Applications of abrasives
Abrasives are used in their forms
As loose powder
To clean the surface prior to coating, abrasive powders.
Example: Quartz and Garnet.
As abrasive paper (or) cloth
Manufacture of abrasive paper or cloth
The roll of paper or cloth is made to pass through a series of rollers, and a thin coating of glue is applied on its upper side. It is then passed under a hopper from which the grit abrasive is allowed to fall and spread evenly on the glued paper or cloth. Then it is dried in warm drying room.. Finally it is allowed to age few days, so that the glue sets firmly.
Uses: It is used to prepare smooth wood metal and plastic surfaces.
Example: Alumina and Silicon carbide.
As grinding wheels
Manufacture of grinding wheel
Grinding wheel is manufactured by mixing abrasive grains with binder.The mixture is moulded into desired shape and heated and cured.
Uses: It is used for the removal of rust from iron surfaces cutting tool sharpening.
Tuesday, November 17, 2009
Lubricants
Lubricants
Introduction
In all type of machines, the moving surfaces rub against each other. Due to this rubbing a resistance is offered to their movement. This resistance is known as Friction. This friction will cause a lot of wear and tear of surfaces of moving parts. Due to the friction large amount of energy is dissipated in the from of heat therby the efficiency of machine gets reduced.
Definitions
Lubricant
Lubricant is a substance used in between two moving surfaces to reduce the friction.
Lubrication
Lubrication is a process of reducing friction and wear between two moving surfaces by adding lubricant in between them.
Functions of a lubricant
1. It prevents the contact between the moving surfaces and reduces wear tear and surface deformation of the concerned parts.
2. It reduces the energy so that efficiency of the machine is enhanced.
3. It reduces the frictional heat and prevent the expansion of metals.
4. It acts as a coolant by removing the frictional heat generated due to the rubbing of surfaces.
5. At sometime it acts as a seal preventing the entry of dust and leakage of gases at high pressure.
6. It reduces the maintenance and running cost of the machine.
7. It minimizes corrosion.
Requirements (or) Characteristics of a lubricant
1. A good lubricant should not undergo any decomposition, oxidation, reduction at high temperature.
2. A good lubricant should have higher flash and fire point than the operating temperature.
3. A good lubricant should have high oiliness, viscosity index aniline point.
4. A good lubricant should not corrode machine parts.
Classification of lubricants
Lubricants are classified on the basis of their physical state as follows.
1. Liquid lubricants
(a) Vegetable oils – Palm oil, castor oil, etc.
(b) Animal oils - Whale oil, tallow oil, etc.
(c ) Mineral oils – Petroleum fractions.
(d) Synthetic lubricant – Silicones, polyglycol ethers, etc.
(e) Blended oils (or) Compounded – Mineral oils with various additives
2. Semi- solid lubricants
Greases, vaselines, etc.
3. Solid lubricants
Graphite, molybdenum-disulphide, etc.
4. Emulsions
(a) Oil in water type – Cutting emulsions.
(b) Water in oil type – Cooling liquids.
Introduction
In all type of machines, the moving surfaces rub against each other. Due to this rubbing a resistance is offered to their movement. This resistance is known as Friction. This friction will cause a lot of wear and tear of surfaces of moving parts. Due to the friction large amount of energy is dissipated in the from of heat therby the efficiency of machine gets reduced.
Definitions
Lubricant
Lubricant is a substance used in between two moving surfaces to reduce the friction.
Lubrication
Lubrication is a process of reducing friction and wear between two moving surfaces by adding lubricant in between them.
Functions of a lubricant
1. It prevents the contact between the moving surfaces and reduces wear tear and surface deformation of the concerned parts.
