MSB, GITAM , HYD Page 1
Lubricants
Lubricant:- Any substance introduced between the two moving and sliding surfaces with a view to reduce frictional resistance is Known as Lubricant.
Functions of Lubricants:
Lubricants help in reducing frictional forces between two sliding surfaces It reduces wear and tear and surface deformation
It act as coolant
It act as seal in internal combustion engines
It reduces running cost and maintenance cost of machines and tools Prevents the accumulation of dirt and foreign matter
It prevent the welding and seizure of moving surfaces.
Mechanism of Lubrication:
Lubrication is affected mainly by three mechanisms 1. Fluid film or hydrodynamic Lubrication 2. Thin film or Boundary Lubrication 3. Extreme Pressure Lubrication
Fluid Film or Hydrodynamic Lubrication:
In this type of Lubrication, the moving surfaces are separated completely by applying a thick uniform film of lubricant between them. The thickness of the film is at least 1000ᴼA, to prevent direct surface to surface contact. Thus, wearing and tearing of metals is minimized.
The liquid lubricant must have sufficient viscosity so as to maintain the fluid film in its place. In Such type of lubrication when the load is
applied, the corresponding pressure developed in the lubricant is sufficient to keep moving surfaces apart and therefore it is known as hydrodynamic lubrication.
This type of lubrication is useful in delicate machinery like watches, clocks, Scientific equipment’s and sewing machines. In a ball bearing, the irregularities of the shaft and bearing surfaces is covered by a thick film of lubricant and do not allow the contact of metallic surfaces with each other.
Hydrocarbon oils blended with long chain polymers are very effective for Fluid lubrication.
THIN FILM OR BOUNDARY LUBRICATION:-
Under high load and low viscosity of oil, a continuous fluid film cannot maintain between the rubbing surfaces. Under such conditions, the thickness of the fluid film should be less than 1000ᴼA, consisting of one or two molecular layers to form a thin film.In this type of lubrication, a thin film of lubricant is absorbed on the metal surface by physical or chemical forces or lubricant may chemically react with the metal surface forming a thin film of metal soap which act as a lubricant. This film is known as boundary film.
A Thin film or boundary lubricant should have the following properties
MSB, GITAM , HYD Page 2 • It should have high oiliness and viscosity index
• It should have polar functional groups to promote spreading and orientation over the metallic surfaces at high pressure
• It should have low pour point and resistance to heat and oxidation. The lubricants which satisfies the above conditions are
• Mineral oils capable of mixing with fatty acids (C17H33COOH) or fatty oils(vegetable , animal oils and their soaps)
• Solid lubricants such as MOS2 and graphite as emulsion in oil EXTREME PRESSURE LUBRICATION:
Under heavy load and high speed conditions a high local temperature is generated due to increase in friction. As result the liquid film may not stick or may decompose and evaporated and become ineffective. For effective lubrication under these conditions, special additives called extreme pressure additives are blended with lubricating oils to form more durable film to with stand high temperatures and pressure are used. While moving instead of metallic surfaces, these additive films come in contact with each other and thus protect the metal surfaces.
Compounds of chlorinated esters, sulfurized oils, tricresyl phosphates are used as extreme pressure additives. These additives combine with the metallic surfaces at high temperature to form metallic chlorides, sulfides, phosphides in the form of durable film.
These films can with stand very high loads and high temperature because of their high melting point. They do not actually reduce friction but prevent the welding between the surfaces. These additives are not effective with chemically inert metal surfaces of silver, chromium, titanium etc.
They are used as lubricants in Wire drawing and machining tough metals.
Differences:-
MSB, GITAM , HYD Page 3 PROPERTIES:-
1. Liquid Lubricants :
Itprovides a continuous fluid film in between metallic surfaces. They act as sealing agents, corrosion preventers, cooling medium. Good lubricant oils have high boiling point, low freezing point, adequate viscosity, high resistance to oxidation and heat, non-corrosive properties & stability to decomposition at operating temperature
a) Vegetable and Animal Oils:
*Vegetable lubricants are based on soybean, corn, castor, cotton seed and rape seed oils.
Vegetable oils are environmentally friendly alternative to mineral oils since they are biodegradable. The main disadvantages of vegetable lubricants are their low oxidation and temperature stabilities. Their oiliness is good but they are expensive can get easily oxidized, hydrolyzed in moist air. Their proportion can be improved by adding some chemicals usually they are used after blending with mineral oils to produce desired effect.
b)Mineral or petroleum oils:
Mineral oils (petroleum oils) are products of refining crude oil (C12 to C50).They are cheap, abundance and stable under service condition. But they are poor in oiliness, which is improved by addition of high mol.wt. compounds (oleic acid, stearic acid, castor oil etc.,)
Purification:-
Impurities are wax (low pour point), easily oxidized compounds & compounds undergoing decomposition at higher temp (sludge)
1. Dewaxing 2. Acid refining 3. Solvent refining methods
c)Blended Oils or Additives to Lubricants:-
Typical properties of petroleum oils are improved by incorporation of specific additives. These are called ‘blended oils’.
