7.Powder Metallurgy 14

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7. POWDER METALLURGYPowder metallurgy is the art and science of producing fine metal powders and then making objects from individual, mixed or alloyed metal powders with or without the inclusion of non-metallic constituent.

ADVANTAGES OF POWDER METALLURGY PROCESS:

LIMITATIONS of POWDER METALLURGY PROCESS:

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APPLICATIONS OF POWDER METALLURGY:

1. Automotive: (ANSWER FOR SELF LUBRICATED BEARINGS) Self-lubricated bearings are made by mixing copper (90%), tin (10%) powder and a small

amount of graphite in correct proportions, cold pressed to the desired shape and then sintered at about 800 C.

Sizing operation is done in order to get correct dimensional accuracy. In Impregnation process the bearings soak up considerable quantity of oil. Hence during service, these bearings produce a constant supply of lubricant to the surface due

to capillary action and as soon as the shaft stops rotating, the oil comes back again and settles down in the pores.

2. Refractory Materials: Metals with high melting points are termed as refractory metals. These basically include four metals tungsten, molybdenum, tantalum and niobium. Refractory

metals as well as their alloys are manufactured by powder metallurgy. The applications are not limited to lamp filaments and heating elements, they also include space

technology and the heavy metal used in radioactive shielding.

3. Cemented Carbides: These are very important products of powder metallurgy and find wide applications as cutting

tools, wire drawing dies and deep drawing dies. These consist of carbides of tungsten, tantalum, titanium and molybdenum. The actual proportions

of various carbides depend upon its applications; either cobalt or nickel is used as the bonding agent while sintering.

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4. Aerospace application: Powder metallurgy pars are used in rockets, missiles, satellite etc. Sintered bronze bearing and other bushing are used in explorer satellite, magnetic materials such

as Alnico.

5. Defense Applications: Powder metallurgy parts and powders are used in rockets, missiles, cartridge cases, gun powder,

fragile bullets etc.

6. Other Applications: (WRITE THIS IF ASKED ONLY FOR 2 MARKS) Typewriters, calculators, permanent magnets etc. Porous nickel electrodes are used in Ni-Cd batteries.

POWDER METALLURGY PROCESSESA. Producing of metal powders B. Mixing / BlendingC. Compacting D. SinteringE. Secondary Operations.F. Inspection/Testing.

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A. PRODUCTION OF METAL POWDERS: Various methods for manufacturing powders are1. Mechanical Processes2. Physical Processes3. Chemical Processes

MECHANICAL PROCESSES: (ANSWER FOR POWDER MAKING PROCESSES)

1. Machining. In this method first chips are produced by filing, turning etc. and subsequently pulverized by

crushing and milling. The powders produced by this method are coarse in size and irregular in shape. Hence, this method is used for special cases such as production of magnesium powder.

2. Crushing.

Process of passing the metal powders against two rollers so that the metal powders are crushed to required size.

Crushing requires equipment such as stamp, hammers, jaw crushers.

3. Milling. Milling is carried out by using

equipment such as ball mill, rod mill, impact mill, disk mill etc.

In ball milling, material to be powdered is collected in a container with a large number of hard steel balls. These balls hit the material and break it in powder form

4. Shotting The process of pouring molten metal through a sieve or orifice and cooling by dropping into water

is known as shotting. This process gives spherical or pear shaped powder particles.

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5. Atomization.

The process of metal spraying against a stream of compressed air or inert gas is Atomization.

It is an excellent means of producing metal powders from many of the low temperature metals such as lead, aluminum, zinc and tin.

PHYSICAL PROCESSES:1. Condensation:

In this method, metals are boiled to produce metal vapours and then condensed to obtain metal powders.

This process is applied to volatile metals such as zinc, magnesium and cadmium.

2. Thermal Decomposition: Fine metal powders of Ni, Mo, Mg, Co etc are manufactured by this method. This method is highly suitable for manufacturing of Fe and Ni powders.

