Lesson 28. Properties and analysis of lubricants

The most important properties of lubricants which are of prime importance in lubrication are viscosity, viscosity temperature relation, viscosity index, volatility etc. In order to get efficient lubrication, there should be no change in these properties during lubrication .Moreover there should be no effect of any chemical change such as decomposition, oxidation, reduction at higher temperature and emulsification during lubrication. The behavior of a lubricating oil or lubricant is determined by its viscosity. Viscosity is defined as the force in dynes necessary for the movement of 1 sq. cm.  layer of fluid with a velocity of 1 cm per sec, past another parallel layer 1 cm away. The unit usually employed is centipoises. The measurement of viscosity is usually done in C.G.S units and the unit is stoked. In case of thin oils, which have very low viscosity, it is measured in centi- stokes. The viscosity measured

28.1 PROPERTIES OF LUBRICATING OILS

28.1.1 Viscosity is the property of a liquid or fluid by virtue of which it offers resistance to its own flow. A liquid in a state of steady flow on a surface may be supposed to consist of a series of parallel layers moving one above the other. Any two layers will move with different velocities; top layer move faster than the next lower layer, due to viscous drag (i.e., internal friction). Consider two layers of a liquid separated by a distance, d and moving with a relative velocity difference, v. Then, force per unit area (F) required maintaining this velocity difference is given by:

F   =  ŋ v /d

Where  ŋ (eta) is a constant of a liquid, called coefficient of viscosity.

Viscosity is the most important single property of any lubricating oil, because it is the main determinant of the operating characteristics of the lubricant: (i) if the viscosity of the oil is too low. a liquid oil film cannot be maintained between two mooing/sliding surfaces, and consequently, excessive wear will take place. On the other hand, (ii) If the viscosity is too high, excessive friction will result. The viscosity is temperature depending property.

Measurement of viscosity of lubricating oil is made with the help of an apparatus called

the viscometer. In a viscometer, a fixed volume of the liquid is allowed to flow, from a given height, through a standard capillary tube under its own weight and the time of flow in seconds is noted. The time in seconds is proportional to true viscosity. Oswald viscometer, Redwood viscometers, Saybolt viscometer are used, for measuring viscosities of lubricating oils. The results are expressed in terms of time taken by oil to flow through particular instrument. For example, if time of flow of oil through Redwood viscometer at 20oC is 100 seconds, then its viscosity is 100 Redwood seconds at 20oC.  Now a day Brookfield viscometer is commonly used for determining the viscosity of lubricating oils.

28.1.2 Flash and fire-points: Flash point is the lowest temperature at which the oil lubricant gives off enough vapours that ignite for a moment, when a tiny flame is brought near it; while fire-point is ' the lowest temperature at which the vapours of the oil burn continuously for at least five seconds, when a tiny flame is brought near it. In most cases, the fire-points are 5 to 40o higher than the flash-points. The flash and fire-points do not have any bearing with the lubricating property of the oil, but these are important when oil is exposed to high temperature service. A good lubricant should have flash point at least above the temperature at which it is to be used. These safeguards against risks of fire, during the use of lubricant. The flash and fire-points are, usually, determined by using Pensky-Marten's apparatus.

28.1.3 Cloud and pour-points: When an oil is cooled slowly, the temperature at which it becomes cloudy or hazy in appearance, is called its cloud-point while the temperature at which the oil ceases to flow or pour, is called its pour-point. Cloud and pour-points indicate the suitability of lubricants in cold conditions. Lubricant used in a machine working at low temperatures should possess low pour-point; otherwise solidification of lubricant will cause jamming of the machine. It has been found that presence of waxes in the lubricating oil raise the pout-point. Determination of pour-point is carried out with help of pour-point apparatus.

28.1.4. Emulsification: It is the property of oils to get intimately mixed with water, forming a mixture, called emulsion. Certain oils form emulsions with water easily. Emulsions have a tendency to collect dirt, grit,. Foreign   matter etc., thereby causing abrasion and wearing out of the lubricated parts of the machinery. So, good lubricating oil should form an emulsion with water, which breaks off quickly. The tendency of lubricant-water emulsion to break is determined by A.S.T.M. test. In this, 20 ml of oil is taken in a test-tube and steam at 100oC is bubbled through it, till the temperature is raised to 90oC. The tube is then placed in a bath maintained at 90oC and the time in  seconds is noted, when the oil and water separate out in distinct layers. The time in second in which oil and water emulsion separates out in distinct layers, is called steam emulsion number (S.E.N.). A goad lubricant should possess a low steam emulsion number.