Oiliness –carriers: vegetable oils & fatty acids
Extreme pressure additives:
1. Fatty esters, acids etc., which forms oxide film with the metal surface.Organic materials which contain sulphur, chlorine and phosphorous compounds. Lead in order to produce thin film of lead sulphide
Pour–point depressing additives- phenol or condensed products of chlorinated wax naphthalene. Paraflow- Ph-CH2-(CH2)30-CH3
Viscosity index improvers – polyisobutylenes, poly methcrylates, hexanols
Thickeners- polystyrene, polyesters etc.,- 300 to 3000 to give higher viscosity
Antioxidants or inhibitors- added in lubricants used in internal combustion engines, turbines etc., where oxidation of oils is serious problem ex. aromatic amino compounds.
Corrosion inhibitors- organic compound of phosphorus or antimony
Abrasion inhibitors- tricresyl phosphates
Antifoaming agents- methylsilicon polymers
Emulsifiers- sodium salts of sulphonic acids
Deposit inhibitors- detergents salts of phenols and carboxylic acids.
2. Greases or Semi-Solid Lubricants
MSB, GITAM , HYD Page 4 semi-solid lube. Semi-fluid lubricants (greases) are produced by
emulsifying oils or fats with metallic soap and water at 204-316°C. The main function of soap is thickening agent, so that grease sticks firmly to the metal surface.
Greases can be classified as
Calcium –based greases-These are the emulsions of petroleum oils with calcium soaps.they are cheap and resistant to water. They are used in low temperature, above 70 C the grease separate into oils and soap. Used in water pumps, tractors etc.,
Sodium – based greases- prepared by thickening petroleum oils by sodium soaps. As sodium salts are soluble in water, these greases are not suitable for bearings exposed to wet conditions.
Lithium based greases-it is superior to other greases, It is resistance to water, breaking down and softening by working conditions. Used as aircraft lubricant.
Advantages:-
Withstands high temperature and pressure under high loads
It stays well at the surfaces and does not contaminated the products
They are high resistance to friction.
Uses
where oil is not suitable for machines spurting of oil is to be avoided. Ex. Paper, textiles, edible industries.
Greases prepared by dispersion of solid (graphite) are used in rail axle box, tractor roller, wire ropes etc.,
3 Solid Lubricants
Solid lubricants are used in high temperature and high load conditions. In certain aerospace devices and other environments in which liquid or semi-solid lubes are prohibited. To minimize friction and wear, under such conditions solid film lubricants are used. Solid lubricates like- Graphite, MoS2 (sandwitch-like structure),CoCl2, CdCl2 CdI2 mica, chalk, wax, soap, etc., which can with stand very heavy load and low speed can be made use of.
Good characteristics of Solid lubricants are
• Low shear strength
• Strong adhesive to the applied surface • Good thermal conductivity
• Chemical inertness
• Stability at operating temperatures.
Graphite-the carbon atoms forms a network of hexagons. The carbon atoms are joined by covalent bonds. The adjacent layers are held together by weak Vander Walls forces which makes them soft and smooth to act as lube. Because of its slippery touch, non-inflammability, resistance to oxidation, graphite is widely used solid lube. It can be used as powder or as colloidal dispersion in water (aqua dag) or oil (oil dag).Graphite is used in IC. Engines, air compression engines, railway track joints, but are ineffective in vacuum conditions and above 375ᴼC.
Molybdenum Disulphide:- MoS2 has a sandwich like structure in which a layer of molybdenum atoms lies between two layers of sulphur atoms. The atom in the layer are held by strong covalent bonds and the layers are held by weak vanderWaals forces of attraction. The powder can be sprinkled on the sliding surfaces to form the film.
MSB, GITAM , HYD Page 5 friction stable in air up to 400ᴼC. Hence it can be used in air frame
lubrication and wire drawing. Other organic solid lubes are copper phthalocyanine and teflon.
Gases:
In recent times gases have also been used as lubes in precious spindles fans, compressors etc. because of their low viscosity, which is independent of temp. pressure. Hence their viscous resistance is minimum and has no effect of variation in temp. On lubrication there is no risk of contamination of gas in enclosed system is used as lubricant.
Properties of Lubricants:-
Viscosity:
It is the property of a liquid or fluid by virtue of which it offers resistance to its own flow. A liquid flowing under steady state conditions may flow in a series of parallel layers moving one above the other with different velocities. Due to internal frication i.e., drag the bottom layer moves slower than next upper layer. If the two layers of a liquid separated by distance‘d’ and moving with a velocity
difference, V, then the frictional force per unit area (F)
where ŋ= coefficient of viscosity. If d = 1 cm and v = 1 cm/sec. Then
The viscosity is expressed in poise or centi poise or centistoke.