CHEMICAL PROCESSES:1. Reduction

Pure metal is obtained by reducing its oxide with a suitable reducing gas at an elevated temperature (below the melting point) in a controlled furnace.

The reduced product is then crushed and milled to a powder. Sponge iron powder is produced this way

Fe3O4 + 4C = 3Fe + 4COFe3O4 + 4CO = 3Fe + 4CO2

Copper powder byCu2O + H2 = 2Cu + H2O

Tungsten, Molybdenum, Ni and Cobalt are made by this method.

2. Electrolysis. This method is commonly used for producing iron and copper powders. This process

is similar to electroplating. For making copper powder, copper plates are placed as anodes in the tank of

electrolyte, whereas the aluminum plates are placed into electrolyte to act as anode.

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When D. C. current is passed through the electrolyte, the copper gets deposited on cathode. The cathode plates are taken out from electrolyte tank and the deposited powder is scrapped off. The powder is washed, dried and pulverized to produce powder of the desired grain size. The powder is further subjected to heat treatment to remove work hardness effect.

The cost of manufacturing is high.

B. MIXING / BLENDING of metal powders Blending: mixing powder of the same chemical composition but

different sizes Mixing: combining powders of different chemistries. The above figure shows the container in which the powder to be mixed

is added The blade continuously rotates and mixes the powder. Figure (b) shows a rotating drum in which powder is filled and the drum

keeps on rotating and simultaneously mixing the powders. Wet or dry mixing is generally employed. For wet mixing water or dry solvent is used to obtain better mixing. Blending and mixing is necessary for

✔Addition of lubricants coats the powders and reduces die wear and lowers pressure required for pressing of powders. ✔Mixing powders of different materials ✔Obtaining uniform distribution of particle sizes.

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C. COMPACTING / PRESSING of metal powders. Compacting: Pressing the powders into desired part shape as closely as possible to final

dimensions. Powders are compacted using high pressure. The final product is known as “Green Compact” Degree of pressure required depends upon:

Required density of final product Ease with which powder particles will weld together.

Compacting processes are:1. Die pressing 2. Roll pressing 3. Extrusion.

1. Die pressing.

It consists of upper punch and lower punch as shown in figure. Powder is filled on the space above lower punch. Upper punch applies high pressure to the powder. Finally the green compact powder is received as shown in figure.

2. Roll pressing.

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Two rolls of appropriate sizes are used.

Stream of powder is guided, so that the rolls are able to apply the necessary compacting pressure in a continuous sequence.

3. Extrusion method.

Ram is used for applying force. Two dies are used for

achieving proper thickness. As seen in the figure the green

compact is extracted which is further sent for sintering process.

D. SINTERING

Sintering is the Heat treatment process, to bond the metallic particles, thereby increasing strength and hardness.

Sintering consists of heating pressed metal compacts in batch or continuous furnaces to a temperature below the melting point of material.

It consists of three steps: Surface Diffusion, Densification, Grain Growth Most metals are sintered at 70% to 80% of melting temperature. Effect of sintering on the properties of material:

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1. Density – Increases2. Strength & Hardness – Increases3. Porosity – Decreases

Types:A) Solid Phase Sintering : Here, the compact is heated above the recrystallization temp of low

melting metal powder.B) Liquid Phase Sintering : Here, the green compact is heated above the melting point of one of the

alloying element

C/D. HOT COMPACTING: Hot pressing involves applying pressure and temperature simultaneously, so that the compacting

and sintering of the powder takes place at the same time in a die. Its application is limited and can be used for compacting. Fe and Brass powders at much lower pressure than conventional pressing and sintering operations.

E. SECONDARY /FINISHING operations: These are secondary operations intended to provide dimensional tolerances, physical and bettersurface finish. A number of secondary and finishing operations can be applied after sintering, some of them are: 1. Sizing 5. Infiltration2. Coining 6. Heat treatment3. Machining 7. Plating4. Impregnation

1. Sizing: It is repressing the sintered component in the die to achieve the required accuracy.

2. Coining: It is repressing the sintered components in the die to increase density, achieve the required accuracy and to give additional strength.

