Properties of Lubricats and Lubrication

i f b iProperties of Lubricants and

Lubrication

Mohd Hanif Dewan Senior Engg Lecturer Mohd. Hanif Dewan, Senior Engg. Lecturer, International Maritime Academy, Bangladesh

3/14/2014 1

Lubricant:Lubricant:The substances which are used to decrease the force of frictionbetween the moving parts of machine in contact are known asLubricants and the process of decreasing the force of frictionbetween the moving parts of machine in contact is known asLubricationLubrication.

Composition of lubricating oils:Lubricating oil fractions extracted from crude oil are a widelyvarying mixture of straight and branched chain paraffinic,napthenic aromatic hydrocarbons having boiling points rangingnapthenic aromatic hydrocarbons having boiling points rangingfrom about 302o to 593oC. Some specialty lubricants may haveboiling point extremes of 177 and 815oC. The choice of grade ofg p glubricating oil base is determined by the expected use.3/14/2014 2

Mohd. Hanif Dewan, Senior Engg. Lecturer, International Maritime Academy, Bangladesh

FRICTION:When on surface of machinery moves over the another ysurface, resistance to relative motion of the surfaces arises. When we look at the solid surface it appears smooth to naked eye but this smooth surface showssmooth to naked eye , but this smooth surface shows irregularities of projections and cavities when viewed under high power microscope.

Mohd. Hanif Dewan, Senior Engg. Lecturer, International Maritime Academy, Bangladesh3/14/2014

Wh h f i l d th itWhen one such surface is placed over another, its projections fall into the cavities of the other and get interlocked Due to this interlocking there isinterlocked .Due to this interlocking , there is resistance to the relative motion of the surfaces. This is called the frictional forces or frictional resistance of friction. In due course of motion, the old projections get broken and deformities arise.

Mohd. Hanif Dewan, Senior Engg. Lecturer, International Maritime Academy, Bangladesh 43/14/2014

So, FRICTION may be defined as the opposing force that is set up between the surface of contact, when pone body moves over the surface of another body.


EFFECT OF FRICTION:

The frictional forces oppose the relative motion betweenthe moving parts of a machine Therefore extra energythe moving parts of a machine. Therefore extra energyhas to be spent to overcome the friction , whichincreases expenses of energy .The friction betweenthe moving parts of machines also produces heatwhich causes damage to the machinery. Thus frictioncauses wear and tear of the moving parts of machinerycauses wear and tear of the moving parts of machineryin contact and due to this cause, the machines losetheir efficiency and become useless.

3/14/2014 6Mohd. Hanif Dewan, Senior Engg. Lecturer, International Maritime Academy, Bangladesh

FUNCTIONS OF LUBRICANTS:I. Lubricants avoid the damage of the moving parts of machines

by minimizing the production of heat.II Lubricants reduce the wear and tear of machinery by keepingII. Lubricants reduce the wear and tear of machinery by keeping

the moving parts of machines apart.III. Lubricants reduce the maintenance and running cost of

machinemachine.IV. Lubricants act as the coolant because it reduces the

production of heat between the moving parts of machine incontactcontact.

V. Lubricants increases the efficiency of machine by reducing the loss of energy.

VI B i th l b i t th l ti ti f th i tVI. By using the lubricants, the relative motion of the moving parts of machine becomes smooth and noise level of running machine reduces.

VII L b i t l t th i tVII. Lubricants also act as the corrosion preventers.

3/14/2014Mohd. Hanif Dewan, Senior Engg. Lecturer, International Maritime Academy, Bangladesh

VIII.Lubricants also act as a seal as in piston. Lubricant used between piston and walls of the container (cylinder) prevents the leakage of hot gases produced by theprevents the leakage of hot gases produced by the internal combustion i.e.it act as seal.


General capabilities expected from an engine lubricant:Di i it it t th ld t f i l- Dispersivity or capacity to the cold parts of an engine clean

- Detergency or capacity to keep hot parts of an engine clean- Thermal strength or capacity to withstand temperature changes- Anti-oxidant or capacity to resist the action of oxygen- Anti-wear or capacity to contain wear- Anti-scuffing or capacity to preserve oil film even in the presence of high pressures.- Alkalinity reserve or capacity to neutralise acids formed during combustion or other sources thereby preventing corrosive wear.- Demulsibility or capacity to separate contaminants.- Resistance to hydrolysis or capacity to withstand the action of water which can affect additives Pumpabilityp y- Centrifugibility and filterability or capacity to separate insoluble elements.- Anti-rust, anti-corrosive and anti-foam are just some of the other , jproperties which protect the metalic object from wear down.


NORMAL PROPERTIES REQUIRED ARE:NORMAL PROPERTIES REQUIRED ARE:1. Adequate viscosity at working temperature so that the oil spreads over the liner surface to provide a tough film which resists th ti f th i t ithe scrapper action of the piston rings.2. The oil must provide an effective seal between the rings and liner.3. Only a soft deposit must be formed when the oil burns,4.Alkalintiy level (total base number or TBN) must match the acidity of the oil being burntacidity of the oil being burnt.5. Detergent and dispersant properties are required in order to hold deposits in suspension and thus keep surfaces clean.


Classification of LubricantsLubricants

Liquid Lubricants

Semi Solid Lubricants Solid Lubricants

Eg.Mineral Oil, P t l Oil Eg. Petroleum Eg. Graphite,

M l bd Petroleum Oil, Vegetable Oil etc

gjellies Molybdenum

Disulphide etc.