Determination of Viscosity:-
Viscosity is determined by the time taken for a given volume of oil to flow from a given height, of a standard capillary under its own weight at a given temperature. This viscosity is called kinematics viscosity.
Viscometer is used to determine the viscosity of lubricating oils. A schematic diagram of Redwood viscometer is shown in the figure.
Viscometer consists of the following parts
1. Oil cup:- It is a brass cylinder, open at the upper end. The bottom of the cylinder is fitted with agate jet with a bore of 1.62 mm. For thin film lubricates a jet of a bore of 1.60 mm and 10 mm length and for thick lubricants a jet of 3.8 mm and length 15 mm. is used. There is a valve rod to open and close the jet. A pointer in the oil cup indicates the level of oil in the cup. A thermometer is fitted in the lid of the cup for measuring the temperature of oil.
2. Heating bath :- Oil cup is surrounded by a cylindrical copper bath filled with water. A thermometer is inserted to measure the temperature of water.
3. Leveling screws:-There are three leveling screw to level the apparatus.
4. Kolrausch’s flask A flask of 50 ml capacity is placed below the jet to receive the oil from the jet out let.
Working
MSB, GITAM , HYD Page 6 cup to note the temp. of oil.
Water bath is heated to a certain temperature with constant stirring to maintain uniform temperature of water. When the oil acquires desired temperature, heating is stopped and the ball valve in lifted. The time taken to fill the kolrausch flask of 50 ml capacity is noted. The valve is immediately closed to prevent the overflow of oil. The experiment is repeated and the mean value of time of flow for 50 ml oil sample is reported and the result is expressed in “Redwood No 1 seconds” at a particular temperature and is noted. Viscosity of liquids decreases with increases in temperature. A good lubricating oil should not undergo change in viscosity with temperature.
To indicate the change in viscosity with temperature an arbitrary scale, known as “viscosity index” is used. The viscosity of sample oil is assigned by comparing the viscosity of oil with viscosity of zero V.I. and with viscosity of 100 V.I. at 38ᴼC both having same viscosity as sample at 99ᴼC (VI).
Viscosity index:-
The viscosity of an oil decreases with increase in temperature as a result of decrease in intermolecular attraction due to expansion. This variation of viscosity with temperature is indicated by viscosity-temperature curves or by means of an arbitrary scale known as viscosity Index(VI).
A high value of viscosity index for an oil indicates that its viscosity is slightly affected by increase in temperature and a low value of V.I. indicate that its viscosity appreciably changes with temperature rise. It is calculated by formulae
U = viscosity of the oil to be tested at 38ºC or 100ºF
L = Viscosity of the standard oil having V.I zero at 100ºF
H = Viscosity of the standard oil having V.I 100 at 100ºF
Viscosity index is a scale based on arbitrarily fixing a value of 100 for Pennsylvanian oils containing paraffins. The viscosity of paraffin oil (VI=100) is little affected by increasing temperature. At lower end of scale viscosity index is fixed as ‘0’ for Gulf coast oil (VI= 0) which contains naphthanic, whose viscosity decrease drastically with increasing temperature.As the lubricants have to function over a wide range of operating temperature, they should have a high VI.
MSB, GITAM , HYD Page 7 Flash Point and Fire Point:-
Good Lubricant oil should not volatilize under the working temperatures. However if volatilized the vapours formed should not catch fire under the condition of lubrication.
Flash point is defined as the minimum temperature at which the lubricating oil gives off its vapours that ignite for a moment, when a flame is brought near it.
Fire point is the lowest temperature at which the vapours of the oil burn continuously for at least 5 seconds, when a flame is brought near it.
The flash point of the lubricating oil is above the operating temperature, because good lubricating oil should not volatilize under the working conditions usually and the fire points are 5 to 40°F higher than flash points. But the flash and fire points do not have any bearing with lubricating property of the oil. The flash point is the rough indication of the tendency of the oil to volatilize. This helps to safeguard against fire hazards.
Determination of Flash and Fire Point:-
The flash and fire points of a lubricating oil are determined experimentally by Pensky - Marten’s apparatus.
The apparatus consists of a small cup of 5 cm diameter and 5.5 cm. height. The cup is closed at the top with a lid containing three openings for inserting a thermometer, stirrer and for introducing test flame. A shutter which can be moved on the top of the container by lever mechanism which can open the lid for introducing the test flame
Working
The container is filled up to the standard mark with the lubricating oil. The cup is gradually heated using a burner. Stirrer is
worked and oil is exposed to the flame for every 1ºC rise in temp. of lubricating oil. The temperature at which the introduced test flame produces a flash is noted as the flash point. Similarly the temperature at which the oil ignites and continues to burn for at least 5 seconds is noted as the fire point of the oil.
