Oil with a high index number, say 160, would look and behave similarly at these two temperatures. The index number indicates the degree of change in viscosity of an oil within a given temperature range, currently 40-100☌. The extent that they change is indicated by their viscosity index (VI). The key is to select a fluid that is not too light and also not too heavy.įluids thicken as their temperatures decrease and thin as their temperatures increase (like candle wax). If you use too high, the fluid friction increases resulting in reduced energy efficiency, higher operating temperatures, and hard starting – particularly at cold temperatures. If you use too low a viscosity oil metal-to-metal contact occurs, poor sealing and increased oil consumption. The fact is, the use of high viscous fluids can be just as detrimental as using too light an oil. Knowing that a fluid’s viscosity is directly related to its ability to carry a load, one would think that the more viscous a fluid is, the better it can lubricate and protect. The fluid must be adequate enough to separate moving parts at operating temperatures of the equipment. The greater a fluid’s viscosity, the greater the loads that it can withstand. Viscosity is very important because it is directly related to a fluid’s load-carrying ability. Terms such as thick, heavy and high suggest the fluid demonstrates a strong resistance to flow in the example such as honey. Other ways to refer to a fluid’s viscosity in more common generic terms are thin, light and low and suggest that the fluid flows readily such as water. The term used to describe this is Apparent Viscosity and it is expressed in units known as centipoise (cP). However, significantly more energy is required to stir honey with that same spoon. It takes little energy to stir water with a spoon.
You can think of this as the energy required to move an object through the fluid. A fluid’s viscosity can also be indicated by measured resistance.
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