VISCOSITY:• It’s a measure of a fluid’s resistance to flow.• Viscosity of the lubricating oil determines its performance under

operating conditions.• A low viscosity oil is thin and flows easily .• A high viscosity oil is thick and flows slowly.• As oil heats up it becomes moreAs oil heats up it becomes more

viscous (Becomes thin)• Too low viscosity of the liquid > Lubricant film cannot be maintained

between the moving surfaces > Excessive wear.between the moving surfaces Excessive wear.• Too high viscosity of the liquid > Excessive friction.• Selected Lubricant must be proper viscous.

Viscosit is s all e pressed in centipoise or centistoke• Viscosity is usually expressed in centipoise or centistoke.

Viscosity Index : • It is “Avg. decrease in viscosity of oil per degree rise in temp between

1000F & 2100F.”• Viscosity of liquids decreases with increasing temperature. • The rate at which viscosity of a lubricant changes with temperature is

measured by a scale called Viscosity Index.• Silicones, polyglycol ethers, Diesters or triesters have high Viscosity Index.

Determination of Viscosity Index : • First the viscosity of the oil under test is determined at 100°F & 210°F.

Let it be U and V respectively.y• Then viscosity of Pennsylvanian oil is determined. Let it be VH.

• Then viscosity of Gulf oil is determined. Let it be VL

viscosity Index = VL- U x 100LUy L

VL- VH

V.I. = 100 (Pennsylvanian oils.)V.I. = Zero (Naphthanic-base gulf oils)

Viscosity

Temp100O 200

H

F( p g )Higher the V.I, lesser is the variation of viscosity with change in temperature.Thus, a good lubricating oil should possess high V.I.

Temp

• Iodine number is the number of Gms equivalent of iodine to amount of IClabsorbed by 100gm of oil.

• Each oil has its specific Iodine Number. • So Iodine Number determines the extent of contamination of oil. • Low Iodine Number is desirable in oils.

Some oils and their Iodine Numbers are given below :

Iodine Number Oil Example

>150 Drying oil Linseed oil, tung oil

100‐150 Semidrying oil Castor oil , Soyabean oil100 150 Semidrying oil Castor oil , Soyabean oil

<100 Non‐Drying oil Coconut oil, Olive oil

• Aniline point is the Min temp at which oil is miscible with equal amt of aniline

• Aniline Point is a measure of aromatic content of the lubricating oil• Aniline Point is a measure of aromatic content of the lubricating oil.• Low Aniline Point oil have high aromatic content which attacks rubber

seals.• Higher Aniline point means low %age of hydrocarbons (desirable).• Thus Aniline Point is used as an indication of possible deterioration of • Thus Aniline Point is used as an indication of possible deterioration of

rubber sealing etc.Determination of Aniline Point :

A ili Homogeneous Aniline + sample oil (equal)

Heated in Test tubeHomogeneous

solution

Coooled

Cloudiness

The temperature at which separation of the two phases (Aniline + oil) takes place is the Aniline Point.

• Emulsification is the property of water to get mixed with water easilyeasily.

• Emulsions can be oil in water emulsion or water in oil emulsion.• A good lubricating oil should form such an emulsion with water

which breaks easily. This property is called demulsification.which breaks easily. This property is called demulsification.• The time in seconds in which a given volume of oil and water

separates out in distinct layers is called steam demulsification number.

• A good lubricating oil should have lower demulsification number.

• Quicker the oil separates out from the emulsion formed, better i th l b i ti ilis the lubricating oil.

• In cutting oils the higher the emulsification number, better the oil is. This is because the emulsion acts as a coolant as well as a lubricanta lubricant.

• Flash Point is the min temp at which the lubricant vaporizes that ignite for a momwhen tiny flame is brought near.

• Fire Point is the Min temp at which the lubricant’s vapours burn constantly• Fire Point is the Min temp at which the lubricant s vapours burn constantly for 5 seconds when tiny flame is brought near.

• Fire point = flashpoint+5 to 400C. • Both should be higher than the max temp of country (for transportation)Both should be higher than the max temp of country (for transportation)• If flash point < 140°F = Flammable liquids

And if flash point > 140°F =Combustible liquids.

The flash and fire points are generally determined by using Pensky-Marten’s apparatus.

•Oil under examination is filled in the oil cup up to the k d h t d b th i b th b bmark and heated by the air bath by a burner.

•Stirrer is worked b/n tests at a rate of about 1 – 2 rev/sec.•Heat is applied so as to raise the oil temp by about 5c/min.5c/min.•The temp at which distinct flash appeared in side the oil cup is recorded as flashpoint.•The heating is continued to record the fire point.

• Drop Point is the Temperature at which grease passes from g pthe semi-solid to the liquid state. So, it determines the upper temp limit for theupper temp limit for the applicability of grease.

Determination :Determination :• Beaker is heated.• Temperature is raised.• Grease sample passes from aGrease sample passes from a

semi-solid to a fluid state.• Temp at which its first drop

falls from the opening isfalls from the opening is recorded as drop-point.

• Cloud Point is the temp at which the lubricant becomes cloudy or hazy when cooled.

• Pour Point is the temp at which the lubricant just ceases to flow when cooled.

• Both indicates suitability of lubricant in cold conditions and thus ot d cates su tab ty o ub ca t co d co d t o s a d t usmust be low.

