The THICK and THIN of VISCOSITY
Of all the characteristics a lubricant may possess, the most important is its viscosity. The viscosity of a fluid and how that fluid reacts to certain variables will determine how well the fluid can perform the basic functions of a lubricant. What is viscosity? When a fluid is subjected to external forces, it resists flow due to internal molecular friction. Viscosity is a measure of that internal friction. Viscosity can be referred to as the measurement of a fluid’s resistance to flow. Viscosity can be viewed in two different ways. The first is a fluid’s tendency to flow as is visually indicated. One can think of this as the time it takes to watch a fluid pour out of a container. The term used to describe this is Kinematic Viscosity and it is expressed in units indicating flow volume over a period of time. The most Commonly used unit of Kinematic Viscosity is the centistoke (cSt). A fluid’s viscosity can also be indicated by measured resistance. You can think of this as the energy required to move an object through the fluid. It takes little energy to stir water with a spoon. However, significantly more energy is required to stir honey with that same spoon. The term used to describe this is Apparent Viscosity and It is expressed in units known as centipoise (cP). There are other ways to refer to a fluid’s viscosity. Some of the more common generic terms are thin, light and low. These terms suggest the fluid flows readily. Water would be an exam-pie of such a fluid. Terms such as thick, heavy and high suggest the fluid demonstrates a strong resistance to flow. An example of such a liquid would be honey.
Viscosity is very important because it is directly related to a fluid’s load-carrying ability. The greater a fluid’s viscosity, the greater the loads that it can withstand. The viscosity of a fluid must be adequate to separate moving parts at the operating temperature of the equipment. 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 fact is, the use of a high viscosity fluid can be just as detrimental as using too light an oil. If you use too low of a viscosity oil, you get metal-to-metal contact, poor sealing and increased oil consumption. If you use too high of a viscosity oil, you get increased fluid friction, reduced energy efficiency, higher operating temperatures, and hard starting — particularly at cold temperatures. The key is to select a fluid that is not too light and also not too heavy.
AAN 7/01
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