. The components may realign or tilt slightly during tightening. That in turn changes thread friction and the effective torque distribution. In short: the scatter of preload force increases - even if the specified torque is hit exactly.
Concretely: during retightening the head seating has already been pressed in and the thread has been altered. The friction behaviour is very different from the initial condition. The actual result is unpredictable - over-elongation, bolt fatigue or merely an apparently correct preload force are all possible outcomes.
What is correct
The correct preload force must be achieved on the first tightening. Capable tools, correct tightening parameters and - for critical joints - torque-angle control with suitable angle-controlled tools are the key.
Where loss of preload due to embedment is structurally unavoidable, this must be addressed by a validated assembly strategy (e.g. multiple tightening steps or yield control) - not by unverified retightening.
Practical consequence: Document which bolted joints in your production are routinely retightened - that is often a strong indicator of a non-capable initial tightening process.
Myth 3: "The friction coefficient is constant"
Why this myth is so persistent
In design and when specifying tightening torques, a fixed friction value (e.g. µ = 0.12) is usually assumed. This value is then built into the assembly specification - and often remains unchanged for years, even when sourcing or surface treatments change.
Why it is wrong
Every single bolt joint exhibits its own specific assembly preload force - even if a complete batch of identical joints is tightened in exactly the same way. The main reason is unavoidable scatter in the friction coefficients in the thread and under the bolt head. On top of that come process and tool variation as well as dimensional and geometric tolerances.
Especially relevant for assembly quality assurance: friction coefficients scatter significantly - particularly when bolts are supplied in "as delivered" condition from different vendors, which is typically required in series production to ensure supply security.
Even seemingly small changes have major effects: modified surface treatments change the friction coefficients substantially, which is often only detected with a delay. Trivalent passivated surfaces, for example, show a much higher scatter in friction values. A change of supplier, a new batch or a different lubricant quality - and your carefully calculated tightening torque suddenly produces incorrect preload forces.
Lubrication with oil, paste or assembly grease lowers friction values. As a result, preload force often increases at the same torque. Anyone who tightens "as always" can suddenly over-tighten - without doing anything "wrong" from their point of view.
What is correct
Regular friction coefficient testing and consistent angle monitoring are necessary to keep preload force scatter in series production under control.
The torque-angle curve, as recorded by QUANTEC MCS®, makes friction anomalies visible as changes in the curve shape - long before a quality problem appears in the field. This type of torque analysis is an effective early-warning system in bolt joint analysis. You will find a detailed explanation of how to interpret these curves in our article Torque-angle analysis: how to read the "fingerprint" of your bolted joint.
Practical consequence: Perform a new process validation whenever you change material suppliers, surface treatments or lubricants. The process capability study (PFU) according to VDI/VDE 2645-3 provides the right framework and metrics.
Myth 4: "Calibrated tools guarantee correct tightening"
Why this myth is so persistent
Calibration is time-consuming, well-documented and required by auditors. It is tempting to see it as proof of quality for the entire tightening process. Many production managers regard a valid calibration certificate as sufficient evidence of standard-compliant tightening.
Why it is wrong
Calibration ensures that the tool measures correctly - not that the process is correct. That is a fundamental difference.
The MFU (machine capability study) is a short-term study under ideal, constant conditions that evaluates only the machine or tightening tool. The PFU (process capability study) is a long-term study under real series conditions that looks at the entire process including all disturbing influences - operator, material, environment and method.
In contrast to the machine capability study (MFU), the process capability study also considers, in addition to the machine influence, the influence categories man, material, method and environment.
A tool with an excellent Cmk value can still deliver defective bolt joints in series production - for example if a component is started crooked by an untrained operator, if the lubricant has changed or if temperature effects shift friction conditions and reaction torque. The calibration protocol says nothing about any of this.
| Criterion | Calibration | MFU (Machine capability) | PFU (Process capability) |
|---|---|---|---|
| What is being tested? | The measuring instrument / tool | The tool under ideal conditions | The entire screw-driving process in series |
| Result quantity | Measurement uncertainty, deviation | Cmk value | Cpk value |
| Conditions | Test stand / laboratory | Controlled conditions, 1 operator | Real production conditions, all influences |
| Includes people, material, environment? | No | No | Yes (all 5M) |
| Standard reference | ISO 6789 / DAkkS | VDI/VDE 2645 Part 2 | VDI/VDE 2645 Part 3 |
| Statement about joint quality? | Indirect (tool accuracy) | Partially (tool capability) | Yes (process capability in production) |
Only when MFU and PFU have both been carried out and documented do you have documented evidence of a truly capable tightening process - as VDI/VDE 2862 requires for category A and B joints. What the capability indices Cmk and Cpk actually mean and how to calculate them is explained in our article on Cmk and Cpk in tightening technology.
What is correct
Calibration is necessary, but not sufficient. Calibration of your tools - ideally by a DAkkS-accredited laboratory such as the GWK calibration lab with the DWPM-1000® test machine of accuracy class 0.2 - is the foundation.
Building on that, the MFU demonstrates tool capability, and the PFU proves process capability under series conditions. Only all three elements together provide a complete quality record for your torque-controlled and angle-controlled tools.
Practical consequence: Check whether your company performs and documents regular MFU and PFU studies in addition to calibration. If not, contact our team for a structured tightening process analysis.