• Pour point of wax can be lowered by dewaxing or adding suitable pour point depressantsuitable pour point depressant.

• Pour point of an oil can be lowered by lowering the viscosity of the oil which is achieved by removing the viscous constituent of th ilthe oil.

• Lubricating oils used in capillary feed systems should have low cloud points, otherwise impurities will clog the capillary.g y

• A high pour point leads to the solidification of the lubricant that may cause jamming of the machine.

• Neutralization Point determines Acidity or Alkalinity of oil. A idit /A id l /A id b i f KOH i d t• Acidity/Acid value/Acid number is mgs of KOH required to neutralize acid in 1 gm of oil.

• Alkalinity/Base value/Base number is mgs of acid required to neutralize all bases in 1 gm oil.

• As Neutralization Point of oil increases, age of oil decreasesdecreases.

• It’s the mgs of KOH required to saponify 1 gm of oil.S ifi ti i h d l i f E t ith KOH t i• Saponification is hydrolysis of an Easter with KOH to give alcohol and Na/K salt of acid.

• Mineral oils do not react with KOH and are not saponifiable. • Vegetable and animal oils have very high saponification

values.

Significance• Saponification value helps us to ascertain whether the oil

d f i i l t bl ilunder reference is mineral or vegetable oil or a compounded oil.

• Each oil has its specific Soaponification Number.Deviation from it indicates the extent of adulteration of oil.

ADDITIVESADDITIVESImprovements in lubricating oil over the last twenty years have come about almost entirely from the useyears have come about almost entirely from the use of additives.

These are added for three main reasons;1.to protect the lubricant in service by limiting the h i l h d d t i tichemical change and deterioration

2.To protect the mechanism from harmful combustion products and malfunctioningcombustion products and malfunctioning lubricating oil3.To improve existing physical properties and to p g p y p pcreate new beneficial characteristics in the oil


Typical additives are: ypBarium, calcium, phosphorus, Sulphur, chlorine, zinc, oxidation inhibitor-increases oil and machinery life, decreases sludge and varnish on metal parts.varnish on metal parts.

Corrosion inhibitor- protects against chemical attack of alloy bearings and metal surfacesbearings and metal surfaces.Antiwear improvers- protects rubbing surfaces operating with this film boundary lubrication. One such antiwear ( and oxidation inhibitor) chemical is Zinc dithiophosphate or ZDDPinhibitor) chemical is Zinc dithiophosphate or ZDDPDetergent- tend to neutralise the deposits before formation under high temperature and pressure conditions, or as a result of using a fuel with high sulphur contentfuel with high sulphur content. Dispersant- used to disperse or suspend the deposits forming contaminants. Typical dispersants, such as polyesters and benzlamides are usually clean burning The molecules have abenzlamides, are usually clean burning. The molecules have a polar charge at one end which attracts and holds the deposits3/14/2014 23


Alkaline agents- neutralises acids, htese form the TBN of the oil and includes additives such as the above dispersants and detergents. An excess of acid neutralising alkalis are present in thedetergents. An excess of acid neutralising alkalis are present in the oil and these help to keep parts clean. Failure to keep an oil alkaline can lead to damage to bearings due to acidic attack as well as increased liner wearwell as increased liner wear.Rust inhibitors- protect to form the oxidation of metal component.Pour point depressants- improves low temperature viscosityOiliness agent- reduces friction seizure point and wear ratesOiliness agent- reduces friction seizure point and wear ratesEP additives- increases film strength and load carrying capabilityAntifoam agents- prevents stable bubble formationViscosity Improvers an additive that improves the viscosity indexViscosity Improvers- an additive that improves the viscosity index of the oil. I.e. reduces the effect of temerpature of the oil. Metal deactivators- prevent catalytic effects of metalAntiseptic bactericideAntiseptic- bactericide.


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LUB OIL ANALYSIS:Regular testing of crankcase lub oil is important to ensure that deterioration has not taken place. The results of in service deterioration could be a reduction in engine protection or actual attack on working points by corrosive deposits. Oil samples are generally tested every 3 to p y p p g y y4 months depending on the system and experience. Shipboard testing is taking a rising prominence to allow monitoring of oil condition between testing. To ensure good representation care should be taken where the sampleTo ensure good representation, care should be taken where the sample is drawn Correct

Main supply line inlet or outlet from lub oil cooler Outlet from main lub oil pump

Incorrect standpipesstandpipes purifier outlet purifier direct sump suction

Samples should be drawn over a period of several minutes


ViscosityViscosityThe viscosity is the most important property of the oil. Oil of correct viscosity will provide optimum film strength withof correct viscosity will provide optimum film strength with minimum friction losses and leakage. The viscosity of a L.O. may fall due to fuel dilution if running on gas oil, and rise if running on heavy f.o. Viscosity may also increase due to heavy soot loading if purifiers and filters not operating efficiently. Oil ageing caused by oxidation and thermal degradation increases viscosity. A simple shipboard test is the Mobil flow stick where p pdrops of new and used oil are placed in separate channels on an inclined 'stick'. The rate the oil flows down the stick is proportional to its viscosity. p p y


Water contentWater contentInitially determined by 'crackle' test. The presence of Na and Mg in a 4:1 ratio indicatespresence of Na and Mg in a 4:1 ratio indicates salt water contamination. Limits are laid down by the manufacturer but asLimits are laid down by the manufacturer, but as a rule of thumb a limit of 0.2% should cause investigation into source and remedial action at 0.5% Gross contamination can be remedied by placing the charge in a separate tank and heating to 70oC and circulating through purifier.


