What Is Torque? Definition, Units & Its Role in Torque Fastening Technology

Imagine this: A wheel bolt on a vehicle is tightened with the wrong torque - too loose, too tight, or without a calibrated tool. According to industry experience, during initial inspections by independent testing organizations, up to three quarters of the torque wrenches tested are found to be outside the permissible tolerances. What may sound like a minor issue is in fact a real safety risk - and in safety-critical sectors such as automotive or aerospace it becomes a major liability issue.

At the heart of every reliable bolted joint is an understanding of a single physical quantity: torque. This article explains what torque is, how it is calculated and measured, which standards apply - and why torque alone is often not enough for robust torque control and repeatability.

Definition: What is torque?

Torque (symbol M; also: tightening torque, turning force, moment of force) is a physical quantity that describes the rotational effect of a force acting on a body. It specifies how strongly a force rotates a body mounted so that it can turn about an axis or pivot point.

The basic formula is:

M = F × r

Where:

• M = torque in newton meters (Nm)
• F = force in newtons (N)
• r = lever arm in meters (m) - the perpendicular distance between the point of force application and the axis of rotation

A simple example: If you apply a force of 100 N to a spanner that is 0.3 m long, you generate a torque of M = 100 N × 0.3 m = 30 Nm. A longer wrench produces a higher torque at the same force - the lever law in practice.

Important: Although torque and mechanical work are both expressed in newton meters (Nm), they are physically completely different quantities. Torque is not a form of energy - the unit name "joule" must not be used for torque.

When engineers ask "what is torque?" in the context of torque fastening technology, this definition is the foundation for every subsequent torque analysis and angle analysis step.

Units of torque

The SI unit of torque is the newton meter (Nm), often written as torque Nm in specifications. In international projects and in the English-speaking world, you will encounter additional units:

Use the interactive calculator above to convert torque values between Nm, ft-lb, in-lb and other units - and to understand how the friction coefficient influences the resulting preload force. This is a practical way to link the theoretical torque definition with real-world torque control.

Torque in torque fastening technology: Why it is so critical

In torque fastening technology, the tightening torque is the central control parameter of the assembly process. The goal of every bolted joint is to generate a defined preload force (clamping force) that securely holds the parts together.

The relationship between torque and preload force

Torque and preload force are linked - but they are not identical. The decisive disturbance variable is friction:

• Approximately 90% of the tightening torque is consumed in overcoming friction in the thread and under the screw head - only around 10% actually becomes the desired axial clamping force.
• The friction coefficient (μ) varies significantly depending on lubricant, surface condition, temperature and the number of reassemblies.
• A change in the friction coefficient from μ = 0.10 to μ = 0.16 can cause a deviation in preload force of more than 30% at the same tightening torque.

This means: If you only control torque, you are monitoring an indirect parameter - not the clamping force that is actually relevant for safety and repeatability. For A-class joints according to VDI/VDE 2862, the combined monitoring of torque and angle is therefore the state of the art.

You can find more details on preload force, how it is calculated, and its influence on joint safety in our dedicated article on preload force in torque fastening technology.

Measurement methods: How is torque captured?

Mechanical tools

Click-type torque wrenches (bending wrenches, Type II) are the classic tools: They provide an acoustic or tactile signal as soon as the preset target tightening torque is reached. They are simple to use, but they do not record curves and offer limited torque wrench accuracy.

Indicating torque wrenches (Type I) display the current torque on a mechanical scale or dial. They are suitable for inspection tasks, but do not offer electronic data storage or advanced torque analysis.

Electronic measurement methods

Electronic torque sensors and torque tools capture torque in real time, store measurement curves and transmit data directly to higher-level systems. Typical measurement principles are:

• Strain gauges (DMS): Measure the elastic deformation of a torsion body proportional to the applied torque.
• Piezoelectric sensors: Convert mechanical stress directly into electrical signals.
• Magnetoelastic sensors: Use changes in magnetic properties under mechanical load.

The QUANTEC MCS® from GWK uses a high-precision electronic torque sensor and achieves an accuracy of ±1% between 10 and 100% of the nominal range - with a measurement range from 3 to 1,000 Nm. This makes it a compact, lab-grade torque and angle meter for development and quality assurance, and a benchmark in precision measurement technology.

The torque-angle method

In the torque-angle method, both quantities are measured simultaneously and the characteristic tightening curve is recorded. This method enables:

• Identification of the joint characteristic (hard, soft, compliance)
• Yield-controlled tightening for maximum utilization of preload force
• Detection of joint faults (missing parts, incorrect tightening behavior)

GWK refers to this as fixed-point-free angle measurement: The angle is captured continuously over the entire tightening process - without any mechanical reference point on the tool. This provides maximum flexibility, even at hard-to-reach fasteners and in complex torque fastening technology setups.

Accuracy classes according to DIN EN ISO 6789

The standard DIN EN ISO 6789 defines the requirements for hand-operated torque tools. It distinguishes between two basic types and several accuracy classes that are key for anyone dealing with torque wrench accuracy or repeatability.

