For normal tension fasteners, we can usually detect thread defects and crossed threads by monitoring the angle after tightening.
However, there are joints where the angle from snug to the final torque and a crossed thread overlap using this method.
An example of this is certain thread forming screws on soft joints and also non-tension joints like pipe pressure plugs.
The prevailing torque algorithm divides the tightening process into 3 distinct zones.
The first zone is the cut-in zone. In the example above, this is set at the lowest torque level. This will detect a crossed thread if it occurs at the start of the tightening process.
The second zone is the prevailing zone. The example above shows that the thread hit a defect or became crossed during the prevailing zone and the tightening cycle was aborted.
The cut-in zone and prevailing zones are limited by angle and can be adjusted for the particular application.
The third zone is the tightening zone. To get to the tightening zone, the fastener must successfully pass through both the cut-in and prevailing zones without exceeding the maximum cut-in and prevailing torques programmed.
The result is that the tightening cycle is monitored throughout the entire tightening cycle to insure a satisfactory error-proof tightening process.