2. It reduces the energy so that efficiency of the machine is enhanced.
3. It reduces the frictional heat and prevent the expansion of metals.
4. It acts as a coolant by removing the frictional heat generated due to the rubbing of surfaces.
5. At sometime it acts as a seal preventing the entry of dust and leakage of gases at high pressure.
6. It reduces the maintenance and running cost of the machine.
7. It minimizes corrosion.
Requirements (or) Characteristics of a lubricant
1. A good lubricant should not undergo any decomposition, oxidation, reduction at high temperature.
2. A good lubricant should have higher flash and fire point than the operating temperature.
3. A good lubricant should have high oiliness, viscosity index aniline point.
4. A good lubricant should not corrode machine parts.
Classification of lubricants
Lubricants are classified on the basis of their physical state as follows.
1. Liquid lubricants
(a) Vegetable oils – Palm oil, castor oil, etc.
(b) Animal oils - Whale oil, tallow oil, etc.
(c ) Mineral oils – Petroleum fractions.
(d) Synthetic lubricant – Silicones, polyglycol ethers, etc.
(e) Blended oils (or) Compounded – Mineral oils with various additives
2. Semi- solid lubricants
Greases, vaselines, etc.
3. Solid lubricants
Graphite, molybdenum-disulphide, etc.
4. Emulsions
(a) Oil in water type – Cutting emulsions.
(b) Water in oil type – Cooling liquids.
Liquid lubricants
Liquid lubricants or lube oils
Vegetable and Animal oils
These are glycerides of higher fatty acids have very good oiliness. However, these oils cannot be used effectively, because
(i) They undergo oxidation at higher temperature and forms gummy and acidic products.
(ii) Also, they get hydrolysed easily under moist conditions. Actually, they are used as “blending agents” with other lubricating oils.
Mineral oils (or) Petroleum oils
It is obtained by fractional distillation of crude petroleum oil. The length of the hydrocarbon chain varies from C12 to C50. It is cheap and quite stable under normal operating condition. But it posses poor oilness. The oilness of which can be improved by mixing it with animal (or) vegetable oils.
The mineral oil obtained cannot be used as such because it contains a lot of impurities such as wax asphalt oxidisable impurities etc. These impurities have to remove from the mineral oil before using it as a lubricant. The impurities are generally removed by the following methods.
(a) Removal of waxes
Waxes get separated at lower temperature and interfere with lubricating properties.
The wax can be removed by dewaxing process in which the petroleum oil is mixed with a suitable solvent (propane trichloroethyl etc.,) and then cooled. The wax crystallises out and is removed by filtration.
(b) Removal of asphalt
Asphaltic and naphthenic materials tend to leave carbon deposits on the engine parts. These materials can be removed by acid refining process, in which the dewaxed oil is treated with con. H2SO4 and then agitated. Some of the unwanted impurities get dissolved in acid while other are converted into sludges. The sludges are removed by filtration. The filtrate is neutralized with calculated quantity of NaOH to neutralize the acid.
(c ) Removal of sulphur
Sulphur can be removed from the oil by desulphurization process in which the oil is treated with hydrogen in the presence of nickel as catalyst. During this process the unsaturated compounds are converted to satured compounds.
(d) Removel of coloured substance
The coloured and micro – crystalline waxes can be removed by filtration through Fuller’s earth.
Synthetic lubricants
Under severe operating condition ( -50 C to 250 C ) for example in air craft the lubricants are pumped at – 50 C but during take off and landing they get heated upto 120 – 150 C. Petroleum oils canot be effectively used because they tend to get oxidized at higher temperature while waxseparation will occur at lower temperatures. So synthetic lubricants have been developed range of ( -50 C to 250 C ) .
Example
Silicones, polyglycol, ethers, etc.
Blended (or) Compounded oils (or) Additives for lubricating oils
To improve the properties of the lubricating oils, certain substances called additives are added to the lubricating oils. The oil thus prepared are known as “ Blended oils (or) Compounded oils” .
Important additives and their functions
(i) Oiliness carriers - Fatty acids such as stearic acid palmatic acid, oleic acid. They increase the oilness adhering property of lubricants.
