Oil Viscosity: Technically, the viscosity of oil is a measure of the oils resistance to shear. Viscosity is more commonly known as resistance to flow. If lubricating oil is considered as a series of fluid layers superimposed on each other, the viscosity of the oil is a measure of the resistance to flow between the individual layers. A high viscosity implies a high resistance to flow while a low viscosity indicates a low resistance to flow. Viscosity varies inversely with temperature. Viscosity is also affected by pressure; higher pressure causes the viscosity to increase, and subsequently the load-carrying capacity of the oil also increases. This property enables use of thin oils to lubricate heavy machinery. The load carrying capacity also increases as operating speed of the lubricated machinery is increased. Two methods for measuring viscosity are commonly employed: shear and time. Shear: When viscosity is determined by directly measuring shear stress and shear rate, it is expressed in centipoise (cp) and is referred to as the absolute or dynamic viscosity. In the oil industry, it is more common to use kinematic viscosity, which is the absolute viscosity divided by the density of the oil being tested. Kinematic viscosity is expressed in centistokes (cst). Viscosity in centistokes is conventionally given at two standard temperatures: 40C and 100C (104F and 212F ). Time: Another method used to determine oil viscosity measures the time required for an oil sample to flow through a standard orifice at a standard temperature. Viscosity is then expressed in SUS (Saybolt Universal Seconds). SUS viscosities are also conventionally given at two standard temperatures: 37C and 98C (100F and 210F). As previously noted, the units of viscosity can be expressed as centipoise (cp), centistokes (cst), or Saybolt Universal Seconds (SUS), depending on the actual test method used to measure the viscosity. Dynamic viscosity Kinematic viscosity Viscosity Index (VI) ISO 3448 Viscosity classification AGMA 9005-D94 Viscosity classification for gear oils SAE J300 Automotive Viscosity classification, engine oils SAE J306 Automotive Viscosity classification, gear oils Comparative Viscosity Classifications Calculators: (Absolute) Dynamic viscosity / temperature Kinematic viscosity / temperature ASTM D341 Viscosity Index (VI) Kinematic viscosity using T @ 40ºC and the Viscosity Index (VI) Kinematic viscosity of a mixture of two base oils Coaxial Cylinder viscometer
Cone on plate viscometer Dynamic viscosity / pressure sensitivity Dynamic viscosity [mpa s = cp] The dynamic viscosity is the viscosity that relates shear stress τ and shear rate du/dz in a fluid, i.e. τ=η du/dz. The viscous shear stress τ is proportional to the shear rate, the dynamic viscosity η being the proportionality factor. So, thicker oils have a higher viscosity value causing relatively higher shear stresses at the same shear rate. Dynamic viscosities are usually measured under high shear conditions, for example, the cone on plate or cylinder viscometer in which the viscous shear torque is measured between two cylinders. With the viscosity known at two reference temperatures the viscosity can be calculated for intermediate temperatures with specific interpolation functions those from Reynolds or Vogel & Cameron. Kinematic viscosity [mm 2 /s = cst] The kinematic viscosity is the quotient of the dynamic viscosity η and the fluid density ρ, ν=η/ρ. The physical principle of measurement is based on the rate at which a fluid flows under gravity through a capillary tube. With the viscosity known at two reference temperatures the viscosity can be calculated for intermediate temperatures using the interpolation function of Ubbelohde-Walther, which is adopted by ASTM D341. Viscosity Index ISO 2909 / ASTM D2270-226 In many cases the temperature dependency is expressed in the Viscosity Index standardized by ISO 2909 / ASTM D2270-226. ISO 3348 Viscosity classification The ISO viscosity classification is recommended for industrial applications. The reference temperature of 40 C represents the operating temperature in machinery. Each subsequent Viscosity grade (VG) within the classification has approximately a 50% higher viscosity, whereas the minimum en maximum values of each grade ranges ±10% from the mid point. For example, ISO VG 22 refers to a viscosity grade of 22 cst ± 10% at 40 C. The viscosity at different temperatures can be calculated using the viscosity at 40 C and the viscosity index (VI), which represents the temperature dependency of the lubricant. ISO 3348 Viscosity class Kinematic viscosity at 40 C [mm 2 /s = cst] Mid-point Minimum Maximum ISO VG 2 2.2 1.98 2.42 ISO VG 3 3.2 2.88 3.52 ISO VG 5 4.6 4.14 5.06 ISO VG 7 6.8 6.12 7.48 ISO VG 10 10 9.0 11.0 ISO VG 15 15 13.5 16.5 ISO VG 22 22 19.8 24.2 ISO VG 32 32 28.8 35.2 ISO VG 46 46 41.4 50.6 ISO VG 68 68 61.2 74.8
