When it comes to automotive engine lubrication, we often prefer oils with a low viscosity spectrum, but a "High Viscosity Index"- in short, this means that the viscosity stays Liquid water has an extremely narrow viscosity spectrum (in the range of 33 F to 211 F) when compared with honey: meaning that the viscosity of water goes relatively unchanged over that temperature range alternatively, honey would pour much slower at 33 F compared to 211 F, which means that its viscosity spectrum is much wider (in said temperature range). But what about water? Whether near freezing or just about to boil, water pours relatively the same. Honey when you heat it up: it immediately becomes easier to pour once it reaches a certain temperature also, when cooled, the consistency of honey thickens. Lubricants resembling this liquid are higher viscosity oils. This is because honey is more viscous than water and has a much higher viscosity.Īlternatively, this fluid is a great deal thicker - somewhat like honey. Immediately - but a glass of honey would slowly spill over, giving you a second to react. If you were to knock over a glass of water, the liquid would spill out Take, for example, this comparison of two common substances with different viscosities: water and honey. In a nutshell, the thicker the fluid is, the more resistance to flow it will demonstrate. Viscosity is the measure of a fluid's resistance to flow (under certain conditions). This liquid's water-like state is reminiscent of a lower-viscosity lubricant. ![]() Viscosity and though the word may sound a bit dense, it's actually pretty simple: it is a number value for the relative thickness (or weight) of your oil or lubricant. These numbers are representative of a lubricant's ![]() Numbers in 5w-20 might signify? Or - should you work with machinery that requires an industrial lubricant - the AW-68 on your pail of hydraulic oil? High-VI fluids can also improve the energy efficiency of a machine.If you've changed your own oil before, then chances are you know what the "w" in 5w-20 means (HINT: it stands for winter grade, or rating). But when conditions are not constant-such as in mobile equipment that must operate from winter cold to desert heat-and loads, speeds and temperatures vary, many OEMs recommend high-VI fluids that help maintain near-optimum viscosity over a wider temperature range. Here, the viscosity index is not so important: Just choose the suitable monograde viscosity recommended for the application. Too thick and the machine is hard to start and has low mechanical efficiency, and can even lead to problems like cavitation.Īlthough oil viscosity changes with temperature, that’s not so important in machines that run at constant load and speed and at constant temperatures. When viscosity is too low and fluid too thin, users will see problems like higher wear and overheating. Experts often recommend shear-stable additives to ensure high-VI fluids work as intended.Īlso remember that it’s important to use a fluid with the recommended viscosity when running a machine. Such action can, over time, break down the polymers and degrade fluid viscosity. However, viscosity improvers can be susceptible to mechanical shearing. But at higher temperatures, the polymers expand and that helps increase viscosity. At low temperatures, the polymer chains in the modifiers contract or fold and don’t have much effect on fluid viscosity. ![]() These modifiers are temperature sensitive. ![]() Synthetic oils usually have a higher VI than do mineral oils.įluid manufacturers can improve the viscosity index of base oils by using polymer additives to form multigrade-viscosity oils. Highly refined mineral oils with few contaminants tend to have higher VIs, all things being equal. Standard ASTM D2270 calculates viscosity index in part by measuring a fluid’s kinematic viscosity at 40° and 100° C. That, in turn, means consistent, high performance from a machine. The best oils with the highest VIs remain stable and don’t vary much in viscosity over a wide temperature range. Typical mineral-oil fluids used in hydraulics have a viscosity index of around 100, although products range from below 100 to well beyond 200. High-VI fluids, in contrast, are less affected by temperature changes. Thus, a fluid with a low viscosity index will experience a relatively large swing in viscosity as temperatures change. (It also applies to fluids like engine and automatic transmission oils, gear lubricants, and power-steering fluids.) The greater the viscosity index (VI), the smaller the change in fluid viscosity for a given change in temperature, and vice versa. Viscosity index is a dimensionless number that represents how the viscosity of a hydraulic fluid changes with temperature.
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