LUBRICANT AND LUBRICATIONS

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LUBRICANT AND LUBRICATION

FUNCTION OF LUBRICATION:
LUBRICATION PRODUCES THE FOLLOWING EFFECTS:
  • REDUCING FRICTION EFFECT
  • COOLING EFFECT
  • SEALING EFFECT
  • CLEANING EFFECT
  1. REDUCING FRICTIONAL EFFECT: THE PRIMARY PURPOSE OF THE LUBRICATION IS TO REDUCE FRICTION AND WEAR BETWEEN TWO RUBBING SURFACES. TWO RUBBING SURFACES ALWAYS PRODUCE FRICTION. THE CONTINUOUS FRICTION PRODUCE HEAT WHICH CAUSES WEARING OF PARTS AND LOSS OF POWER. TO AVOID FRICTION, THE CONTACT OF TWO SLIDING SURFACES MUST BE REDUCED AS FAR AS POSSIBLE. THIS CAN BE DONE BY PROPER LUBRICATION ONLY. LUBRICATION FORMS AN OIL FILM BETWEEN TWO MOVING SURFACES. LUBRICATION ALSO REDUCES NOISE PRODUCED BY THE MOVEMENT OF TWO METAL SURFACES OVER EACH OTHER.
  2. COOLING EFFECT: THE HEAT, GENERATED BY PISTON, CYLINDER, AND BEARINGS IS REMOVED BY LUBRICATION TO A GREAT EXTENT. LUBRICATION CREATES COOLING EFFECT ON THE ENGINE PARTS.
  3. SEALING EFFECT: THE LUBRICANT ENTERS THE GAP BETWEEN THE CYLINDER LINER, PISTON, AND PISTON RINGS. THUS, IT PREVENTS LEAKAGE OF GASES FROM THE ENGINE CYLINDER. 
  4. CLEANING EFFECT: LUBRICATION KEEPS THE ENGINE CLEAN BY REMOVING DIRT OR CARBON FROM INSIDE OF THE ENGINE ALONG WITH THE OIL.

LUBRICATION THEORY:
THERE ARE TWO THEORIES IN EXISTENCE REGARDING THE APPLICATION OF LUBRICANTS ON A SURFACE
  • FLUID FILM THEORY
  • BOUNDARY LAYER THEORY
  1. FLUID FILM THEORY: ACCORDING TO THIS THEORY, THE LUBRICANT IS, SUPPOSED TO ACT LIKE MASS OF GLOBULES, ROLLING IN BETWEEN TWO SURFACES. IT PRODUCES A ROLLING EFFECT, WHICH REDUCES FRICTION.
  2. BOUNDARY LAYER THEORY: ACCORDING TO THIS THEORY, THE LUBRICANT IS SOAKED IN RUBBING SURFACES AND FORMS OILY SURFACE OVER IT. THUS THE SLIDING SURFACES ARE KEPT APART FROM EACH OTHER, THEREBY REDUCING FRICTION. 