3. Machining: Machining operation is carried out on sintered part to provide under cuts, holes, threads etc. which cannot be removed on the part in the powder metallurgy process.4. Impregnation:

It is filling of oil, grease or other lubricants in a sintered component such as bearing.

Sintered parts may be impregnated with oil, grease, wax or other lubricating materials is self-lubricating properties are required.

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Parts are immersed in lubricants and are heated upto approx. 93 C. The porous structure gets completely impregnated in 10-20 mins. The lubricant is retained in the part by capillary action.

It is done in order to:a. Improve corrosion resistanceb. Seal prior to platingc. Improve machinabilityd. Introduce pressure tightnesse. Reduce the friction between parts.

5. Infiltration: It is filling of pores of sintered product with molten metal to improve physical properties. Once the part is sintered, it has about 77% theoretical density. Then an infiltration plank of Copper or Brass is placed over the part which is sent through the

furnace. The infiltration melts and soaks through the porous part, producing high density product.

It provides high strength, hardness and density which straight sintering cannot.

6. Heat Treatment: The processes of heating and cooling sintered parts are to improve Wear Resistance Grain Structure Strength

7. Plating: Plating is carried out in order to: Import a pleasing appearance (Cr plating) Protect from corrosion (Ni plating) Improve electrical conductivity (Cu and Ag plating)

F. INSPECTION / TESTING: Before the component is put to the service, it should be tested for any inaccuracy or defect. The various tests to be conducted are comp. & tensile strength, porosity, density, hardness. Non-Destructive Testing methods can also be used to find defects. Once the component satisfies the properties, it is ready for use.

CHARACTERISTICS OF METAL POWDERS / POWDER TESTING: Powders can be defined as a finely divided particulate solid. Engineering powders include metals and ceramics. The main purpose of powder testing is to ensure whether or not the powder is

suitable for further processing.Principle characteristics of metal powders are:1. Chemical composition. (Purity)

It is the term to understand the type and percentage of impurities that the powder contains.

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2. Particle size and its distribution. The particle size is defined by the mesh count Mesh count refers to the number of openings per linear inch of mesh.

3. Particle shape. Various shapes of metal powders observed according to the method of production.

4. Particle porosity :- Ratio of the volume of the pores (empty spaces) in the powder to the bulk volume

(filled space). Porosity increases stress in the part so it is not desirable. Difference between destructive and non destructive testing

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6. NON METALLIC MATERIALS1. Polymer:

The word polymer is derived from two Greek words ‘Poly’ and ‘Meros’. Poly means multiple and meros means parts. Polymers are organic materials having carbon as common element linked with other elements. The process of linking the monomers together to form a polymer is called as polymerisation.

2. Plastics:

Characteristics/Properties Uses/Applications

1. These are having low density. 1. Water pot, mugs, Buckets.

2. Corrosion resistance is high. 2. Drums, Container for storage.

3. These are having good mould ability.

3. Bottles, containers for storing food stuffs or kitchen items.

4. They can be used as transparent or with colours.

4. Decorative articles like show piece, artificial flowers.

5. They have excellent surface finish.5. Air-tight boxes.

6. They are economical. 6. Insulators, electric shock proof grips and heat proof grips.

7. They are having poor strength. 7. Tables, chairs, cupboards.8. They are having poor temp.

resistance.9. They are bad conductors of heat

and electricity.

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3. Types of Plastics:

Thermoplastics:

Thermoplastics are those which soften on heating, can be heated any number of time and then subsequently cooled for obtaining desired shape.

3 a. Types of Thermoplastics:

Names Characteristics/Properties Uses/Applications

1. Acrylonitrile butadiene styrene

1. High tensile strength.2. High wear and tear, abrasion

resistance.3. High impact load resistance.

