2. Kinematic Viscosity and Viscosity Index
For any oil lubrication system, oil viscosity is considered as the most
important parameter.
Viscosity can be defined as measurement of fluid internal resistance to
flow at a specified temperature.
Values of kinematic viscosity for pure liquid hydrocarbons are usually
measured and reported at two reference temperatures of 40 oC and 100
oC in cSt.
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η= factor × time (sec)
η= kinematic viscosity
3. Viscosity Index
The viscosity of lubricants changes with temperature. As temperature rises, viscosity decreases, and
vice versa.
A measure of the relative rate of change of viscosity with temperature is referred to
as the fluid’s Viscosity Index or VI.
Viscosity @40℃ & viscosity @ 100℃ must be determined to calculate the VI of any oil.
A low VI means the change in viscosity of the oil with temperature is high,
a high VI number indicates a relatively small change in viscosity with temperature.
4. The degree to which viscosity changes with temperature depends on:
1- The base oil type, such as mineral oil (This change for paraffin base oils is
relatively less than that for naphthenic base stock
2- as well as on the VI improver additives contained in the lubricant.
5. Solvent extraction is the traditional process for adjusting the viscosity index
of the oil by partially removing the high molecular weight aromatic
hydrocarbons, which are high viscosity and low viscosity index components.
• Viscosity Index improvers (VII): These additives are basically organic
polymers precisely blended with the base oil to improve the viscosity-
temperature characteristics of the lubricant.
How improve Viscosity and Viscosity index
6. Procedure
1- Measure the viscosity of sample at 40℃ by viscometer in viscometer bath. (U)
2- Measure the viscosity of sample at 100℃ by viscometer in viscometer bath.
3- Calculate the viscosity index (VI).
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VI = (L − U) / (L − H) × 100
7. VI = (L − U) / (L − H) × 100
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Where
VI is viscosity index of a sample
U is kinematic viscosity of the examined
oil at 40 ℃ (mm²/s) or Cst.
L is kinematic viscosity of the oil at 40 ℃
(mm²/s) with viscosity index 0 and
kinematic viscosity at 100 C equals with
that of the examined oil.
H is kinematic viscosity of the oil at 40 ℃
(mm²/s) with viscosity index 100 and
kinematic viscosity at 100 C equals with
that of the examined oil.
