Torque wrenches apply controlled tightening force to fasteners and are critical in aerospace, automotive, and manufacturing. Calibration verifies that the indicated torque matches a reference standard across the working range. Proper calibration prevents under- or over-torqued fasteners that can cause joint failures.
Inspect the torque wrench for damage, corrosion, or worn drive components. Verify the adjustment mechanism moves freely and the scale or display is legible. Check that the ratchet head engages properly in both directions.
Mount the torque wrench and reference transducer in a calibration fixture that applies a pure torque load without side forces. Ensure the loading point is at the marked handle center and the wrench is horizontal.
Before taking data, exercise the wrench by loading it three times to the maximum test value. This eliminates the effect of static friction and conditions the internal mechanism for consistent results.
Test at a minimum of three points: 20%, 60%, and 100% of the wrench's rated capacity. At each point, apply torque smoothly until the wrench clicks or reaches the set value. Record the reference transducer reading. Repeat each point three times.
For each test point, calculate the average of three readings and the percent error relative to the set value. Determine the measurement uncertainty using the reference standard uncertainty, repeatability, and resolution contributions.
If errors exceed acceptance limits, adjust the wrench per the manufacturer's procedure and repeat the full calibration. Record both as-found and as-left data on the certificate.
Complete the calibration certificate with all readings, errors, uncertainty, and pass/fail status. Apply a calibration label. If the wrench fails and cannot be adjusted, mark it as out of tolerance and remove from service.
Per ISO 6789, error must not exceed ±4% of the reading at each test point for click-type wrenches (±6% for indicating-type). The repeatability at each point must not exceed 1% of the reading.
12 months or 5,000 cycles
For click-type torque wrenches with a coil spring mechanism, manufacturers generally recommend backing the setting down to the lowest scale value (not zero) for storage. This reduces spring fatigue while keeping the mechanism under minimal load.
ISO 6789 requires testing at a minimum of three points: 20%, 60%, and 100% of the marked capacity. Each point must be measured at least three times. Some quality systems require additional points at 40% and 80%.
A beam-type torque wrench can serve as a reference only if it has been calibrated with traceable uncertainty better than the unit under test. In practice, a calibrated torque transducer with digital readout provides much lower uncertainty and is preferred.
CalibrationOS tracks due dates, stores certificates, and generates audit-ready reports.
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