SpectrometrySpectrometryIndicates the presence of metal element composition and identifies additive and contaminant levels. Zinc(Zn),Phosphorus(P)- are components of many oils such as diesel engine oils, hydraulic oils and gear oils, to enhance antiwear and over properties of the oil p pCalcium(Ca)- primarily a component of engine oils, provides detergency,alkalinity and resistance to oxidation. Residual fuel engine oils have higher Ca levels Nickel(Ni)- Bearings, Valves, gear plating, fuel derivative Barium(Ba)- Multi purpose additive, declining importance Magnessium(Mg)- as for Ca may also be due to seaMagnessium(Mg) as for Ca, may also be due to sea water contamination if found in Ratio of 1:4 of Na Chromium(Cr)- Piston rings, hydraulic actuator cylinders Manganese(Mn)- Cylinder wearManganese(Mn)- Cylinder wear


Aluminium(Al)- generally comes from wearing piston skirts, ( ) g y g plevels rise where new piston fitted to old engine. Typically 10ppm, but rises during bedding in. May also indicate the presence of catylytic fines in residual fuels. Iron(Fe) Molybdenum(Mo) Chromium(Cr) metals alloyedIron(Fe), Molybdenum(Mo), Chromium(Cr)- metals alloyed for piston ring etc, a rise in level may indicate ring pack/liner wear. Copper(Cu), Lead(Pb) , Tin(Sn), Silver(Ag) - soft metalsCopper(Cu), Lead(Pb) , Tin(Sn), Silver(Ag) soft metals used in the overlay of shell bearings, and phosphor bronze gears.Note that high copper content can also occur when samples are drawn from copper pipes which have not been flushed as well as gear wearflushed as well as gear wear. Silicon(Si)- Indicates poor air filtration, possible fuel derivative Sulphur(S)- May indicate the presence of clay based (bentonite) greases(bentonite) greases Sodium(Na)- With Mg indicates the presence of sea water contamination, possible coolant system and fuel derivative Vanadium(V)- Usually indicates the presence of fuel oilVanadium(V) Usually indicates the presence of fuel oil


Alkalinity and acidityAlkalinity and acidityTBN-TOTAL BASE NUMBER- measure of alkaline additives available for the neutralisation of acids from combustion products and oxidation. Level governed by type of fuelLevel governed by type of fuel. For crosshead engines the TBN will tend to rise due to contamination by liner lubrication, it should not be allowed to raise more than twice that of the new charge. A id th TBN f f h il h ld b t l tAs a guide, the TBN of fresh oil should be at least:

10 x fuel sulphur content (%) for trunk piston engines (10mgKOH/g) 20 x fuel sulphur content (%) for cyl oil in x-head engines20 x fuel sulphur content (%) for cyl oil in x head engines (20mgKOH/g)

A simple shipboard go,no-go test is available for measuring the TBN, it involves the addition of an indicator and acid reagent to a 30ml sample The quantify of acid reagent added is determined by thesample. The quantify of acid reagent added is determined by the required level of TBN, for TBN2.5 0.5ml are added, for TBN20 4ml is added. After three minutes the colour is checked against a chart

Purple:Good level of TBN Green:BorderlineYellow:Low level of TBN


TAN TOTAL ACID NUMBER f i id d tTAN-TOTAL ACID NUMBER-measure of organic acid and strong acid content of oil. Where SAN is nil, the TAN represents the acidity in the oil due to both the acids in the additives and the oxidation of the hydrocarbons in the oil. The TAN of fresh oils varies with oil type,

d t d t li b ith A hi h TAN i di t th t iland tends to climb with age. A high TAN may indicate that an oil should be changed or freshened by top up. A high TAN may be accompanied with increased viscosity. SAN-STRONG ACID NUMBER-indicates the presence of strong, p g,highly corrosive (inorganic) acids, usually formed from combustion products. If SAN is not zero the oil should be changed immediately Oil cleanlinessIC INDEX OF COMBUSTION measures soot loading of oilIC-INDEX OF COMBUSTION-measures soot loading of oil MD-MERIT OF DISPERSANCY-Ability of an oil to disperse contaminants, such as soot, wear debris and water and thereby carry them away from the critical areas. Measured by oil blot test and h ld t b ll d t f ll b l 50should not be allowed to fall below 50

DP-DEMERIT POINTS- combination of IC and MD: the lower the value, the healthier is the condition of the oil


Shipboard water content test1 Th fl k i fill d t k 'A' ith k1. The flask is filled to mark 'A' with kerosene2. A capsule of reagent (calcium hydride) is added.

Any water in the kerosene will react with the calcium hydride and any gas vented off.y y g

3. he container is topped to mark 'B' with sample oil4. The screw valve and cap are closed.5. The flask is inverted and shaken6. After 2 minutes the screw valve is opened. The

hydrogen produced by the reaction between the reagent and water exerts a pressure which forces the kerosene through the open valve into thethe kerosene through the open valve into the graduated cylinder. The amount discharged is proportional to the water content in the oil sample.

7. If the water content is greater than 1.5% then the t t h ld b t d thi ti i lltest should be repeated this time using a smaller sample by filling only to mark 'C'.The second scale on the graduated cylinder should then be used.

