Roofing plant beats bearing contamination

by Roger Burtnette, Rexnord Corporation

Auxiliary cap seals keep contaminants out to cut bearing replacement costs.

At the Pabco Roofing plant in Tacoma, Wash., bearings on the many rolls used to handle the web of asphalt-coated fiberglass were failing in as little as two to four weeks, due to a harsh environment that included fine sand used in the process to prevent the backs of the shingles from sticking together. By switching to Rex roller bearings with auxiliary end cap seals, the company was able to exclude contaminants and extend bearing life by an average of 10 to 12 times or even considerably more.

In 1984, Pacific Coast Building Products purchased the West Coast manufacturing facilities of CertainTeed to begin Pabco Roofing, a quality supplier of asphalt shingle products to the western United States. The plant, built in 1955, produces approximately 250,000 tons of product annually.

All shingles made at the plant are UL Class A fire rated, the highest rating possible. In the web-based process, a fiberglass core is impregnated on both sides with a mixture of asphalt and calcium carbonate (limestone dust) that is heated to about 425 F. The top surface is then covered with 3M ceramic-coated granules, while the back is coated with fine backing sand to prevent the shingles from sticking together.  After being cooled with a water spray and dried, the web is die cut into shingles and packaged.

In this punishing atmosphere, bearing life varied from short to shorter. At the time the plant was built, premium ball bearings had been used to support the rolls. As speeds and loads increased over the years, contamination also took its toll. The increased maintenance workload became impossible to handle.

About six years ago, when we started running seven days a week, we had a tough time finishing all the maintenance we needed in the eight hours a week that we were shut down, said Dennis Williams, division operations manager. One of the things that was the most difficult was replacing the bearings, because we had to change so many.

The web process uses a large number of rolls, each with two bearings, most of which are supporting 2-7/16-inch diameter shafts. Williams estimates the number of bearings in the plant is in the hundreds. The fine backing sand would work its way into the bearings and accelerate wear that led to premature failure. And, the heavy moisture from the cooling process caused some lubricant dilution and contamination. Williams reported that bearings would often fail in a month or less.

We changed some of them every couple of weeks in the really bad spots, explained Williams. The contaminants would get right in and kill them.

Extra Sealing Power Extends Bearing Life
To solve the problem, the company first added lubrication systems to ensure an adequate supply of grease for the bearings.

 Then we started looking for another bearing, and we decided on a Rex roller bearing with a sealed end cap, said Williams.

Working with local distributor Kaman Industrial Supply, he replaced a number of the bearings and found that the combination of improved lubrication, added bearing capacity and more effective sealing extended bearing life dramatically. According to Williams, the previous bearings often had to be replaced monthly, while the new bearings last 10 to 12 months or even much longer. 

We started this about five years ago, he said. And, some of the original bearings are still running. We used to change five to 10 bearings every week, and now we hardly ever have to change them.

The bearings are made by Rexnord Corporation at its Downers Grove, Ill., plant. The double-row concave roller bearings are fully self-aligning, and most are mounted in cast iron pillow block housings. All are equipped with end caps that provide supplemental protection for the primary bearing seal. They are available with open ends, for through-shafts, and with closed ends for ends of shafts. Both types are used at Pabco.

In addition to the primary bearing seal, auxiliary caps provide four points of sealing protection. They include a V-ring seal, which rides against the machined cap surface; a labyrinth seal, which is machined into the auxiliary cap; a cavity, which can be filled with grease to provide a barrier to keep contaminants away from the primary bearing seal; and a gasket, which seals between the auxiliary cap and bearing housing.

Installation was easy, with minimal rework required.

On a few of the rolls, we had to cut the excess end off a shaft to make room for the cap, Williams explained. But, most of them bolted right in.

Bearing replacement is made easier and bearing life is extended by the Twist Lock feature that Pabco now specifies on the Rex capped roller bearings. The design incorporates a pair of cam surfaces, one on the inner race and the other on the face of the collar. The user twists the collar when installing the bearing, causing the mating surfaces of the cams to move in opposite directions and create a clamping force on the shaft.

Williams said that, after changing to the Rex bearings, the only time we lost any of them was when a setscrew backed off, so now weve gone to the Twist Lock eccentric locking collar.  The added holding power, combined with the sealing benefits of the auxiliary caps and the capacity advantage of the roller bearings, has dramatically reduced downtime and improved bearing reliability in the plants challenging environment.

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