1. Resistance to acids and alkalies.

1. Used in automobile industry for panel work.

2. Used for television and camera back bodies.

3. ABS with Chromium plating can be used in

Radiator grills.

2. Acrylics-Polymethyl Metha crylate (PMMA)

1. High insulation properties.2. High index of refraction.3. Light in weight, tough but not

scratch-proof.

1. Sanitary ware.2. Contact lenses.3. Sinks, Wash Basins, other

bathroom fittings.4. Hospital Equipment for

patient’s hygiene and health care.

3. Nylons (Polyamides)

1. Along with being tough and strong, these are flexible too.

2. High impact resistance.3. Poor resistance to hot water,

1. Nylon ropes have wide domestic and commercial uses.

2. Tooth-brush and other

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alcohol and ultra violet rays. washing brush bristles.

4. Poly Vinyl chloride (PVC)

1. Good dimensional stability.2. Water resistant.3. It is self-extinguishing when ignited.

Becomes soft above 80oC.

1. Water pipes, chemical tubing.

2. Cable jackets.3. Lead wire insulation.

Thermosetting Plastics:Thermosetting plastics are those which become soft during their first heating and

become permanently hard when cooled thus cannot be recycled.

3 b. Types of Thermosetting Plastics:

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1. Epoxides1. These are light and transparent in

colour.2. Excellent chemical resistant.3. Good electrical insulating

properties.4. Extremely high bonding strength.

1. In automobile panels and light covers.

2. Insulating materials in cables, transformers.

3. Used as adhesive for joining and repair work.

2. Melamine Formalde

h-yde (M.F.)

1. They have high resistance to chemicals, heat and moisture.

2. Better electrical resistance3. Lightly affected by sunlight.

1. Used in paints, surface coating.2. Used as glues, adhesive for

joining.3. Plugs, switches, buttons are

made from M.F.

3. Bakelite

1. It is Black and Brown in colour.2. Strong, rigid and dimensionally

stable.3. It gets discoloured on ageing by

sunlight.

1. Used in non-conducting parts of radios and bases & sockets for light bulbs & vacuum tubes.

2. Automobile distributor caps and other insulators, brake pads and related automotive components.

3. Kitchenware such as canisters and tableware.

Rubbers:Rubbers are highly elastic and resilient polymeric materials, they are organic

polymers those are capable of being subjected to large elastic strains without plastic yielding natural and synthetic rubbers are called as elastomers.

4. Types of Rubbers:

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1. Neoprene 1. It is resistant to oils, chemicals, sunlight, ozone, etc.

2. It retains properties up to 120oC.3. Excellent resistance to permeability

by gases.4. It is having mechanical properties

similar to those of natural rubber.

1. Heavy duty conveys or belts.

2. V-belts, horse covers.3. Foot wears, shoe soles.4. Brake diaphragms , gaskets.

2. Styrene Butadiene Rubber (Buna S)

1. Low oil and chemical resistance.2. High impact and abrasion resistance.3. Low tensile strength and resilience

than natural rubber.

1. Vehicle tyres are made of Buna S.

3. Nitrile Butadiene Rubber (Buna N)

1. Good resistance to oil, fuel, other chemicals.

2. Low in mechanical properties.3. Low in tensile strength.4. Good resistance to weathering.

1. Used in flexible couplings, printing blankets, rubber rollers.

2. Washing machine parts .

5. SILICONES:There are unique polymers which are Silicon based instead of carbon based. They have -

SI-O-SI- bonding, are widely used is in organic polymer.Properties:

1. Excellent resistance to high & low temperature.2. Excellent resistance to oils & chemicals.3. Poor tensile strength.

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4. Excellent electrical properties.5. Poor physical properties.

6. OTHER ENGINEERING MATERIALS:


1. CERAMICS

1. They are very hard, strong and dense.2. They have a very high resistance to

the action of chemicals & weathering.3. They have a very high resistance to

high temperature creep.4. They have a very high strength, both

in tension as well as compression.

