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Consistency, penetration, and National
Lubricating Grease Institute (NLGI) numbers: The most
important feature of a grease is its rigidity or
consistency. A grease that is too stiff may not feed into
areas requiring lubrication, while a grease that is too
fluid may leak out. Grease consistency depends on the type
and amount of thickener used and the viscosity of its base
oil. A grease’s consistency is its resistance to
deformation by an applied force. The measure of consistency
is called penetration. Penetration depends on whether the
consistency has been altered by handling or working. ASTM D
217 and D 1403 methods measure penetration of unworked and
worked greases. To measure penetration, a cone of given
weight is allowed to sink into a grease for 5 seconds at a
standard temperature of 25°C (77°F). The depth, in tenths
of a millimeter, to which the cone sinks into the grease is
the penetration. A penetration of 100 would represent a
solid grease while one of 450 would be semifluid. The NLGI
has established consistency numbers or grade numbers,
ranging from 000 to 6, corresponding to specified ranges of
penetration numbers. Table A, below lists the NLGI grease
classifications along with a description of the consistency
of each classification.
Table A
| NLGI
Grease Classification |
| NLGI
Number |
ASTM
Worked Penetration
0.1 mm (3.28 × 10 ft) at
25°C (77°F) |
Consistency |
| 0000 |
445 - 475 |
Semifluid |
| 00 |
400 - 430 |
Semifluid |
| 0 |
355 - 385 |
Very soft |
| 1 |
310 - 340 |
Soft |
| 2 |
265 - 295 |
Common grease |
| 3 |
220 - 250 |
Semihard |
| 4 |
175 - 205 |
Hard |
| 5 |
130 - 160 |
Very hard |
| 6 |
85 - 115 |
Solid |
|