28.1.5. Volatility: When lubricating oil is used in heavy machinery working at high temperature, a portion of oil may vaporize; leaving behind a residual oil, which have different lubricating properties (like increased viscosity). Good lubricant should have low, volatility. The volatility of oil is determined by an apparatus, called vaporimeter, which consists essentially of a furnace heated by some fuel gas. In the centre of the furnace passes a coiled-form of copper tube, through which air can be passed. A known weight of oil under examination is taken in a platinum crucible, which is then introduced into the copper tube. Dry air at a rate of   2 litres/min is passed through the copper tube. After 1 hour of heating, the crucible is taken out, cooled and weighed. The loss is weight is calculated as percentage of the original weight of oil taken.

28.1.6. Carbon residue: Lubricating oils contain high percentage of carbon in combined form. On heating, they decompose depositing a certain amount of carbon. The deposition of such carbon in machine is intolerable, particularly in inert combustion engines and air-compressors. A good lubricant should deposit least amount of the carbon in use. The estimation of carbon residue is, generally, carried out by Conradson method. A weighed quantity of oil is taken in a silica crucible (about 65-85 ml capacity). The skidmore crucible is provided with a lid, having a small tube-type opening for the escape of volatile matter. The combination is then placed in a wrought Iron crucible (about 8 cm in dia and 6 mm high) covered with chimney-shaped iron hood (of about 10 cm diameter). The wrought iron crucible is heated slowly for 10 minutes, till flame appears. Slow heating is continued for 5 minutes more. Finally, strong heating is done for about 15 minutes, till vapours of all volatile matter are burnt completely. Apparatus is then allowed to cool and weight of residue left is determined. The result is expressed as percentage of the original weight of oil taken.

28.1.7 Corrosion stability of lubricating oil is estimated by carrying out corrosion test. A polished copper strip is placed in the lubricating oil for a specified time at a particular temperature. After the stipulated time, the strip is taken out and examined for corrosion effects. If the copper strip has tarnished, it shows that oil contains any chemically active substance. A good lubricant should not affect the copper strip. To retard corrosion effects of oil, certain inhibitors are added to them. Commonly used inhibitors are organic compounds containing phosphorus, arsenic, antimony, chromium, bismuth or lead.

28.1.8 Aniline point of oil is defined as the minimum equilibrium solution temperature. For equal volume of aniline and oil sample. Aniline point gives an indication of the possible deterioration of oil in contact with rubber sealing’s, pickings, etc. Aromatic hydrocarbons have a tendency to dissolve natural rubber and certain types of synthetic rubbers. Consequently, low aromatic content in the lubricants is desirable. A higher aniline-point means a higher percentage of paraffinic hydrocarbons and hence, a lower percentage of aromatic hydrocarbons.

Aniline point is determined by mixing mechanically equal volumes of the oil sample and aniline in a test-tube. The mixture is heated, till homogeneous solution is obtained. Then, the tube is allowed to cool at a controlled rate. The temperature at which the two phases (oil and aniline) separate out is recorded at the aniline point.

28.1.9 Precipitation number is the percentage of asphalt present in oil. A known weight of the lubricant is dissolved in petroleum ether and centrifuged. The precipitated asphalt, if any, is filtered, washed, dried and weighed. It is the then expressed as percentage of weight of oil taken. Precipitation number is used to differentiate the differentiate classes of the lubricants

28.1.10 Specific gravity is very useful in identifying oil, as it gives an indication of the type of crude from which the lubricant has been prepared.

28.1.11 Ash content of oil is determined by heating strongly in air a known weight of oil in a crucible to a constant weight. The percentage of ash in lubricating oil, particularly in used-oil, is very useful in determining the materials that may cause abrasion and wear.

28.1.12 Neutralization number refers to the determination of acidic or basic constituents of an oil. Determination of acidic constituents is more common and it is referred to as Acid number or value, which is defined as the number of milligrams of KOH required to neutralize the free acids in 1 g of the oil. Generally, free acids are not present in the lubricants, unless refined in faulty manner. Lubricating oil should possess acid value less than 0.1. Value greater than 0.1 indicates that oil has been oxidized. This will, consequently, lead to corrosion, besides gum and sludge formation.

