The test lab and roll inspection together might not be enough for defects that occur less frequently than say a few times an hour as we saw in the last clip. The reason is in part due to variabilities that can be as important as the averages or means.
In the case of web breaks, variation might be even more important than targets. Why does a 10 PLI strong web break when tensioned at only 1 PLI? In universal units, why might a web be damaged at tensions of only 10% of its strength?
The answer has to do with variation. Strength varies across the width and down the length. Tension changes every time the web hits a new roller in both the MD (due to bearing drag as well as drives and inertia during speed changes) and CD (due to in-plane roller misalignment, etc.) and with time (especially during accel/decel).
Web201.60bYouTube clip link: