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Oil Viscosity: Technically, the
viscosity of an oil is a measure of the oil’s resistance
to shear. Viscosity is more commonly known as resistance to
flow. If a lubricating oil is considered as a series of
fluid layers superimposed on each other, the viscosity of
the oil is a measure of the resistance to flow between the
individual layers. A high viscosity implies a high
resistance to flow while a low viscosity indicates a low
resistance to flow. Viscosity varies inversely with
temperature. Viscosity is also affected by pressure; higher
pressure causes the viscosity to increase, and subsequently
the load-carrying capacity of the oil also increases. This
property enables use of thin oils to lubricate heavy
machinery. The loadcarrying capacity also increases as
operating speed of the lubricated machinery is increased.
Two methods for measuring viscosity are commonly employed:
shear and time.
Shear: When viscosity is determined by
directly measuring shear stress and shear rate, it is
expressed in centipoise (cP) and is referred to as the
absolute or dynamic viscosity. In the oil industry, it is
more common to use kinematic viscosity, which is the
absolute viscosity divided by the density of the oil being
tested. Kinematic viscosity is expressed in centistokes (cSt).
Viscosity in centistokes is conventionally given at two
standard temperatures: 40°C and 100°C (104°F and 212°F
).
Time: Another method used to determine oil
viscosity measures the time required for an oil sample to
flow through a standard orifice at a standard temperature.
Viscosity is then expressed in SUS (Saybolt Universal
Seconds). SUS viscosities are also conventionally given at
two standard temperatures: 37°C and 98°C (100°F and
210°F). As previously noted, the units of viscosity can be
expressed as centipoise (cP), centistokes (cST), or Saybolt
Universal Seconds (SUS), depending on the actual test method
used to measure the viscosity.
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