BearingTrends

Bearings & Power Transmission Distributors Community

Back Home Knowledge More Technical... Physical and Chemical Properties of Mineral Oils

Physical and Chemical Properties of Mineral Oils

This paper is a review of the physical and chemical properties of mineral oils that affect lubrication. A few properties may have both physical and chemical effects. Recognition of these properties is useful for designing lubrication systems, choosing lubricating oils, diagnosing lubrication, friction and wear problems, and selecting appropriate testing methods. The material is limited to industrial lubricating and hydraulic mineral oils, and is written for people entering the field of tribology.

 

In each section the authors have:

Defined each property according to

the Organization For Economic Co

operation and Development [1]. Stated how the property is measured

by ASTM methods [2]. Described its application to industrial

lubrication. Used SI units given in "Standard

Practice For The Use Of The

International System of Units -

Modernized" [3].

The information came from many unnamed authors. However, we specifically wish to acknowledge the authors cited in the references, from which most of the information was obtained. The reader is referred to the references for further details. We also acknowledge obtaining specific lubricant properties from the bulletins of several lubricant suppliers.

Viscosity

Viscosity is the property of a fluid that causes it to resist flow, which mechanically is the ratio of shear stress to 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 (HDL) and in elastohydrodynamic lubrication (EHL). Viscosity is also a significant factor in predicting the performance and fatigue life of rolling element bearings and gears that are lubricated by oil. 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.

Physical and Chemical Properties of Mineral Oils

Units of Viscosity Measurements

The unit of absolute or dynamic viscosity is force divided by area times time. The SI unit is Pascal times second Pa s (or Ns m-2). Typically, mineral oils are between 0.02 and 0.05 Pa.s at 40 degree Celsius.

1 mPa.s = 1 centiPoise (cP) cP is commonly used for absolute viscosity. The symbol for viscosity is usually mu.

When gravity is used to cause flow for the viscosity measurement, the density p of the oil is involved and kinematic viscosity is reported =mu/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 (ISO VG) 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 an oil's change of viscosity 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 ISOVG 32

Viscosity, cSt 40°C

Viscosity, cSt 100°C

Viscosity Index

Visc-Temp Coefficient

Machine Oil

30.4

4.8

58

0.842

Turbine Oil

32

5.4

102

0.831

Hydraulic Oil

30.4

6.1

154

0.799

A less arbitrary indication of the change in viscosity with temperature is the viscosity-temperature coefficient. For 40 to 100 degree C it is defined as Viscosity (cSt) at 40 degree C minus Viscosity (cSt) at 100 degrees C, divided by the Viscosity (cSt) at 40 degrees C. 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. The chemistry of VI improvers, as well as other additives, is described in References 4 and 5.

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. (See Reference 6 for details of viscosity temperature relations).

Viscosity Pressure Coefficient

Viscosity increases with pressure because the molecules are squeezed together forcing greater interaction. In an EHL 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. Reference 7 gives the pressure viscosity coefficients of several mineral oil showing a variation from 1.6 to

2.68 X 10-8 Pa-1. The coefficient increases with viscosity, and can vary by a factor of 3.

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 EHL 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 viscosities 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. Results are reported as: viscosity in cP (at 150 degree C and a shear rate of 106 s-1).

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.

Many original equipment manufacturers (OEMs) now require a minimum shear stability. Some OEMs now require a viscosity of 2.9 cP (at 150 degree C and 106 s-1).

Pour Point

Pour point is a viscosity temperature phenomenon. It is defined as the lowest temperature at which a lubricant will flow under specified conditions. Most lubricant suppliers give the pour point of their oils so that the user can determine if it can be pumped and would be fluid in low temperature applications.

The pour point increases with viscosity. For example, an ISOVG 46 mineral oil might have a pour point of -39 degree C, whereas an ISOVG 460 would have a pour point of -15 degree C. For mineral oils, the increase in viscosity as temperature is reduced, is due to gelling of the oil by the precipitation of crystalline wax.

Although this paper is a review of mineral oil properties, it should be noted that one advantage of many synthetic oils is their very low pour points because of the absence of wax. Pour point depressants for mineral oils are additives that lower the pour point by interfering with wax crystallization.

Pour point is measured by ASTM D 97, which describes the procedure for cooling an oil until it will not pour out of a vessel. Pour point, flash point, VI, and other properties of 81 mineral oils are given in Reference 7. Cloud point is defined as the temperature at which a wax cloud first appears on cooling mineral oil under specified conditions.

Density

Density is the mass of a unit volume of a substance. Oil density is used to determine the mass of a given volume, or the volume of a given mass. Density is used in lubrication to identify an oil, or oil fractions, and in the measurement of kinematic viscosity (absolute viscosity divided by density). Also, density is in the equations for the calculation of temperature rise in an oil film, and the equation for Reynolds Number (which determines if flow of an oil film is laminar (smooth layers) or turbulent (tumbling)).

