Torque calibration
Torque is force times distance, so it is calibrated with a lever arm and weights, or against a reference transducer. Hand tools such as click wrenches are the biggest group of tested instruments.
Instruments and typical accuracy
| Instrument | Typical range | Typical accuracy | Where it is used |
|---|---|---|---|
| Dead-weight lever and arm | Up to thousands of N·m | 0.01 to 0.1 % of reading | Primary standard |
| Reference torque transducer | 0.1 N·m to over 10 kN·m | 0.05 to 0.2 % of reading | Working standard for calibration labs |
| Torque tester (wrench analyser) | Ranges of 1 to 1000 N·m | 0.5 to 1 % of reading | Testing click wrenches in service |
| Click-type torque wrench | 1 to 1000 N·m | Often ±4 % (±6 % at low torque), by class and standard | Assembly and maintenance |
| Beam or dial torque wrench | 1 to 1000 N·m | ±2 to ±4 % | Checks and fine work |
| Electronic torque wrench | 1 to 1000 N·m | ±1 to ±2 % | Critical fasteners with record keeping |
The tolerance of a wrench depends on its class and the edition of the standard. The accuracy figures are typical ranges to help you compare instruments. Always use the specification of the instrument in front of you.
Reference standards and traceability
- Primary realisation. A known mass on a lever arm gives torque from force and length, so it traces to mass, length and gravity.
- Transducers. Reference transducers are calibrated against the lever and checked with a shunt calibration.
- Standards. ISO 6789 covers hand torque tools, with ISO 6789-2 for calibration. ISO 376 covers force-proving instruments, which helps for lever-based systems.
- Quality system. ISO/IEC 17025, scope listing torque with range and direction.
How it is calibrated
Typical procedure
- Exercise the wrench by taking it to its maximum a few times, then let it rest.
- Mount it on the tester at the right drive size and with the load applied in the intended direction.
- Check at about 20 %, 60 % and 100 % of the range (typical for ISO 6789), several readings each.
- Apply torque smoothly and slowly. Stop at the click and read.
- Calculate the error at each point and compare with the tolerance.
- Adjust, retest and record as-found and as-left.
Where it goes wrong
- Load applied too fast. Click wrenches overshoot.
- Wrong handle position. The effective length is from the hand-hold, not the end.
- Extensions and adapters. They change the torque at the fastener unless corrected.
- Left at a high setting. Storing a click wrench loaded weakens the spring: return it to the lowest setting.
- Side loads. Misalignment creates bending that the transducer also senses.
What goes in the uncertainty budget
- Reference transducer uncertainty and drift.
- Arm length and force for lever-based references.
- Alignment, side loads and adapters.
- Rate of loading and repeatability.
- Resolution of the wrench and the operator.
Combine the contributions as in the uncertainty budget calculator, then decide pass or fail with a guard band as explained on the calculators page.
Calculators
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