An important quality of a lubricant is its Viscosity. Viscosity is a measurement of the internal cohesiveness of an oil, better known as its resistance to flow. Viscosity is defined as being equal to shear stress/ shear rate. High Viscosity oils have molecules with greater cohesion ability (Higher resistance to flow) were as low viscosity oils have lower cohesion ability (Low resistance to flow) allowing faster flow rates to occur.
Lubricants are typically used to separate moving parts in a system. They reduce friction, surface fatigue, heat, noise and vibrations. Lubricants are able to do this by forming a physical barrier between moving parts.
When oil is heated its viscosity is reduced, this reduces its load carrying capacity. Viscosity changes based on load and temperature. When temperature increases, the lubricant becomes thinner and the viscosity becomes lower. Inversely as the temperature decreases, the lubricant thickens and viscosity increases, making it more difficult to pour or pump. The graph illustrates what impacts temperature can have on viscosity.
Viscosity Index (VI) is a measurement of oil's viscosity change due to temperature. The higher the VI the less it will thicken when cold, and the less it will thin out when heated. Oil's that have high VI are more resistant to viscosity change and are more effective over a wide temperature range. Oils can be rated or group by their VI values.
Testing of Viscosity is most commonly analysed via ASTM D445, known as the Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids. This is done by measuring the time for a volume of liquid to flow under gravity through a calibrated glass capillary viscometer at a set temperature, usually either 40˚C or 100˚C. Kinematic Viscosity is reported in Centistokes (cSt) at temperatures 40˚C and 100˚C. 1 centistoke (cSt) = 1mm ²/S.
Viscosity is an important indicator in your condition monitoring program. It identifies the condition of the oil or the ability the oil has to lubricate internal components, separate contact and reduce friction. New oils (Virgin Oil) is tested at both 40˚C and 100˚C to establish the viscosity grade and later used as base line data against the used oil. It is plotting against the used oils viscosity to identify any increase or reduction in the used oil. This identifies whether or not the oil still has the capabilities of providing adequate lubrication to the internal components. Viscosity generally have standard limit set for increases and decreases: Upper limits are usually set at a 10% increase for Caution and a 20% increase for Problem or Critical. Lower limits are usually set around 5 or 10% decrease for Caution and 10 or 20% decrease for Problem. An example can be seen below.
What causes viscosity to increase or decrease increases may be caused but not limited to oxidation, polymerisation, carbon build-up (Soot), contaminants, anti-freeze, water ingress and/ or the addition of a wrong oil type. Decreases in viscosity could indicate fuel dilution, sheer down of the VI, thermal cracking, over extended oil drain periods and again the addition of a wrong oil type.
There are so many important factors about viscosity, viscosity index, temperature, dynamic and absolute viscosity, base oils, additives, monogrades, multigrades, synthetics, testing, trending, limits and more, there is so much that there simply isn't enough time to place all the information into this document, I have explained the basics and you should now have an understanding of what viscosity is and how important it is, if you would like to know more?
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