CLOUD AND POUR POINTS:-
The lubricating oil is derived from petroleum; it contains dissolved paraffin wax and other resinous impurities. These impurities tend to separate out of oil at lower temperatures. The temperature at which the impurities begin to separate out from the solution and lubricating oil becomes cloudy or hazy in appearance is called cloud point. The temperature at which the oil ceases to flow or pour is called pour point.
Cloud point and pour points indicate the stability of lubricants in cold conditions. Machines working with low temperatures like refrigerator plants, aircraft engines, lubricants with low cloud and pour points are preferred.
MSB, GITAM , HYD Page 8 thermometer is introduced in the oil. As the cooling proceeds slowly,
the temp. falls continuously. For every 1ºC fall of temperature, the tube is withdrawn from air jacket for a moment and observed for cloudiness.
The temperature at which cloudiness is noticed is recorded as cloud point. Similarly after sometime the temperature at which the lubricating oil solidifies and resists to flow is recorded as the pour point.
Oiliness:-
Oiliness is one of the important characteristics of a lubricant particularly for extreme pressure lubrication. It is a measure of its capacity to stick to the surface of machine parts under heavy pressure or load. . A lubricant with poor oiliness in subjected to high pressure, the oil gets squeezed out of the machine parts. Lubricant with good oiliness stay in between lubricated surfaces at high pressure.Mineral oils have low oiliness, while the oils of vegetable origin have good oiliness. To improve oiliness, additives like vegetable or animals oils or higher fatty acids are added to them
ANILINE POINT:-
The aniline point of oil is the temperature at which equal volumes of
oil and aniline are just miscible. The Aniline point of oil gives an indication of the deterioration of an oil when it comes in contact with rubber sealings, packing etc., .Aromatic hydrocarbons have a tendency to dissolve natural rubber and certain synthetic rubbers; low aromatic content is therefore desirable in a lubricant. A lower aniline point of an oil means a higher percentage of aromatic hydrocarbons in it, so more chances of deterioration of an oil when it comes in contact with rubber sealing, packing etc., A higher aniline point is therefore desirable.
Emulsification Number:-
It is the ability of oil to get intimately mixed with water forming an emulsion. These emulsions can easily gather dirt, dust and causes abrasion and waving of machine parts. A good lubricant can form emulsion which could break off easily, quickly. It is expressed in steam emulsion number (S.E.M.). A good lubricant must possess low SEM.
S.E.N Test:- 20 ml of oil is taken in a test tube and a steam at 100ᴼC is bubbled through the test tube until the temperature raises to 90ᴼC. Then the test is placed in a bath maintained at 90ᴼC and time in seconds is noted, when oil and water separate out. The time in seconds in which oil and water separate out in distinct layers is called
steam emulsion number.
Saponification Number:-
MSB, GITAM , HYD Page 9 minerial or compounded oil.
Carbon Residue:-
Lubricating oils contain high percentage of carbon in combined form. Certain oils on heating tend to deposit carbon in certain parts of the machine or engines. A known weight of oil sample is heated to vaporize and pyrolise under set conditions. The residual carbon left behind is expressed as % weight of sample.
The experiment is carried out in Conradson’s carbon residue tester. A good lubricating oil deposit least amount of carbon.
% of carbon residue =
Precipitation Number:-
Precipitation number is the % of asphalt present in oil. A known weight of the lubricant is dissolved in petroleum ether and centrifuged. The precipitated asphalt is filtered washed, dried and weighted. It is expressed as percentage of weight of oil taken. Precipitation number is used to differentiate the different classes of the lubricants.
Neutralization number:-
The neutralization number is an indication of acidic or basic impurities in the lubricating oil. Determination of acidity is more common and is expressed in terms of acid value or acid number.
A known weight of the oil is dissolved in neutral alcohol and titrated against KOH using phenolphthalein as indicator. The number of milligrams of KOH required to neutralize the acidic constituents in 1 gram of the oil is known as neutralization number or acid number
Acid number =
The acid number greater than 0.1 is usually taken as an indication of oxidation of the oil.
The source of acidity in the oil may be
• Products of oxidation of oil
• Contamination of oil by SO2 from combustion of the fuel • Additives used in improving properties of the oil.
Volatility:-
When lubricating oil is used in heavy machinery working at high temperature a portion of the oil may vaporize, leaving behind a residual oil, which have different lubricating properties. Good lubricant should have low volatility.
Assignment Questions:-
1. What is lubrication? Explain the mechanism involved in fluid-film lubrication and Thin- fluid-film lubrications
2. Write a brief note on flash and fire points of a lubricant
3. Explain the properties of viscosity, oiliness, and aniline point of lubricants
4. Present the classification of lubricants.
5. Write a brief not on
a. Carbon residue
