For driven rollers, if one drive roller runs faster or slower than other drive roller by a percent outside the reasonable strain of the web it must be slipping. This may be anywhere from 0.1% for foils, 0.5% for paper, or 5% for stretchier webs.
Idler roller slip is rarely measured, but a simple handheld contact tachometer can be used to check for web vs. roller speed differentials of greater than one percent.
More advanced systems use targets and optical tachometers or laser velocimeters to monitor idler roller speeds without contact.
The torque applied between some or all layers within a wound or winding roll exceeds the traction available to transmit that torque (a.k.a. torque transmission capacity), causing layers to slide in the machine direction.
This machine direction slippage is called ‘cinching,’ much like the motion of tightening a belt.
At unwinding, mark a spoke line on the side of the roll before tensioning. If any part of the spoke line shifts, MD slip has occurred.
MD slip in winding rolls is more difficult to measure. An inkjet printer synchronized with the winding roll will form a spoke line (no slip) or D-line (indicating MD slip).
Throw ‘flags’ into the winding roll, sticking out one side. Video tape the winding process. Play back the video and note if flags near the core change their angular position relative to flag in the roll’s outside layers.
MD slip is nearly always accompanied by CD slip. If a winding roll begins winding with well-aligned sidewalls, but later they are misaligned, slippage is highly likely to have occurred.