With enough torque, web slipping past a roller is inevitable. I have seen web slipping on paper machine forming sections, coating rolls and many other places.
Slippage occurs when the forces on the web exceed the traction available between the roller and web. Slippage can be aggravated by wet surfaces (water, coating), high differential tension in and out of a roller, between layers of a winding roll, dry powders such as talc or starch. Some web surfaces such as silicon are very prone to slipping.
The forming sections of paper machines are also very prone to slipping because a synthetic screen (or wire) is driven long distances to form a web out of a slurry containing 97% water. Showers are used to clean the wire as it moves. To reduce slippage and provide the required torque, many motors are used to drive rollers around the length of the wire.
Coating sections are also prone to slipping. Coating rollers have a smooth rubber surface and the web is wet while passing the coating roller. It is often impossible to prevent slippage until the coating is right and the blade pressure is correct. Even then some coatings may run at substantially different speeds than other coatings with slippage being the only explanation.
The drive can detect gross slippage in several ways. Where a tension regulator is used, the output limit high and output limit low signals (part of the tension regulator) indicate the web is probably slipping. It may also indicate the draw is incorrectly set, or the wrong roller diameter is entered in the system during the past roll change.
When no tension regulator is used, slippage can be detected by comparing the speed of two or more rollers. This requires a tolerance of two times the speed variation of each drive speed regulator plus a several second time delay.
Another tool available is to monitor speed and torque changes. Jumps (rate of change) in speed and torque do not necessarily indicate slippage, but are a good indication.
To date, drive engineers have not spent much effort on detecting slip. This could be very valuable to operators and process engineers.
Cars have effective traction control today, but only after decades of development. In web handling, we cannot supply the same real time solutions. That is we can’t make web while pulsing a brake for one or more rollers or by driving from a redundant roller that is not slipping. We can plan ahead and improve traction in the same way we can change our 3 season radials for snow tires during the months snow and ice are guaranteed.