Bearings Withstand Harsh-Run Conditions In Paper Plant
When Wausau Paper Mills Co., Jay, ME, suffered repeated failures of the original bronze bearings furnished with equipment, the maintenance staff switched to graphite metal-alloy bearings. The move was made to self-lubricating bushings designed specially to withstand the intermittent run-dry conditions of the paper mill's stock tank agitators, where varying stock levels sometimes eliminate all lubrication. The new bearings have already withstood 12 to 24 months of service without exhibiting significant wear, and have saved $40,000 per year by reducing replacement parts and labor expense.
Run-dry conditions
The Jay plant's 280 employees produce 70,000 tons of paper per year. Products include lightweight, dense, technical specialty papers typically used for pressure-sensitive products, medical and food packaging and imaging papers.
The mill uses 15 pulp-stock agitators to produce manufacturing papers and labels. Each unit has a horizontal shaft driving a 54-in., 4-blade shockless propeller made of stainless steel. Original bushings were made of bronze and had graphite plugs that were expected to provide self-lubricating properties. During normal operation, the bearings are supposed to be continuously submerged in pulp stock in order to provide bearing lubrication; water is piped directly to the bearings for cooling and lubrication. However, the stock level occasionally drops below the bearings when the water lines are plugged with stock in the tank, leading to run-dry conditions.
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 the spider adapters, shaft and sleeves.
The total cost of rebuilding one agitator, including parts and labor, was $3,700. The time required on the part of the maintenance crew to simply inspect and change the bearing is four hours. If the sleeve is damaged, repairs take eight to 12 hours. If the shaft itself is damaged, two to three days of work is required.
Taking an agitator out of service did not usually cause downtime because the plant had 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.
The mill maintenance staff tested several alternative synthetic nylon bearing materials, but the new materials did not display the ability to consistently withstand the demanding rundry conditions of the application. A distributor of bearings and drive components recommended Graphalloy graphite-metal alloy from Graphite Metallizing Co., Yonkers, NY. This self-lubricating bearing material requires no grease or oil, survives rundry conditions and eliminates galling and seizing in hot and dry conditions. The material works at temperatures up to 1,000°F, 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. Providing a constant, low coefficient of friction rather than just a surface layer, the alloy helps protect against catastrophic failure.
Dramatic life improvements
One Graphalloy bushing was installed in January 1996. During the plant's July 1996 shutdown, this bushing was inspected, and showed no wear. At the January 1997 inspection, again the bushing showed no wear. In February 1997, eight additional Grade GM105.3 bushings were ordered and the new bushings were installed on all remaining agitators.
As of January 1998, the bearings were still operating well and no measurable wear was apparent on those installed 18 and 24 months prior.
Mill 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. Additional benefits are expected by freeing up maintenance staff who previously spent a significant portion of their time addressing the agitator problem.