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Stock tank agitators present an especially challenging bearing application because varying stock levels sometimes eliminate all lubrication. The bronze bushings furnished as part of the original equipment failed frequently between six-month preventive maintenance inspections, often causing damage to the spider adapter and shaft sleeve and sometimes the shaft itself. On the recommendation of a distributor, Wausau maintenance staff switched to a special proprietary grade of self-lubricating graphite-metal alloy bushings specially designed to withstand intermittent run-dry conditions. The new bearings have already withstood 12 to 24 months of service without exhibiting significant wear to date. The switch from bronze to graphite saved $40,000 per year by reducing replacement parts and labor expense.
Wausau Paper Mills Co. is a leading paper producer. The company has three operating divisions: the Printing and Writing Division, which operates a paper mill in Groveton, N.H.; and the Technical Specialty Division, which has mills in Rhinelander, Wisc., and Jay, Maine. The Otis mill at Jay, Maine was acquired by Wausau on May 12, 1997. The mill employs 280 people and has the capacity to produce 70,000 tons per year. The Otis mill manufactures lightweight, dense, technical specialty papers, which are primarily sold directly to converters and end users throughout the U.S. Typical end uses for these papers are pressure-sensitive products, silicone coated products, medical packaging, food packaging, multiple laminated products, electrographics and imaging papers.
Run-dry conditions. The Otis mill uses 15 pulp stock agitators in the production of manufacturing papers and labels. The agitators were manufactured by Improved Machinery Inc. (IMPCO) between the early 1950s and late 1960s. Each unit has a horizontal shaft driving a 54-inch, 4-blade shockless propeller made of 316 stainless steel. The original bushings were made of bronze and had graphite plugs which were expected to provide self-lubricating properties. During normal operation the bearings are supposed to be contin-uously submerged in pulp stock in order to provide bearing lubrication. In addition, water is piped directly to the bearings for purposes of cooling and lubrication. However, on occasion the stock level drops below the bearings when the water lines are plugged with stock in the tank; the result is a run-dry condition.
For this reason, the original bronze bearings failed frequently without warning in between the six month preventive maintenance inspections. The failures not only destroyed the bearings but usually damaged the spider adapters, shaft, and sleeves as well. The cost for replacement parts in this case was approximately $400 for the old bearing, $1,000 for the spider adapter, $800 for the sleeves, $200 for the shaft, and $250 for machine shop work. It required four hours to simply inspect and change the bearing. If the sleeve was damaged, then repairs took eight to 12 hours. If the shaft itself was damaged, then two to three days of work were required. Therefore, the total cost of rebuilding one agitator including parts and labor was $3,700.
Taking an agitator out of service did not usually cause downtime because the plant had some extra capacity in this area. However, any time an agitator went down it was necessary for the stock prep department to spend a fair amount of time revalving and cleaning tanks to take over for the one that was out of service.
Trialing alternatives. In an effort to solve the problem, Otis mill maintenance staff trialed several alternative synthetic nylon bearing materials. The trials were conducted by replacing the bearings on a single agitator and monitoring the device to see how they stood up. Some improvements were seen but the new materials still did not exhibit the ability to consistently withstand the demanding run-dry conditions of the application. Motion Industries of Rumford, Maine, a distributor of bearings and drive components, informed maintenance staff about graphite-metal alloys that take advantage of the special properties of graphite. A graphite structure can be compared to a deck of cards with individual layers that can easily slide off the top of the deck. This phenomenon gives the material a self-lubricating ability unmatched by any 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.
Shown is a graphite metal alloy bearing housed inside the tank. The bearing is the dark ring between the "gold" ring (press-fitted onto six-inch shaft) and the six-holed stainless steel housing that the bearing is pressed into.
Note: Cylindrical object containing impellers, shaft, etc., is called "mid-feather" and is housed inside the tank, which is rectangular. Shaft and mid-feather, etc., are horizontal.
Stock tank agitators present an especially challenging bearing application because varying stock levels sometimes eliminate all lubrication.
The distributor recommended using Graphalloy graphite-metal alloy from Graphite Metallizing Corporation, Yonkers, N.Y. This unique self-lubricating bearing 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 1,000ºF (525ºC) where oil-based lubricants burn off or oxidize, and plastics fail. It maintains its integrity even when submerged in hostile liquids such as acids, alkalies, hydrocarbons, black liquor, and liquid gases. The alloy 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. Wear components made of this material also improve reliability under conditions such as low speed operation, frequent starts and stops, and switch-overs from standby to continuous running. Bushings are available in over 100 grades of material in any desired size or geometry, including cylindrical with or without grooves, flange or double flange, split and metal-backed. Graphite Metallizing Corporation application specialists recommended Grade GM105.3 babbit-impregnated Graphalloy bushings, which cost less than $700 each.
Dramatic life improvements. One bushing was installed during the January 1996 inspection. During the July 1996 shutdown this bushing was inspected. There was no wear at all and the operation was running smoothly. During the January 1997 inspection, again the bushing showed no wear. Time permitted the installation of only one additional bushing during this inspection. In February 1997, eight additional Grade GM105.3 bushings were ordered and the new bushings were installed on all the remaining agitators during the July 1997 shutdown. This inspection showed that there was still no wear on the two previously installed bushings. The January 1998 inspection showed that one bearing, installed in the most recent shutdown, had failed for unexplained reasons, without damaging the adapter, sleeve, or shaft. All of the other bearings were still operating well and no measurable wear was apparent on the bearings that had been installed 18 and 24 months previously.
Otis mill management is extremely pleased to have solved this challenging bearing problem. Management expects to save at least $40,000 per year by reducing purchases of replacement parts and labor required for replacements of the previous bearings. In addition, management expects to achieve additional benefits by freeing up maintenance staff who previously spent a significant portion of their time addressing the agitator problem. The success of this graphite metal component application convinced management to seek out new applications for this promising bushing material. At this site there are 15 additional internally-built agitators that are also experiencing premature failures due to run-dry conditions. The maintenance staff intends to change over to Graphalloy split bushings mounted in the existing bronze housings. The staff is also examining several other applications, such as paper drying equipment, where they see the possibility of achieving further savings and freeing up additional maintenance resources.
Mr. Castonguay is mechanical maintenance superintendent of Wausau Paper Mills Company/Otis Mill, Jay, Maine.
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