Q. What are lubricants? Explain properties of lubricating oil.
Ans. Lubricants: The metal to metal frictional resistance can be minimized by using certain suitable substances which form a thin layers in between the moving parts, which keep them apart; such substance are called lubricants.
Any substance introduced between moving or sliding surfaces to reduce the friction in order to avoid or reduce the wear and tear is known as lubricant.
Lubrication: The process of reducing friction between two sliding surfaces or moving bodies by the introduction of lubricant is known as lubrication.
Types of lubricants
(a) Lubricating oils: Lubricating oils are of 4 types.
1. Animal and vegetable oil
2. Mineral oils
3. Blended oil
4. Synthetic oil
Animal oil and vegetable oil: The most important quality of these oils is the oiliness. This property causes sticking of oil to machine parts. These oils are also known as fixed oils. The name fixed oil is given because these oils mainly esters of higher fatty acid with glycerol and decomposes on heating instead of distillation.
Mineral oils: Produced by distillation and refining treatment.
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Light oil
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Medium oil
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Heavy oil
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Residue
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spindle
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turbines
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Internal combustion engine
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Steam cylinder oil
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refrigeration
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compressors
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Heavy machinery
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Extreme pressure
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transformers
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Light machinery
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Gear oil
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Lubrication gear oil
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Cup grease
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Pressure gun greases
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Heavy greases
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Blended oil:
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Name of additives
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Chemical used
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Functions
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1
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Oiliness carriers
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Vegetable oil and fatty acid
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Increase oiliness and strength of the oil film
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2
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Extreme pressure additives
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Fatty ester acids sulphurized fats, organic chlorine and phosphorus compounds.
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Absorbed on metal forming surface layer of low shear strength to prevent tearing welding.
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3
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Corrosion prevents
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Organic compounds of phosphorus and antimony
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Protect the metal from corrosion.
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4
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Emulsifiers
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Sodium salts of sulphuric acids, fatty amine salts
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Help in formation of emulsions.
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5
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Antifoaming
Additives
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Silicone polymers, glycols and glycerol
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Prevent formation of stable foams.
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6
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Pour point depressing
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Phenol wax alkylated naphthalene
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Prevent separation of wax from oil
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7
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Viscosity index improvers
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High molecular weight compounds.
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Reduce damage of viscosity with temperature.
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8
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Antioxidants or inhibitors
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Aromatic, phenol or amino compounds
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Retard the oxidation of foils.
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9
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Thickness improvers detergents
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Polystyrenes polyesters salts of phenol and carboxylic acids
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Give higher viscosity, dispense and clean the deposits formed in IC engines due to imperfect combustion.
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Synthetic and lubricating oils: These are used where operating condition are at extremely high temperature and high pressure or most drastic and severe conditions like in craft engines.
1. It works over a wide range of temperatures, such as from -500C to 2600C.
2. Very high viscosity index values.
3. Non-inflammable.
4. High-chemical stability, even in corrosive environment.
5. Resistance to hydrolysis and oxidations.
6. High flash and fire points.
7. Rust preventive qualities.
8. Very low freezing point.
Greases or semi solid lubricants: Semi solid lubricants are combinations of liquid lubricating oil and soap or a mixture of soaps. Soaps are mainly used as thickening agent so that grease sticks firmly to the metal surfaces.
Greases are employed in following situations:
1. When a machine is worked at slowly speed, high load or with sudden jerks etc. For example, rail axle boxes.
2. In bearing and gears working at high temperature.
3. In bearing where sealing is necessary against dust.
4. When dripping or spurting of oil is undesirable.
5. When a machine is worked at slow speed, high load or with sudden jerks etc. For example rail axle boxes.
6. In bearing and gears working at high temperature.
7. In bearing where sealing is necessary against dust, dirt or moisture.
8. When dripping or spurting of oil is undesirable, e.g. machines preparing papers, textiles, edible articles.
(b) Solid lubricants are used:
1. Where liquid or semi solid lubricants cannot be used.
2. Where contamination of lubricating oil or grease due to entry of dust particles is unacceptable.
3. Where operating temperature or load is too high to use even a semisolid lubricant.
Graphite and molybdenum disulphide are most commonly used solid lubricants. Other examples are talc, mica, soapstone.
Emulsions: in milling, threading, turning and boring, tools employed get highly heated. This causes damage to tools. In order to prevent overheating and damage to tools, efficient cooling and lubrication have to be provided. This is done usually by using emulsions of oil droplets in water. Such emulsions are called cutting fluids or cutting oils.
Extreme pressure emulsions: Extreme pressure lubricants are used under the following two conditions:
1. Where moving or sliding surfaces are under the very high load and speed.
2. Where a high local temperature develops at the surface due to frictional heat.
For such conditions additives are called "extreme pressure additives" are added to mineral oils. Chlorinated esters, some sulphurized oils are and tracery! Phosphates are some example of extreme pressure additives. These form more durable film of metallic chlorides, sulphides or phosphates by reacting with metallic surfaces.
Properties of lubricating oils: Liquid lubricants are also known as lubricating oils. Lubricating oils reduce frictional and wear between two metallic moving bodies by providing a continuous fluid film in between them. A good liquid lubricant must possess:
1. Low vapour pressure or high boiling point.
2. Adequate viscosity for particular service conditions.
3. Low freezing point.
4. High oxidation resistance and heat stability.
5. Non corrosive properties.
6. Resistance to decomposition at higher and elevated temperatures.
Liquid lubricants also act as:
(a) Cooling medium
(b) Sealing agent
(c) Corrosion preventers.