Most blade coater beams are fastened to structures called pivot frames. When the coating head is loaded, it is the pivot frame that moves the head into position until it is firmly held against mechanical hard stops on each end. These hard stops are adjusted after every backing roll change to guarantee the proper blade beam geometry when in the run position.
For tube loaded coaters, the only requirement of the pivot frame is to be loaded up evenly against the hard stops. The tube will provide the coat weight control and blade tip angle.
However, for rigid support profile bar coaters, it is not enough that the pivot frame is loaded against the hard stop. The pivot frame must be able to float as the blade load is changed, both in and out. If the pivot frame binds up for any reason, blade load control will be affected resulting in loss of MD coat weight control, poor CD coat weight profiles and sheet breaks.
A possible cause of this phenomenon is binding pivot frame pins and bushings. These pins and bushings are located at the bottom of the pivot frame. Some have grease fittings on them, some do not. If provided, it is good practice to grease them every maintenance outage.
If you suspect that the pivot pins and bushings are binding, Valmet can perform a hysteresis test of the pivot frame – essentially measuring how the actual blade beam position changes when the blade load is increased and decreased by the control system. If there is binding going on, it will be seen on the hysteresis graph. This is done using dial indicators.
As the control system changes the blade loading setpoint, the dial indicators should move by the same amount. The setpoints are increased from 0 to 5 mm, then back down to 0 mm again. If there is no binding in the pivot pins, the actual blade load will match that of the computer setpoint.
If the actual loading differs from the setpoint, there may be binding in the main pivot pins. To fix this problem, Valmet will first check the pivot frame hydraulic circuit to determine if the cylinders are working properly (pistons not blowing by, adequate pressure, etc.). If they are working properly, there is a good chance that the pivot frame pins and bushings may need to be replaced to eliminate the binding.
If you need service for your coating equipment, contact your Valmet representative.