(ii) Extreme pressure additives - Organic chlorine compounds, organic sulphur compounds, organic phosphorous compounds. They react with metal surface film of lower shear strenge and high melting point.
(iii) Viscosity index improvers - n-hexanol, poly isobutlene, poly alkyl benzene. They prevent the oil from thinning higher temperatures and thickening at lower temperatures.
(iv) Pour-point depressants - Phenols, poly alkyl benzene. They prevent separation of wax from the lubricating oil.
(v) Thickeners - Polyesters, polystyrene. They increase the viscosity of the lubricants.
(vi) Anti oxidants - Aromatic amoino compounds, phenoilc, compounds. They retard the oxidation of the oil and prevent the formation of gum-like substances.
(vii) Deflocculents and detergents (or) Deposit inhibitors - Salts of phenols salts of carboxylic acids, sulphonates. They prevent foreign particles and prevent the formation of gum-like substances.
(vii) Deflocculents and detergents (or) Deposit inhibitors - Salts of phenls, salts of carboxylic acids, sulphonates. They prevent foreign particles and carbon deposits in engines which block the passage of oil.
(viii) Corrosion preventors or Corrosion inhibitors - Tricaresyl phosphates, organic compounds phosphorous antimony. They are adsorded on metal surfaces there by protecting the surface from attack by moisture.
Vegetable and Animal oils
These are glycerides of higher fatty acids have very good oiliness. However, these oils cannot be used effectively, because
(i) They undergo oxidation at higher temperature and forms gummy and acidic products.
(ii) Also, they get hydrolysed easily under moist conditions. Actually, they are used as “blending agents” with other lubricating oils.
Mineral oils (or) Petroleum oils
It is obtained by fractional distillation of crude petroleum oil. The length of the hydrocarbon chain varies from C12 to C50. It is cheap and quite stable under normal operating condition. But it posses poor oilness. The oilness of which can be improved by mixing it with animal (or) vegetable oils.
The mineral oil obtained cannot be used as such because it contains a lot of impurities such as wax asphalt oxidisable impurities etc. These impurities have to remove from the mineral oil before using it as a lubricant. The impurities are generally removed by the following methods.
(a) Removal of waxes
Waxes get separated at lower temperature and interfere with lubricating properties.
The wax can be removed by dewaxing process in which the petroleum oil is mixed with a suitable solvent (propane trichloroethyl etc.,) and then cooled. The wax crystallises out and is removed by filtration.
(b) Removal of asphalt
Asphaltic and naphthenic materials tend to leave carbon deposits on the engine parts. These materials can be removed by acid refining process, in which the dewaxed oil is treated with con. H2SO4 and then agitated. Some of the unwanted impurities get dissolved in acid while other are converted into sludges. The sludges are removed by filtration. The filtrate is neutralized with calculated quantity of NaOH to neutralize the acid.
(c ) Removal of sulphur
Sulphur can be removed from the oil by desulphurization process in which the oil is treated with hydrogen in the presence of nickel as catalyst. During this process the unsaturated compounds are converted to satured compounds.
(d) Removel of coloured substance
The coloured and micro – crystalline waxes can be removed by filtration through Fuller’s earth.
Synthetic lubricants
Under severe operating condition ( -50 C to 250 C ) for example in air craft the lubricants are pumped at – 50 C but during take off and landing they get heated upto 120 – 150 C. Petroleum oils canot be effectively used because they tend to get oxidized at higher temperature while waxseparation will occur at lower temperatures. So synthetic lubricants have been developed range of ( -50 C to 250 C ) .
Example
Silicones, polyglycol, ethers, etc.
Blended (or) Compounded oils (or) Additives for lubricating oils
To improve the properties of the lubricating oils, certain substances called additives are added to the lubricating oils. The oil thus prepared are known as “ Blended oils (or) Compounded oils” .