8. If water is detected its type, sea or fresh , should yp , ,then be determined by use of a special reagent the water

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Types of LubricationC id i th t f ti b t iConsidering the nature of motion between moving or sliding surfaces, there are different types of mechanisms by which the lubrication is done They are:by which the lubrication is done. They are:1. Hydrodynamic lubrication or thick film lubrication2. Hydrostatic lubricationy3. Boundary lubrication or thin film lubrication4. Extreme pressure lubrication


1. Hydrodynamic Lubrication or Thick Film LubricationHydrodynamic lubrication is said to exist when the moving surfaces are separated by the pressure of a

i b k fil l f l b i i hicontinuous unbroken film or layer of lubrication. In this type of lubrication, the load is taken completely by the oil filmfilm.The basis of hydrodynamic lubrication is the formation of an oil wedge. When the journal rotates, it creates an oil g j ,taper or wedge between the two surfaces, and the pressure build up with the oil film supports the load.


Hydrodynamic lubrication depends on:relative speed between the surfaces, ‐ oil viscosity, ‐ load, andload, and clearance between the moving or sliding surfaces.In hydrodynamic lubrication the lube oil film thickness is greater than outlet pressure at the inlet increases quickly remains fairlythan outlet, pressure at the inlet increases quickly, remains fairly steady having a maximum value a little to the outside of the bearing center line, and then decreases quickly to zero at the outlet.


Application of hydrodynamic lubricationDelicate instruments.

fScientific instruments.Large plain bearings like pedestal bearings, main bearing of diesel engines.g

Fig: Hydrodynamic Lubrication3/14/2014 37


Hydrodynamic LubricationHydrodynamic Lubrication


Elastohydrodynamic,This is the type of lubrication used with rollingThis is the type of lubrication used with rolling element bearings. To clarify, the material of the running surface deforms under high pressure as the g g prolling element passes over it. The oil wedge forms in this deformation.(i) Deformation and increased viscosity with pressure are involved(ii) Frictional coefficient = 0 05(ii) Frictional coefficient = 0.05(iii) film thickness less than Hydrodynamic


2. Hydrostatic Lubrication:Hydrostatic lubrication is essentially a form of hydrodynamic l b i ti i hi h th t l f t d blubrication in which the metal surfaces are separated by a complete film of oil, but instead of being self‐generated, the separating pressure is supplied by an external oil pump. Hydrostatic lubrication depends on the inlet pressure of lube oil and clearance between the metal surfaces, whereashydrodynamic lubrication it depends on the relative speedhydrodynamic lubrication it depends on the relative speed between the surfaces, oil viscosity, load on the surfaces, and clearance between the moving surfaces.


Example: the cross head pin bearing or gudgeon pin bearing in two stroke engines employs this hydrostatic lubrication mechanism. In the cross head bearing, the load is very high and the motion is not continuous as the bearing oscillation is a d t e ot o s ot co t uous as t e bea g osc at o sfairly short. Thus hydrodynamic lubrication cannot be achieved. Under such conditions, hydrostatic lubrication offers the advantagethe advantage.

Hydrostatic Lubrication3/14/2014 41


3 Boundary Lubrication or Thin Film Lubrication3. Boundary Lubrication or Thin Film LubricationBoundary lubrication exists when the operating condition are such that it is not possible to establish a full fluid condition,

ti l l t l l ti d b t th iparticularly at low relative speeds between the moving or sliding surfaces.The oil film thickness may be reduced to such a degree y gthat metal to metal contact occurs between the moving urfaces. The oil film thickness is so small that oiliness becomes predominant for boundary lubricationbecomes predominant for boundary lubrication.Boundary lubrication happens when,

•A shaft starts moving from rest.Th d i l•The speed is very low.•The load is very high.•Viscosity of the lubricant is too low.y


Examples for boundary lubrication:Guide and guide shoe in two stroke engine.Lubrication of the journal bearing in diesel engines (mainly duringLubrication of the journal bearing in diesel engines (mainly during starting and stopping of engine).Piston rings and when cylinder liner is at TDC and BDC position h th i t di ti h d if th l ti d iwhen the piston direction changes and if the relative speed is very

slow.

Boundary Lubrication


A thin lubricant should have high viscosity indexA thin lubricant should have high viscosity index,good resistance to heat and oxidation, goodoiliness and low pour point.

Velocity


4. Extreme Pressure LubricationWhen the moving or sliding surfaces are under very high pressure and speed, a high local temperature is attained. Under such condition liquid lubricant fails to stick to the moving parts and maycondition, liquid lubricant fails to stick to the moving parts and may decompose and even vaporize. To meet this extreme pressure condition, special additives are added to the minerals oils. These are called “extreme pressure lubrication.” These additives form on the metal surfaces more durable films capable of withstanding high loads and high temperature. Additives are organic compounds like chlorine g p g p(as in chlorinated esters), sulphur (as in sulphurized oils), and phosphorus (as in tricresyl phosphate).

The Extreme pressure Additives are the organic compounds possessing the active radicals or groups such as chlorine, sulphur , phosphorus etc. These compounds react with metallic surfaces at high temperature to form metallic chlorides.


Extreme Pressure Lubricationsulphides or phosphides which can form surface layers on the moving or sliding metallic surfaces and act as good lubricantsgood lubricants.


