![]() Higher molecular weight polymers make better thickeners but tend to have less resistance to mechanical shear. The various polymers have different shear stability characteristics. These include, amongst others, poly alkyl methacrylates (PMA), olefin copolymers (OCP) and hydrogenated styrene-diene copolymers (SDP). Various types of polymers are used as VIIs in lubricating oil formulations. Consequently, it affects the ability of the polymer molecules to add to the viscosity of the fluid at elevated temperatures. This “physical breakage” cannot be reversed when the shear stresses are removed. There are areas in engines, gearboxes, hydraulic pumps, etcetera that have very tight clearances and this can shear the polymers (viscosity index improver molecules) into smaller pieces. The primary disadvantage is that polymers are susceptible to shearing when subjected to high mechanical stresses in severe service. Unfortunately, viscosity index improvers have some downsides as well. In addition to multigrade engine oils, Viscosity Index Improvers are also used in multigrade gear oils, automatic transmission fluids, power-steering fluids and high viscosity index hydraulic oils. It should be noted that there is actually no SAE 40 base oil in an SAE 15W40 formulation. The outcome is an SAE 15W40 multigrade oil that will provide adequate protection over a wide temperature range (Figure 2). The result is that when these polymer additives are blended in the correct proportion with for example SAE15W base oil, the oil flows like an SAE 15W at low temperatures and similar to an SAE 40 oil at high temperatures. When the oil cools down, the polymers go back to their original shape. As the temperature increases, they expand into large stringy structures that restrict the normal oil flow, which has a thickening effect on the oil. ![]() When the oil temperature is low, these polymers curl up into tight balls that flow readily with the oil molecules (Figure 1). Viscosity Index Improvers (sometimes referred to as Viscosity Modifiers) are polymers that provide “thickening characteristics” to oil at elevated temperatures. This is when the use of a Viscosity Index Improver (VII) becomes essential. The oil also needs to be viscous (thick) enough, SAE 40 for example, to protect the engine when it reaches operating temperature. SAE 15W (“W” for winter), is needed to enable the oil pump to push the oil through the engine during cold morning starts. For instance in automotive engines, an oil with a low viscosity at low temperature, i.e. Many applications require the lubricant to perform satisfactory across a wide range of temperatures. VI has no units and is calculated using the viscosity of the oil at 40˚C and 100˚C. The lower the VI, the more the viscosity will change with variation in temperature. It is most commonly used to portray the viscosity-temperature behavior of lubricating oils. If you have the viscosity in centiPoise, divide the centiPoise value by the oil's viscosity to get centiStokes.Viscosity Index (VI) is a measure of the change in viscosity with change in temperature. ![]() This calculator uses centiStokes (cSt), which are equivalent to mm^2/s. Higher quality oils tend to have higher viscosity indices, and while there are ranges in every category, a synthetic oil will tend to have a much higher viscosity index than its conventional counterpart. If you know the viscosity of your oil at 40✬ and 100✬ (100✯ and 212✯) you can easily calculate its Viscosity Index. Oil needs to flow when cold, but not get too thin and runny when it's warm. That is, oil with a high VI will retain their viscosity over a wider temperature range and work as designed in both hot and cold environments.Ī higher VI is important when your machine will experience a big change in operating temperatures. The higher the viscosity index, the lower the change in viscosity. The degree to which the viscosity decreases determines the Viscosity Index (VI). The viscosity of an oil decreases as temperature increases. ![]() Viscosity Index and what you need to know ![]()
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