Lubrication of synovial joints

• The extremely low value of coefficient of friction found in bone joints (~0.01) is due to the combined effect of the cartilage tissue as well as the synovial fluid present.

• Cartilage tissue at the ends of bones are soft, slippery and deformable.

• Synovial fluid contains certain molecules that cause the low COF.

http://www.shoppingtrolley.net/images/anatomy/synovial-joint.jpg

Lamina splendens

Collagen fibers

Proteoglycan aggregate

Chondrocyte cells

Calcified cartilage

Subchondral bone

Superficial tangential zone (STZ)

Middle zone

Deep zone

Zone of calcified cartilage

• Cartilage is made up of a porous material.• It contains 70% by volume water.• It is therefore considered to be a bi-phasic material.• Within the cartilage there are hydrophilic molecules that hold

the water within.

Lubrication regimes in a bone joint

• The bone joint undergoes a combination of lubrication regimes.

• Due to the deformable nature of cartilage, there is elastohydrodynamic lubrication (EHL).

• Under standing conditions there is normal squeeze film lubrication.

• When there is heavy load, the cartilage undergoes weeping lubrication.

• In weeping lubrication, the fluid that is present within the cartilage is squeezed out into the joint space to provide lubrication.

Synovial fluid- characteristics• The lubricant in a bone joint is called synovial fluid.

• Syn means like and ovial means egg white.

• The lubricant is therefore similar to egg white.

• It is non-newtonian in nature.

• This non-newtonian behavior is provided by a molecule known as Hyaluronic acid containing a chain of the COO- group approximately 1.5 nm apart.

• In a healthy joint, the molecular weight of the hyaluronic acid is approximately 5 x 105 and its concentration is 0.1-5 mg/ml.

• The hyaluronic acid of diseased joints gets depolymerized and the fluid does not have non-newtonian behavior.

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Production of basic grades of lubricating oils

6Ref: http://www.energyinst.org.uk/education/coryton/images/column.gif

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Atmospheric distillation

• In an oil refinery, the principal process for converting crude oil into a range of marketable products is distillation

• After removal of gaseous products, the crude is distilled in a column at atmospheric pressure

• All materials boiling at temperatures below 340 oC (approximately) are vaporized in stages depending on their boiling temperature

• These materials are then condensed in order of increasing boiling range, at successively lower points in the column, and are drawn off

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Atmospheric distillation (contd.)

• The distillates obtained are the raw materials for production of gas oils, kerosines and motor oils.

• The residue from the bottom of this column contains materials boiling above about 340 oC, and is the raw material for manufacture of lubricating oils, fuel oil, waxes and bitumen.

• This residue is generally known as ‘atmospheric residue’.

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Vacuum distillation• The temperature to which the residue from the atmospheric

column can be raised is limited by its tendency to decompose or ‘crack’.

• Thus, further separation is effected in a vacuum distillation column where boiling is achieved at moderate temperatures by operating under reduced pressures.

• The products from the vacuum distillation column comprise a series of three or four distillate fractions separated according to boiling range, and a vacuum residue.

• The distillate fractions provide a number of materials with different viscosities, and an oil with a higher viscosity than the distillates is produced from the residue.

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Vacuum distillation (contd.)

• Some heavy gas oil is also produced in this process and is removed from the top of the column.

• The distillate and residual fractions from the vacuum column are not, at this stage, suitable for blending into finished lubricating oils, but require to be treated to remove undesirable constituents.

• The treatments applied vary according to the properties of the crude.

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Bitumen• Bitumen is a mixture of organic liquids that are highly viscous, black,

sticky, entirely soluble in carbon disulfide, and composed primarily of highly condensed polycyclic aromatic hydrocarbons.

• Naturally occurring or crude bitumen is a sticky, tar-like form of petroleum which is so thick and heavy that it must be heated or diluted before it will flow.

• At room temperature, it is much like cold molasses. Refined bitumen is the residual (bottom) fraction obtained by fractional distillation of crude oil.

• It is the heaviest fraction and the one with the highest boiling point, boiling at 525 °C (977 °F).

• Bitumen is primarily used for paving roads.

• It is also used for water proofing products, blending with other oils, and preservation of fossils.

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Solvent de-asphalting

• The residue from the vacuum distillation column can be used as a blend component for fuel oils, or for bitumen manufacture.

• It can also be processed to yield a high viscosity lubricating oil.

• For lubricant production, the vacuum residue is treated with liquid propane, which dissolves the oily and waxy components and leaves the asphaltic residue as a precipitate.

• After separating the precipitate, the propane is stripped off for re-use and the treated residual oil proceeds to further processing, after which it is generally called ‘Bright stock’.

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Solvent extraction or acid treatment

• All of the oils, including the de-asphalted vacuum residue, contains resins and aromatic hydrocarbons which impair stability and oxidation resistance and lower the viscosity index.

• These materials are removed, either by treatment with concentrated sulphuric acid, or by the more modern process of extraction with solvents.

• The principal solvents used in the extraction process are furfural and phenol.

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Solvent extraction

• The disadvantage of the sulphuric acid process is that acid sludges are formed, which present a disposal problem.

• Extract from the solvent treatment can be stripped to recover solvent and the residue is suitable for fuel oil blending.

• Very heavy acid treatments are used to produce speciality products of highly paraffinic character such as medicinal ‘liquid paraffin’.

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Solvent dewaxing

• Wax refers to crystalline substance in crude oil which are solids at room temperature.

• The waxes are normally dissolved in the lubricating oil stocks, but are readily precipitated at low temperatures to form a solid matrix which holds the liquid oil in the interstices.

• Thus, wax has the undesirable property of preventing oil from flowing freely at low temperatures, and is therefore removed.

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Solvent dewaxing

• In this process the oil is dissolved in a mixture of solvents, such as methyl ethyl ketone and toluene, and chilled

• The wax is precipitated as a crystalline solid and is filtered off

• The solvent is stripped from the dewaxed oil for re-use, and the wax is treated separately to produce a marketable product

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Finishing treatments• The lubricating oil cuts receive a final treatment to

neutralize or remove any acidic materials, to improve the color and increase the oxidation stability.

• Normally the oil is either percolated (passed through porous substance or small holes) through a bed of clay or mixed with finely divided clay, heated and filtered.

• The spent clay is removed and either discarded or reactivated for further use, and the oil is passed to storage.

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