Calender Barring - Self Excited Vibration Causes Excessive
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
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
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.