Talking Torque with HTC #4: Service and calibration of torque wrenches

Following on from our previous articles, we now understand the significance of accurately tightening fasteners to manufacturers’ specifications, in order to prevent downtime, noncompliance, or injury to ourselves or others.

To maintain precision, tools need to be well serviced and calibrated by trained individuals using the correct testing equipment which meets international standards.

For the most part, torque wrenches use springs, gears, washers, and pins that need to be in good condition to ensure repeatable results. If something binds internally, this can cause the operator to believe they have achieved the correct torque, when the real value may be far from ideal. Servicing these tools involves disassembling, cleaning, inspecting, lubricating, reassembling, and most importantly, testing the wrench, as readings can change simply from movement of the internal parts. Hence, it’s critical that servicing is carried out in a calibration workshop.

Along with servicing, annual calibration is generally recommended by manufacturers, local law, and foreign auditing bodies. However, in high use situations, we may suggest shorter service/ recalibration intervals. The test process involves setting up the equipment, performing the test procedure and recording the results, adjusting/ servicing the tool as necessary, then re-testing.

Powered torque wrenches need specialised equipment with purpose-made transducers to simulate gradual bolt tightening. They are then taken through their working range as mentioned earlier and a record is produced. Powered tools can generate extreme forces — 33,000Nm, for example— so it is crucial that the correct procedure and precautions are adhered to in a suitable workshop.

Manual torque wrenches also require specialised equipment that meets the current international standard. This involves a ‘loading device’ which applies a known and measurable force to a torque wrench using a high accuracy transducer and counterbalanced reaction points to perform the test. We no longer test by manually clicking the wrench on a transducer, as internationally this is not considered to be accurate enough anymore.

In summary, whether you use your tooling on a road vehicle or a multimillion-dollar piece of machinery, these tools can read incorrectly and