This apparent loss of viscosity results from the deformation of the spherical shape of the polymer molecules. Finished lubricants generally contain additives that lead to structural viscosity, called apparent viscosity, which decreases progressively under the influence of shear. However, shear forces can significantly affect the viscosity of finished lubricants. Pure mineral oils are unaffected by these shear forces and retain their viscosity. These forces can cause a degradation of these molecules, leading to loss of viscosity. Hydrocarbon molecules or molecular aggregates that make up the oil experience shear forces while in a container or during flow. One effect that the polymer has on the oil is a change in viscosity as a function of the shear ratio. Viscosity modifiers find great use in engine and gear oils of various classes, transmission fluids, power steering fluids, greases and some hydraulic fluids. Another way of describing this phenomenon is that, at higher temperatures, the polymer becomes more soluble, causing an increase in viscosity. The result is an effective increase in viscosity. This increases the association of the polymer with the oil due to an increase in its surface area. At high temperatures, however, the situation is reversed because the polymer chains extend or expand. At low temperatures, the polymer molecules occupy a small volume and therefore have minimal association with the oil. This is a practical way to extend the operating range of mineral oils to higher temperatures, without adversely affecting their low temperature fluidity. They are high molecular weight polymeric chemical compounds that are added to low viscosity oils to improve their high temperature lubrication characteristics. VI improvers are performance additives that make it possible to formulate multiple grade oils. To ensure that the lubricant performs better at elevated temperatures, viscosity index (VI) enhancing additives are added. For oils with similar kinematic viscosity, the higher the viscosity index, the less the effect of temperature on kinematic viscosity ”. In order to know how much the temperature affects the viscosity of a given lubricant, we use the viscosity index as a common measure to define these viscosity-temperature characteristics in the lubricants.Īccording to ASTM D-2770: Standard Practice for Calculating the Viscosity Index of the Kinematic Viscosity at 40 ° C and 100 ° C, the viscosity index is defined as “an arbitrary number used to characterize the variation in the kinematic viscosity of a oil product with temperature. The viscosity of liquids decreases with increasing temperature and at high temperatures it can drop to a very low level, which will cause it to lose the ability to maintain a satisfactory lubricating film on the surfaces. Therefore, the greater the resistance to flow, the greater the viscosity of the oil. Viscosity can be defined as the internal resistance to flow. The thickness of the film is a function of viscosity. To achieve this, the lubricant forms a film to separate the surfaces and minimize metal-to-metal contact. One of the main functions of the lubricant is to reduce friction and, consequently, heating and wear between moving parts.
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