28.1.13 Saponification number is the number of milligrams of KOH required to saponify 1 g of oil. Mineral oils do not saponify at all, but vegetable and animal oils do. Consequently, this test helps us to ascertain whether the oil under reference is animal and vegetable oil or mineral or a compounded oil containing mineral and vegetable oils.

28.2 PROPERTIES OF GREASES

28.2.1 Consistency or yield value is expressed in terms of penetration, which is defined as the distance in tenth of millimetre that a standard cone penetrates vertically into the sample, under the standard conditions of load, temperature and time. The value of load, temperature and time are taken respectively as 150 g, 25oC, and 5 seconds. Consistency of grease depends on the structure and Interaction of the gelling elements in it and to some extent on the viscosity of oil used. The consistency is determined by using Penetrometer.

28.2.2 Drop-point is the temperature at which it passes from the semi-solid to the liquid state. So this temperature determines the upper temperature limit of the applicability of the grease. The sample is taken in a metal cup, which has an opening of standard size in its bottom. It is then enclosed in a glass case, having a tight lid. A thermometer is also inserted in the cup, so that the bulb of the thermometer is just above the surface of grease sample. The combination is then placed in a glass beaker, containing water and provided with a stirrer. The beaker is heated slowly at a rate of 1oC/minute. As the temperature is raised, the grease sample passes from a semi-solid to a fluid state. The temperature at which its first drop falls from the opening is recorded as drop-point.

28.3 CUTTING FLUIDS

Any liquid (such as oil, water or oil emulsion) or a gas used to cool as well as to lubricate is called a cutting fluid. Emulsions of oil-in-water are mostly used as cutting fluids. Cutting fluids are required for tools used in the machine shop for cutting, threading, sawing, planning, turning, drilling, etc. ; and the cutting fluid performs either of the fractions ; (i) to cool the tools, or (ii) to lubricate the tools, or (iii) to cool as well as to lubricate the tools.

In such machining operations, the friction is very high, due to close contact between the work-piece and the tool; and this generates large amounts of local heat, thereby the tool gets overheated and it may even lose its temper and hardness. Consequently, in such a case, the cutting fluid provides cooling, besides lubrication. In order to provide satisfactory service, the cutting fluid should possess :

1. good lubricating property
2. Low viscosity, so that the lubricant can easily fill in the cracks formed on the work-piece
3. Chemical Stability
4. Non-corrosive nature towards the metals of the work-piece as well as tool
5. High thermal conductivity.

28.3.1 Oil as cutting fluid: In fine work, a stream of oil (like lard or rape or sperm oil) is directed over the work-piece. Here the cutting fluid (the oil) acts more as lubricant and less as heat-carrying agent. When the speed is low and the pressure is high (e.g., in broaching), the oils are mostly used as cutting fluids. Compounded oils obtained by mixing petroleum oils with vegetable and animal oils are also used for such purposes.

28.3.2 Water as cutting fluid: in rough situation, like rough grinding and turning; only cooling action is sufficient. Water being good cooling agent and cheap, so it is mostly employed as cutting fluid. Water is primarily used, when only roaring is required, e.g., in high-speed operations

28.3.3 Emulsion as cutting fluid: in most cases, use of either water or oil alone is not satisfactory; and both a cooling agent and a lubricant are needed simultaneously. For such situations, oil-in-water emulsions are employed. These are employed by mixing small amount of petroleum oil in water, and then stabilizing by adding emulsifier (like soap, tallow oil, lard oil, sperm oil, sulphonated oils, e.g., H2SO4 -treated castor oil, chloro-sulphonated organic compounds, etc.). Moreover, to prevent gelling (or thickening) of the concentrate, glycols or

alcohols are added. In addition to these, certain germicide (like sodium nitrite, triethanol amine) is added. Cutting emulsions serves as :  Lubricant to reduce frictional heat and medium for the transfer of heat produced. In other words, use of cutting emulsions reduce power consumption as well as wear of tool. Before choosing a cutting fluid for a job it is, however, essential that it does not cause rusting of the metals involved.

Last modified: Friday, 11 April 2014, 9:52 AM