Machinery Lubrication magazine published by Noria Corporation
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Viscosity LESSONS IN LUBRICATION BENNETT FITCH NORIA CORPORATION ANATOMY of a VISCOMETER This is the third part of a series of "anatomy" lessons within Machinery Lubrication. In this issue, the various styles of viscometers will be dissected and explained to uncover all of their internal characteristics along with the factors that go into performing a precise and repeatable viscosity measurement. In tribology and the study of lubrication, viscosity has often been referred to as the most important characteristic of a base oil. Why is viscosity so important in the lubrication of a machine? For one, too low of a viscosity would allow interfacial contact between internal components in motion to generate wear. On the other hand, too high of a viscosity would force the machine to work harder to overcome the lubricant's internal resistance to flow. For this reason, it is important to understand not only the viscosity of the base oil but also how it may change due to fluctuating operating or environmental conditions. Viscometers vs. Rheometers Viscosity is the measure of a fluid's resistance to flow (shear stress) at a given temperature. As flow conditions vary, not all fluids retain an unchanged viscosity. A fluid in which the viscosity changes with flow conditions is called a non-Newtonian fluid. The viscosity of these types of fluids is measured by rheometers. Viscometers measure the viscosity of Newtonian fluids. Kinematic vs. Absolute Viscosity Among the various viscometer methodologies, there are two distinct ways in which viscosity can be expressed: kinematic viscosity and absolute viscosity. The key difference between the two is that kinematic viscosity is measured by observing the fluid's resistance to flow under the force of gravity, while absolute viscosity is measured by observing the fluid's resistance to flow under an external and controlled force, either through a capillary or by movement of a body through the fluid. Kinematic viscosity is reported in centistokes (cSt), but absolute viscosity is reported in centipoise (cP). For the sake of comparison, absolute viscosity is often converted into kinematic viscosity by dividing it by the fluid's specific gravity (SG). cSt = cP/SG or the equation's inverse: cP = cSt x SG 48 | July - August 2013 | www.machinerylubrication.com Viscosity Testing Methods Several methods have been established for viscosity testing, and each has its advantages. Below is a list of the most common techniques used in base oil viscosity testing. Capillary (Glass) Viscometer Test The main apparatus used in a capillary viscometer test is a glass tube in the general shape of a "U," which gives it its commonly associated name, the U-tube. The procedure for a U-tube requires the Gravity Flow tube to be submerged in a U-shaped Glass Tube temperature-controlled bath (usually 40 or 100 degrees C) and a precise time reading (in seconds) for the time it takes a fixed amount of fluid to flow within the tube from one marked point to another by suction or through the force of gravity. This measured time is then multiplied by a constant (associated to the particular tube) to calculate the absolute viscosity (suction) or the kinematic viscosity (force of gravity). Rotational Viscometer Test The key characteristic of a rotational viscometer Capillary Viscometer involves a rotating appaASTM D445-97 ratus, which is referred to as the spindle, being submerged within the test fluid. The torque on the rotating shaft is then used to measure the fluid's resistance to flow. Since this measurement does not involve the force of gravity but rather a function of the fluid's internal shear stress, the rotational viscometer calculates the fluid's absolute viscosity. A common variation of this type of viscometer is called the Brookfield viscometer.