General terminology for Lubricants
Viscosity:
Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. The less viscous the fluid is, the greater its ease of movement (fluidity). Viscosity is defined in kinematic and dynamic forms.
Viscosity Index:
Viscosity index (VI) is a dimensionless measure for the change of viscosity with temperature. It highlights how a lubricant's viscosity changes with variations in temperature. A high viscosity index number indicates a good quality lubricating oil with low viscosity variations with temperature.
Flash Point:
Flash point of a volatile material is the lowest temperature at which it vaporizes to form an ignitable mixture with air and in the presence of a flame burns and then dies out immediately.
Pour Point:
The pour point of a liquid is the lowest temperature at which it will pour or flow.
Consistency (NLGI grade) and Cone Penetration of a Grease:
The consistency (NLGI grade) is a widely used measure for the softness of lubricating greases based on their cone penetration at a certain temperature (normally 25 C°) defined in mm/10. NLGI grade classifies greases in one of nine grades from fluid (grade 000 with 445-475 mm/10 penetration range) to very hard (grade 6 with 85-115 mm/10 penetration range).
Dropping Point:
The dropping point of a soap-thickened lubricating grease is the temperature at which it passes from a semi-solid to a liquid state under specific test conditions and drops from the hole of the test apparatus.
Water Washout Resistance:
Greases show different resistance in contact with water, some are completely resistant and stable and can be used at humid conditions and the presence of water while some are not stable and dissolve in water or form emulsion mixtures with water.
Reversibility:
The capabilty of a grease to keep its structure and formidability under sucessive heating and cooling is called reversibility. Some types of greases (e.g. calcium greases) are not reversible and by increasing operation temperature and reaching their drop point lose their structure and decompose and even after cooling cannot return to their initial form and shall be replaced immediately.
4-Ball Test:
This test method covers the determination of the load carrying properties of lubricating greases according to DIN 51 350 (parts 4-5) or ASTM D2596 standards.
Timken Test:
This test method covers the determination of the load carrying capacity of lubricating greases according to ASTM D2509 standard by means of the Timken Extreme Pressure Tester.