Tolerances and calibration intervals

• Tolerance Type I/II, Class A-D: ±4% of the set torque over the measurement range from 20 to 100% of the nominal value
• Tolerance Class E-G: ±6%
• Calibration interval: According to DIN EN ISO 6789, torque tools must be calibrated at least once a year or after 5,000 operations - whichever occurs first.

Practical tip: In series production with 5 fasteners per component and 50 components per shift, the 5,000-operation mark is reached within just a few weeks. Plan your calibration schedule accordingly to maintain torque control and process capability.

GWK operates its own DAkkS-accredited calibration lab with the fully automated DWPM-1000c test machine (accuracy class 0.2). This corresponds to four times higher measurement accuracy than the ±1% tolerance required of the calibration equipment itself - a foundation for high-end, norm-compliant calibration of torque and angle tools.

Torque vs. angle: Why torque alone is not enough

A common misconception in torque fastening technology is that the correct tightening torque automatically guarantees the correct preload force.

Why is this wrong? Because friction conditions vary. Even with identical screws, the same lubricant and the same tool, the actual preload force can scatter significantly - purely due to unavoidable fluctuations in the friction coefficient.

The torque-angle method significantly reduces this uncertainty:

For A-class joints - safety-critical connections according to VDI/VDE 2862 - complete documentation of both quantities is no longer optional. Tools such as QUANTEC MCS® capture torque and angle simultaneously, store the full tightening curve and transmit the data via WLAN to your quality assurance systems. This is state-of-the-art torque control and angle control in a single system.

Common torque errors in practice

Even with calibrated tools, systematic errors can occur. The most frequent issues are:

1. Incorrect handling of the tool
The point at which the operator applies force on the handle must match the intended position. If the operator grips too far forward or backward, the effective lever arm changes - the displayed torque and the torque actually applied to the fastener will differ.

2. Ignoring friction differences
Fastenings with lock washers, liquid sealants, coated screws or undefined lubricant quantities can lead to major deviations between target and actual preload force. Before defining tightening parameters, friction tests under real conditions are essential for reliable torque analysis and repeatability.

3. Worn or uncalibrated tools
Mechanical torque wrenches are subject to wear on the spring and release mechanism, causing indication accuracy to deteriorate over time. An overdue calibration interval constitutes a direct nonconformity in audits according to IATF 16949.

4. Missing process documentation
Measuring torque without documenting the value is a critical error for A-class joints. In the event of damage, you must be able to prove that the joint was executed correctly. You can find more information on the documentation requirements for tightening processes in our detailed article.

5. No distinction between testing and calibration
The Q-CHECK® from GWK is a quality and audit tool for sample-based residual torque measurements and process capability studies (PFU) according to VDI/VDE 2645-3 - it is not a calibration device. DAkkS-accredited calibration is performed in the GWK DAkkS calibration lab using the DWPM-1000c. This distinction is essential for standards-compliant processes and for anyone working under DIN EN ISO 6789 requirements.

GWK tools for precise torque control

As a hidden champion in German precision measurement technology, GWK offers a coordinated portfolio of torque tools for every phase of torque control and angle analysis:

• QUANTEC MCS® - The compact torque lab for development and quality assurance. Measures torque, angle and optionally preload force simultaneously. Accuracy ±1% between 10 and 100% of the nominal range. Fixed-point-free angle measurement, WLAN data transmission, robust aluminum-titanium construction. Frequently referenced as Quantec MCS in international projects, it sets the benchmark for precision measurement technology in torque and angle control.

• Q-CHECK® - The quality and audit tool for sample-based residual torque measurements and process capability studies (PFU) according to VDI/VDE 2645-3 in ongoing production.

• OPERATOR® - The modular production tool with an innovative interchangeable square-drive system for maximum flexibility on assembly lines. PLC communication via the OPERATOR® EST01 enables seamless integration with your production equipment.

• DWPM-1000c - The fully automated test machine in the DAkkS-accredited calibration lab for the calibration of torque and angle wrenches in accuracy class 0.2.

All GWK tools are Made in Germany, modular in design with individually replaceable components, and engineered for low life-cycle and service costs. Combined with GWK's DAkkS calibration lab and mobile services, they form a complete system for high-end torque control, angle control and repeatability in industrial applications.

Conclusion: Understand torque correctly - and measure it correctly

Torque is the physical key to the quality of every bolted joint. Those who understand the relationship between tightening torque, friction and preload force can make better decisions about tool selection, process design and calibration strategies.

The key takeaways at a glance:

• M = F × r - torque is force times lever arm, measured in newton meters (Nm)
• Up to 90% of the tightening torque is lost due to friction
• DIN EN ISO 6789 defines accuracy classes (±4% or ±6%) and calibration intervals (12 months or 5,000 operations)
• Torque + angle measured together delivers significantly higher process capability and safety than torque alone
• For A-class joints, electronic measurement and complete documentation are mandatory

Together with you, we develop the optimal measurement technology solution for your specific requirements - from the first joint analysis through to standards-compliant series production with stable torque control, angle control and repeatability.

Frequently asked questions (FAQ)