At the Stora Enso North America (formerly Consolidated Papers, Inc.) manufacturing plant in Whiting, Wisconsin, SKF Compact Aligning Roller Bearings (CARB^®) are taming a classic case of rock ’n roll fever.One of the world’s largest manufacturers of lightweight coated paper, Stora Enso North America has been focused on papermaking for close to a century. The company’s Whiting Mill produces high-quality publication-grade papers on two paper machines, one a 22-year-old machine installed in 1978.

   During the paper drying stage on this machine, the paper runs through a line of 60 dryer cans. It is here that bearing-related vibration and bearing housing movement caused a domino effect of issues that lowered productivity due to the need for continual maintenance. Today, those dryer cans are spinning to a smoother beat thanks, in part, to SKF CARB bearings.

Rock ’n roll dryers
Each of the paper machine dryer cans is a hollow iron cylinder, 60 inches in diameter, weighing 21,000 pounds. Steam injected into the can heats the shell, which in turn dries the paper moving across the can at a machine speed of 3,800 fpm. Dryer can surface temperatures range from 150°F to 240°F.

   The dryer cans were supported by spherical roller bearings on both the locating and non-locating sides. Rocker-supported housings on the tending side of each dryer can were designed to move to take up axial displacement caused by thermal expansion. But according to plant mechanical engineer Dan Higgins, “the housings vibrated left and right, and up and down, shifting and shuffling all over. Our vibration readings indicated high vibration levels.” In fact, Higgins noted, the loosely mounted housings would sometimes actually fall off the rockers. But that was just the beginning of the problems.

Swinging siphon tubes
A key component of the paper machine dryer cylinders are stationary siphon tubes that run through the inside of the front side dryer journals. These siphons help maintain consistent dryer surface temperatures by evacuating condensation from the cylinders. “When the housing would rock and roll, it caused an amplified movement of the stationary siphon tubes,” said Higgins. “The siphon shoe at the end of the siphon tube would contact the inside surface of the dryer shell and sometimes catch on internal components and break off. In some dryers, the siphon shoe was pushed against the dryer shell until it created a seal which prevented evacuation of condensation.” More problematic was the effect on the drying process. “Damage to the siphon tubes prohibited them from pulling condensation out of the cans. That caused the dryer temperatures to fall and become cold, which in turn resulted in quality issues and poor machine runnability,” said Higgins.

Rattling rope sheaves
Bearing housing movement and bearing vibration also caused major problems with free-wheeling rope sheaves – huge wheels measuring five feet in diameter and weighing 300 pounds. The sheaves guide carrier ropes that help thread tails of paper through the machine at various points. These sheaves were mounted to the same rocker-style bearing housings as the dryer can bearings. According to Maintenance Superintendent John Adamski, “With the rocker-supported housings, nothing was holding the housing down except for the weight of the dryer can. If the bearing has any misalignment, the weight of the rope sheave tended to tip the housing. When it started to tip, it would cause the sheave to hit the dryer can, causing misalignment, rubbing and fraying of the rope. Sometimes the rope would be cut through, other times it would just fall off.” Misalignment of the ropes also contributed to increased tailing times, which in turn, lowered paper machine efficiencies. And once again, the Whiting Mill’s maintenance staff was faced with more downtime for repairs.

Spinnin’ smooth
With 60 dryer cans all experiencing the same chain of problems, the Whiting Mill’s maintenance crew was continually challenged by a never-ending cycle of dryer cans, siphon tubes and rope sheaves literally rockin’ around the clock – until they became aware of SKF Compact Aligning Roller Bearings (CARB).

   CARB is a unique toroidal design that combines some of the best aspects of spherical, cylindrical and needle roller bearings. Intended for use at the non-locating end of a two-bearing arrangement, CARB has a single row of broad rollers with a slightly curved profile – a unique design that enables the bearing to accommodate both angular misalignment and axial adjustment internally and without frictional resistance. This arrangement allows the housing to be firmly bolted to the foundation, reducing vibration, allowing higher running speed and contributing to longer bearing service life.
   
According to SKF Sales Representative Jeff Priebe, “In a toroidal roller bearing the rollers always find the position at which the applied load is at a minimum. In a sense, the rollers behave like a wet bar of soap between a pair of cupped hands. If you press on one edge or the other, the soap automatically glides to a new position in order to escape edge loading, finding a place where the stress is minimized.”
   
Stora Enso’s John Adamski immediately saw a solution with CARB. “We liked the fact that we could have a very secure mounting because the alignment and stability problems were handled by the bearing itself,” he said.
   
Initially, Stora Enso mounted CARB bearings in modified versions of the rocker-style housings. However, the modifications weren’t optimal and they later switched to a custom-designed SKF pillow block housing.
   
SKF Senior Industry Consultant Wayne Miller notes that “the housing is extremely important to the performance of the bearing. For optimal performance and proper lubrication, it really needs to match precisely with the OEM design, while accommodating the new bearing. For this application, we kept the old housing covers, but started from scratch with everything else to create a perfect fit.”

   The result? “The installations were highly successful,” notes Stora Enso’s John Adamski. “Our stationary siphon and free-wheeling rope sheave problems went away.” Before and after vibration readings graphically tell the story, as vibration spikes prior to CARB dropped to a virtual flat line immediately after the CARB installation. By eliminating bearing housing movement, problems with the siphon tubes and rope sheaves were eliminated. Dryer can temperatures can be maintained, productivity has increased and downtime and maintenance problems have been reduced.

   To date, Stora Enso has installed CARB bearings on 20 dryer cans, and has plans
to replace the bearings on all 60 paper machine dryers
during scheduled maintenance downtime
For Adamski, the change from shake, rattle and roll to smooth operations has meant more than just enhanced operations. It’s also paid off in peace of mind. “We run a 24-hour operation,” he notes. “I sleep better at night knowing that the CARB bearings have solved the problem.”