Important additives and their functions
(i) Oiliness carriers - Fatty acids such as stearic acid palmatic acid, oleic acid. They increase the oilness adhering property of lubricants.
(ii) Extreme pressure additives - Organic chlorine compounds, organic sulphur compounds, organic phosphorous compounds. They react with metal surface film of lower shear strenge and high melting point.
(iii) Viscosity index improvers - n-hexanol, poly isobutlene, poly alkyl benzene. They prevent the oil from thinning higher temperatures and thickening at lower temperatures.
(iv) Pour-point depressants - Phenols, poly alkyl benzene. They prevent separation of wax from the lubricating oil.
(v) Thickeners - Polyesters, polystyrene. They increase the viscosity of the lubricants.
(vi) Anti oxidants - Aromatic amoino compounds, phenoilc, compounds. They retard the oxidation of the oil and prevent the formation of gum-like substances.
(vii) Deflocculents and detergents (or) Deposit inhibitors - Salts of phenols salts of carboxylic acids, sulphonates. They prevent foreign particles and prevent the formation of gum-like substances.
(vii) Deflocculents and detergents (or) Deposit inhibitors - Salts of phenls, salts of carboxylic acids, sulphonates. They prevent foreign particles and carbon deposits in engines which block the passage of oil.
(viii) Corrosion preventors or Corrosion inhibitors - Tricaresyl phosphates, organic compounds phosphorous antimony. They are adsorded on metal surfaces there by protecting the surface from attack by moisture.
Semi-solid lubricants
Semi-solid lubricants (Greases)
Preparation
Greases are semi – solid lubricants obtained by thickening of lubricating oil by the addition of metallic soaps. Soaps are prepared by saponification of vegetable oils (or) fats with alkali (like as thickner (or) gelling agents).
Example
Preparation of Lithum grease
1step
Lithium soap is prepared by the saponification of vegetable oils (or) fats with lithium hydroxide.
II Step
Lubricating oil thickened by adding lithium soap.
The nature of the soap determines,
(a) The temperature upto which the grease can be used .
(b) It acts as a thickener.
(c) It is water and oxidation resistant.
(d) It enables the grease to stick to the metal surface firmly.
Properties and Uses of different greases
1. Sodium – soap grease (soda – base grease)
(i) Slighty soluble in water.
(ii) Dropping point is very high. It can be upto 175 deg C in ball bearing.
2. Calcium – soap grease ( lime – bases grease, cup grease)
(i) It is water resistant.
(ii) Beyond 70o deg C the grease separates into soap and oil.
(iii) Lesser dropping point than soda – base grease. It is a general purpose grease, used for
lubricating water pumps, tractors, etc.
3. Barium – soap grease
(i) It is water – resistant.
(ii) Possesses good adhesiveness. Used for lubricating automotives.
4. Lithium – soap grease
(i) It is a resistant to water and heat.
(ii) Expensive and superior to all other types. It is used at lower temperature: used for
lubricating engines.
5. Aluminium – soap grease
(i) It possess very high adhesiveness.
(ii) Expensive and water resistance. Used for lubricating chains and oscillating surfaces.
6. Axle – grease (Resin grease)
It is cheap and water resistant. Used for less delicate equipments under high load and low speed.
Grease are used under the following situations
(i) Where oil is squeezed out due to heavier load or low speed.
(ii) Where the bearing and gears that work at high temperature.
(iii) Where the bearing need to be sealed against the entry of dirt and dust etc.
(iv) Where frequent application of lubricant is inconvenient, as in automobile wheel bearings.
Preparation
Greases are semi – solid lubricants obtained by thickening of lubricating oil by the addition of metallic soaps. Soaps are prepared by saponification of vegetable oils (or) fats with alkali (like as thickner (or) gelling agents).
Example
Preparation of Lithum grease
1step
Lithium soap is prepared by the saponification of vegetable oils (or) fats with lithium hydroxide.
