VISCOSITY. The most important physical property of a lubricant is its viscosity. Viscosity, which may be defined as a fluid’s resistance to flow, is the characteristic most generally stipulated by equipment manufacturers when making lubricant suggestions. The selection of proper lubricant viscosity is often a compromise between selecting one high enough to prevent metal to metal (wear) contact, and one low enough to let sufficient heat dissipation. In the past, viscosity was measured in such units as Saybolt Universal Seconds (SUS), Redwood No. 1 Seconds, and Engler Degrees. The recommended system of measurement for the U. S. Navy is the centistokes (cSt). Kinematic viscosity in centistokes is acquired by measuring the time required for a particular volume of fluid to pass through a calibrated capillary tube at a specific temperature. Various industry standards exist for the characterization of lubricant viscosity. The most familiar of these is the Society of Automotive Engineers (S. A. E. ) category of automotive engine and gear case oils. This system qualities lubricants in accordance to their viscosity qualities at either -18°C (0°F) or100°C (212°F). Oils reaching low temperature viscosity conditions are assigned a ″W″ after the grade number(for example, SAE grade 10W). Oils meeting high temperature requirements are assigned a grade number such as SAE grade 30. Multigrade oils may be formulated to meet both low and high temperature requirements (for example, SAE grade 10W-30). However, these viscosity designations are relevant primarily for the lubrication of inner combustion engines. By international agreement, all nations now recognize a globally applicable system of viscosity classification known as the International Standards Organization (ISO)/American Society of Testing and Materials (ASTM) Viscosity System for Industrial Lubricants. This system assigns viscosity grades from ISO VG2 through VG1500, where the number shows the midpoint viscosity in centistokes of the lubricant at 40°C (104°F).
VISCOSITY INDEX (VI). The effect of temperature on a lubricant’s viscosity is a measurement of its Viscosity Index (VI). When the VI scale was introduced in 1929, a reference paraffinic base stock was assigned a VI of 100, and a naphthenic base stock a VI of 0. Most naval oils of paraffinic base stock have VI’s in the 95-100 range. Naval oils ready from synthetic stock, and multigrade engine oils generally have VI’s in excess of 100. The higher the VI, the less a given lubricant’s viscosity will change with a subsequent change in temperature.
CLOUD POINT AND POUR POINT. Since petroleum stock consists of a mixture of molecular components, lubricants do not exhibit sharp freezing points. Rather, as a lubricant is cooled, particular components such as waxes will begin to precipitate out and become evident in the liquid as a cloud. The temperature at which this happens is known as the cloud point of the lubricant. If the product is further cooled, a point will be achieved at which the lubricant will no longer flow or be effectively pumped. The temperature at which this occurs is termed the pour point of the lubricant. Both properties are related to the wax material of the base stock. The pour points of high-wax lubricants may be depressed by the element of pour point depressant additives. Pour point behavior turns into important in functions such as refrigerant compressor lubrication where the oil is exposed to low temperatures.
FLASH POINT AND FIRE POINT. As a lubricant is heated, lighter components begin to vaporize.
The temperature at which sufficient vapor concentration exists over the surface of the lubricant so that ignition with a test flame is possible is known as the flash point of the product. Flash point is beneficial for both product storage demands and for the detection of contaminants of one product with another. The fire point of a lubricant is that temperature at which sufficient vapors are present above the surface of the lubricant to support combustion upon ignition. This parameter is useful for storage and safety considerations.
VISCOSITY INDEX (VI). The effect of temperature on a lubricant’s viscosity is a measurement of its Viscosity Index (VI). When the VI scale was introduced in 1929, a reference paraffinic base stock was assigned a VI of 100, and a naphthenic base stock a VI of 0. Most naval oils of paraffinic base stock have VI’s in the 95-100 range. Naval oils ready from synthetic stock, and multigrade engine oils generally have VI’s in excess of 100. The higher the VI, the less a given lubricant’s viscosity will change with a subsequent change in temperature.
CLOUD POINT AND POUR POINT. Since petroleum stock consists of a mixture of molecular components, lubricants do not exhibit sharp freezing points. Rather, as a lubricant is cooled, particular components such as waxes will begin to precipitate out and become evident in the liquid as a cloud. The temperature at which this happens is known as the cloud point of the lubricant. If the product is further cooled, a point will be achieved at which the lubricant will no longer flow or be effectively pumped. The temperature at which this occurs is termed the pour point of the lubricant. Both properties are related to the wax material of the base stock. The pour points of high-wax lubricants may be depressed by the element of pour point depressant additives. Pour point behavior turns into important in functions such as refrigerant compressor lubrication where the oil is exposed to low temperatures.
FLASH POINT AND FIRE POINT. As a lubricant is heated, lighter components begin to vaporize.
The temperature at which sufficient vapor concentration exists over the surface of the lubricant so that ignition with a test flame is possible is known as the flash point of the product. Flash point is beneficial for both product storage demands and for the detection of contaminants of one product with another. The fire point of a lubricant is that temperature at which sufficient vapors are present above the surface of the lubricant to support combustion upon ignition. This parameter is useful for storage and safety considerations.
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