Stresses on Lube oilStresses on Lube oilThe main stresses experienced by Lube oils in diesel engines operating on heavy fuel oils are expressed as followsA id St C d b l h i d id ti id Thi l d tAcid Stress‐ Caused by sulphuric and oxidation acids. This leads to increased corrosive wear, deposits, reduced Base Number and shorter oil life.Rapid depletion of the BN is the clearest sign of oil stressh l/ d h d b l d l dThermal/Oxidative stress‐This caused by elevated temperatures leading to increased rates of thermal/oxidative breakdown of lubricant and fuel. This leads to increased levels of deposits, sludges, corrosive wear of bearing material, oil thickening and reduced oil life. In addition deposits on the under crown side of the piston can lead to increased hot corosionon the piston.Asphaltene Stress‐This caused by fuel contamination of the lube oil and can lead to increased levels of deposits, sludges, lacquers, oil thickening and reduced oil life. In addition deposits on the under crown side of the piston can lead to increased hot corosion on the piston


Various Factors of Lubricating Oil:gOXIDATION‐ Oxidation degrades the lube oil producing sludges, varnishes and resins Presence of moisture and some metals particularly copperresins. Presence of moisture, and some metals particularly copper tend to act as a catalyst. Once oxidation starts, deterioration of the properties of the oil is rapid.‐ Oxidation reduces its effectiveness as a lubricant. Oxidation will also cause deposits which can block passage ways and coat working parts. The rate of oxidation will depend upon temperature, the p p p p ,higher the temperature the more rapid the rate. Anti oxidants are available which reduce the rate, also additional properties can be achieved by the use of additivesachieved by the use of additives.


‐‐Most of the chemicals found in an oil will react more or less with oxygen, The effects of this oxidation is always undesirable. Hence, a major objective of the refining process of a mineral oil is to remove j j g pthose hydrocarbons i.e. the aromatics, the small amount of unsaturates together with molecules containing sulphur, oxygen and nitrogennitrogen. ‐ The use of anti‐oxidants make a slightly better balance although there usefulness is limited.Tin based white metal is susceptible to hardening as an oxide layers‐ Tin based white metal is susceptible to hardening as an oxide layers

from on the surface. These tin oxides are a grey‐black in appearance and are extremely hard. There formation reduces the bearing l h id l i hi k h h i i l hi lclearance as the oxide layer is thicker than the original white metal material from which it formed. The oxide has a lower coefficient of friction than the original white metal but it will cause problems if it brakes up as fragments will become embedded edge on in the white metal and can score the pin. .3/14/2014 49


Emulsification‐ This occurs due to water contamination; also, contamination with grease, fatty oils, varnish, paint and rust preventers containing fatty products can also promote emulsification.- The presence of an emulsion can be detected by a general cloudiness of the sample. Salt water emulsifies very easily and should be avoided.- Water entrained in the oil supplied to a journal bearing can lead to loss of oil wedge, rub and failure.Fresh water contamination whilst not in itself dangerous can lead to rusting. The iron oxides catalyses the oil to form sludge's. The g y gadditives in the oil can leach out to change the water into an electrolyte.- Salt water contamination is very serious as it causes tin oxide ycorrosion, and also leads to electrochemical attack on the tin matrix in the white metal. The sea water act as then electrolyte.A major problem of water within a lub oil is where the mix enters aA major problem of water within a lub oil is where the mix enters a bearing, here it is possible for the water to be adiabatically heated causing it to flash off collapsing the oil wedge.3/14/2014 50


THERMAL DEGRADATION:Under high temperatures an oil is liable to thermal degradation which causes discoloration and changes the viscosity. Additives cannot change an oils susceptibility to this degradation.cannot change an oils susceptibility to this degradation.

RECHARGINGWhen recharging no more than 10 % of the working charge shouldWhen recharging no more than 10 % of the working charge should be topped up due to heavy sludgeing that can occur due to the heavy precipitation of the sludge.EP ADDITIVE OILSCan assist in healing of damaged gear surfaces but should be used as a temporary measure only due to risk of side effects.as a temporary measure only due to risk of side effects.


Contamination of Lube oil in Diesel Engine:Contamination of Lube oil in Diesel Engine:WaterWater from,

1 bil '1.bilge's2.Jackets3.Sea via coolers4.leaky seals or washing in purifiers5.Condensation

Problems caused by water contamination,Problems caused by water contamination,•Water leads to corrosion especially if there is sulphur present due to fuel contaminationforms emulsions which are not capable of withstanding•forms emulsions which are not capable of withstanding

high loads•removes water soluble additives when centrifuged out•leads to possible bacterial attack


FuelMay be heavy residual or light diesel/gas oil and can be sourced toMay be heavy residual or light diesel/gas oil and can be sourced to faulty to cylinder combustion or faulty seals on fuel p/ps.Problems

Increases viscosity for hfo reduces viscosity for D O- Increases viscosity for hfo, reduces viscosity for D.O.- Reduces flashpoint- Introduces impurities such as sulphur

Dil t l b il h i l titi- Dilutes lub oil when in large quantities.

Solid impuritiesb f th li d b ti ti l l fcarbon from the cylinder combustion process, particularly of

importance with trunk piston engines but also for crosshead engines with inefficient diaphragm. The carbon can lead to restrictions and

f f Sblockages of oil ways causing bearing failure. Straight mineral oils hold 1% carbon in suspension, dispersant oils hold about 5%.


