What is the NLGI grade or consistency of a grease?
One of the most common and important factors to consider when selecting the appropriate lubricating grease is consistency, regardless of the intended application. If the consistency of the grease is too hard, it may not reach all contact areas, while if it is too soft, it may migrate away from them, which would result in an insufficient lubricating layer.
The consistency value of a grease is indicated by its penetration value and is assessed in accordance with the National Lubricating Grease Institute (NLGI) standardised grade chart. The NLGI number is a measure of the consistency of the grease in accordance with its worked penetration value.
The penetration test measures the depth to which a standard cone penetrates a grease sample in tenths of a millimetre. Each NLGI grade corresponds to a specific range of worked penetration values. Higher penetration values, such as above 355, indicate a lower NLGI grade number. The NLGI scale ranges from 000 - more fluid - to 6 - more solid.
The viscosity of the base oil and the amount of thickener greatly influence the NLGI grade of the finished lubricating grease. Grease thickeners operate in much the same way as a sponge, releasing the lubricating fluid (base oil and additives) when force is applied.
Greater consistency results in greater resistance to the release of lubricating fluid when force is applied. A grease with a low consistency will release lubricating fluid more easily. It is essential to ensure the correct consistency of grease in order to guarantee the optimal amount of lubricating fluid is provided and maintained within the system, thus ensuring proper lubrication.
NLGI Grade | Consistency | Worked penetration (DIN ISO 2137) |
000 | Fluid | 475 – 445 |
00 | Semi-fluid | 430 – 400 |
0 | Very soft | 385 – 355 |
1 | Soft | 340 – 310 |
2 | Creamy | 295 – 265 |
3 | Firm | 250 – 220 |
4 | Moderately hard | 205 – 175 |
5 | Hard | 160 - 130 |
6 | Very hard | 115 – 85 |
NLGI 000-0 grades
Greases of these grades are classified as fluid and semi-fluid and tend to have a lower viscosity. This range of consistency can be exemplified by comparing it to that of ketchup and mustard. These grease grades can be beneficial in closed, centralised applications where grease migration is not an issue. For instance, a gearbox requires a grease within this NLGI range to maintain a consistent supply of lubricant in the contact area. Alternatively, grades 0 can be utilised in central lubrication systems with multiple application points where grease creep is a concern.
NLGI 1- 3 grades
A grease with an NLGI grade of 1 has a consistency similar to tomato paste, while a grease with an NLGI grade of 3 has a consistency similar to that of butter. The most common consistency for a lubricating grease is NLGI grade 2, which would be similar in consistency to a spreadable cheese. Greases of this grade can operate at higher temperatures and speeds than NLGI 000-0 grades, making them the optimal choice for use in bearings.
NLGI 4- 6 grades
NLGI grades classified in the 4-6 range are firm and hard greases, ranging in consistency from ice cream-like to that of cheddar cheese. Grade 4 greases are used in very specific applications such as bearings at extreme speeds (over 15,000 rpm), as these experience increased friction and heat build-up, requiring a hard grease. Greases with an NLGI grade of 5 or 6 are considered sealants and are not used as lubricants.
