Lubrication

Introduction

Lubricants are a special category of materials, usually in either liquid or semi-solid state, applied between two or more moving parts in order to reduce the friction between them as well as dissipate the heat produced in them.

General Description of Lubricants

Lubricants are solid, liquid or semi-solid materials applied between moving parts to make their movement friction free. Right from the day of their discovery, a wide variety of lubricants have been used, depending upon the requirements of the application. The most commonly used lubricants are grease, oils, graphite etc. These materials, when applied over mechanical parts, allow easy movement of another component over it. This helps to prevent the damage caused by abrasion and friction as well as uneven expansion of the components due to heat.

Methods of Lubrication

The lubricants form a thin-film or thick-film over the surface of the components between which it is applied. When the parts move, the thick film aids in easy movement by providing a smooth surface. Also, they absorb friction and heat that is generated between moving parts. The difference between thick and thin films is that thick films completely separate the two moving layers while thin-films maintain some contact between the moving parts. Thin-film lubrication is used when the loads are heavy whereas a thick-film is used for high-speed machines, especially reciprocating and oscillating types.

LUBRICATING OILS

• Oil lubricants are liquid lubricants which are mainly used in automobiles and industrial machines which require a continuous and rather controlled supply of lubricants to the system. Some of them are:

• Animal and Vegetable Oils

• Mineral Oils

• Blended Oils

ANIMAL AND VEGETABLE OIL

• Animal oils are extracted from crude fat by Rendering process in which the enclosing tissue is broken by treatment with steam.

• Lard oil and Neats foot oil are the example of Animal oil.• Vegetable oils such as cotton seed oil and castor oil are

obtained by crushing the seeds.• Castor Oil and Olive Oil are the examples of vegetable oil.

Vegetable Lubricants

•Examples of vegetable lubricants are:–Castor oil–Olive oil–Cottonseed oil

•Animal and vegetable oils have a lower coefficient of friction than most mineral oils but they rapidly wear away steel

MINERAL OR PETROLEUM OILS

• These are low molecular weight hydrocarbons with about 12-50 carbon atoms. Mineral oil are obtained by distillation of petroleum. These are widely used as they are cheap, available in abundance. Only drawback is that oiliness of these oils is less. However addition of oelic acid and stearic acid increases the oiliness of mineral oil.

PURIFICATION OF MINERAL OIL

Dewaxing: It means removal of wax presence of which increase the pour point so lubricant can not be used at low temperature. For dewaxing, oil is mixed with propane and then refrigerated for the precipitation of wax. Wax is then removed from oil by passing the oil wax suspension through a centrifugal working at 1700 rpm. To recover the solution distillation is done.

Acid Refining: Impurities like asphaltic and napthenic acid is removed by acid refining in which dewaxed oil is treated with conc. H2SO4. Impurities either get dissolved in acid or converted into tarry sludges. Sludge is removed by filtration.

Solvent Refining: It involves mixing of oil with nitrobenzene or some other solvent(like dichloro ethyl ether or mixture of propane and cresol or sulphur dioxide and benzene). Solvent is such in which oil immiscible but impurities are highly soluble. After proper mixing with such solvent, the oil is left undisturbed for some time so that liquid separates into two layers. ‘Solvent Layer’ containing impurities and ‘Oil Layer’ free from impurities. By distillation of solvent layer impurities can be separate out to get pure solvent.

Synthetic Lubricants

•Because of the high operating temperatures of gas-turbine engines, it became necessary to develop lubricants which would retain their characteristics at temperatures that cause petroleum lubricants to evaporate and break down•Synthetic lubricants do not break down easily and do not produce coke or other deposits

BLENDED OILS

• Characterstics(like decrease the pour point or increase the anti oxidation ability or change in viscosity index) of lubricating oil can be improved by adding some additives, the oil obtained after the addition of additives is called blended oil.

• An additive is a material that either impart a new desired property to lubricant oil or enhance the degree of property already possess by oil.

TYPES OF ADDITIVE

• Chemically Active Additives: These are those which chemically interact with metals(to form protective film) and with polar oxidation and degradation products. Dispersants, Detergents Anti-Wear agents,oxidation inhibitors, Rust and Corrosion inhibitors are the examples of chemically active additives.

