Spectrum Technologies Ltd
Soft Nip Calender Vibration
Self Excited Vibration Causes Excessive Maintenance
Single nip soft nip calenders can bar affecting paper quality and the life of the calender rolls.
Soft nip calenders do not have paper caliper variations from upper nips acting as a vibration excitation source. Instead the soft roll acts as the feedback path. This path can be temperature induced cover property variations, or creep in the cover causing diameter variations.
The barring frequency is always at or close to a resonance in the calender stack or rolls. The resonance is excited by an integer number of bars around the circumference of the soft roll.
Measurements to help understand the problem include vibration of the calender stack along with the rotational speed of the rolls, and the soft cover temperature are used to determine the operating conditions. Modal measurements are used to determine the resonances within the stack. Finite element calculations are used to relate the machine design to the resonances measured.
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.
Source: http://www.spectrum-tec.com/english/pulpandpaper/softnipcalendervibration.html
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