Twin roller hearth heat-treating furnaces at transformer manufacturer ABB Power T&D Co., Jefferson City, Mo., need dependable conveyor drives to keep them operating 24 hr/day, 7 days/wk. The drives have a combined total of 442 (zirk-fitted) ball bearings and 100 paired carbon steel roller chains. A furnace temperature of approximately 1,500 F makes it hot for the bearings, which operate at 200 to 400 F.
Until recently, maintenance workers lubricated the bearings manually every 14 days, using high-temperature grease. They also applied a spray lubricant to the chains on the same schedule.
The lubrication process required two workers for one full shift. Despite a high skill level, lubricating the bearings correctly and consistently was difficult. Inevitably, some received too much lubricant, others not enough. Also, machine structure and plumbing limited access to the lubrication points, causing the workers ergonomic stress.
As a result, bearing performance was a chronic problem: in 1995, 162 of them failed. When a bearing failed, workers had to stop production and replace it. To do otherwise would put the expensive transformer shells in the furnace at risk.
In July 1996, ABB maintenance engineer George Faherty decided to find an alternative to manual lubrication, with product quality as a primary goal. He sought to significantly lengthen bearing life, reduce labor costs and lubricant consumption, and, if possible, eliminate the disposal problems associated with traditional lubricating greases.
The answer: centralize
After examining numerous alternatives, the company chose a centralized lubrication system, custom-engineered and manufactured by Digilube Systems Inc., Springboro, Ohio. The system consists of a 65-gal reservoir, a programmable controller for setting lubrication intervals, and injectors for dispensing a measured amount of lubricating fluid into each bearing housing. Flowmeters dispense the same lubricant to the desired positions on the chains.
The lubricant, GED 50-1, is a waterbased, low-viscosity fluid for high-temperature applications such as furnaces and motors. The company chose a waterbased fluid because of concerns about fire in the high-temperature furnace environment. The lubricating agent, consisting of molybdenum disulfide and synthetic lubricants, is suspended in the water. When injected into the bearing, the water quickly evaporates leaving a protective lubricant residue on the bearing surfaces. The fluid is free of solvents and fumes, which improved both the workplace environment and disposal conditions.
Mr. Faherty initially set the bearing lubrication interval at 10 min. As the lubricant worked into the bearings, the interval was stretched to 20 min, and eventually to 1 hr. The chains are lubricated at a rate of three drops every 12 hr.
The savings achieved by ABB since installation of the centralized lubrication system are impressive. The 162 replacement bearings in 1995 cost a total of $9,947. Replacing each failed bearing required two workers and took 1.5 hr, for a labor cost of $96 per replacement, or $15,552 for 1 yr. Thus, the total replacement cost for one year was $9,947 + $15,552 = $25,499.
ABB estimates that the new lubricating system will eliminate 75% of the failures, saving $19,124 per year. Eliminating manual lubrication of all the bearings and chain saves the company another $12,000 per year, for a total savings of over $31,000. Actually, this estimate appears to be conservative as only one bearing failed during the first 7 mo, and it had been in service for over 1 yr (5 mo with original lubrication system, 7 mo with new system).
In addition to these direct savings, utility costs have declined due to the elimination of heat loss caused by unnecessary unloading and loading of the furnace during bearing replacement.
The cost of the centralized lubrication system, including the engineering modifications ABB required, will allow a payback of less than 2 yr.