Oil Technical Glossary
Definitions of shear and viscosity:
shear rate - the rate at which adjacent layers of a fluid move with respect to each other, usually expressed as reciprocal seconds (also see shear stress). When the fluid is placed between two parallel surfaces moving relative to each other: shear rate (seconds)-1 = relative velocity of surface (meters/second) distance between surfaces (meters)
shear stress - the frictional force overcome in sliding one "layer" of fluid along another, as in any fluid flow. The shear stress of a petroleum oil or other Newtonian fluid at a given temperature varies directly with shear rate (velocity). The ratio between shear stress and shear rate is constant; this ratio is termed viscosity. The higher the viscosity of a Newtonian fluid, the greater the shear stress as a function of rate of shear. In a non-Newtonian fluid -- such as a grease or a polymer-containing oil (e.g., multi-grade oil) -- shear stress is not proportional to the rate of shear. A non-Newtonian fluid may be said to have an apparent viscosity, a viscosity that holds only for the shear rate (and temperature) at which the viscosity is determined. The shear failure point in most oils is between 4,000 and 8,000 psi. The failure rate for Royal Purple is 100,000-plus.
Newtonian fluid -- any fluid, such as a straight mineral oil, whose viscosity does not change with rate of flow Non-Newtonian fluid - fluid, such as a grease or a polymer-containing oil (e.g., multi-grade oil), in which shear stress is not proportional to shear rate. One of the interesting characteristics of a multi-grade oil is that is tends to act in reverse of Newtonian fluidics (i.e. 10W30 has a viscosity of 10 when cold ? ?W? means ?Winter? -- and 30 when hot -- as the fluid gets hotter the viscosity index increases). But when the temperature is constant the fluid acts more Newtonian.
stick-slip motion - erratic motion due to starting friction encountered by a machine part at each end of its back-and-forth (reciprocating) movement.
thixotropy - the tendency of grease or other material to soften or flow when subjected to shearing action. Grease will usually return to its normal consistency when the action stops. Thixotropy is also an important characteristic of drilling fluids, which must thicken when not in motion so that the cuttings in the fluid will remain in suspension.
tribology - the science of the interactions between surfaces moving relative to each other. Such interactions usually involve the interplay of two primary factors: the load, or force, perpendicular to the surfaces, and the frictional force that impedes movement. Tribological research on friction reduction has important energy conservation applications, since friction increases energy consumption.
viscosity - a fluid's resistance to flow. The common metric unit of absolute viscosity is the poise, which is defined as the force in dynes required to move a surface one square centimeter in area past a parallel surface at a speed of one centimeter per second with the surfaces separated by a fluid film one centimeter thick. For convenience, the centipoise (cP) - one one-hundredth of a poise -- is the unit customarily used in the petroleum industry. Laboratory measurements of viscosity normally use the force of gravity to produce flow through a capillary tube (viscometer) at a controlled temperature. This measurement is called kinematic viscosity. The unit of kinematic viscosity is the stoke, expressed in square centimeters per second. The more customary unit is the centistoke (cSt) - one one-hundredth of a stoke. Kinematic viscosity can be related to absolute viscosity by the equation: cSt = cP * fluid density In addition to kinematic viscosity, there are other methods for determining viscosity, including Saybolt Universal viscosity, Saybolt Furol viscosity, Engler viscosity, and Redwood viscosity. Since viscosity varies inversely with temperature, its value is meaningless unless the temperature at which it is determined is reported.
viscosity index (VI) - an empirical, unitless number indicating the effect of temperature on the kinematic viscosity of a Newtonian fluid. Liquids change viscosity with temperature, becoming less viscous when heated; the higher the VI of an oil, the lower its change in viscosity with temperature. The VI of an oil -- with known viscosity at 40*C -- is determined by comparing the oil with two standard oils having an arbitrary VI of 0 and 100, respectively, and both having the same viscosity at 100°C as the test oil. The following formula is used, in accordance with test method ASTM D 2270: V.I. = L-U/L-H x 100 where L is the viscosity at 40°C of the 0-VI oil, H is the viscosity at 40°C of the 100-VI oil, and U is the viscosity at 40°C of the test oil. There is an alternative calculation, also in ASTM D 2270, for oils with VI's above 100. The VI. of paraffinic oils is inherently high, but is low in naphthenic oils, and even lower in aromatic oils (often below 0). The VI of any petroleum oil can be increased by adding a viscosity index improver. High-VI lubricants are needed wherever relatively constant viscosity is required at widely varying temperatures.
Saybolt Universal viscosity - the efflux time in Saybolt Universal Seconds (SUS) required for 60 milliliters of a petroleum product to flow through the calibrated orifice of a Saybolt Universal viscometer, under carefully controlled temperature, as prescribed by test method ASTM D 88. In the petroleum industry, this method has largely been replaced by the kinematic viscosity method ASTM D 455. Note: weld point - the lowest applied load in kilograms at which the rotating ball in the Four Ball EP test either seizes and welds to the three stationary balls, or at which extreme scoring of the three balls results.
SI - from the French name, Système International d'Unités, the international system of units developed and maintained by the General Conference on Weights and Measures. This is the modern metric system based on the fundamental units of meters, kilograms, and seconds (MKS). The fundamental units of the older (cgs) metric system are centimeters, grams and seconds.
additive - Common petroleum product additives are: antifoam agent, anti-wear additive, corrosion inhibitor, demulsifier, detergent, dispersant, emulsifier, EP additive, oiliness agent, oxidation inhibitor, pour point depressant, rust inhibitor, tackiness agent, viscosity index (VI.) improver.
molybdenum disulfide - a black, lustrous powder (MoS2) that serves as a dry-film lubricant in certain high-temperature and high-vacuum applications. It is also used in the form of pastes to prevent scoring when assembling press-fit parts, and as an additive to impart residual lubrication properties to oils and greases. Molybdenum disulfide is often called moly or molysulfide.
polyolefin - a polymer derived by polymerization of relatively simple olefins. Polyethylene and polyisoprene are important polyolefins.
polyol ester - a synthetic lubricant base, formed by reacting fatty acids with a polyol (such as a glycol) derived from petroleum. Properties include good oxidation stability at high temperatures and low volatility. Used in formulating lubricants for turbines, compressors, jet engines, and automotive engines.
All definitions courtesy of the Society for Maintenance & Reliability Professionals