The graph shown above is the torque vs. angle trace for a thread forming fastener. One of the typical outputs of this test is the average drive torque value, which is the highest torque necessary to form threads in a through-drilled hole (Point “a”). The other output of this sort of testing is the average strip torque, which is highest torque reached before destroying or stripping the threads in the fastened component (Point “b”).

Most OEM’s will specify a minimum ratio between these two values that needs to be achieved in order to deem the joint acceptable. This ratio, commonly referred to as the drive/strip ratio is usually 1:3 or 1:4 depending on the application. For example, the sample graph above shows a drive torque of 3.7 Nm and a strip torque of 21.3 Nm. This results in an acceptable drive/strip ratio of about 1:6.

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