1. Glassware, Windows, Pottery, Dinner ware.

2. Catalytic converters, Ceramic filters, Air bag sensors, Spark plug.

3. Orthopaedic joint replacement, Dental restoration, Bone implants.

4. Refractory bricks, Lab equipment’s, cutting tools.

2. GLASSES

1. No definite crystalline structure. 2. It absorbs, refracts or transmits light.3. It can be made available in beautiful

colours.4. It is capable of being worked in

several ways.

1. In electric lamps & electrons tubes.

2. In windows glass, doors and other decorative articles.

3. Laboratory equipment’s and apparatus resistant to acids and other chemicals.

3. GLASS - WOOL

1. It usually consists of soda lime glass fibres.

2. It provides excellent insulator against heat and cold.

3. It has a very high tensile as well as well as compressive strength.

4. It is having a uniform as well as

1. It is widely used in exterior walls and in ceilings of houses.

2. Extremely low density wool is used as thermal as well as sound insulator in aeroplanes.

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definite formulation not there in mineral wool.

3. Used as insulators in refrigerators and freezing.

4. LAMINATED COMPOSITE

1. They are having two or more layers of different materials bonded physically to each other.

2. High corrosion resistance, electrical and magnetic properties can be obtained.

3. High stiffness and high strength.

1. Cladded aluminium alloy is used for various automotive applications.

2. Cladded stainless steel with mild steel with mild steel is used for heat exchangers.

5.REINFORCED COMPOSITES

1. Fiber reinforced plastic have excellent strength toughness and corrosion resistance.

2. Metal matrix fiber composites (MMFC) have excellent resistance to high temperature creep and impact strength.

3. Reinforced cement concrete (RCC) have high tensile strength of steel as well as high compressive strength of concrete.

1. Water as well as chemical storage tanks, luggage bags, helmets, fans blades of wind turbines.

2. Marine and wind aerospace components.

3. Used in columns and beams of various structures, such as bridges and buildings.

6. NANO MATERIALS

1. Particle size ranges from 0.1mm to 100mm which are extremely small.

2. At low temperatures hardness and strength of Nano material increases with reduction in particle size.

3. With decrease in particle size even non-magnetic properties.

1. Cutting tools like tungsten carbide are Nano-technology made; micro drills of hair size can also be made.

2. Zirconia is a ceramic based material Nano made used

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4. The use of Nano-materials reduces the size of a component by more than 50%.

for springs, ball bearings and other automotive applications.

3. Nano crystals like zinc sulphide are used for flat panel electronic display in T.V and computer screens.

4. Nano materials are also used for high power magnets used in MRI and other diagnostic purpose.

Applications of Polyester: (2 MARKS)Paints, Binders for glass fibers, Safety helmets, Fiber glass boats, Joining and repair works, Cladding panels, Automobile body components, Fans, Chairs.Applications of Asbestos: (2 MARKS)Fire proof textiles, Fire proof theatre curtain, Boiler insulation, Brake shoes, Pipe insulation, gaskets, Protective pads.Classification of polymer materials: (2 MARKS)1. ACCORDING TO ORIGINa. Natural Polymer b. Synthetic or artificial Polymer2. ACCORDING THERMAL BEHAVIOURa. Thermoplastic polymer b. Thermosetting plastic polymer3. ACCORDING TO STRUCTUREa. Link polymer. b. Branched polymer c. Crossed link polymer4. ACCORDING TO FORMa. Plastic b. Elastomers c. Fiber and resins

Properties of composite materials: (EACH POINT ½ MARK)

