Machinery Lubrication

Machinery Lubrication Jan Feb 2014

Machinery Lubrication magazine published by Noria Corporation

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18 | January - February 2014 | While traveling and teaching lubrication fundamentals courses at various facilities across the country, I am constantly reminded of the misconceptions that are rampant in our industry. Recently, I was asked the question, "How do you deter- mine the proper circulating oil flow to a journal bearing if you know the shaft speed, clearance, oil type and temperature?" Like many things in lubrication, the answer is not nearly as simple as it may seem. Several concepts must be explained, including some of the design processes that are involved in deter- mining the correct journal and bearing dimensions, materials and lubrication requirements. Several names are used for these types of bearings, such as journal bearings, sleeve bearings and plane bearings, just to name a few. The basic function of a journal bearing is generally to support a shaft. This type of bearing usually is chosen for applications that are not subject to changes in shaft speed or load. There are three major components of this type of system: the stationary part or the bearing, the moving part or the journal, and the lubricant. The system's metal components may consist of any number of materials. The bearing normally is made of a softer metal than that of the journal to prevent wearing of the moving element. Typically, the lubricant enters the bearing from the center and passes through to the ends where it leaves the bearing. The lubri- cant performs several functions including providing controls for friction, wear, corrosion, temperature and contamination, as well as a power transmission component. In journal bearing lubrication, three basic lubrication regimes apply: hydrodynamic or full film, mixed film, and boundary. It is important to note that regardless of how well a metal surface is machined, imperfections still exist. These little peaks and valleys are known as asperities. The three lubrication regimes essentially refer to the amount of contact between these asperities. The vast majority of journal bearings are designed to operate in the hydrodynamic (full-film) regime. However, these bearings spend a portion of their operating life in the other two regimes as well, so they also deserve an explanation. In his article "Boundary Lubrication, an Appraisal of World Literature," W.E. Campbell defines boundary lubrication as: "… lubrication by a liquid under conditions where the solid surfaces are so close together that appreciable contact between opposing asperities is possible." In short, boundary lubrication is the regime where metal-to-metal contact occurs and the largest portion of wear is generated. The vast majority of the load is being carried by these asperities with very little, if any, being carried by the lubricant. This typically takes place upon equipment startup. In mixed-film lubrication, a little contact between the asperities still exists, but the lubricant is also supporting some of the load. This transpires shortly after startup but prior to reaching normal operating speed. Harry C. Rippel's book, The Cast Bronze Bearing Design Manual, describes mixed-film lubrication as: "… part of the total load carried by the bearing is being supported by individual load-carrying pools of self-pressurized lubricant and the remaining part by the very thin contaminating film associated with boundary lubrication." PersPeCtIVe loreN GreeN | NorIA CorPorAtIoN B e a r i n g l u b r i c a t i o n determining Proper oIl Flow to Journal BeArINGs

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