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Viscosity is the measure of the resistance to flow or internal friction of the fluid. Viscosity changes with the temperature so the temperature at which the measure was made must always be specified. In other words viscosity is the property of a fluid that causes it to resist flow, which mechanically is the ratio of

Shear stress: Shear rate
Viscosity may be visualized as a result of physical interaction of molecules when subjected to flow. Lubricating oils have long chain hydrocarbon structures, and viscosity increases with chain length. Viscosity of an oil film, or a flowing column of oil, is dependent upon the strong absorption of the first layer adjacent to the solid surfaces, and the shear of adjacent layers.
Viscosity is by far the most significant property for establishing the thickness, pressure, and temperature of an oil film in hydrodynamic lubrication and in elasto hydrodynamic lubrication . Viscosity is also a significant factor in predicting the performance and fatigue life of rolling element bearings and gears. Plastohydrodynamic lubrication accounts for the existence of hydrodynamic effects in metalworking.
Calculations for oil film thickness require knowledge of the viscosity of the oil film at the temperature, pressure, and shear rate in the component. Viscosity is in the numerator of all equations predicting oil film thickness, fluid friction or hydraulic pressure. Oil film thickness increases with viscosity. Viscosity is also in equations for calculating the Sommerfeld Number, velocity in an oil film, shear stress, fluid friction force, and power loss for hydrodynamic bearings.
Units of Viscosity Measurements

The unit of absolute or dynamic viscosity is

Force / Area X Time
The basic SI unit is Pascal X second Pa s (or Ns m-2). Mineral oils are typically 0.02 to 0.05 Pa.s at 40 degree C.
1 mPa.s = 1 Centipoise (cP) cP is commonly used for absolute viscosity. The symbol for viscosity is usually u.
When gravity is used to cause flow for the viscosity measurement, the density p of the oil is involved and kinematic viscosity is reported =u/p. The basic SI unit is meter2/second (m2 s-1). Also
1 cm2 s-1 = 1 Stoke (St)

and 1 mm2 s-1 = 1 centiStoke (cSt)

cSt is commonly used for kinematic viscosity.

Viscosity of industrial lubricants is commonly classified using the International Standard Organization Viscosity Grade ( ISOVG ) system, which is the average viscosity in centiStokes (cSt) at 40 degree C. For example, ISOVG 32 is assigned to oils with viscosity between 28.8 and 35.2 cSt at 40 degree C.
The viscosity of oils is dependent upon temperature, pressure, and shear rate. Viscosity decreases as temperature increases because the molecules vibrate more and interact less. Conversely, the viscosity of oil increases as temperature decreases and can become grease-like at very low temperatures.
Viscosity Index (VI)
VI is a commonly used expression of relative magnitude of viscosity change with temperature. VI is based on two hypothetical oils with arbitrarily assigned VI's of 0 and 100. The higher the viscosity index the smaller the relative change in viscosity with temperature. Most industrial mineral lubricating oils have a VI between 55 and 100, but VI varies from 0 to "high VI" oils with VI up to 175. Viscosity-Temperature-VI relationship is shown in the following table:
Industrial Oil Viscosity, cSt Viscosity, cSt Viscosity Visc-Temp
ISOVG 32 40°C (A) 100°C (B) Index Coefficient
Machine Oil 31 4.8 58 0.845
Turbine Oil 32.5 5.4 102 0.833
Hydraulic Oil 33.6 6.6 154 0.803
A less arbitrary indication of the change in viscosity with temperature is the viscosity temperature coefficient
Viscosity-Temp Coefficient (VTC) = Viscosity (cSt) at 40 °C -Viscosity (cSt) 100 °C
VTC = (A-B)/A
Calculated values of the viscosity-temperature coefficient are also shown in the table. The lower the value of the coefficient, the higher the VI. The coefficient for mineral oils can vary by a factor of 10 depending on the temperatures.
VI Improvers
VI improvers are used in a few industrial oils, such as gear oils, by the addition of high molecular weight polymers and are called multi-grade oils. They reduce change in viscosity with temperature
Viscosity Measurements
Viscosity is measured by ASTM method D 445 using a common cross arm viscometer. The sample is introduced into a "U" shaped, calibrated, glass tube, submerged in a constant temperature bath. The oil is warmed to the desired temperature (usually 40 degree C for industrial oils) and allowed to flow via gravity down the tube and up the opposite side. The number of seconds the oil takes to flow through the calibrated region is measured. The oil's viscosity in cSt is the flow time in seconds multiplied by the apparatus constant.
Viscosity is also measured in the Brookfield viscometer by measuring the resistance to rotation of a spindle in a container of oil at a specified temperature. Brookfield viscosity is useful for low temperature measurements. For example, a gear oil for arctic use is 120,000 cP at -40 degree C.
Viscosity Pressure Coefficient
Viscosity increases with pressure because the molecules are squeezed together forcing greater interaction. In an elasto hydrodynamic lubrication contact where the pressure can be 2.1 GPa (300,000 psi) the viscosity is so high that the oil is considered a plastic-like solid. Viscosity at high pressures is measured by flow through pressurized capillary tubes, or a ball falling down a pressurized tube. The higher the temperature the lower the viscosity increase due to pressure.
Viscosity pressure coefficient is the slope of lines on graphs of the log of viscosity vs. pressure. The unit for pressure viscosity coefficient is the reciprocal of pressure. The SI units are 1/Pa or m2 N-1. Pressure viscosity coefficient can also be measured from oil film thickness and other parameters from a transparent disk-on-ball apparatus. Pressure viscosity coefficient is used in the calculation of oil film thickness in tribological contacts. For example, in elasto hydrodynamic lubrication contacts, oil film thickness is directly proportional to the 0.74 power of the pressure viscosity coefficient.
Viscosity Shear Rates
Mineral oil viscosity does not change much with shear rate, that is, they are Newtonian fluids. However, the viscosity of multi-grade, non-Newtonian oils usually decrease with shear rate because of the temporary alignment or breaking down of long chain hydrocarbon molecules to form shorter molecules. Shear rate is speed divided by oil film thickness: Shear rate =ms-1/m = s-1, or reciprocal seconds. For example, with a speed of 1 ms-1 and an oil film 1 micrometer thick, the shear rate is 106 s-1.
Shear stability is defined as the ability of a lubricant to withstand shearing without breaking of the long chain hydrocarbon molecules. In lubrication, the viscosity of an oil at high shear rates is important to understanding performance in high speed, thin oil film equipment. An example is a large tilting pad thrust bearing in an hydroelectric generator.
Viscosity, as a function of shear rate, is measured by various rotating instruments. The instruments measure the force resisting the flow of oil films of known thickness and speeds. ASTM method D 4683-90 prescribes a tapered roller rotating in a matched tapered stator with a known oil film thickness between them.
Another rotating apparatus is the Couette Rheometer, where a precision cylinder rotates at high speed in a larger cylinder with an oil film of known thickness between them. Viscosity at high shear rates is also measured with an ultrasonic shear tester, and a high shear rate capillary at specified frequency, temperature and time.