What is viscosity ?
Viscosity is a measure of a fluid’s resistance to flow. For lubricating oil in general, viscosity is the most important physical property. It is viscosity, as well as the pressure and speed of movement, which determines the thickness of an oil film between two moving surfaces. This in turn determines the ability of the oil film to keep the two surfaces apart, the rate heat is generated by friction and the rate the oil flows between the surfaces and thus conveys the heat away. The oil should have a viscosity at the operating temperature that is correct for maintaining a fluid film between the bearing surfaces, despite the pressure tending to squeeze it out. While a reasonable factor of safety is usually desirable, excessive viscosity should be avoided because this can create more drag and therefore unnecessary heat generation. Viscosity is also useful for identification of grades of oil and for following the performance of oils in service. An increase in the oil’s viscosity during use u
Many people have the misconception that SAE numbers are a continuous series, and that the higher the SAE number, the higher the viscosity. They are unaware that, in general, the viscosity of SAE 30 motor oil is similar in viscosity to SAE 90 gear oil. Another point of interest is that the minimum viscosities for both the SAE motor oil series and SAE gear oil series are essentially the same: 30 SUS @ 210°F for motor oil and 40 SUS @ 210°F for gear oils. It would seem logical to ask why motor oil and gear oil cannot be interchanged, or the two systems combined into one. The real difference between engine oils and gear oils is not viscosity, but additives. Viscosity can be defined simply as the internal resistance of a liquid to flow. Viscosity is stated relative to a given temperature. Tests are generally done at 100°F and 210°F. Viscosity Index (VI) is an empirical number which indicates the rate of change in the viscosity of an oil within a given temperature range. For example, a high
Viscosity is a scientific term describing the internal friction of a fluid or gas. Both have adjacent layers, and when pressure is applied, the friction between layers affects how much the substance will respond to external force. Viscosity, in its simplest form, can be evaluated by the thickness of a substance. A general rule is that gases are less viscous than liquids, and thicker liquids exhibit higher viscosity than thin liquids. Viscosity may also be described as resistance of a liquid to penetration. Some refer to viscosity as the density of a liquid or gas. The term fluidity is opposite to viscosity, as it measures lack of resistance instead of amount of resistance. Within each substance, the molecules reduce flow. They collide with each other, and also exhibit a degree of attraction. Molecular analysis can help determine specific viscosity measurements, thus helping to determine which substances will be of most use in a given application. Temperature also effects viscosity. Rai
Viscosity is resistance to deformation and can be sub-divided into shear and elongation. Shear viscosity is envisaged as the resistance of material laminates sliding past each other in a flowing fluid. Since polymers in the melt phase are often visco-elastic, they also stretch, and the resistance to stretching is termed the elongational or extensional viscosity. Both viscosity measurements are important as they are sensitive to different aspects of molecular architecture.
This question is often best answered by example. Imagine a styrofoam cup with a hole in the bottom. If I then pour honey into the cup I will find that the cup drains very slowly. That is because honey’s viscosity is large compared to other liquids’ viscosities. If I fill the same cup with water, for example, the cup will drain much more quickly. Viscosity is a measure of a fluid’s resistance to flow. It describes the internal friction of a moving fluid. A fluidwith large viscosity resists motion because its molecular makeup gives it a lot of internal friction. A fluid with low viscosity flows easily because its molecular makeup results in very little friction when it is in motion. Gases also have viscosity, although it is a little harder to notice it in ordinary circumstances.