II Step
Lubricating oil thickened by adding lithium soap.
The nature of the soap determines,
(a) The temperature upto which the grease can be used .
(b) It acts as a thickener.
(c) It is water and oxidation resistant.
(d) It enables the grease to stick to the metal surface firmly.
Properties and Uses of different greases
1. Sodium – soap grease (soda – base grease)
(i) Slighty soluble in water.
(ii) Dropping point is very high. It can be upto 175 deg C in ball bearing.
2. Calcium – soap grease ( lime – bases grease, cup grease)
(i) It is water resistant.
(ii) Beyond 70o deg C the grease separates into soap and oil.
(iii) Lesser dropping point than soda – base grease. It is a general purpose grease, used for
lubricating water pumps, tractors, etc.
3. Barium – soap grease
(i) It is water – resistant.
(ii) Possesses good adhesiveness. Used for lubricating automotives.
4. Lithium – soap grease
(i) It is a resistant to water and heat.
(ii) Expensive and superior to all other types. It is used at lower temperature: used for
lubricating engines.
5. Aluminium – soap grease
(i) It possess very high adhesiveness.
(ii) Expensive and water resistance. Used for lubricating chains and oscillating surfaces.
6. Axle – grease (Resin grease)
It is cheap and water resistant. Used for less delicate equipments under high load and low speed.
Grease are used under the following situations
(i) Where oil is squeezed out due to heavier load or low speed.
(ii) Where the bearing and gears that work at high temperature.
(iii) Where the bearing need to be sealed against the entry of dirt and dust etc.
(iv) Where frequent application of lubricant is inconvenient, as in automobile wheel bearings.
Solid lubricants
SOLID LUBRICANTS
Solid lubricants are used the following situations.
(i) where the operating temperature and load is too high.
(ii) where contamination of oils or greases by the entry of dust or grit particles are avoied.
(iii) where combustible lubricants must be avoided.
The most widely used solid lubricants are graphite and molybdenum disulphide
8.7.1. Graphite
Graphite consists of flat layers of hexagonal arrangement of carbon atoms.The carbon atmos in hexagone are bonded together by strong covalent bonds. The adjacents layers are held together by weak Vander Walls forces . Since the distance between the adjacent layers can slide easily one over the other co-efficent of friction .This property makes use of graphite as a lubricant.
Graphite is very soapy to touch and non – inflammable. It can be used upto 375oC above this temperature it gets oxidized. Graphite is used as a dry powder or as a colloidal dispersion . A dispersion of graphite in oil is called oil Aqua dag and a dispersion of graphite in oil is called oil dag.
Used
(i) Oil dag is used in internal combustion engines.
(ii) Aqua dag is used in air compressors and in food processing equipments.s
(iii) Graphite is as a lubricant generally used in lathes, machine shop work railway track joints, open gears chains.
8.7.2. Molybdeenum disulphide
Molydbenum disulphide has a sandwich like structure in which a layer of molybdenum atoms lies between two layers of sulphur atoms. The atoms in the layer are bonded together by strong covalent bonds but the layers are held together by weak Vander Waals forces of attraction
The molybdenum layers and sulphur layers silde over one another .As a result MoS2 possess very low co-efficient of friction. It is also used either as a dry powder or as a colloidal dispersion. It gets oxdised above 800oC. It is mainly in heavy machineries working under heavy load and high temperatures.
Uses
(i) Pure MoS2 is used in the vacuum of outer space.
(ii) It is used in heavy machinery working at higher temperatures.
Solid lubricants are used the following situations.
(i) where the operating temperature and load is too high.
(ii) where contamination of oils or greases by the entry of dust or grit particles are avoied.
(iii) where combustible lubricants must be avoided.