Myth 5: "Category C joints don't need to be tested"
Why this myth is so persistent
According to VDI/VDE 2862, category C refers to bolted joints where failure does not result in personal injury or significant functional impairment. In practice, "non-critical" is easily equated with "no control" - especially when quality assurance resources are tight.
Why it is wrong
The German Product Liability Act (ProdHaftG) applies to all products - regardless of the classification of the bolted joints used. In the event of damage, the manufacturer must prove that it complied with the state of the art. Quality assurance is the foundation of product liability.
Product liability means liability for compensation for delivering a defective product and for damage caused to other legal assets. Liability initially exists towards any customer if the manufacturer's fault can be proven.
If, in the event of a complaint - even for a seemingly minor category C bolt joint - you cannot document that tools were checked regularly and the process was monitored, you face a serious liability problem. Pointing out that "it was only a C joint" offers no protection. Our article Liability risk in screw assembly: why the 'state of the art' is not optional describes the full liability chain and its consequences in detail.
What is correct
Category C joints also require basic tool accuracy and regular testing as part of systematic assembly quality assurance.
For many category C applications, the Q-CHECK® QS and audit tool from GWK is the most economical solution: With a measuring range of 3-1000 Nm, an accuracy of ±1% between 10 and 100% of the nominal range and 2 GB of memory for up to 1,000 bolted joints it enables fast, documented residual torque measurements directly in production - without the need for a complex lab setup.
Practical consequence: Classify all bolted joints in your production systematically according to VDI/VDE 2862. Even for C-class joints, the rule is: perform tests, document them, and keep evidence ready. For support, see our audit checklist: 10 points your auditor will check in tightening processes.
Summary: The 5 myths at a glance
All five myths follow a common pattern: they reduce complex physical and process-engineering relationships to a seemingly practical rule of thumb - while ignoring the decisive influencing factors.
| # | Myth | Reality |
|---|---|---|
| 1 | High torque = secure joint | Torque is only an auxiliary variable; friction scatter causes up to 44% variation in clamping force |
| 2 | Retightening makes it safe | Changed friction conditions make the result unpredictable; quality must be achieved on the first tightening |
| 3 | Friction coefficient is constant | Batch, coating, lubricant and temperature can cause 20-40% friction variation |
| 4 | Calibrated tools guarantee quality | Calibration validates the tool - not the process; only MFU + PFU together prove process capability |
| 5 | Category C needs no testing | Product liability law applies to all bolted joints; missing documentation creates liability risk |
The good news: all five myths can be controlled with the right know-how, systematic torque monitoring and the right tools.
What you can do now - concrete next steps
You do not need to rebuild your tightening process from scratch. But it is worth taking an honest look at your current practice and tightening technology:
- Capture both torque and angle - QUANTEC MCS® with reference-free angle measurement makes this possible without additional fixtures and is ideal where you need a mobile angle torque meter for advanced bolt joint analysis.
- Perform MFU and PFU together - only then do you obtain fully documented capability evidence according to VDI/VDE 2645 and can reliably prove the stability of your tightening and assembly testing processes.
- Test all bolt joint categories regularly - including category C. The Q-CHECK® QS and audit tool is the efficient solution for ongoing production.
- Document without gaps - the OPERATOR® with WLAN data transmission and optional barcode scanner sends tightening results directly into your quality documentation and supports continuous torque monitoring and assembly quality assurance.
If you are unsure where blind spots exist in your process, we will gladly support you with a structured tightening process analysis - together we will develop the optimal solution for your specific requirements.
Why is torque control alone not sufficient for safety-critical bolted connections?
Torque is a surrogate measure: Only about 10% of the applied torque actually generates clamping force - the rest is lost to thread friction and head bearing friction. If the coefficient of friction varies (due to coating variance, lubrication, temperature), the resulting clamping force changes significantly, even though the torque target was met exactly. The torque-angle analysis makes this relationship visible and allows early detection of anomalies.
What is the difference between MFU and PFU?
MFU (Machine Capability Study) according to VDI/VDE 2645 Part 2 tests only the screwdriving tool under controlled laboratory conditions and provides the Cmk value. The PFU (Process Capability Study) according to VDI/VDE 2645 Part 3 additionally captures all real-world production conditions - operator, material, method and environment - and provides the Cpk value. Only together do they demonstrate a verifiably capable fastening process.
Do Category-C fastenings also require regular inspections?
Yes. Even non-critical Category-C fastenings fall under the Product Liability Act (ProdHaftG). In the event of a claim, the manufacturer must prove that they worked according to the state of the art. The Q-CHECK® QS and Audit tool enables fast, documented audit checks also for C-class fastening cases, thereby ensuring your ability to provide evidence.
What does reference-point-free torque-angle measurement mean on the QUANTEC MCS®?
With conventional torque-angle tools, the angle is measured from a defined fixed reference point (e.g., head contact). The reference-point-free torque-angle measurement of the QUANTEC MCS® requires no such reference point and can measure the torque-angle directly at the tool across the entire fastening process - without additional fixtures. This enables a more precise analysis of the torque-rotation curve even in hard-to-reach installation situations.
Can I rent QUANTEC MCS® instead of buying?
Yes. With the GWK ToolRent® rental system you receive calibrated QUANTEC MCS® analysis tools on demand - weekly, monthly, or yearly, with worldwide shipping. Ideal for one-off process analyses, audits, or capacity peaks, without tying up capital.
