Spectrum Technologies Ltd
Calender Barring
Self Excited Vibration Causes Excessive Maintenance
Calender barring is a vibration problem which not only affects paper quality, but also the life of the calender rolls. When serious enough it causes severe noise, worsening work place health and safety. 
When calender barring occurs, there is a vibration feedback mechanism, where the vibration, once started is sustained. The mechanism can be caliper variation induced in the upper nips of the calender stack, forcing the lower nips to vibrate, or it can be barring patterns worn into the calender rolls acting as the excitation mechanism. Both mechanisms can also work simultaneously.
The manner in which caliper variation induced in an upper nip feeds back into a lower nip can be compared to the way a swing works in a back yard. When the swing is pushed in phase with its movement, the swing goes higher. When it is pushed against its movement, the swing very quickly stops moving. In a similar manner, if a calender roll is offset to change the nip-to-nip wrap distance, the caliper can push against the roll vibration and stop the vibration tendency. If the calender roll is in just the wrong position the vibration will be amplified and create problems.
The normal first step in solving a calender barring problem is to perform offset calculations on the calender stack. These predict the optimum calender offsets to minimize the vibration. This procedure often gives good results. At other times the results are less than ideal, due to some of the assumptions inherent in this procedure being violated. When this happens a more in depth study is suggested.
The in depth study involves considerably more data being collected to ensure that the dynamics of the calender stack are accurately modeled. A mathematical model is built which accurately reflects the characteristics of the calender stack. From this model, the best course of action to eliminate the barring problem is taken.
Customers have compared the performance of our offset recommendations with other vendors and have found that ours give better results.
Willie Pruys from Fort Frances says:
Our stack has had a barring problem for as long as we can remember. Finally we asked Jake Zwart of Spectrum Technologies to recommend offsets for our stack. After installing the offsets using these recommendations on Dec 12, 2007, we have run our stack under a variety of operating conditions, changing both grade and speed. The stack has been running very smoothly since that time, without a trace of barring. We are really satisfied, and when it comes time to change the next stack in this mill, I will be pushing to have Jake calculate offsets for it.
Source: http://www.spectrum-tec.com/english/pulpandpaper/calenderbarring.html
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