Solutions to multiple poly roll cover failures, a case study

The night-shift operator began picking up the chunks of polyurethane lying useless on the mill floor. The routine was familiar – for the second time in a month, the cover on the third press poly covered grooved roll on No. 4 had disintegrated. The catastrophic failure of two roll covers hinted at impact damage resulting from a foreign object passing through the nip. Yet, no one had seen or heard evidence that anything other than paper had come in contact with the roll.

The first roll cover had run for several years, requiring only routine maintenance. Then, seemingly without reason, a large portion blew off the roll. Suspecting a faulty product, the mill replaced the roll cover with a new one from a different supplier. The switch was to no avail. The replacement roll cover failed a mere three weeks after installation.

The mill called a Valmet roll specialist and requested immediate delivery of another roll cover. But the mill’s dilemma went beyond getting No. 4 up and running. The question still remained as to why two roll covers, one which was new, inexplicably failed with no sign of impact damage to either?

Investigation - roll records, nip impressions... coolant?

The specialist reviewed roll records to determine if the cover had exhibited any questionable characteristics such as hardening or unusual wear patterns. The records revealed dates when the roll was changed, but little else.

Nip impressions confirmed even pressure across the roll face and an alignment well within the allowable variance. The specialist also examined the third press rolls on No. 3, which had an identical setup to No. 4, yet had run for seven months without incident.

The Valmet roll expert noted that both rolls used a water cooling system that required a 40 gallon-per-minute flow through the roll to offset the heat generated by operating speeds of 3500 fpm and pressure up to 650 pli. Gauges on each machine indicated both rolls were receiving the required water flow. Still, the roll specialist requested that the water flow be manually measured.

Just as he had suspected, the water flow to No. 4 was discovered to be only 3 gpm – well below the 40 gpm requirement. The resulting heat build-up in the roll created a bonding problem between the roll cover and the roll. Under the duress of high speeds and pressure, portions of the roll cover which had lost bond with the overheated roll had torn away.

The solution - new gauge and water system check

The Valmet specialist recommended the mill replace the faulty gauge and check the water delivery system to ensure the proper flow rate into the roll. He also recommended the water inlet temperature for this polyurethane application be a minimum of 90 °F to a maximum of 150 °F, with a maximum of 10 °F variance between inlet and outlet temperature. This would ensure even temperature throughout the roll, eliminating the bonding problems that had cost the mill two roll covers.

By implementing the roll specialist’s recommendations, the mill eliminated the six-figure expenditures required to replace roll covers which had fallen victim to an inexpensive gauge and a faulty water-delivery system.

For more information and assistance with solving roll cover failures, and the most complete and accurate information on roll cover care - contact your Valmet representative.