Properties of lubricant

PROPERTIES OF LUBRICANTS
Viscosity
Viscosity is a measure of the internal resistance of a liquid during its flow. It is expressed in centipoise.The viscosity of an oil is the time in seconds for a given quantity of a oil to pass through a standard orifice under the specified conditions.

Determination
The viscosity of an oil is determined by
(i) Red wood viscometer.
(ii) Say bolt viscometer.
The time required for 50ml of the liquid to pass through the orifice of a red wood viscometer is called as Red wood seconds. The time required for 60 ml of the liquid to pass through the orifice of a say bolt viscometer is called Say bolt universal seconds.

Significance
(i) If the viscosity of the lubricating oil is too high, the movement of the machine is restricted due to excessive friction.
(ii) If the viscosity of the lubricating oil is too low, the liquid oil film can’t be maintained and excessive wear will take place.
A good lubricating oil must have moderate viscosity.

Viscosity index
The viscosity of an oil decreases with increase in temperature. The rate of change of viscosity with temperature is indicated by viscosity temperature curves or by a scale known as viscosity index(V.I). Viscosity index is defined as “ The average decreases in viscosity of an oil per degree rise in temperature between 100°F and 210°F”.
(i) If the viscosity of an oil decreases rapidly with the increase in temperature it has low V.I.
Example
Gulf coast oil (consists of naphthenic hydrocarbons) exhibits a larger change in viscosity with a increase in temperature and its V.I value is arbitrarily assigned as zer0.

(ii) If the viscosity of an oil is slightly affected with the increase in temperature, it has higher V.I.
Example
Pennsylvanian oil (consists of paraffinic hydrocarbons) exhibits a relatively a smaller change in viscosity with a increase in temperature and its V.I value is arbitrarily assigned as 100.

Determination
The V.I of a test oil is calculated by comparing with the above two standard oil. The test oil is compared at 38°C (100°F) with zero V.I oil (gulf coast ) and 100 V.I oil (pensylvanian oil ) both having the same viscosity as the test oil aty 99°C (210°F).
The V.I of the test oil is given by the following formula
V.I=L-U/L-H ×100
Where,
U is the viscosity of the test oil at 38°C.
L is the viscosity of the low V.I oil at 38°C
H is the viscosity of the high V.I oil at 38°C
The viscosity temperature curve is flatter for the oil of high V.I than the oil of low V.I

Significance
A good lubricant should have minimum change in viscosity for a wide range of temperature. A good lubricant should have a high V.I.

Improving viscosity index
V.I of a lubricant can be increased by the addition of linear polymers such as polyisobutylene, n-hexanol, etc.