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LUBRICANTS

(c) Dr. Payal Joshi, NMIMS

Tribology


Lubricants and Lubrication

•Any substance introduced between two moving/sliding surfaces to reduce frictional resistance between them is known as lubricant •Process of reducing frictional resistance between moving/sliding surfaces by introduction of lubricants in between them is called lubrication


Greases • Semi-solid lubricants with combination of petroleum product

(fraction) and soap • Used when machine is at low speed and under high pressure • Greases do not spill- ideal in bearings ‘stay-put’ • Grease is prepared be dispersing a gelling agent (soaps of Ca,

Al, Li) in petroleum oil (lubricating oil) • Role of soaps: Acts as a thickener, enables grease to stick to

the machine parts firmly, nature of soap determines the temperature at which grease can be used-consistency, resistance to water, oxidation


Functions of a lubricant

• Reduces surface deformation, wear & tear because direct contact between the rubbing surfaces is avoided

• Reduces loss of energy in form of heat --- lubricant acts like a coolant --- reduces frictional heat thereby controlling expansion of metals --- maintains shape, size & dimensions of metal parts in contact

• Reduces waste of energy-- Efficiency of machine enhanced

• Acts as a sealant --- preventing leakage of gases under high pressure


• Controls corrosion of moving machine parts ---prevents attack due to moisture

• Cleaning and Suspending—Lubricant facilitates smooth operation of the equipment by removing & suspending potentially harmful products, such as carbon, dirt and wear debris


Mechanism of Lubrication

Hydrodynamic/Thick film/ Fluid

Film

Boundary/ Thin Film

Extreme Pressure


Hydrodynamic lubrication •Moving/Sliding surfaces are separated from each other by a thick lubricant film (1000A0 thick) •This prevents direct surface to surface contact so that the small peaks and valleys do not interlock • Sewing machines, clocks, scientific instruments


• Lubricant is applied in the annular space

• Stationary position – Two surfaces remain in contact

• In Motion – Shaft begins to rotate and also the lubricant film rotates between metallic surfaces

• As shaft gains velocity, liquid film flows at greater rate

• Due to thick oil film, all asperities of metal surfaces are filled up and pressure is developed that keeps the two surfaces separated----thereby reducing wear

Operation of a Journal Bearing Bearing consists of shaft rotating at a fair speed with moderate load


Boundary lubrication • Thick film cannot persist when

machines are on low speed and high load with a lubricant of low viscosity

• Thin layer (100A) of lubricant is adsorbed on the metallic surfaces which avoids direct metal to metal contact

• Soaps of vegetable oil or animal oil, mineral oils blended with fatty acids, solid lubricants like graphite, semi-solid lubricants like greases

Applied in machines like gears, rollers


Extreme pressure lubrication

• Under heavy load, high speed operating conditions, machines attain higher temperatures

• Due to frictional heat, liquid lubricants fail to stick to the surfaces & may decompose or vaporize

• Additives like P, S or chloro compounds are added to mineral oils. Such additives improve specific characteristics of lubricating oil---Extreme pressure additives

• Additives react with metal surfaces => form durable films like phosphides, sulfides that act as good lubricant under extreme pressure & temperature


Properties of lubricants Viscosity and Viscosity Index • Property by virtue of which a liquid or fluid (oil) offers

resistance to its own flowÆ Viscosity • Rate at which viscosity of oil changes with temperature is

measured by an empirical number, known asÆ Viscosity Index

• Oils become thin on heating– viscosity decreases; Decrease in viscosity is rapid ---oil has low viscosity index & vice versa

• Oils should possess adequate viscosity to function as a good lubricant

• Redwood viscometer


Flash and Fire point • Measures of flammability • Flash Point : Minimum temperature at which liquid gives

off vapours that will ignite for a moment when a small flame is brought near it

• Fire Point : Minimum temperature at which vapors of the oil burn continuously for 5 seconds when a small flame is brought near it

• Knowledge of flash and fire points in lubricating oil helps to take preventive measures against fire hazards

• Pensky-Marten’s Apparatus


Cloud and Pour Point • Indicate the suitability of lubricants in cold conditions • Cloud Point: Temperature at which oil becomes cloudy

or hazy in appearance; temperature at which solidifiable compounds, like paraffin wax, present in oil begin to crystallize or separate from solution

• Pour Point : Temperature at which oil ceases to flow or pour

• A good lubricating oil must have low cloud and pour point or else it may solidy at lower temp causing jamming of machine parts affecting speed of working machine


•Oiliness: Ability of lubricant oil to stick to the machine parts under heavy load or pressure • Poor oiliness causes easy squeeze out of lubricants when

machines are operated under heavy load • Petroleum oil has poor oiliness, but Vegetable oils have

good oiliness • Petroleum oils are mixed with animal/vegetable oils or

fatty acids like oleic acid, stearic acid, etc


Saponification & Acid Value • Saponification value: Number of milligrams of potassium

hydroxide required to saponify one gram of oil • Characteristic property of vegetable/animal oil;

