![]() ![]() Excessively high viscosity will prevent metal-to-metal contact, but will generate unnecessary heat and loss of power as the rolling elements are forced to plow through the oil film. When viscosity is too low, it can cause mixed or boundary lubrication with resulting heat generation and component wear. The functionality of the lubricant is negatively affected when the incorrect viscosity is used. The optimum fluid film is sufficiently thick to allow the rolling elements to function without metal-to-metal contact with minimum friction. Use of the correct viscosity lubricant for the speed, temperature and loads ensures the development of a full oil film between rotating parts. If temperature also exceeds 100 C (212 F), a synthetic is recommended because the oxidation rate of mineral oil accelerates at higher temperatures. The viscosity of synthetic oil is less sensitive to temperature changes and more widely used when temperature fluctuations exist. The oil can be hydrocarbon oil, although synthetic oils are used for specific lubrication applications. These numbers relate to the kinematic viscosity in centistokes. Oil in process pumps is typically an ISO grade 32, 46, 68 or 100. Conversely, if low speeds, high loads and high temperatures are the parameters, then a high-viscosity lubricant should be chosen. If an application involves high speeds, low loads and low temperatures, then a low-viscosity lubricant is adequate. Oil or grease base viscosity selection is influenced by load, temperature and speed. Although lubrication is present, there is not enough to prevent metal-to-metal contact. Boundary layer is sometimes referred to as thin film lubrication and is usually the result of insufficient lubricant supply.They are predominantly found in rolling element bearings. Elastohydrodynamic (EHD) is when a hydrodynamic film is formed by applied pressure or load.They are also known as hydrodynamic full film. Full film denotes the presence of enough lubricant to ensure complete separation of the moving surfaces.The three basic oil film conditions are referred to as: A protective oil film is required to perform these functions. Viscosity is an important property of a lubricant, which supports the function of a lubricant in reducing friction and wear. When selecting a lubricant, it is important to consider that the primary purpose of oil, or the oil constituent of grease, is to separate the rolling elements and raceway contact surfaces, lubricate sliding surfaces within the bearings and provide corrosion protection and cooling. ![]() ![]() Rolling element bearings (Image 1) in process pumps are lubricated by grease, mineral oil or synthetic oil. The viscosity and type of lubricant used will depend on the bearing design and service. Rolling element bearings (Image courtesy of Hydraulic Institute) ![]()
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