| Viscosity Explanation |
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The strength of the cohesive force in an oil is related to its viscosity. Viscosity, in turn, determines the amount of fluid friction. Lubricants are make in a wide range of viscosities to meet the great variety of mechanical equipment designs and operating conditions found in modern industrial machinery. Viscosity is, therfore, the most important property to consider in choosing a lubricant for a given application. Other physical and chemical properties govern the ability of an oil to resist deterioration in service but the viscosity of the oil at the operating temperature determines the bearing friction, rise in temperature, and rate of oil flow through the bearing. To a certain extent, viscosity also determines the load-carrying capacity of the bearing. The Viscosity of the lubricant determines the drag exerted by the moving parts, which pulls the lubricant between the metal surfaces. An oil of the correct viscosity will be distributed quickly to the moving surface, thereby decreasing friction when starting or stopping. In general, the slower the motion, the higher the fluid viscosity needed to produce a fll load-supporting film. Also, heavier loads require viscosity fluids capable of preventing such loads from breaking through the film layers. A Word About Viscosity Test Methods and ISO Classification System  Older viscosity test methods practiced in the United States described and measured the viscosity of most industrial lubricants in either Saybolt Universal Seconds (SUS) or centistrokes (cSt) at reference temperatures of 100'F (37.8°C) and 210'F (98.9°C). Most petroleum testing laboratories now measure the viscosity in cSt an 40 'C and 100 'C, according to ASTM D445, the method specified by the International Standards Organization (ISO).The ISO Classification System for lubricants has been the industry standard since January 1, 1978. In addition to specifying the standard method for measuring viscosity, the system also establishes eighteen viscosity grades so that users, suppliers, and equipment designers can select or specify the viscosity of industrial fluid lubricants on a common and uniform basis. Each grade is designated by its nominal viscosity at 40°C, with specified viscosity range tolerance, and a nomenclature system which identifies each grade. For example, an oil with a nominal viscosity of 100 cSt/40°C is identified as an ISO VG 100 oil. |
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