Graphite-alloy Bearing Endures Hot Corrosive Chemicals
At the Les Papiers Perkins facility in Candiac, Quebec, three belt washers feed washed recycled pulp stock into a 35-ft-long screw conveyor. The conveyor consists of a U-shaped trough, 20 inches in diameter, with a screw at the bottom that pushes the stock along the conveyor to feed the washed pulp to a reservoir. From there the pulp goes to a bleaching tower. The consistency of the stock at this point is about 13% and the temperature is 100F. Operating at a constant 175 rpm 24 hours a day, the conveyor is so long it must use two screws mounted end to end and joined by a hanger bearing. This interface uses a split-hanger bearing to support the five-inch long stub shaft joining the two screws.
"Finding a suitable bearing for this application was difficult," says Les Papiers Perkins Design/Drafting Coordinator Sven Henriksen. First, the bearing location in the fluid stream created an interruption in the flow from one screw to the other. If this obstruction were too large, it could cause the stock to overflow the trough. Therefore, engineers wanted the design to be as small and compact as possible. Second, the bearing was submerged in the stock and exposed to its thick consistency, process heat, and chemical additives. This hostile environment can substantially reduce service life, so the chosen material must resist deterioration. Lastly, dismantling the conveyor each time the bearing was replaced was both costly and time-consuming. To minimize the expense, the bearing design should be split type in order to slide it under the steel shaft while in situ.
Material problems. Engineers tried a variety of materials for the bushing including Teflon, PVC, urethanes, and several types of nylons. Although these materials held up in the corrosive environment, they were not strong enough to support the weight of the rotating parts and the heavy stock. Under this constant load, the plastic deformed, allowing the shaft to sag and eventually eat into the bearing support housing. If the problem went undetected for too long, the outside diameter of the screw started to damage the trough since there is only 1/2 inch of clearance. Plastic materials lasted anywhere from weeks to a couple of months before excessive wear became a problem.
The problems with this particular bearing caused Les Papiers Perkins to spend a great deal of time and money on its maintenance, and even threatened to cause downtime at the mill. When the stub shaft wore through the bushing and into the housing, maintenance personnel had to take the drive off the end of the conveyor, remove the entire screw, and put in anew bearing and stub shaft. This process required the labor of four people working for six to eight hours. The labor charge for this repair was approximately $3,300. Typically, such work was scheduled for a maintenance day when there was enough reserve stock to keep paper machines running. However, if the bearing malfunctioned unexpectedly, it could shut down pulping operations with the additional high costs of production loss.
Engineers also tried an anti-friction split roller bearing, which theoretically should have worked. The environment of the conveyor was so wet, however, that it was impossible to keep the bearing sealed well enough to keep water out. Constant applications of grease were required, but in the tight quarters where the bearing is located, this was impractical. The anti-friction bearing lasted between three and six months, which was the best performance the mill had experienced at that point. But the time and inconvenience required to keep the bearing lubricated ruled it out as a feasible solution.
Graphite-metal bearings. Engineers continued to search for a better solution to the bushing problem. An article in a trade journal led them to GRAPHALLOY-graphite-metal alloy from Graphite Metallizing Corporation, (Yonkers, NYC. Graphite-metal alloys take advantage of the special properties of graphite, the structure of which can be compared to a deck of cards with individual layers able to easily slide off the deck. This phenomenon gives the material a self-lubricating ability that is matched by no other material. External lubricants are simply not necessary. The graphite matrix can be filled with a variety of impregnants to enhance chemical, mechanical, and tribological properties.
Les Papiers Perkins engineers asked Graphite Metallizing to recommend a graphite-alloy bushing that would address their problem. From its many material grades, the company recommended a Grade 211 graphite-copper bushing material that cost approximately $2,000. This self-lubricating bushing material requires no grease or oil, survives run-dry conditions, and eliminates galling and seizing in hot and dry conditions. The material works at higher temperatures up to 1,OOOF (525C) where oil-based lubricants burn off or oxidize and plastics fail. It maintains its integrity even when submerged in hostile liquids such as acids, alkalis, hydrocarbons, black liquor, and liquid gases. The material provides a constant, low coefficient of friction rather than just a surface layer, helping to protect against catastrophic failure.
Lubrication is maintained even during linear motion; lubricant is not drawn out and dust is not pulled in. GRAPHALLOY wear components also improve reliability under conditions such as low-speed operation, frequent starts and stops, and switch-overs from standby to continuous running, according to Graphite Metallizing Corp. GRAPHALLOY bushings are available in more than 100 grades of material, in any desired size or geometry, including cylindrical with or without grooves, flange or double flange, split, and metal-backed.
Sturdy material. The new GRAPHALLOY bushing was installed in March 1998. A new stub shaft was installed at that time as well. In August, engineers inspected the bushing and found only 0.05 inch of wear on the 4.43-inch diameter bore. The wear was located in a 3:30 to 4 o'clock position, so the engineers rotated that bushing 180 degrees. Several visual inspections following that showed a similar wear pattern, and engineers planned to rotate the bushing again during the next shutdown of the pulping department. However, the bushing was still running in July 1999 without a problem, so the second rotation was not performed.
One benefit of having the GRAPHALLOY bushing is that engineers reportedly no longer have to worry about an unexpected bearing malfunction causing downtime in the plant. Although there is some wear, it is minimal enough that it can be addressed in plenty of time before it causes a problem, they say. There has also been a significant labor savings, they say, since it isn't necessary to replace the bearing every few months. Since its installation, replacement labor savings over plastic bearings is estimated at more than $20,000. In addition, having this problem solved frees up maintenance people for other tasks within the mill.