Valmet encountered an issue where a size press was having wrinkling when the nip closed and before the applicator beam was loaded.
Recall that wrinkling going into a size press is caused by uneven nip load profiles, with the wrinkling seen on where the nip load is lower. On this sizer press, once the applicator beam was loaded, the wrinkling went away. More specifically, the applicator beam on the pivoting roll is the one that was involved with stopping the wrinkles.
The problem turned out to be an issue with the applicator beam’s turning hydraulic circuit. On this particular size press, the applicator beams are prevented from opening all the way by means of a block valve. If the beams are allowed to open all the way, unbalanced forces due to differences in where the cylinders bottom out can be transferred to the roll nip. Since this size press had an automatic nip tare program, the required nip loading cylinder pressures were different during nip loading and would not even out until the applicator beam was loaded.
Most SymSizers don’t have this nip tare function so they don’t have to worry about this type of problem. When a SymSizer nip is closing, a pressure setting is sent to the cylinders and the nip closes uniformly. When the applicator beam on the pivoting roll is loaded, the synchronizing cross-shaft balances any uneven forces created by the applicator beam. It is good practice to stroke this cross shaft periodically to make sure it doesn’t freeze up.
If you have any wrinkling issues on your size press, first try adding pressure to the side where the wrinkles are or lower pressure to the side that is not wrinkling. Keep the bias front to back loading pli to a maximum of 28 pli. If you experience premature cover wear, the synchronizing shaft should be checked for free motion.
If you have any size press questions or need assistance, contact your Valmet representative.