The most widely used solid lubricants are graphite and molybdenum disulphide
8.7.1. Graphite
Graphite consists of flat layers of hexagonal arrangement of carbon atoms.The carbon atmos in hexagone are bonded together by strong covalent bonds. The adjacents layers are held together by weak Vander Walls forces . Since the distance between the adjacent layers can slide easily one over the other co-efficent of friction .This property makes use of graphite as a lubricant.
Graphite is very soapy to touch and non – inflammable. It can be used upto 375oC above this temperature it gets oxidized. Graphite is used as a dry powder or as a colloidal dispersion . A dispersion of graphite in oil is called oil Aqua dag and a dispersion of graphite in oil is called oil dag.
Used
(i) Oil dag is used in internal combustion engines.
(ii) Aqua dag is used in air compressors and in food processing equipments.s
(iii) Graphite is as a lubricant generally used in lathes, machine shop work railway track joints, open gears chains.
8.7.2. Molybdeenum disulphide
Molydbenum disulphide has a sandwich like structure in which a layer of molybdenum atoms lies between two layers of sulphur atoms. The atoms in the layer are bonded together by strong covalent bonds but the layers are held together by weak Vander Waals forces of attraction
The molybdenum layers and sulphur layers silde over one another .As a result MoS2 possess very low co-efficient of friction. It is also used either as a dry powder or as a colloidal dispersion. It gets oxdised above 800oC. It is mainly in heavy machineries working under heavy load and high temperatures.
Uses
(i) Pure MoS2 is used in the vacuum of outer space.
(ii) It is used in heavy machinery working at higher temperatures.
Mechanism of lubrication (Types of lubrication)
Mechanism of lubrication(Types of lubrication)
Fluid film (or) Thick film (or) Hydrodynamic lubrication
Condition - Under low load and high speed
Under the condition of low load and high speed a thick fluid film of lubricant is maintained between the two solid surfaces. The thickness of fluid film is atleast 1000A. Since the thick fluid film separates the two solid surfaces there is no direct contact between solid surfaces this reduces wear and tear. The co-efficient of friction in such is as low as 0.001 to 0.03
Example
Consider the rotation of a shaft with respect to a stationary bearing.
When a lubricant is added to the system, It occupies the annular space between the shaft and the bearing and forms a hydrodynamic wedge so long as the shaft rotates, the hydrodynamic wedge will remain and prevent contact between the two solid surfaces. When the load becomes very high, the lubricant will be sequeezed out of the wedge and friction will occur.
Boundary lubrication (or) Thin film lubrication
Condition: under high load and slow speed
Under the conditions of high load and slow speed, a continuous fluid film cannot be maintained between the moving surfaces. Under such conditions, the thickness of the fluid film should be less than 1000A. Such a thin film, consists of 2 or 3 molecules thick. To form a thin film the lubricant has to be adsorbed on the metal surface by physical or chemical forces. In some cases, the lubricant will react chemically with the metal surface forming a thin film of metal soap, which will act as a lubricant. This thin film is known as boundary film. The co-efficient of friction in such cases is around 0.05 to 1.15.
The effectiveness of boundary lubrication depends on the oiliness of the lubricant. Oiliness is the ability of a lubricant to stick on to the surface. Vegetable oils and their fatty acids have more oiliness. (eg) Oleic acid (C17H33COOH), stearic acid (C17H35COOH) etc. The polar carbonyl group (-COOH ) of these oils reacts with the metal surface to form a continuous thin film of lubricant. Hydrocarbon chain of the fatty acid gets oriented outwards in a perpendicular direction.
Extreme pressure lubrication
Condition: Under high load and high pressure
Under the conditions of high load ( high pressure) and high speed, more heat is generated between the moving surfaces. As a result of this, the liquid lubricant fails to stick and undergoes decomposition or evaporation. Under these conditions, for effective lubrication, special additives known as extreme pressure additives are used along with the lubricants.
Import extreme pressure additives are organic compounds having active radicals or groups such as chlorine (e.g Sulphurized oils) etc. These compounds react with metallic surfaces to form metallic chlorides, sulphides etc. These metallic compounds possess high melting points and serve as good under extreme pressure conditions.