Bacterial attack:Certain bacteria will attack oil but water must be present TheCertain bacteria will attack oil but water must be present. The bacteria may exist in a dormant state in the oil but water is required if they are to reproduce.. The bacteria digest the oil causing breakdown emulsions to be formed, acidity increases, dead bacteria block filters and corrosive films form on working surfaces.on working surfaces.In summary their must be three essential conditions for microbiological growth;1 There must be a source of carbon present in the oil1. There must be a source of carbon- present in the oil2. There must be some bacteria or fungal spores present-these are almost universally present in the atmosphere.3. There must be free water present.


Two other factors which encourage the growth of bacteria: 1. A slight acidity in the water (pH 5 or 6) and2. A slightly raised temperature (20 to 40oC) which can lead to2. A slightly raised temperature (20 to 40 C) which can lead to rapid growth.Biocide additives are available but they are not always compatible with other desired additives and can lead to large organicwith other desired additives and can lead to large organic blockages if treated in the machinery. The best solution is to avoid the presence of water. If mild attack takes place the oil may be heated in the renovating tank to above 90oC for 24hrs before being returned to the sump via the centrifugal separator. For a severe attack the only solution is complete replacement of the charge y p p gfollowed by sterilization of the system. It may be noted that on replenishment the bacteria may be present in a dormant state in the new chargethe new charge.


LUBRICATION FEATURES OF A LARGE DIESEL ENGINELUBRICATION FEATURES OF A LARGE DIESEL ENGINE

• In some engines such as long and super long stroke engines, the piston is not directly connected to the crank pin via a connectingpiston is not directly connected to the crank pin via a connecting rod.

• The piston has a piston rod extending from the bottom of theThe piston has a piston rod extending from the bottom of the piston.

• The piston rod is then connected to the connecting rod at the h d b icrosshead bearing.

• The crosshead bearing has a to and fro motion and therefore a continuous hydrodynamic film cannot formcontinuous hydrodynamic film cannot form.

• Therefore oil has to be pumped to the crosshead bearing at a predetermined pressure in order to take the loads of compression

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and combustion.

• The crosshead is connected to the crank pin via a connecting rod.3/14/2014Mohd. Hanif Dewan, Senior Engg. Lecturer, International Maritime Academy, Bangladesh

Piston

Piston rod

Piston rings

Piston skirt

Pl tf ti li d Piston rod

Stuffing box

Platform separating cylinder from crank case

Crosshead, crosshead bearing (reciprocating)

Connecting rod

Oil pumped at a certain pressure

Crank pin, bottom end bearing (rotatory motion)

Journal journal bearing

57

Journal, journal bearing (rotatory motion)

Web3/14/2014


Problems caused by stuffing box leakage oil enteringProblems caused by stuffing box leakage oil entering crankcase:Low speed engines are particularly at risk from crankcase l b i t t i ti d b li d il d i t thlubricant contamination caused by cylinder oil drainage past the piston rod gland and combustion products. This can lead to severe damage of engine crankcase components and reduction of life of oil which is normally expected to last the lifetime.There has been a general increase in the viscosity and Base number of crankcase oils over recent years particularly for y p yengines built since the early 1980's. Increased alkalinity, viscosity and insolubles, fuel derived elements such as vanadium and oil additive derived elements such as calcium, suggest that theadditive derived elements such as calcium, suggest that the contamination is from the cylinder oil drainage.Deterioration of the crankcase oil has led to the expensive necessity of replacing up to 50% of the sump this is particularlynecessity of replacing up to 50% of the sump, this is particularly of concern as it is often only a temporary measure.


Four causes are put forward:Four causes are put forward:1. New crankcase oil contaminated with new cylinder oil-unlikely2. Cylinder oil drainings being recycled and returned to the sump-very likely as it is a common practice to purify oil leaking through the gland, y p p y g g g ,tests done on this purified oil found high amounts of insolubles.3. Leakage past rod gland- very likely, high pressure scavenge air can blow cylinder oil and dirt past the top scrapper ring and sealing rings y p p pp g g ginto the piston rod drain tank, and even possibly directly into the sump. A problem that worsens with age and wear.4. Leakage of exhaust valve lubrication system-unlikely

The most likely cause for contamination is leakage past the piston rod. It is seen that maintenance of the stuffing box is of the utmost importance. Tell tales and drainage lines should be proved free and use of oil drained from the uppermost drain should not be allowed even after purification due to the high level of contamination which can destroy the properties of the oil in the sump.


Cylinder liner lubricationCylinder liner lubricationThe type of Cyl. Lub oil required will depend upon the cylinder conditions and the engine design e.g crosshead or trunk piston. Ho e er the propert req irements are basicall the same b t illHowever, the property requirements are basically the same but will vary in degree depending upon the fuel and operating conditions.In some engines, lubricating oil in the cylinder is different from the oil supplied to the other bearings.The cylinder oil contains additives to withstand the high temperatures and contaminants from combustion products.The oil is slightly basic in nature to counter the acids formed from combustioncombustion.Scraper rings spread the oil over the liner surface.Lub. oil is usually injected between the two scraper rings.Oil is injected at a predetermined period during the downward stroke.O s jected at a p edete ed pe od du g t e do a d st o eBefore starting, oil is pumped into the liner by manual primingmethods.After starting, the oil pump is driven by the engine through a cam shaft

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shaft.


