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Lubricating Oil
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Background
Since the Roman era, many liquids, including water, have been used as lubricants to minimize
the friction, heat, and wear between mechanical parts in contact with each other. Today,
lubricating oil, or lube oil, is the most commonly used lubricant because of its wide range of
possible applications. The two basic categories of lube oil are mineraland synthetic. Mineral oils
are refined from naturally occurring petroleum, or crude oil. Synthetic oils are manufactured
polyalphaolefins, which are hydrocarbon-based polyglycols or ester oils.
Although there are many types of lube oils to choose from, mineral oils are the most commonly
used because the supply of crude oil has rendered them inexpensive; moreover, a large body of
data on their properties and use already exists. Another advantage of mineral-based lube oils is
that they can be produced in a wide range of viscositiesviscosity refers to the substance's
resistance to flowfor diverse applications. They range from low-viscosity oils, which consist of
hydrogen-carbon chains with molecular weights of around 200 atomic mass units (amu), tohighly viscous lubricants with molecular weights as high as 1000 amu. Mineral-based oils with
different viscosities can even be blended together to improve their performance in a given
application. The common 1OW-30 motor oil, for example, is a blend of low viscous oil (for easy
starting at low temperatures) and highly viscous oil (for better motor protection at normal
running temperatures).
First used in the aerospace industry, synthetic lubricants are usually formulated for a specific
application to which mineral oils are ill-suited. For example,
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synthetics are used where extremely high operating temperatures
are encountered or where the lube oil must be fire resistant. This
article will focus on mineral-based lube oil.
Raw Materials
Lube oils are just one of many fractions, or components, that can be
derived from raw petroleum, which emerges from an oil well as a
yellow-to-black, flammable, liquid mixture of thousands of
hydrocarbons (organic compounds containing only carbon and
hydrogen atoms, these occur in all fossil fuels). Petroleum deposits
were formed by the decomposition of tiny plants and animals that
lived about 400 million years ago. Due to climatic and geographical
changes occurring at that time in the Earth's history, the breakdown
of these organisms varied from region to region.
Because of the different rates at which organic material decomposed
in various places, the nature and percentage of the resulting
hydrocarbons vary widely. Consequently, so do the physical and
chemical characteristics of the crude oils extracted from different
sites. For example, while California crude has a specific gravity of
0.92 grams/milliliter, the lighter Pennsylvania crude has a specific
gravity of 0.81 grams/milliliter. (Specific gravity,which refers to the
ratio of a substance's weight to that of an equal volume of water, is an
important aspect of crude oil.) Overall, the specific gravity of crudes
ranges between 0.80 and 0.97 grams/milliliter.
Depending on the application, chemicals called additives may be mixed with the
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Lubricating oil is refined from crude oil. After undergoing a purifying process colled
sedimentation, the crude oil is heated in huge fractionating towers. The various vaporswhich
can be used to make fuel, waxes, or propane, among other substancesboil off and are collected
at different points in the tower. The lube oil that is collected is filtered, and then additives are
mixed in.
refined oil to give it desired physical properties. Common additives include metals such as lead or
metal sulphide, which enhance lube oil's ability to prevent galling and scoring when metalsurfaces come in contact under extremely high pressures. High-molecular weight polymerics are
another common additive: they improve viscosity, counteracting the tendency of oils to thin at
high temperatures. Nitrosomines are employed as antioxidants and corrosion inhibitors because
they neutralize acids and form protective films on metal surfaces.
The Manufacturing
Process
Lube oil is extracted from crude oil, which undergoes a preliminary purification process(sedimentation) before it is pumped into fractionating towers. A typical high-efficiency
fractionating tower, 25 to 35 feet (7.6 to 10.6 meters) in diameter and up to 400 feet (122
meters) tall, is constructed of high grade steels to resist the corrosive compounds present in
crude oils; inside, it is fitted with an ascending series of condensate collecting trays. Within a
tower, the thousands of hydrocarbons in crude oil are separated from each other by a process
calledfractional distillation.As the vapors rise up through the tower, the various fractions cool,
condense, and return to liquid form at different rates determined by their respective boiling
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points (the lower the boiling point of the fraction, the higher it rises before condensing). Natural
gas reaches its boiling point first, followed by gasoline, kerosene, fuel oil, lubricants, and tars.
Sedimentation
1 The crude oil is transported from the oil well to the refinery by pipeline or tanker ship. At
the refinery, the oil undergoes sedimentation to remove any water and solid contaminants,
such as sand and rock, that maybe suspended in it. During this process, the crude is
pumped into large holding tanks, where the water and oil are allowed to separate and the
contaminants settle out of the oil.
