Web tension is complicated whenever the web path changes. This may happen when nips open or close. It has a huge impact on tension during an index or cutover in a turret winder.
Whenever the web length changes, the speed of at least one roll or roller must change to maintain tension. If the web length shortens, the speed of downstream sections must increase. If the web length increases, the speed of downstream sections must decrease.
It is difficult to maintain tension control when speed is changing. There are inertia effects which are generally not modeled. Also, the diameter calculator often depends on knowing the line speed and spindle roller speed. If the spindle RPM is changing to compensate for a web path change, the diameter calculation based on the ratio of spindle RPM to line speed is invalid and the calculated diameter is garbage.
Things that lengthen the web length are raising the layon roll, turret index.
Things that shorten the web length are lowering the layon roll, extending the knife assembly.
Necessary techniques to minimize tension variations during turret cutovers are:
1) Reduce the time for an index – less off-spec material is wound
2) Diameter hold (an old diameter is better than a wrong diameter)
3) Torque hold (the best we can do without complicated modeling)
4) Model the speed change required for each instant of the cutover