Fluid film (or) Thick film (or) Hydrodynamic lubrication
Condition - Under low load and high speed
Under the condition of low load and high speed a thick fluid film of lubricant is maintained between the two solid surfaces. The thickness of fluid film is atleast 1000A. Since the thick fluid film separates the two solid surfaces there is no direct contact between solid surfaces this reduces wear and tear. The co-efficient of friction in such is as low as 0.001 to 0.03
Example
Consider the rotation of a shaft with respect to a stationary bearing.
When a lubricant is added to the system, It occupies the annular space between the shaft and the bearing and forms a hydrodynamic wedge so long as the shaft rotates, the hydrodynamic wedge will remain and prevent contact between the two solid surfaces. When the load becomes very high, the lubricant will be sequeezed out of the wedge and friction will occur.
Boundary lubrication (or) Thin film lubrication
Condition: under high load and slow speed
Under the conditions of high load and slow speed, a continuous fluid film cannot be maintained between the moving surfaces. Under such conditions, the thickness of the fluid film should be less than 1000A. Such a thin film, consists of 2 or 3 molecules thick. To form a thin film the lubricant has to be adsorbed on the metal surface by physical or chemical forces. In some cases, the lubricant will react chemically with the metal surface forming a thin film of metal soap, which will act as a lubricant. This thin film is known as boundary film. The co-efficient of friction in such cases is around 0.05 to 1.15.
The effectiveness of boundary lubrication depends on the oiliness of the lubricant. Oiliness is the ability of a lubricant to stick on to the surface. Vegetable oils and their fatty acids have more oiliness. (eg) Oleic acid (C17H33COOH), stearic acid (C17H35COOH) etc. The polar carbonyl group (-COOH ) of these oils reacts with the metal surface to form a continuous thin film of lubricant. Hydrocarbon chain of the fatty acid gets oriented outwards in a perpendicular direction.
Extreme pressure lubrication
Condition: Under high load and high pressure
Under the conditions of high load ( high pressure) and high speed, more heat is generated between the moving surfaces. As a result of this, the liquid lubricant fails to stick and undergoes decomposition or evaporation. Under these conditions, for effective lubrication, special additives known as extreme pressure additives are used along with the lubricants.
Import extreme pressure additives are organic compounds having active radicals or groups such as chlorine (e.g Sulphurized oils) etc. These compounds react with metallic surfaces to form metallic chlorides, sulphides etc. These metallic compounds possess high melting points and serve as good under extreme pressure conditions.
Properties of lubricant
PROPERTIES OF LUBRICANTS
Viscosity
Viscosity is a measure of the internal resistance of a liquid during its flow. It is expressed in centipoise.The viscosity of an oil is the time in seconds for a given quantity of a oil to pass through a standard orifice under the specified conditions.
Determination
The viscosity of an oil is determined by
(i) Red wood viscometer.
(ii) Say bolt viscometer.
The time required for 50ml of the liquid to pass through the orifice of a red wood viscometer is called as Red wood seconds. The time required for 60 ml of the liquid to pass through the orifice of a say bolt viscometer is called Say bolt universal seconds.
Significance
(i) If the viscosity of the lubricating oil is too high, the movement of the machine is restricted due to excessive friction.
(ii) If the viscosity of the lubricating oil is too low, the liquid oil film can’t be maintained and excessive wear will take place.
A good lubricating oil must have moderate viscosity.
Viscosity index
The viscosity of an oil decreases with increase in temperature. The rate of change of viscosity with temperature is indicated by viscosity temperature curves or by a scale known as viscosity index(V.I). Viscosity index is defined as “ The average decreases in viscosity of an oil per degree rise in temperature between 100°F and 210°F”.
(i) If the viscosity of an oil decreases rapidly with the increase in temperature it has low V.I.