• Chemically Inert Additives: These are those which improve the physical properties. These include viscosity index improvers, foam inhibitors, Pour point depressents, demulsifiers and emulsifiers etc.

2. Classification of lubricants according to their physical state

•Solid

•Semi-solid

•Liquid

Solid Lubricants

Solid lubricants e.g., graphite, are in the form of powders and can be applied on machine surfaces where they orient themselves parallel to the surface. When the machine parts move, the graphite particles or lamellas shave over each other, reducing the friction over the machine parts. In graphite lubricant sprays, a thin film of the lubricant is sprayed using the sprayer over the machine surface. After some time, the sprayed lubricant forms a thin coating or film over the surface, which protects the moving parts from friction and heat.

Semi-solid Lubricants

Greases are commonly used semi-solid lubricants. They are used when liquid lubricants such as lubricating oils cannot be supplied continuously during the operation of the moving machine parts. Greases of various types are available with varying operating temperatures, moisture resistance, and stability over oxidation, etc. Grease actually consists of a base lubricant such as oil, a thickening agent whose job is to keep the lubricant in semi-solid state and additives to improve the performance of the lubricant. They are used in applications requiring low viscosity and are easier to store and handle. Grease is ideal for open gears, chain and cable mechanisms, high temperature and extreme pressure applications.

Liquid Lubricants

Oil lubricants are liquid lubricants which are mainly used in automobiles and industrial machines which require a continuous and rather controlled supply of lubricants to the system. Some of them are:

•Hydraulic Oils

•Way Oils

•Spindle Oils

Lubricating Oil Properties

•Flash Point•Viscosity•Cloud Point•Pour Point•Carbon-Residue Test•Ash Test

•Precipitation Number•Corrosion and Neutralization Number•Oiliness•Extreme-Pressure (Hypoid) Lubricants•Chemical and Physical Stability

Flash Point

The flash point of an oil is the temperature to which the oil must be heated in order to give off enough vapor to form a combustible mixture above the surface that will momentarily flash or burn when the vapor is brought into contact with a very small flame

Viscosity

•Viscosity is technically defined as the fluid friction of an oil•To put it more simply, it is the resistance an oil offers to flowing•Heavy-bodied oil is high in viscosity and pours or flows slowly

Cloud PointThe cloud point is the temperature at which the separation of wax becomes visible in certain oils under prescribed testing conditionsWhen such oils are tested, the cloud point is slightly above the solidification point

Pour Point

•The pour point of an oil is the temperature at which the oil will just flow without disturbance when chilled

ANILINE POINT

• It is the minimum equilibrium solution temperature for equal volume of aniline and oil sample.

• Determined by mixing equal volume of aniline and oil sample in test tube. Heat it to get homogeneous solution. Then allowed to cool it and note the temp. at which oil and aniline phases separate out that temp. is called as Aniline point.

• Lower aniline point means higher percentage of aromatic hydrocarbons which means more are the chances of deterioration of oil when it comes in contact with rubber sealing, packing etc.

ASH TEST

The ash test is an extension of the carbon-residue testIf an unused oil leaves almost no ash, it is regarded as pureThe ash content is a percentage (by weight) of the residue after all carbon and all carbonaceous matter have been evaporated and burned

Carbon-Residue Test

•The purpose of the carbon-residue test is to study the carbon-forming properties of a lubricating oil•There are two methods:–The Ramsbottom carbon-residue test–The Conradson test

Precipitation Number

•The precipitation number recommended by the ASTM is the number of milliliters of precipitate formed when 10 mL of lubricating oil is mixed with 90 mL of petroleum naphtha under specific conditions and then centrifuged.

Applications of Lubricants

• Lubricate parts like gears, chains, wheels, bearings etc.

• Lubricate the piston movement in engine cylinders.

• Lubricate the vanes of turbines and blowers.

• Cool engines and pumps by dissipating heat effectively.

• Lubricate and cool compressors.

• Lubricate spring systems and rollers.