Fractionating
2 Next, the crude oil is heated to about 700 degrees Fahrenheit (371 degrees Celsius). At
this temperature it breaks down into a mixture of hot vapor and liquid that is then pumped
into the bottom of the first of two fractionating towers. Here, the hot hydrocarbon vapors
float upward. As they cool, they condense and are collected in different trays installed at
different levels in the tower. In this tower, normal atmospheric pressure is maintained
continuously, and about 80 percent of the crude oil vaporizes.
3 The remaining 20 percent of the oil is then reheated and pumped into a second tower,
wherein vacuum pressure lowers the residual oil's boiling point so that it can be made to
vaporize at a lower temperature. The heavier compounds with higher boiling points, such
as tar and the inorganic compounds, remain behind for further processing.
Filtering and solvent extraction
4 After further processing to remove unwanted compounds, the lube oil that has been
collected in the two fractionating towers is passed through several ultrafine filters, which
remove remaining impurities. Aromatics, one such contaminant, contain six-carbon rings
that would affect the lube oil's viscosity if they weren't removed in a process called solvent
extraction. Solvent extraction is possible because aromatics are more soluble in the solvent
than the lube oil fraction is. When the lube oil is treated with the solvent, the aromatics
dissolve; later, after the solvent has been removed, the aromatics can be recovered from it.
Additives, inspection, and packaging
5 Finally, the oil is mixed with additives to give it the desired physical properties (such as
the ability to withstand low temperatures). At this point, the lube oil is subjected to a
variety of quality control tests that assess its viscosity , specific gravity , color, flash, and fire
points. Oil that meets quality standards is then packaged for sale and distribution.
Quality Control
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Most applications of lube oils require that they be nonresinous, pale-colored, odorless, and
oxidation-resistant. Over a dozen physical and chemical tests are used to classify and determine
the grade of lubricating oils. Common physical tests include measurements for viscosity, specific
gravity, and color, while typical chemical tests include those for flash and fire points.
Of all the properties, viscosity, a lube oil's resistance to flow at specific temperatures and
pressures, is probably the single most important one. The application and operating temperature
range are key factors in determining the proper viscosity for an oil. For example, if the oil is too
viscous, it offers too much resistance to the metal parts moving against each other. On the other
hand, if it not viscous enough, it will be squeezed out from between the mating surfaces and will
not be able to lubricate them sufficiently. T he Saybolt Standard Universal Viscometer is the
standard instrument for determining viscosity of petroleum lubricants between 70 and 210
degrees Fahrenheit (21 and 99 degrees Celsius). Viscosity is measured in theSay bolt Universal
second,which is the time in seconds required for 50 milliliters of oil to empty out of a Saybolt
viscometer cup through a calibrated tube orifice at a given temperature.
The specific gravity of an oil depends on the refining method and the types of additives present,
such as lead, which gives the lube oil the ability to resist extreme mating surface pressure and
cold temperatures. The lube oil's color indicates the uniformity of a particular grade or brand.
The oil's flash and fire points vary with the crude oil's origin. Theflash pointis the temperature
to which an oil has to be heated until sufficient flammable vapor is driven off so that it will flash
when brought into contact with a flame. Thefire pointis the higher temperature at which the oil
vapor will continue to burn when ignited.
Common engine oils are classified by viscosity and performance according to specifications
established by the Society of Automotive Engineers (SAE). Performance factors include wear
prevention, oil sludge deposit formation, and oil thickening.
The Future
The future of mineral-based lubricating oil is limited, because the natural supplies of petroleum
are both finite and non-renewable. Experts estimate the total recoverable light to medium
petroleum reserves at 1.6 trillion barrels, of which a third has been used. Thus, synthetic-based
oils will probably be increasingly important as natural reserves dwindle. This is true not only for
lubricating oil but also for the other products that result from petroleum refining.
Where To Learn More
Books
Fuels, Lubricants, and Coolants, 7th ed. Deere & Company Service Publications, 1992.
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Malone, L. J.Basic Concepts of Chemistry. John Wiley & Sons, Inc., 1989.
Nadkarni, R. A., ed.Analysis of Petroleum Products & Lubricants.American Society for Testing
& Materials, 1991.
Seal, Shirley C., ed.Fluids, Lubricants & Sealing Devices. National Fluid Power Association,
1989.
Periodicals
Bienkowski, Keith. "Coolants and Lubricants: The Truth."Manufacturing Engineering.March,
1993.
"System Provides Real-Time Lube Oil Blending."Design News. February 26, 1990, p. 39.
O'Lenick, Anthony and Raymond E. Bilbo. "Saturated Liquid Lubricant Withstands Aluminum
Forming."Research & Development. February, 1989, p. 162.
Peterson, Ivars. "Friction Features."Science News.April 30, 1988, p. 283.
Templeton, Fleur. "The Right Lube Job for Superhot Ceramic Engines?"Business Week. May 18,
1992, p. 11 3.
Vogel, Todd, John Rossant, and Sarah Miller. "Oil's Rude Awakening."Business
Week.September 26, 1988, p. 44.
Craig F. Whitlow