Best Practices in Residual Torque Measurement
- Frank Skog
- Jun 25, 2013
- 2 min read
This blog post describes the best practice for taking residual torque measurements using the Angle Restart algorithm. Even though the Angle Restart algorithm is vastly less subjective to operator induced variability than-peak measurements, it is not completely immune as there are other factors that affect the reported residual value. In order to minimize those factors, the following practices are recommended:
Settle time. Particularly on soft joints, try to keep the time duration between installation and residual measurement as close to constant as possible. Different joints have different relaxation rates and the greater the variability in settle time the greater the variability in residual measurement.
Wrench orientation. The axis of the handle of the wrench should be exactly perpendicular to the axis of rotation of the fastener. If not perpendicular, then there will be an upward bias in residual measurements.
Socket contact. The wrench should be held so that the socket contacts only the fastener and not the underlying work piece. If the socked is in contact, then there will be an upward bias in residual measurements.
Wrench interference. If the body of the wrench is in contact with the work piece or other interference, then there will be an upward bias in residual measurements.
Tools. The same socket and extension should be used by all auditors. The use of a taller or cracked socket or a longer or thinner extension will create an upward bias on residual measurements.
Jerking. The pull on the wrench should be smooth and steadily increasing.
Pull speed. The typical pull on the wrench should have duration of about 0.8 seconds to about 1.2 seconds. Pulling slower is at worst a waste of time. However, pulling faster than the fastener can mechanically respond will create an upward bias in residual measurements. The torque/time curve in Excel can be a useful training aid.
This curve shows good time duration and is a bit jerky. Not bad Overall.
This curve while smooth is being pulled so fast that it is likely exceeds the fastener’s mechanical response time.
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