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a) To reduce power consumed, caused by friction, by converting friction between solid surfaces (sliding friction, rolling friction), into fluid friction; b) To carry away the heat produced by friction; and c) To prevent wear and tear of moving parts.
PETROLEUM LUBRICANTS: Petroleum products are called hydrocarbons because they are formed mainly of compounds consisting of the elements hydrogen and carbon combined together to form molecules. The simplest hydrocarbon is one atom of carbon linked to four atoms of hydrogen. It is called methane (CH4); it belongs to the paraffin family of hydrocarbons and is a gas. The next in the series is ethane (C2H6) and is also a gas. The number of carbon atoms which can form a chain is endless e.g. C3H8; C4H10....C20H42; C21H44 etc. As the number of carbon atoms in the chain increases so the molecule becomes heavier and from C5H12 (Pentane) the compounds are liquid. The greater the number of carbon atoms the heavier or more viscous the liquid. To complicate matters the chains are not all simple straight chain compounds but may have side chains attached to the main chain e.g. iso-pentane(C5H12), cyclo-pentane(C5H10), benzene (C6H6), naphthalene (C10H8) etc. Crude oils are classified into 3 types: i)
Paraffin base crudes contain little or no asphaltic matter and give
high yields of paraffin wax. ii) Naphthanic base - also called asphaltic
base. They generally have low wax contents but give high yields
of bitumen. The lubricating oils derived from them are more viscosity/temperature
sensitive than those from paraffin base crude, that is, they have
a lower viscosity index. iii) Mixed base crudes have proportions
of both paraffins and naphthenes with some aromatics. Have characteristics
between those of paraffin and Naphthanic based crudes. They yield
either Paraffinic or Naphthanic lubricating oils, according to the
predominating type of hydrocarbon present. The
process of refining crude oil involves separating the complex chemical
compounds into groups by means of distillation. Compounds with few
carbon atoms in the molecule have low boiling points compounds with
a large number of carbon atoms have high boiling points. This results
in a range of products (fractions) of increasing boiling points
(liquid petroleum gas, gasoline-benzene kerosene and gas-oil). These
are called distillates. The part of the crude oil remaining after
the distillation (the residue) may be used as fuel oil or treated
further to produce lubricating oil wax and bitumen. Lubricating
oil is produced by vacuum distillation of the residue from the primary
distillation. The lubricating oil is separated into a number of
fractions, which we can call "light", "medium"
and "heavy". These fractions are the base oils/feedstocks,
for production of finished lubricating oils. The
definition of viscosity is a measure of the resistance to flow of
a liquid. Stoke (ST)is kinematic measurement unit of a fluid's resistance
to flow, defined by the ratio of a fluids viscosity to its density,
usually measured as centistokes (cSt) Viscosity of oil is related
to the complexity of its molecule. The fractions containing compounds
of low boiling point have a low viscosity and those with high boiling
point have a high viscosity Commonly used units of viscosity are,
SAE (Society of Automotive Engineers, developed in the USA), Redwood-I
in the UK, Saybolt Universal Seconds in the USA and Engler in many
European countries. The International Standards Organizations viscosity
grade (ISO-VG) number is the number corresponding to the centi-Stoke(cSt)
value measured at 400 C. Footnote. Unfortunately no simple or reliable
test has yet been devised that will enable the motorist to determine
the condition of the oil in the crankcase or provide an indication
as to when the oil should be changed. To ensure satisfactory and
safe lubrication of the engine, the oil change recommendation of
the manufacturers should always be observed. They have based their
judgment on many years of experience and have an intimate knowledge
of engine performance in relation to the quality of lubricant recommended
with particular frequency of oil change.
Consistency
is the term used to describe the hardness or softness of a grease
is measured by allowing a cone to drop in to the grease under standard
conditions. The consistency of the grease is the depth to which
the cone penetrates in units of 0.1 mm. Penetrations are usually
determined on the unworked grease, that is in its original condition
and again after working the grease for 60 strokes by a perforated
disc in a standard worker pot. The National Lubricating Grease Institute
- NLGI in the USA has formulated a penetration classification covering
greases from semi-fluid to hard block greases. That is from "000",
"00", "0", "1", "2" .....
"6"
These embrace chemical products manufactured with or without additives, which are essentially composed of synthetic base oils such as organic esters and glycols.
The usual definition of lubricating grease is a solid to semi-fluid product of dispersion of a thickening agent in a liquid lubricant. Ingredients imparting special properties are included to enhance its quality. Synthetic liquids such as Esters, Silicones and Glycol are employed for the manufacture of Synthetic Greases. Grease lubrication becomes necessary in many lubricated bearings and places where retention of oil at the bearing surface would otherwise be difficult such as chassis of cars and trucks, paper mills, textile plants, etc., where dripping and creeping must be overcome and where application of the lubricants can be made only at relatively long intervals of time, beyond the control of the users. Two distinct advantages of grease lubrication over oil, one being the better retention in the bearings during operation and the other being the inability of the journal to squeeze all of the greases out, when at rest. The latter permits easier starting of heavily loaded journals, while the former makes the lubrication of many bearings more certain and convenient.
In
general, do not to use grease where oil will do the job conveniently
as because Oil forms a better film, it can dissipate heat much faster,
it keeps bearings cleaner under certain conditions, and it is more
satisfactory in high speed services and at high temperatures.
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