The SI unit for density is kg m-3, but usually reported as grams/ml (g/ml-1). For example, an oil could be 850 kg m-3 or 0.850 g ml-1. The density of mineral oil lubricants varies from 0.86 to 0.98 g ml-1

Specific Gravity

For many liquids, specific gravity is used which is ratio of the mass of a given volume to the mass of an equal volume of water. Therefore, specific gravity is dimensionless. The specific gravity of mineral oils also varies from 0.86 to 0.98 since the specific gravity of water is 1 at 15.6 degree C. Specific gravity decreases with increased temperature and decreases slightly as viscosity decreases for similar compositions. Reference 5 (pp. 482- 484) gives the specific gravity of 81 mineral oils at 15.6 degree C.

Most lubricant supplier's typical data bulletins give A.P.I. (American Petroleum Institute) Gravity in degrees for lubricating oils instead of specific gravity. A.P.I. gravity is an expression of density measured with a hydrometer. A.P.I. gravity has an inverse relationship with specific gravity, as shown in the following table from Reference 8.

A. P. I. Gravity

Specific Gravity

15

0.97

34.9

0.85

Many mineral oil lubricants have an A.P.I. gravity value of around 27 degrees. Reference 8 gives the equation for converting A.P.I. gravity to specific gravity.

Density, specific gravity, and A.P.I. gravity are measured by ASTM D-1298, using a calibrated, glass hydrometer and a glass cylinder. The cylinder is partially filled with the sample oil and the hydrometer is set into the oil and allowed to stabilize. A reading of the gravity is taken from the markings on the stem of the hydrometer at the surface of the oil. The temperature of the oil is measured and the final result is converted to 15.6 degree C (60 degrees F) and reported as A. P. I. gravity at 60 degrees F.

Two other oil properties related to density are thermal expansion and bulk modulus or compressibility.

Thermal Expansion

The volume of a given oil mass increases with temperature, therefore, its density decreases. The degree of expansion is expressed as the coefficient of thermal expansion. Thermal expansion is useful to determine the size of a container needed when the oil will be heated. Inexperienced people often have an oil overflow because of a surprising amount of thermal expansion.

In HDL, the thermal expansion of the oil in the clearance of a bearing increases the hydraulic pressure. Some researchers discuss the "thermal wedge" mechanism of film formation and apply it to parallel sliding surfaces, especially flat, non-tilting, thrust bearings.

The coefficient of thermal expansion is the ratio of the relative change of volume to a change in temperature. Thermal expansion is expressed as the ratio of volume change to the initial volume after heating 1 degree C. Therefore, the unit is reciprocal degree C, or degree C-1. The values of the coefficient of thermal expansion for mineral oil are near 6.4 X 10-4 degree C-1.

Thermal expansion (or contraction) determinations require the measurement of the volume of a given mass of oil at various temperatures. The sample is placed in a graduated cylinder and the volume is observed as the temperature is either increased or decreased. A simplified method of calculating the thermal expansion of petroleum products can be found in ASTM D 1250, Petroleum Measurement Tables, "Volume Corrections Factors".

Bulk Modulus or Compressibility

Bulk modulus expresses the resistance of a fluid to a decrease in volume due to compression. A decrease in volume would increase density. Compressibility is the reciprocal of bulk modulus or the tendency to be compressed. Bulk modulus varies with pressure, temperature, molecular structure and gas content. Generally, mineral oils are thought to be incompressible. In high-pressure hydraulic systems a high bulk modulus or low compressibility is required to transmit power efficiently and dynamically.

In EHL, bulk modulus is a factor used in some film thickness calculations. Bulk modulus is a consideration in some viscosity-pressure relationships. (Low viscosity polysiloxane fluids have a low bulk modulus or high compressibility compared to mineral oils). Dissolved gases decrease bulk modulus of mineral oils.

The unit for bulk modulus is pressure and the unit for compressibility is the reciprocal of pressure. The SI units are N m-2, and m2 N-1 respectively.

Bulk modulus is determined by measuring the volume of an oil at various pressures or derived from density measurements at various pressures. Bulk modulus can also be measured by the speed of sound in oils under various pressures. A discussion of bulk modulus and values are given in References 9 and 10. Since a graph of pressure versus volume gives a curve, the secant to the curve is used and is called Isothermal Secant Bulk Modulus.



BearingTrends Sitemap

Bearing Brands

JapanESE brands

Europe

  • Distributors1
  • Distributors2
  • Distributors3
  • Distributors4

Asia

  • Distributors5
  • Distributors6
  • Distributors7
  • Distributors8

America

  • Distributors9
  • Distributors10
  • Distributors11
  • Distributors12

Others

  • Distributors13
  • Distributors14
  • Distributors15
  • Distributors16