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Machinery and Equipment MRO
September 2020
HEAVY INDUSTRY LUBRICATION Calling this article, “heavy industry greasing” or “heavy industry oiling” would work, as the real issue is not “lubricating” rolling element bearings, but about preventing the effects of a contaminated environment, or controlling the bearing operating temperature. BY DOUGLAS MARTIN
about lubrication, the motor bearings should get greased more o¨en than the unit blocks, but it is the opposite, why? Since we are not lubricating the unit block bearing, we are enhancing the “sealing” of the unit block bearing by re-greasing. Therefore, what are the key “re-greasing” factors?
Sealing
There are essentially two methodologies of sealing a bearing and/or its cavity; lip contact seal, and labyrinth seal. With a lip contact seal, the primary role is to keep the good grease (or oil) in the bearing (area). With a lip seal integrated into the bearing its purpose is to keep the good grease in the rolling contact area. In most cases, an external sealing mechanism like a labyrinth or flinger provides protection against contamination and a space between the bearing and outer seals that contains grease. With a labyrinth seal, the grease is expected to flow though the bearing (or housing) and out the labyrinths carrying away contamination. If properly greased, the labyrinth seal should always have a bead of fresh grease at its external opening. There are no common calculations that can determine how much grease is needed for labyrinth seals as there are many variables. Such as: operating temperature, NLGI number of the grease used, speed of the application, degree of contamination, and amount of vibration in the machine application.
Photo: bjdlzx / E+ / Getty Images
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n example that defines the difference between “lubricating” and “greasing” is: an experiment done by E.R. Booser, research scientist, who operated a ball bearing with only two initial drops of oil. The bearing was running at 36,000 rpm for two weeks at a temperature of 100C before encountering failures. The question then is, how much oil is needed for lubrication? In this case, the bearing would have continued to run if one drop of oil was added once a week. An example, once the grease has been added to the bearing, and the rolling element has created its “channel” in the grease, there is a very small amount of oil that bleeds into the lubricating gap. If the channel remains stable, and is always able bleed oil into the gap, all is good. There are many “sealed for life” bearings in industrial se ings (such as electric motors). Looking at a 50hp motor that has sealed for life bearings, and an 1-15/16” sha¨ unit block ball bearing on a conveyor. Both bearings have the same insides, 6210.2RSR bearing in the motor has the same size balls and race diameters as P2BL-115WF unit block. The unit block rotates at 100rpm on a dusty conveyor and the motor that drives the same conveyor rotates at 1750 rpm, but bearings are inside a cavity with a labyrinth seal, and a grease pack protecting the sealed bearing. The slow-moving unit block bearing gets greased once a month, and the motor bearing never gets greased. If this was
