A mill was having machine direction coat weight barring. High speed coat weight data was obtained by single pointing the coat weight scanner. The barring frequency was determined to be one times the backing roll rotation.
Much work was done on the backing roll to determine why it was "whipping". All backing rolls were checked for runout in the grinder before and after grinding – no signs of runout were seen. The backing roll was checked for runout in the machine as well using a special dial indicator. A small amount of runout was seen after the roll sat idle for a long time. However, as soon as the roll was rotated for a while, that runout went away.
After much work with no clear cause for the MD coat weight variations, a Valmet vibration team was brought in to conduct a rigorous analysis of the coater. They checked vibration of the backing rolls, dryer cans, felt rolls and reel section. What they found was very interesting…
It turned out that the vibration at the reel was actually being transmitted through a common sole plate to the coater backing rolls and exciting the backing rolls, causing the MD coat weight oscillation. This machine had a history of high vibration at the reel. Conversion from pneumatic to hydraulic controls in the past reduced the vibration, but didn’t eliminate it.
In a novel attempt to reduce the vibration at the reel, Valmet recommended that the steel reel drum be replaced with a compliant cover. This cover was made of rubber and was double grooved – venta nip grooving and chevron grooving.
As soon as the machine started up, the MD coat weight problem was significantly much better. The vibration at the reel was reduced so much that the machine was able to speed up another 300 fpm.
The system saw a huge increase in yield off the supercalenders because of the compliant reel drum, as the mill had been seeing up to 8" of lost paper due to reel drum vibration at the coater reel. Based on these results, Valmet has successfully converted several machine reels to rubber covered drums to reduce crepe wrinkles and bottom losses at turnup.
If you are interested in finding out more about compliant reel drums, contact your Valmet representative.