Various types of mills are used to grind coal, limestone and other materials. Most of these mills are driven by large pinion and girth gears, which operate in high load conditions. These mills often are critical to a plant’s operation and cannot afford to be down except for scheduled maintenance. Therefore, lubrication of these gears with the most advanced lubricant available is crucial in assuring continuous operation of the mills and protecting the expensive gears.
Yet many of the typical lubrication solutions for mills, particularly asphaltic and molybdenum-based products, have drawbacks. These include health, safety and disposal problems; needing extra labour to keep their required drip pans clean; and frequent build-up in the bearings and shrouds. In response to some of these challenges, Lubrication Engineers (LE) developed a high-performance Pyroshield® line of open gear lubricants specifically for applications subject to extreme conditions, such as immense pressure, shock loading and the high contact point temperatures experienced during boundary lubrication.
Asphaltic and molybdenum-based product limitations
In many mining, electric utility and industrial plants, metallic and non-metallic minerals are pulverized or ground so they can be used as solid fuels or ingredients in products. For example, coal as a solid fuel requires some processing before its latent energy can be fully exploited. At a coal-fired power plant, the coal is pulverized in a mill, then fed into a boiler and used as fuel to heat water and generate steam to drive the turbine. Likewise, limestone must be ground to a fine size so it can be used to manufacture various types of cement or made into a slurry for use in scrubbers at power plants.
Historically, asphaltic products had been used in these types of applications, serving more as “cushioning compounds,” than lubricants. Some unleaded asphaltic-based lubes are considered cancer-causing because they contain polycyclic aromatic hydrocarbons, which are listed as potential carcinogens by the International Association for Research on Cancer.
Asphaltic products remaining in the bottom of the drums and in the drip pans pose possible health hazards and must be disposed of at an approved hazardous waste disposal site. Approximately 75% of the product remains to be disposed of after use. This is both inconvenient and costly.
In addition to health and disposal concerns, users of asphaltics have been plagued with housekeeping problems. The sticky black asphaltic product builds up in the gear tooth roots. Callum Ford, National Marketing Manager at LE South Africa, explains that, because of this, plants experience problems such as worn pinions due to misalignment, pinion bearing failure, breakage of pinion pedestal bolts, and disintegration of the pedestal concrete foundation.
“Most users of asphaltic materials have remained so because of a lack of knowledge of alternatives,” he says. While molybdenum-based fluid grease lubricants have offered some improvements, they usually result in increased consumption.
LE formulated its Pyroshield® lubricants with its proprietary wear-reducing additive Almasol®, along with a unique combination of EP additives, to ensure outstanding load-carrying ability. Asphaltic lubricants have a Timken OK load of only nine to 15 kilograms as compared to 40 kilograms for Pyroshield® 9000 and Pyroshield® 5180, and an even higher weight for Pyroshield® 9011. “Plants around the world that have switched from asphaltics to one of LE’s Pyroshield® lubricants have eliminated the problems with their previous gear lubricant and obtained additional benefits,” Ford says.
Some of the many benefits of Pyroshield® include lower consumption, no disposal problems, no build-up and lower pinion temperatures which mean less friction and wear and result in longer lifespans for equipment components.
“Another key aspect for effective lubrication of equipment like mills which operates under extreme conditions, is to ensure that the correct amount of lubrication is applied,” says Ford. Automatic lubrication systems use an air-operated pumping unit that feeds the system directly from the drum of the lubricant. This reduces wastage and prevents build up, contributing to better performance and longevity for milling components.