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NON FERROUS METALSCOPPER

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PROPERTIES APPLICATION

Relatively soft gears

Malleability Automobiles, utensils

Ductile Wires

High toughness Gears

Electric conductivity Wires

Thermal conductivity Utensils

Corrosion resistance Piping

Machinibility Bolts and nuts

COPPER/ZINC/TIN ALLOYS

NAME COMPOSION PROPERTIES APPLICATION

BRASS Zinc-50%

Copper-50%

1. Ductile2. High tensile

strength3. Non corrosive 4. Power

electricity conductor

1 Pipes2 Bearings3 Bushes

NAVEL METAL COPPER-60%

ZINC-39%

TIN- 1%

1 Corrosion resistance

2 High tensile strength

3 Good hardness

1 Navel construction

2 Piston rods3 Bushes4 Ornament

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MUNTZ METAL COPPER-60%

ZINC-40%

1 REDDISH IN COLOUR

2 HIGH TENSILE STRENGTH

3 HOT WORKBALITY

4 CORROSION RESISTANCE

1 Utensils2 Bushes ,bolts3 Marine

BRONZE 1 ZINC2 COPPER3 TIN

1 Corrosion resistance

2 Anti friction 3 High hardness4 malleable5 ductile

1 coins 2 ship building3 bushes 4 clutch disk

PHOSPHORUS BRONZE

COPPER-97.7%

PHOSPHOURUS-0.3%

TIN-0.6%

1 high tensile strength

2 corrosion resistance

3 high hardness4 can be forged,

ductile&melleable

1 disk of clutch2 marine,

worm& worm wheel

3 pipes4 foundry

SILICON BRONZE COPPER-97%

SILICON-3%

MAGNESIUM-1%


2 corrosion resistance

3 high hardness4 can be forged,

ductile&malleable

5 good casting6 conductivity

1 marine pipes2 bolts, nuts3 foundry4 tanks

ALUMINIUM COPPER-88% 1 high tensile 1 marine

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BRONZE

Al-8%

Fe-3%

Tin-0.5%

strength2 high hardness3 corrosion

resistance4 difficult to cast5 magnetic

2 bolts3 winding

GUN METAL COPPER-88%

TIN-10%

ZINC-2%


2 very strong3 resistance to

corrosion4 castable5 machiniable6 can be forged

1 bushes2 nuts3 marine4 piping5 gears

ALLUMINIUM

PROPERTIES APPLICATION

HIGH HEAT CONDUCTIVITY CABLES

GOOD CONDUCTOR OF ELECTRICITY AIRCRAFTS

RESISTANCE TO CORROSION UTENSILS

DUCTILE AND HIGH TENSILE STRENGTH CONTAINERS

LIGHT WEIGHT Aircraft

EASILY MACHINED

NON TOXIC Vessel

ALLOY COMPOSITION PROPERTIES APPLICATION

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DURALUMIN Al- 94%

Cu-4%

Mg-0.5%

Mn-0.6%

Silicon-0.4%

1. Light weigh 2. High tensile

strength3. High electric

conductor4. Good

machining5. Shock

resistance

1 cable 2 automobile 3 rivets4 aircraft

Y-ALLOY Cu-4%

Mg-1.5%

Ni-2%

Al-92.5%

1. high tensile strength

2. shock resistance

3. high heat conductor

4. easily machined

1 engine parts2 Piston and

cylinder heads.

HINDALIUM Aluminium

Magnesium

Manganese

Chromium

Silicon

1 easily machined2 non toxic3 high tensile

strength 4 corrosion

resistance

1 container2 automobile

BEARING MATERIALS

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Materials used to manufacture bearings are called as bearing materials.

BEARING MATERIALS:

1 COPPER RED ALLOY2 SILVER BEARING3 ALLUMINIUM ALLOY.

PROPERTIES OF BEARING MATERIALS:

1 High tensile strength2 Good corrosion resistance 3 Good compression resistance4 Hard and wear resistance5 Low cost and easily available6 High hardness7 Low coefficient of friction8 Shock resistance and ductile

POROUS SELF LUBRICATING BEARING:

1 PRODUCED BY POWEDER METULLERGY2 IT CONSIST 40-50% POROSITY3 PORES ARE SATURATED WITH OIL AN ON APPLYING PRESSURE OIL COMES OUT OF PORES .THUS NO

EXTERNAL LUBRICATION IS NOT REQUIRED.

7.Powder Metallurgy 14

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