ISO VG 100 100* 90 110 ISO VG 150 150 135 165 ISO VG 220 220 198 242 ISO VG 320 320 288 352 ISO VG 460 460 414 506 ISO VG 680 680 612 748 ISO VG 1000 1000 900 1100 ISO VG 1500 1500 1350 1650 Any viscosity can be obtained with a mixture of two ISO VG base oils AGMA 9005-D94 Viscosity classification for gear oils viscosity Equivalent ISO EP gear oils AGMA mpa.s at 40 C viscosity class AGMA lubricant no. min max (ISO 2448) lub. no. 0 28.8 35.2 32 1 41.4 50.6 46 2 61.2 74.8 68 2 EP 3 90 110 100 3 EP 4 135 165 150 4 EP 5 198 242 220 5 EP 6 288 352 320 6 EP 7C 1) 414 506 460 7 EP 8C 1) 612 748 680 8 EP 8AC 1) 900 1100 1000 8 A EP SAE Viscosity Grades for Engine Oils 1 SAE J300 Dec 99 The actual viscosity grade of a lubricant is determined by the Society of Automotive Engineers, for example SAE-15W40 for a multigrade oil and SAE-40 for a monograde oil. The first number (15W) refers to the viscosity grade at low temperatures (W from winter), whereas the second number (40) refers to the viscosity grade at high temperature. Automotive Lubricant Viscosity Grades1 Engine Oils SAE J 300, Dec. 1999 SAE Low Temperature Viscosities High-Temperature Viscosities Viscosity Grade Cranking 2 (mpa.s) max at temp C Pumping 3 (mpa.s) max at temp C Kinematic 4 (mm 2 /s) at 100 C High Shear 5 Rate (mpa.s) at 150 C, 10/s min max min 0W 6200 at -35 60 000 at -40 3.8 5W 6600 at -30 60 000 at -35 3.8 10W 7000 at -25 60 000 at -30 4.1 15W 7000 at -20 60 000 at -25 5.6 20W 9500 at -15 60 000 at -20 5.6 25W 13 000 at -10 60 000 at -15 9.3 20 5.6 <9.3 2.6 30 9.3 <12.5 2.9 40 12.5 <16.3 2.9 6 40 12.5 <16.3 3.7 7 50 16.3 <21.9 3.7 60 21.9 <26.1 3.7 1 All values are critical specifications as 4 ASTM D445
defined by ASTM D3244 2 ASTM D5293 3 ASTM D4684. Note that the presence of any yield stress detectable by this method constitutes a failure regardless of viscosity. 5 ASTM D4683, CEC L-36-A-90 (ASTM D 4741) or ASTM DS481 6 0W-40, 5W-40 & 10W-40 grades 7 15W-40, 20W-40, 25W-40 & 40 grades SAE Viscosity of Automotive Gear Oils a SAE J306 Jan 2005 SAE Viscosity Grade Automotive Lubricant Viscosity Grades Gear Oils Except SAE J 306, 1998 Maximum Temperature for a viscosity of 150,000 cp ( C) Minimum Viscosity at (cst) a 100 C Maximum Viscosity at (cst) a 100 C ASTM D 2983 ASTM D 445 ASTM D 445 70W -55 4.1 -- 75W -40 4.1 -- 80W -26 7.0 -- 85W -12 11.0 -- 80 -- 7.0 <11.0 85 -- 11.0 <13.5 90 -- 13.5 <18.5 110 -- 18.5 <24.0 140 -- 24.0 <32.5 190 -- 32.5 <41.0 250 -- 41.0 -- 1 3 The precision of ASTM D 2983 has Using ASTM D 2983, additional low not been established for temperature viscosity requirements may be determinations made at appropriate for fluids intended for use in lightduty synchronized manual transmission. 2 fact should be taken into temperatures below 40 C. This Limit must also be met after testing in CEC consideration in any producerconsumer l-45-t-93, Method C (20 hours) relationship. Comparative Viscosity Classifications ISO 3348 Industrial oils 1500 AGMA 9005-D94 Gear oils SAE J300 Engine oils SAE J306 Gear oils 1000 8A 250 680 8 460 7 140 320 6 220 5 60 50 90 150 4 40 85 W 100 3 30 80W
68 2 46 1 32 0 22 20 15W 10W 75W 15 5W, 10W 10 7 3 2 ISO and AGMA are specified at 40 C. SAE 75w, 80w, 85, 5w, & 10w are specified at low temperature. SAE 90 to 250 and 20 to 50 are specified at 100 deg C. Viscosities can be related horizontally assuming 96 VI single grade oils. Rule of Thumb: SUS @ 100ºF / 5 = cst @ 40ºC. Nomenclature for Viscosity Result Units Equation Relative Viscosity dimensionless η r = η/η 0 Specific Viscosity dimensionless η sp = η r - 1 = (η- η 0 )/η 0 Inherent Viscosity dl/g η inh = (ln η r )/c Reduced Viscosity dl/g η red = η sp /c Intrinsic Viscosity dl/g [η] = (η sp /c) c 0 Absolute Viscosity cp η = η r x η solvent Kinematic Viscosity cs η k = η / density