4.
OIL LUBRICATION
A lubricating oil is composed of a base stock
blended with various performance enhancing
additives. The base stock may be a petroleum
oil, a synthetic oil, or in rare specialized in-
stances, vegetable oil. Petroleum oils are
usually classified as either paraffinic or
naphthenic. Paraffinic oils, as the name implies,
contains paraffin wax, and are the most widely
used type of lubricating oil base stock. In com-
parison to naphthenic, paraffinic oils are more
resistant to oxidation, have a lower volatility, a
higher viscosity index, and are generally a better
lubricant. Since naphthenic oils are essentially
wax free, they have a naturally low pour points.
blended with various performance enhancing
additives. The base stock may be a petroleum
oil, a synthetic oil, or in rare specialized in-
stances, vegetable oil. Petroleum oils are
usually classified as either paraffinic or
naphthenic. Paraffinic oils, as the name implies,
contains paraffin wax, and are the most widely
used type of lubricating oil base stock. In com-
parison to naphthenic, paraffinic oils are more
resistant to oxidation, have a lower volatility, a
higher viscosity index, and are generally a better
lubricant. Since naphthenic oils are essentially
wax free, they have a naturally low pour points.
Synthetic based lubricants, that is, lubricants
produced from man made products rather than
from vegetable or refined petroleum oils, are
generally superior to petroleum lubricants in
most circumstances. Despite the superior per-
formance of synthetic lubricants, their use is
usually limited to severe or unusual applications
due to their cost, which can be many times more
than a similar petroleum product.
produced from man made products rather than
from vegetable or refined petroleum oils, are
generally superior to petroleum lubricants in
most circumstances. Despite the superior per-
formance of synthetic lubricants, their use is
usually limited to severe or unusual applications
due to their cost, which can be many times more
than a similar petroleum product.
4.1 Oil Characteristics
Viscosity. Probably the single most important
characteristic of a lubricant Is its viscosity. As
mentioned earlier, viscosity is a measure of a
fluids internal friction or resistance to flow. The
higher viscosity a fluid has, the greater the inter-
nal resistance, and the greater its load capacity,
but with the higher internal resistance, operating
temperatures can rise. An oil with the correct
viscosity for a particular application will be thick
enough to support the load while not being so
thick to cause excessive fluid friction and a
corresponding increase in operating tem-
perature.
characteristic of a lubricant Is its viscosity. As
mentioned earlier, viscosity is a measure of a
fluids internal friction or resistance to flow. The
higher viscosity a fluid has, the greater the inter-
nal resistance, and the greater its load capacity,
but with the higher internal resistance, operating
temperatures can rise. An oil with the correct
viscosity for a particular application will be thick
enough to support the load while not being so
thick to cause excessive fluid friction and a
corresponding increase in operating tem-
perature.
Dynamic or absolute viscosity is defined as the
ratio of shear stress to shear rate and is most
commonly measured in poise or centipoise.
ratio of shear stress to shear rate and is most
commonly measured in poise or centipoise.
the dynamic viscosity divided by the density of
the lubricant and is most commonly measured in
centistokes. The kinematic viscosity is related to
the time required for a fixed volume of lubricant
to flow through a capillary tube under the
influence of gravity. The kinematic viscosity Is
the most common method of expressing a
lubricant's viscosity.
the lubricant and is most commonly measured in
centistokes. The kinematic viscosity is related to
the time required for a fixed volume of lubricant
to flow through a capillary tube under the
influence of gravity. The kinematic viscosity Is
the most common method of expressing a
lubricant's viscosity.
There are many other methods for measuring
and expressing the viscosity of lubricants. One
common viscosity measuring system, although
considered by many to be obsolete, is Saybolt
Universal Seconds (SUS), or Saybolt Seconds
Universal (SSU). In this system the viscosity is
the amount of time, in seconds, it takes for 60
cubic centimeters of the lubricant to flow through
a standard orifice at a given test temperature,
usually 100 or 210° F.
and expressing the viscosity of lubricants. One
common viscosity measuring system, although
considered by many to be obsolete, is Saybolt
Universal Seconds (SUS), or Saybolt Seconds
Universal (SSU). In this system the viscosity is
the amount of time, in seconds, it takes for 60
cubic centimeters of the lubricant to flow through
a standard orifice at a given test temperature,
usually 100 or 210° F.
Various organizations also have grading sys-
tems for lubricant viscosity. The International
Standards Organization (ISO) grades are com-
mon for turbine and hydraulic oils. The Society
of Automotive Engineers (SAE) has developed
grades for crankcase and gear oils. It should be
noted that these grades do not correspond to a
specific Kinematic or Saybolt viscosity value,
but to a viscosity range.
tems for lubricant viscosity. The International
Standards Organization (ISO) grades are com-
mon for turbine and hydraulic oils. The Society
of Automotive Engineers (SAE) has developed
grades for crankcase and gear oils. It should be
noted that these grades do not correspond to a
specific Kinematic or Saybolt viscosity value,
but to a viscosity range.
some of the more common viscosity grading
systems to Kinematic and Saybolt viscosities.
systems to Kinematic and Saybolt viscosities.
Viscosity Index. An oil's viscosity index (VI) is a
empirical number used to describe its viscosity-
temperature relationship. An oil with a high VI
indicates a relatively low change in viscosity
with a change in temperature.
empirical number used to describe its viscosity-
temperature relationship. An oil with a high VI
indicates a relatively low change in viscosity
with a change in temperature.
Pour Point. A fluid's pour point is the lowest
temperature at which the fluid will flow. In
paraffinic oils, the pour point is the result of the
crystallization of waxy particles. In naphthenic
oils, the pour point is the result of the decrease
in viscosity caused by a decrease in tempera-
temperature at which the fluid will flow. In
paraffinic oils, the pour point is the result of the
crystallization of waxy particles. In naphthenic
oils, the pour point is the result of the decrease
in viscosity caused by a decrease in tempera-
(FIST 2-4 11/90)
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