Lubrication

Of

Roller Bearings

Tasks of Roller Bearings

� reducing friction

� transfer of loads

� guiding rotating parts

Friction in Roller Bearings

� small portion of rolling friction

� but considerable share of sliding friction at:

roller bearing ringscages and / orrolling elements against each other

Kinds of Lubricationof Roller Bearings

9,5% by oils 0,5% by solid lubricants only

90% by greases

The most important Parameters for selecting the suitable Lubricant

revolution or DN - factor

D = external bearing diameter [mm]d = internal bearing diameter [mm]n = revolution per minute [rpm]dm= medium bearing diameter [mm] = ———

D + d2

DN = ——— • nD + d2

temperature range

Lubrication by greases

Grease pump

Contamination maycause bearing failures.Seals protect to a largeextent from outside contaminations. Frequent relubrication is required when

the hazard of grease contamination exists.Same applies if the grease should seal alsoagainst penetrating moisture.

Contamination penetrating throughlabyrinth channels.

Newly supplied grease pushes out bothused up and contaminated grease through the labyrinth packing seals.

Relubrication Intervals for Grease lubricated Roller Bearings

Small bearings with lateral seals or cover plates do not need any relubrication. They are already for - life lubricated.

For all other roller bearings relubrication intervals become valid.

Guidelines forRelubrication Intervalls

10 000

6 000

4 000

3 000

2 000

1 500

1 000

500

300

200

150

100

50

20 000

15 000

10 000

7 500

5 000

2 500

1 500

1 000

750

500

250

20 000

15 000

10 000

5 000

3 000

2 000

1 500

1 000

500

100 200 500 1 000 2 000 5 000 10 000n [U/min]

tfa

tfb

tfcLubrication interval,given by service hours

tfa = radial ball bearingtfb = cylindrical roller bearing / needle bearingtfc = self aligning roller bearing / taper rolling bearing / axial roller bearing massive cylindrical roller bearing (0,2 tfc), axial cylindrical roller bearing (0,5 tfc)

500 420 360 280 240 200 160 120 100 80 6040

20

d=10

Example: A deep-groove ball bearing with an internal diameter d= 100mm running at a speed of n = 1000 rpm. The operating temperature lies between 60 and 70 °C.Which is the expected relubrication interval?

The vertical in point n= 1000 rpm on the abscissacuts the curve for d= 100mm. From there the horizontal leads to the outer left-hand scale (deep- groove ball bearings)indicating a relubrication interval of 10000 service hours

Correction of Relubrication Intervals

The assessed intervals are valid only for:

� bearings in stationary machines

� normal load

� ageing-proofed greases

� temperature < + 70°C at the outer ring

Dependance of Relubrication Intervals on Tempearture

100

85

70

55

40

Lubrication interval°C

Temperature at the outer ring

� above 70°C the factor 0,5 appliesfor each temperature rise of 15°C

i.e. with a temperature rise from e.g.70°C up to 85°C the service life ofthe grease is cut by half.

� below 70°C the factor 2 appliesfor each temperature drop of 15°C

i.e. with a temperature drop from e.g.70°C down to 55 °C the service life of the grease is doubled.

Lubrication by Oil

is used at high speeds resp. at DN-factors exceeding1.000.000 and in case heat conduction is necessary.

Oil bath lubrication

Suitable only for bearings with low speeds.During stand-stills the oil level should notquite reach the middle of the rolling elementat the bottom. .

Oil injection lubrication

At high speed it becomes necessary, that the air turbulence circulating withthe bearing is penetrated by the oil.This is ensured by injecting oil underhigh pressure (min.15 m/s) from the sideinto the bearing.

Circulatory oil lubrication

With increasing speed the service temperaturerises and accelerates the ageing of thelubricating oil.By circulatory lubrication the oil can be cooled and consequentlythe service temperature be reduced.

Cooling

pump

Lubrication with Solid Lubricants

Deep groove ball bearings with increased clearance required at:

� high temperatures > + 200 °C

� high temperature variations

� low speed

Lubrication with Solid Lubricants - Exsample

Lubrication by Solid Lubricants only

at temperatures > + 200 °C

OKS 230 MoS2-High Temperature Paste(for thin-film lubrication during assembly and maintenance)

in connection with

OKS 310 MoS2-High-Temperature Lubricant(for relubrication during operation)

The carrying fluid evaporates residue-free above 200°C and dry MoS2 films remain, which assure lubrication up to 450°C.

Kiln-car bearings in the ceramics and brick industry

Bibliography

www.google.com

Wikipedia.org

Sciencemania.com

Thanks…