CYLINDER LUB OIL PROPERTIESCYLINDER LUB OIL PROPERTIES

Normal properties required are:1 adequate viscosity at working temperature so that the oil1. adequate viscosity at working temperature so that the oil

spreads over the liner surface to provide a tough film which resists the scraper action of the piston rings

2. the oil must provide an effective seal between the rings and liliner

3. only a soft deposit must be formed when the oil burns 4. alkalinity level (total base number or TBN) must match the

acidity of the oil being burntacidity of the oil being burnt 5. detergent and dispersant properties are required in order to hold

deposits in suspension and thus keep surfaces clean Behaviour depends upon the temperature of the liner, piston crown

d i t i TBN d d t l l li k d Thiand piston rings. TBN and detergency are closely linked. This can have an adverse effect when running on lighter fuels with lower sulphur content for any period of time. Coke deposits are can increase.


Cylinder liner

Compression rings

Scraper ringsOil i j i

p gOil injection passage

Piston

Injection pointsCylinder oil pump/lubricator

Handle

62

Camshaft


Trunk type engine (no piston rod)‐Splash type lubricationSplash type lubrication

Piston ringsCylinder liner Piston rings

Gudgeon pin

y

Oil is picked up by the webs while rotating, and splashed onto the piston and liner

Crank pin, bottom end bearing (rotatory motion)

Connecting rod

Journal, journal bearingJournal, journal bearing (rotatory motion)

Web extension

63

WebWeb extension

Oil3/14/2014


Piston rodCROSSHEAD LUBRICATION

Telescopic pipes (one moves inside the other)

Movement of crossheadOil supply

Crosshead bearing

Movement of bearing

Connecting rodStationary pipe

64

Connecting rody p p


Journal bearingJournal

Bearing

Oil supply

The journal bearing may undergo hydrodynamic lubricationor a combination of hydrodynamic and hydrostatic (externally pressurized) lubricationpressurized) lubrication.

The oil supply may be from any one or number of positions, depending on the design.

65

p g g


Distribution within a journal bearingIf the maximum radial clearance is Crthen Cr = e + Hmwhere e is the eccentricity between the shaft and bearing centre line and Hm is the minimumand bearing centre line and Hm is the minimum clearance (oil film thickness)an eccentricity factor can be calculated fromn = e / CrFactors involved with the eccentricity factor n are:

minimum oil film thickness,journal attitude angle- journal attitude angle,

pressure distribution, peak pressure angle, friction,friction, horsepower loss and oil flow through the loaded region.

The latter three determine the temperature of the bearing which for high speed bearings can be a limiting factor.3/14/2014 66


Oil passage between bearings in a unit

TRUNK TYPE ENGINE

Gudgeon pin

k

Connecting rod

Web

Crank pin

JournalOil passage (drilled)


Lubrication systemLubrication system• TG‐ Temperature gauge• PG‐ Pressure gauge

ENGINE ShaftTG PG

Bearings

Cooler

PG

PG

TG

Storage tank PumpFilter

PG

6868

Filter


Engine sump

ENGINE Shaft Bearing

C ti f

Storage tank/sumpPump

Connection for filling the tank

• The storage tank usually forms the bottom-most compartment of the engine.

strainer

• It is also sometimes known as the sump.

• Oil from the sump is usually transported to the bearings by an engine driven pump or an independently electric motor driven pump that transports the oil to p p p y p p pthe journal bearings.

• Through passages drilled in the crank shaft and webs, it is transported to the crank pin.

69

p

• Usually a strainer is provided on the suction side of the pump to prevent large contaminant particles from damaging the pump and bearings.


Oil cooler‐ tube and shell typeOil in

TG TGTG

PG

Water in Water out

Oil out

TGPG PG

PG

Oil out

• In this case, cooling water flows through the tubes.

• Oil flows in the shell around the tubes and passes the heat to the water• Oil flows in the shell around the tubes and passes the heat to the water.

• The in/out temperatures of the oil and water are to be monitored.

• Oil pressure is always kept above water pressure to prevent water

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contamination of oil

• However, if there is a leak oil is lost and the sump level is therefore to be monitored regularly3/14/2014


Engine lubrication systemSome adverse situations:

• TG- Temperature gauge• PG- Pressure gauge

Some adverse situations:

•Oil inlet pressure to engine LOW

•Oil outlet temperature from engine HIGH

ENGINE ShaftTG PG

•Oil outlet temperature from cooler HIGH

Cooler

BearingsPG

TG

PG

TG

7171

Storage tank PumpFilter


Properties ideal for bearings: 1 Soluble for high speed fluid film hydrodynamic1. Soluble for high speed fluid film hydrodynamic

lubrication, hence, low viscosity with reduced oil film friction.

2. moderate bearing loads 3. improved heat transfer behavior 4. corrosion protection 5. cooling 6 low friction6. low friction 7. good low temperature viscosity 8. good high temperature viscosity8. good high temperature viscosity


PROPERTIES IDEAL FOR GEAR CASE:• high film strength to prevent metal to metal contact. H hi h i i dh i i lidi dHence, high viscosity adhesive to resist sliding and centrifugal forces• corrosion protectionp• cooling• reduces friction• good low tempo viscosity• good low tempo viscosity• good high tempo viscosity The thicker the oil film the greater the cushioning against shocks. Also less tendency f it f ti b h d li ti i kfor pit formation by hydraulic action in cracks,• sound damping properties with cushioning effects• antifoam propertiesp p


Turbine oilTurbine oil

Compromise between above two requirements 1. Generally a good quality refined mineral oil derived

f ff i b t k d ith ifrom paraffanic base stock used with various additives including EP additives for highly loaded gearing.

2. Anti-foaming properties important


Date post:	18-Jan-2015
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Properties of Lubricats and Lubrication

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