Example
Gulf coast oil (consists of naphthenic hydrocarbons) exhibits a larger change in viscosity with a increase in temperature and its V.I value is arbitrarily assigned as zer0.
(ii) If the viscosity of an oil is slightly affected with the increase in temperature, it has higher V.I.
Example
Pennsylvanian oil (consists of paraffinic hydrocarbons) exhibits a relatively a smaller change in viscosity with a increase in temperature and its V.I value is arbitrarily assigned as 100.
Determination
The V.I of a test oil is calculated by comparing with the above two standard oil. The test oil is compared at 38°C (100°F) with zero V.I oil (gulf coast ) and 100 V.I oil (pensylvanian oil ) both having the same viscosity as the test oil aty 99°C (210°F).
The V.I of the test oil is given by the following formula
V.I=L-U/L-H ×100
Where,
U is the viscosity of the test oil at 38°C.
L is the viscosity of the low V.I oil at 38°C
H is the viscosity of the high V.I oil at 38°C
The viscosity temperature curve is flatter for the oil of high V.I than the oil of low V.I
Significance
A good lubricant should have minimum change in viscosity for a wide range of temperature. A good lubricant should have a high V.I.
Improving viscosity index
V.I of a lubricant can be increased by the addition of linear polymers such as polyisobutylene, n-hexanol, etc.
Viscosity
Viscosity is a measure of the internal resistance of a liquid during its flow. It is expressed in centipoise.The viscosity of an oil is the time in seconds for a given quantity of a oil to pass through a standard orifice under the specified conditions.
Determination
The viscosity of an oil is determined by
(i) Red wood viscometer.
(ii) Say bolt viscometer.
The time required for 50ml of the liquid to pass through the orifice of a red wood viscometer is called as Red wood seconds. The time required for 60 ml of the liquid to pass through the orifice of a say bolt viscometer is called Say bolt universal seconds.
Significance
(i) If the viscosity of the lubricating oil is too high, the movement of the machine is restricted due to excessive friction.
(ii) If the viscosity of the lubricating oil is too low, the liquid oil film can’t be maintained and excessive wear will take place.
A good lubricating oil must have moderate viscosity.
Viscosity index
The viscosity of an oil decreases with increase in temperature. The rate of change of viscosity with temperature is indicated by viscosity temperature curves or by a scale known as viscosity index(V.I). Viscosity index is defined as “ The average decreases in viscosity of an oil per degree rise in temperature between 100°F and 210°F”.
(i) If the viscosity of an oil decreases rapidly with the increase in temperature it has low V.I.
Example
Gulf coast oil (consists of naphthenic hydrocarbons) exhibits a larger change in viscosity with a increase in temperature and its V.I value is arbitrarily assigned as zer0.
(ii) If the viscosity of an oil is slightly affected with the increase in temperature, it has higher V.I.
Example
Pennsylvanian oil (consists of paraffinic hydrocarbons) exhibits a relatively a smaller change in viscosity with a increase in temperature and its V.I value is arbitrarily assigned as 100.
Determination
The V.I of a test oil is calculated by comparing with the above two standard oil. The test oil is compared at 38°C (100°F) with zero V.I oil (gulf coast ) and 100 V.I oil (pensylvanian oil ) both having the same viscosity as the test oil aty 99°C (210°F).
The V.I of the test oil is given by the following formula
V.I=L-U/L-H ×100
Where,
U is the viscosity of the test oil at 38°C.
L is the viscosity of the low V.I oil at 38°C
H is the viscosity of the high V.I oil at 38°C
The viscosity temperature curve is flatter for the oil of high V.I than the oil of low V.I
Significance
A good lubricant should have minimum change in viscosity for a wide range of temperature. A good lubricant should have a high V.I.
Improving viscosity index
V.I of a lubricant can be increased by the addition of linear polymers such as polyisobutylene, n-hexanol, etc.
Subscribe to:
Posts (Atom)