Mineral/synthetic oils do not undergo saponification • Alkaline hydrolysis of pure oil giving soap and glycerol • Helps us to estimate stability of an oil in aqueous/alkaline

medium • Signifies composition of oil, thereby checks suitability of

oils for lubrication--- Drying property of oil is a harmful phenomena which can also be checked


Oil (W g) is mixed with known excess of alc. KOH (0.5 N)

Stirred vigorously and reflux mixture for 2 hr in water bath using water condenser

Fatty acids form potassium salts (soaps) & glycerol is released Oil + alc. KOH = Soap + glycerol + unreacted KOH

Unreacted KOH titrated against HCl (0.5N). Amount of unreacted KOH is known from amount of HCl consumed

Saponification value = Volume of KOH consumed (blank – back) x NKOH (0.5) x 56/W of oil (Wg)

Expressed in mg of KOH


• Acid value: Number of milligrams of potassium hydroxide required to neutralize free fatty acids present in one gram of oil

• To determine content of free acids in an oil; their presence can harm machines during lubrication

• Mineral oils on prolonged exposure to oxygen, results in oxidation--- formation of carboxylic acids--- making oil unsuitable for lubrication

• Acid value should be minimum (< 0.1) • Higher the acid value, greater the corrosion of machine

parts, more wear & tear----------- more maintenance cost


• Accurately weigh 5 g of the oil under test into a 250 ml conical flask and add 50 ml of alcohol.

• Heat the flask over a water bath for about 30 minutes. • Add few drops of phenolphthalein indicator. • Titrate with the standard KOH solution until a pale

permanent pink color appears at end point.

• Acid Value = VKOH consumed x NKOH x 56 Weight of oil (W gm)


Numerical Problems Based on Saponification value

• 5 g of vegetable oil was saponified using excess of 0.5N alcoholic KOH. The mixture required 17 ml of 0.5N HCl. Blank titration reading was 44 ml. Calculate saponification value of the oil.

• Data: Weight of oil= 5 g Blank titration reading =44 ml Back titration reading = 17 ml • Solution: Since, Volume of 0.5N KOH required by oil for

saponification in terms of 0.5N HCl = (44-17) ml = 27 ml


Conclusion: Saponification value of oil is 151.2 mg of KOH.


• 1.55 g of oil is saponified with 20 ml of N/2 alcoholic KOH. After refluxing the mixture, it requires 15 ml of N/2 HCl solution. Calculate the saponification value of oil.

Data: Weight of oil = 1.55 g Blank titration reading = 20 ml Back titration reading = 15 ml Normality of KOH and HCl = N/2 = 0.5N Solution: Volume of 0.5N KOH required by oil for saponification in terms of 0.5N HCl = (20-15) ml = 5 ml


• Oil sample of saponification value 168 mg of KOH was saponified using 0.4 N alcoholic KOH. Blank titration reading was 50 ml of 0.4N HCl solution. Find the quantity of alcoholic KOH required while saponification reaction.

• Data: Saponification value of oil = 168 mg of KOH Blank titration reading = 50 ml Weight of oil = 1g Normality of KOH= 0.4N • Consider the back titration reading as x ml,


168 = (Blank – Back) x 0.4 x 56/1 168 = (50 – x ) x 0.4 x 56/1 168 = [50 – x] x 22.4 168 = 1120 – 22.4x 22.4 x = 1120 – 168 Hence, x = 1120 – 168/22.4 = 42.5 ml of alcoholic KOH.


Numerical Problems based on Acid Value

• Vegetable oil was tested for its acid value. 10 g of oil required 2.4 ml of 0.02 N KOH. Calculate the acid value and determine whether the oil is suitable for lubrication or not.

• Data: Weight of oil = 10 g Volume of KOH = 2.4 ml Normality of KOH = 0.02 N


Acid value of vegetable oil is 0.268 As the acid value of vegetable oil is greater than 0.1, oil is not suitable for lubrication.


• 9.0 ml of oil is taken from the machine and it required 1.5 ml of 0.04N KOH. Calculate its acid value if density of oil is 0.81 g/ml.

• Data: Volume of oil = 9 ml Volume of KOH = 1.5 ml Density of oil = 0.81 g/ml Normality of KOH = 0.04 N • Solution: Mass of oil = Volume x density = 9 x 0.81 = 7.29 g


Acid value of vegetable oil is 0.461. As the acid value of vegetable oil is greater than 0.1, oil is not suitable for lubrication.


• 1.3 g of oil required 0.8 ml of 0.001 N KOH for neutralization. Calculate acid value and mention whether the oil is suitable to be used as a lubricant or not. •Data: Weight of oil = 1.3 g Volume of KOH = 0.8 Normality of KOH = 0.001N


Acid value of vegetable oil is 0.034. As the acid value of vegetable oil is lesser than 0.1, oil is suitable for lubrication.


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