PROPERTIES OF LUBRICANT: 
  • VISCOSITY: VISCOSITY IS A MEASURE OF THE RESISTANCE TO FLOW OR THE INTERNAL FRICTION OF THE LUBRICANT.  -USUALLY MEASURED BY SAYBOLT UNIVERSAL SECONDS (SUS) AND REDWOOD VISCOMETER. ALSO IT IS EXPRESSED WITH CENTISTOKE (UNIT OF KINEMATIC VISCOSITY) AND CENTIPOISE (UNIT OF ABSOLUTE VISCOSITY) -EXPRESSED IN TWO TEMPERATURE I.E. -18ᵒC (0ᵒF) AND 99ᵒC (210ᵒF).
  • VISCOSITY INDEX: IT IS USED TO GRADE LUBRICANTS. VISCOSITY IS INVERSELY PROPORTIONAL TO TEMP. -IF TEMP. INCREASES, THE VISCOSITY OF THE LUBRICANT DECREASES AND IF TEMP. DECREASES, THE VISCOSITY OF THE LUBRICANT INCREASES.  -THE VARIATION OF VISCOSITY OF OIL WITH CHANGES IN TEMPERATURE IS MEASURED BY VISCOSITY INDEX -OIL TO MEASURE IS COMPARED WITH 2 REFERENCE OIL HAVING SAME VISCOSITY AT 99ᵒC. ONE IS PARAFFINIC BASE OIL INDEX OF ZERO AND ANOTHER NAPHTHENIC BASE OIL INDEX OF 100 -HIGH VISCOSITY INDEX NUMBER INDICATES RELATIVELY SMALLER CHANGE IN VISCOSITY OF THE OIL WITH TEMPERATURE.  -LOW VISCOS OIL IS RECOMMENDED FOR AUTOMOBILE ENGINES IN WINTER THAN SUMMER. THE VISCOSITY OF A LUBRICANT SHOULD BE JUST SUFFICIENT TO ENSURE LUBRICATION. IF IT IS MORE THAN THIS VALUE, POWER LOSS WILL BE HIGHER DUE TO INCREASED OIL RESISTANCE. -VI IMPROVER ARE ADDED TO IMPROVE VISCOSITY INDEX.
  • OILINESS: IT IS THE PROPERTY OF A LUBRICATING OIL TO SPREAD & ATTACH ITSELF FIRMLY TO THE BEARING SURFACES AS WELL AS PROVIDE LUBRICITY. GENERALLY, THE OILINESS OF THE LUBRICATING OIL SHOULD BE HIGH PARTICULARLY WHEN IT IS TO BE USED FOR MATING SURFACES SUBJECTED TO A HIGH INTENSITY OF PRESSURE AND SMALLER CLEARANCE PORTION TO AVOID THE SQUEEZING OUT OF THE OIL. SUCH A WAY THAT THE METAL IS PROTECTED BY A THIN LAYER OF THE OIL AND THE WEAR IS ALSO CONSIDERABLY REDUCED. IT IS MEASURED BY CO-EFFICIENT OF FRICTION AT EXTREME OPERATING CONDITION.
  • FLASH POINT: FLASH POINT OF OIL IS THE MIN. TEMP. AT WHICH THE VAPOURS OF LUBRICATING OIL WILL FLASH WHEN A SMALL FLAME IS PASSED ACROSS ITS SURFACE. IT IS OF TWO TYPE OPEN FLASH POINT AND CLOSED FLASH POINT. THE FLASH POINT OF THE LUBRICATING OIL MUST BE HIGHER THAN THE TEMP. LIKELY TO BE DEVELOPED IN THE BEARINGS IN ORDER TO AVOID THE POSSIBILITY OF FIRE HAZARDS.
  • FIRE POINT: IF THE LUBRICATING OIL IS FURTHER HEATED AFTER THE FLASH POINT HAS BEEN REACHED, THE LOWEST TEMP. AT WHICH THE OIL WILL BURN CONTINUOUSLY FOR 5 SECONDS IS CALLED FIRE POINT. -USUALLY 11ᵒC HIGHER THAN OPEN FLASH POINT AND VARIES FROM 190ᵒC TO 290ᵒC FOR THE LUBRICANTS USED FOR IC ENGINES -THE FIRE POINT OF A LUBRICANT ALSO MUST BE HIGH SO THAT THE OIL DOES NOT BURN IN SERVICE.
  • CLOUD POINT: IT IS THE TEMP. AT WHICH THE LUBRICATING OIL CHANGES ITS STATE FROM LIQUID TO SOLID. ITS TEMP. MUST HIGH FOR THE LOW TEMP. OPERABILITY OF THE LUBRICATING OIL DURING WINTER.
  • POUR POINT: IT IS THE LOWEST TEMP. AT WHICH THE LUBRICATING OIL WILL NOT FLOW OR TOTALLY FORM WAX OR SOLIDIFY. THIS PROPERTY MUST BE CONSIDERED BECAUSE OF ITS EFFECT ON STARTING AN ENGINE IN COLD WEATHER. OIL DERIVED FROM PARAFFINIC CRUDES TENDS TO HAVE HIGHER POUR POINTS THAN THOSE DERIVED FROM NAPHTHENIC CRUDES. THE POUR POINTS CAN BE LOWER BY THE ADDITION OF POUR POINT DEPRESSANT USUALLY A POLYMERISED PHENOL OR ESTER. POUR POINT MUST BE AT LEAST 15ᵒF LOWER THAN THE OPERATING TEMPERATURE TO ENSURE MAXIMUM CIRCULATION.
  • CORROSIVENESS: THE PRESENT OF ACID (MINERAL ACID, PETROLEUM ACID) IS HARMFUL TO THE METAL SURFACES. THE LUBRICATION OIL SHOULD NOT ATTACK CHEMICALLY THE MATERIALS OF THE ENGINE. THE LUBRICANT SHOULD NOT BE CORROSIVE, BUT IT SHOULD GIVE PROTECTION AGAINST CORROSION. NEW OIL HAS LOW NEUTRALIZATION NUMBER I.E. IT MAINTAINS THE ALKALINE AND ACID SOLUTION TO MAKE THE OIL NEUTRAL.
  • OXIDATION STABILITY: IT IS RESISTANCE TO OXIDATION. DUE TO OXIDATION THE OIL WILL FORM DEPOSITS ON THE PISTON RINGS AND LOSE ITS LUBRICATING PROPERTY. LOW TEMPERATURE OPERATION AVOIDING THE HOT-AREA CONTACT AND CRANKCASE VENTILATION CAN HELP IN PRESERVING THE STABILITY OF OIL OVER LONGER PERIODS. OXIDATION INHIBITORS ARE USED TO IMPROVE OXIDATION STABILITY. THESE ARE COMPLEX COMPOUNDS OF SULPHUR AND PHOSPHORUS OR AMINE AND PHENOL DERIVATIVES. 
  • CLEANLINESS: LUBRICATING OIL MUST BE CLEAN. IT SHOULD NOT CONTAIN DUST AND DIRT PARTICLES AS WELL AS WATER CONTENT WHICH PROMOTE CORROSION. 
  • CARBON RESIDUE: AFTER EVAPORATION OF A MASS SAMPLE OF LUBRICATING OIL UNDER SPECIFIC CONDITION MAY REMAIN AS CARBONACEOUS RESIDUE. IT INDICATES THE DEPOSIT CHARACTERISTICS OF OIL. PARAFFINIC OIL HAS HIGHER CARBON RESIDUES THAN THE NAPHTHENIC BASE OIL. 

