A SymSizer was having intermittent problems with sheet breaks when the rolls were nipped up. As the rolls touched, the sheet would steer to one side, wrinkle badly, and then break. The problem did not occur every time, so it was difficult to troubleshoot.
Mill maintenance had changed or swapped several hydraulic controls in an attempt to rectify the situation: both proportional valves and cards, and a directional valve. The problem remained.
During a machine outage, more time was spent troubleshooting the hydraulic circuits since so many changes were made to it. The speed of closing the nip was adjusted to balance each end.
No problems were found until an e-nip impression blanket was used to determine target end pressures. The first e-nip impression showed a slightly wider nip on the drive side. On another attempt, there was no pressure at all on the tending side. The nip was actually open by about 1/4” inches.
The nip was cycled several times and it was obvious that the nip would close all the way, and then open back up, but only on one side. Hydraulics appeared to be ok – high pressure on the cap end, low pressure on the rod end. Still the nip would not close.
Along with the nip not closing all the way, it was observed that the tending side pressure was slow to reach setpoint. As a way to test the nip loading circuit, shutoff valves to the cylinders were closed and the proximity switch that confirms the rolls are loaded was flashed, indicating a closed nip situation to the PLC. With a closed nip signal, the proportional valves were allowed to go to pressure control mode. Both tending and drive side pressures reached their setpoints and maintained them. This proved that the control system was not the problem.
The next logical move was to focus on the main nip loading cylinders. It was decided that the tending side main cylinder was suspect and was replaced with a new spare cylinder. After the change and air was bled from the cylinders, the roll was nipped again. This time, the e-nip impression was even across all the time.
The cylinder was the root cause of the nip loading problem. The piston seals were suspected of not seating properly every time it stroked. No more issues were reported after the cylinder was changed.
It should be emphasized that whenever a hydraulic cylinder is replaced, especially large ones like this, they should be allowed to fill with oil prior to activating the equipment. Air in hydraulic cylinders can cause many problems and uncontrolled movement of equipment because it is so compressible.
For more assistance with nip loading of your devices, contact your Valmet representative.