Machinery's Handbook, 31st Edition
2504 Viscosity Viscosity: The viscosity of a fluid is measured as its resistance to flow and shear; resis tance caused by fluid friction is set up along the molecular shear planes of the lubricant as depicted in Fig. 6. Thin or light lubricants, such as machine and spindle oils shear at a faster rate than thick lubricants such as gear oils, and are said to be less viscous. Although lower viscosity oil is desirable for reducing energy (less drag), it likely would not be “stiff” or viscous enough to withstand the demands of a heavily loaded gearbox. Bearing
Shaft
Viscous Shear Planes
Fig. 6. Viscosity Shear Planes Kinematic Viscosity: Oil viscosity is measured by a variety of classifications. The two generally accepted industrial standards are: Saybolt Universal Seconds or SUS (impe- rial measure), and ISO VG-centisokes-cSt (metric measure). These two standards rate oil by their kinematic viscosity values. The ratings, based on a fluid temperature of 100 ° F (40 ° C) and 212 ° F (100 ° C), relate the time taken for a fluid to flow through a viscosimeter capillary apparatus and directly measure oil’s resistance to flow and shear by the forces of gravity. Other common viscosity classifications and comparison equivalents are shown in Table 2. (2) where h = absolute or dynamic viscosity in centipoise Absolute Viscosity: The absolute or dynamic viscosity is measured in poise (metric) or centipoise (cP) and reyn (imperial), where one reyn is equivalent to 68,950 poise. One- poise is equivalent to a one-dyne force required to move a plane surface (shear plane) of unit area a distance of one centimeter with unit speed (one centimeter per second) over a second plane at a unit distance (one centimeter) from it. Absolute viscosity is calculated by multiplying the kinematic viscosity value by the density of the lubricant measured at the test temperature, and is the measure of oil’s resistance to flow and shear caused by internal friction. Absolute viscosity is the viscosity measured through oil analysis. (3) where h = absolute or dynamic viscosity in centipoise g/cc = lubricant density (specific gravity) cSt = kinematic viscosity in centistokes To convert cSt to SUS at 100 ° F (40 ° C), multiply by 4.632 To convert cSt to SUS at 210 ° F (100 ° C), multiply by 4.664 cSt g cc ⁄ η cP ( ) = -------- at 60 ° F η cP ( ) g cc ⁄ at 60 ° F cSt × =
g/cc = lubricant density (specific gravity) cSt = kinematic viscosity in centistokes
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