TYPES OF LUBRICANTS
LUBRICANTS ARE AT FOLLOWING THREE TYPES:
  • SOLID: GRAPHIC, MICA ETC
  • SEMI SOLID: GREASE
  • LIQUID: LUBRICANTS ARE OBTAINED FROM ANIMAL FAT, VEGETABLES AND MINERALS. LUBRICANTS MADE OF ANIMAL FAT, DOES NOT STAND MUCH HEAT. IT BECOMES WAXY AND GUMMY WHICH IS NOT VERY SUITABLE FOR MACHINES. VEGETABLE LUBRICANTS ARE OBTAINED FROM SEEDS, FRUITS AND PLANTS. COTTONSEED OIL, OLIVE OIL, LINSEED OIL AND CASTOR OIL ARE USED AS LUBRICANT IN SMALL MACHINES. MINERAL LUBRICANTS ARE MOST POPULAR FOR ENGINES AND MACHINES. IT IS OBTAINED FROM CRUDE PETROLEUM FOUND IN NATURE. PETROLEUM LUBRICANTS ARE LESS EXPENSIVE AND SUITABLE FOR INTERNAL COMBUSTION ENGINES. 
  • GRAPHITE IS OFTEN MIXED WITH OIL TO LUBRICATE AUTOMOBILE SPRING. GRAPHITE IS ALSO USED AS A CYLINDER LUBRICANT. -GREASE IS USED FOR CHASSIS LUBRICATION. 

GRADE OF LUBRICANTS:
GENERALLY LUBRICATING OILS ARE GRADED BY SAE (SOCIETY OF AUTOMOTIVE ENGINEERS) METHOD BY ASSIGN A NUMBER TO OIL WHOSE VISCOSITY AT GIVEN TEMPERATURES FALLS IN CERTAIN RANGE. -TWO TEMPERATURES -18ᵒC (0ᵒF) AND 99ᵒC (210ᵒF) ARE USED TO ASSIGN THE NUMBER EX-SINGLE GRADE TYPE: (A) SAE 5W,10W AND 20W GRADES ARE VISCOSITY AT -18ᵒC (0ᵒF) AND FOR WINTER USE. (B) SAE 20, 30, 40 AND 50 GRADES LUBRICATING OIL ARE VISCOSITY AT 99ᵒC (210ᵒF) AND FOR SUMMER USE. MULTI-GRADE TYPE: EX- SAE 20W/50 OIL HAS VISCOSITY EQUAL TO THAT AT SAE 20W AT -18ᵒC AND VISCOSITY EQUAL TO THAT AT SAE 50 AT 99ᵒC 

SAE GRADES OF OIL ARE BASED ON VISCOSITY BUT NOT QUALITY BASED. API (AMERICAN PETROLEUM INSTITUTE) USED REGULAR, PREMIUM, AND HEAVY DUTY TYPE OIL WHICH ARE BASED UPON PROPERTIES OF OIL AND OPERATING CONDITIONS. GENERALLY REGULAR TYPE OIL IS STRAIGHT MINERAL OIL, PREMIUM TYPE CONTAINED OXIDATION INHIBITORS AND HEAVY TYPE CONTAINED OXIDATION INHIBITORS PLUS DETERGENT DISPERSANT ADDITIVES. ACCORDING TO API, FOR GASOLINE ENGINE 5 SERVICE RATINGS OIL ARE USED: SA, SB, SC, SD AND SE FOR DIESEL ENGINES 4 SERVICE RATINGS ARE USE: CA, CB, CC AND CD WHERE S AND C STANDS FOR SI AND CI ENGINES RATING A IS FOR LIGHT-DUTY SERVICE, THE SEVERITY OF SERVICE INCREASING TOWARDS RATING D AND E WHICH IS SEVERE DUTY.

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