Vibration and wear in a multi-roll steel calender stack can lead to a phenomenon known as calender barring.
It results in a machine-direction periodic caliper variation. In a mild case it is visible as dark bars in the sheet, and in an extreme case it can cause the calender stack to "sing", if the vibration is at an audible frequency.
A mother pushing her child on a playground swing is a good analogy to help understand what's happening in a barring calender stack.
When mom pushes her child on a swing, the force of her push is in phase with the velocity of the swing, thus increasing the amplitude of the swing. On the other hand, if she pushes on the swing as it's coming towards her, the swing quickly stops swinging.
If we consider only the rolls' vertical motion when a calender is barring, the top nip induces caliper variations in the paper. As this paper travels to the next nip, this caliper variation acts as an input force at that nip. If the caliper is increasing as the rolls in that nip are moving apart, it is acting like the swing when mom pushes it away from her, increasing the vibration amplitude, also known as self-excited vibration.
On the other hand, if we can offset the calender roll slightly in the horizontal direction, we change the wrap length from nip to nip so that the caliper is decreasing as the rolls are moving apart, and the paper acts like the mother pushing on the swing as it comes towards her. The vibration is damped.
Offsetting calender rolls is the process of selecting the right wrap length for your calender, paper and operating conditions to prevent self-excited vibration. Calender barring programs use the vibration characteristics of the calender stack to build a dynamic model of the calender stack and predict the best set of offsets for that calender stack. This takes into account caliper variations induced by vibration in the upper nips acting as exciters in all subsequent nips.
Over a long period of time, if calender barring is not addressed, the calender rolls can wear into a barred pattern. In this case they need to be removed and the barring pattern ground out.
A new calender barring gauge, modeled after the Bowater Electronic Curvature Gauge from decades ago, and enhanced with a wheel to measure bar spacing and digital signal processing, can be used to measure the barring in calender rolls. This gauge is helpful to determine when rolls should be removed from the calender stack for grinding. It can also be used to assess if the roll grinder has removed the barring from the roll during the grinding process.
Jake Zwart, president of Spectrum Technologies, brings advanced vibration test and analysis techniques to solving problems in the pulp and paper industry. www.spectrum-tec.com