Physical Properties of Lubricants

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Following are the important properties of a lubricant. 1. Density

Density is fundamental physical property of oil. It is the mass of liquid per unit volume. It is measured at specific temperature by Hydrometer or Automatic Density Meter. Density measurement is necessary for conversion of measured volumes to volumes at standard temperature. The desired volume is then converted to mass of the product and filling is carried out on weight basis. e.g., if typical density of Power Supreme is 0.887, to fill 210 lit barrels, the weight of product is maintained as Weight (Kg) = 210 x 0.887 = 186.27 Kg 2. Kinematic Viscosity It is a measurement of liquid’s flow under the influence of gravity. It is expressed as “Centistokes” or cSt (mm2/sec). A viscometer containing test sample is placed in constant temperature bath, normally 40OC or 100OC. Numbers of seconds are noted, required for certain amount of oil to travel fix distance in a capillary tube.

Thin oil will take less time compared to thick oil, e.g., at 40OC, AWH VG 46 will take less time to travel the fix distance in capillary tube compare to AWH VG 68 or at 100OC, All Fleet 30 will take less time compared against All Fleet 40.

Physical Properties of Lubricants

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3. Viscosity Index Liquids have a tendency to thin out when heated and to thicken when cooled. However, this response of viscosity to temperature changes is more pronounced in some liquids than in others. The property of resisting changes in viscosity due to changes in temperature can be expressed as the viscosity index (V.I.). Lubricating oils are subjected to wide ranges of temperatures in service. At high temperature, the viscosity of oil may drop to a point where the lubricating film is broken, resulting in metal to metal contact and severe wear. At the other extreme, the oil may become too viscous for proper circulation, or may set up such high viscous forces that proper operation of machinery is difficult. Consequently, many applications require oil with a high viscosity index. In an automobile, for example, the crank case oil must not be so thick at low starting temperatures as to impose excessive drag on the engine and to make cranking difficult. During the warm up period, the oil must flow freely to provide full lubrication to all engine parts. After the oil has reached operating temperature, it must not thin out to the point where consumption is high or where the lubricating film can no longer carry its load.

Physical Properties of Lubricants

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4. Flash point (ASTM D-92, D-93): the temperature to which a combustible liquid must be heated to give off sufficient vapor to form a flammable mixture with air; this mixture should burn momentarily without sustaining combustion when a small flame is applied under specific conditions. Because it indicates the temperature at which a flammable vapor is produced, flash point is generally the most useful single index of fire hazard potential. Flash Point can be checked by two Test methods: Cleveland open cup (COC) or Pensky-Marten Closed Cup test (PMCC)

5. TBN (ASTM D 2896)

The alkalinity of oil is measured by titration through an acid, and expressed in mg KOH/g. The comparison between the TBN volume of the fresh oil and that of the used oil allows the determination to be made of whether the used oil is still capable of neutralizing acid residues. These acids are produced by combustion (sulphur in fuel) and oxidation of the oil and oil additives. When the oil is in service too long, the TBN will drop significantly.

Too low a TBN can be due to: heavy oxidation of the oil, when the oil has been in service for too long, of the oil level was insufficient, or due to a defective cooling system, producing overheating; use of a fuel containing a high sulphur content; use of an inappropriate lubricant; or contamination of the oil by fuel or water.

Typical TBN of few of our grades and their usage against fuel type is given below Grade TBN Fuel Type Power Supreme 10.5 Diesel Fuel GEO 5.5 Natural Gas Champ 4T 6.5 Petrol Marine Oil 3040 30.5 High Sulphur Fuel

Physical Properties of Lubricants

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6. TAN (ASTM D664)

The acid number (TAN) is a measure of the amount of acidic substance in the oil. TAN is used as a guide in the quality of oil formulations. It is also used as a measure of oil degradation in sevice.

This method is used to indicate the relative changed that occur in oil during use under oxidizing conditions regardless of the color or other properties of oil.

TAN is also represented as mg KOH/gm.

New and used oils may contain acidic constituents that are present as additives or as degradation product formed during service, such as oxidation products. The relative amount of these materials can be determined by this method.

Sharp increase in TAN may indicate contamination or severely oxidized oil.

7. Pour Point (ASTM D 97) This method is intended for use on any petroleum products to measure liquid’s fluidity at low temperatures Pour point is defined as the lowest temperature at which the oil is observed to flow when cooled and observed as prescribed manner. In application to a lubricating system, limiting low temperature levels are established corresponding to maximum viscosity for proper circulation. This property of oil helps to predict and improve fluidity and pumpability of the oil at low temperatures.

Physical Properties of Lubricants

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8. Foaming Characteristics (ATM D 892) This test method covers the determination of foaming characteristics of lubricating oil at specified temperatures. The test is carried in 3 sequences: Seq. I: The sample, maintained at the temperature of 24 degree C is blown with air at a constant rate (94ml/min) for 5 minutes then allowed settling for 10 minutes. The volume of foam is measured at the end of both periods. Seq. II and III: The test is repeated on a second sample at 93.5 degree C, and than collapsing the foam at 24 degree C

The primary causes of foaming are mechanical, essentially an operating condition that tends to produce turbulence in the oil in the presence of air. Foaming in an industrial oil system is a serious

Physical Properties of Lubricants

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service condition that may interfere with satisfactory system performance and even lead to mechanical damage. Antifoaming additives may be used in oils to decrease foaming tendencies of the oil. However, many such additives tend to increase the air entrainment characteristics of oil, and their use requires striking a balance between these two undesirable phenomena. 9. CONSISTENCY OF GREASE (ASTM D 217) The consistency of lubricating grease is defined as its resistance to deformation under an applied force; in other words,its relative stiffness or hardness. The consistency of grease is often important in determining its suitability for a given application. If the grease is too soft ,it may not stay in place, resulting in poor lubrication .If the grease is too hard, it will not flow properly, and either fails to provide proper lubrication or cause difficulties in dispensing equipment. These statements sum up the reasons for classifying greases by consistency. Grease consistency is given a quantitative basis through measurement with the ASTM Cone Penetrometer, the pictorial description of which is given here.