NLGI grade is a standardized capacity of grease consistency; that is, how hard or soft is the grease. National Lubricating Grease Institute (NLGI) developed this classification as 000 (very fluid) to 6 (extremely firm). Notably, the NLGI grade does not provide a measure of the quality of greases or their performance in regard to usage, but the consistency of the grease and the manner in which the grease performs at its application state and usefulness.
The usual fallacy is that the higher NLGI grade, the higher the lubrication. It is fact that it simply means that the grease is harder. The flow of grease, its pumps and its ability to remain in place is not determined by how well it lubricates under load or temperature; rather, this is determined by NLGI grade.
What Is the NLGI Grading System?
The NLGI system of grading greases offers an international method by which grease can be categorised so that it is compatible in any manufacturer as well as in any application globally.
This system, which was set by the National lubricating Grease Institute, has been the standard of the world decades ago. It enables the maintenance teams and the operator of the equipment to define the greese according to physical actions or not by generalized explanations such as soft and stiff. The classification of consistency is important in that it directly determines the way grease is applied, retained and replenished in the real operation condition.
How NLGI Grade Is Measured: Penetration Explained
The determination of NLGI grade is based on a standard penetration test, which is a measurement of the distance of a cone penetration into worked grease in controlled conditions.
The test is a measure of the penetration of a worked cone into grease; the working depth (in tenths of a millimeter) in which the standard cone has penetrated after 60 strokes of mechanical working. The smaller penetration values denote harder grease with the larger values reflecting soft consistency. This penetration test is a repeatable objective method of assigning NLGI grades.
| NLGI Grade | Penetration Range (Worked, 0.1 mm) | Consistency Description |
| 000 | 445–475 | Very soft, semi-fluid |
| 00 | 400–430 | Very soft, flowable |
| 0 | 355–385 | Soft, semi-soft |
| 1 | 310–340 | Medium soft, pumpable |
| 2 | 265–295 | Medium, general-purpose |
| 3 | 220–250 | Firm, high retention |
| 4 | 175–205 | Very firm |
| 5 | 130–160 | Very firm, block-like |
| 6 | 85–115 | Extremely firm, hard block |
What Each NLGI Grade Is Typically Used For
Various NLGI grades are appropriate in the certain kind of equipment design and lubrication procedures, relying on decades of field expertise.
Lower grades are much easier and they will work in systems that need simple distribution and higher grades cannot be pushed aside in free or high frequency systems.
| NLGI Grade | Typical Applications |
| 000–00 | Centralized lubrication systems, enclosed gearboxes in cold climates |
| 0–1 | Low-temperature gearboxes, automatic lubrication lines |
| 2 | Rolling element bearings, chassis points, general industrial machinery |
| 3 | High-temperature bearings, vertical shafts, conveyor systems |
| 4–6 | Block greases for slow-moving or specialty sealed applications |
Most maintenance crews when choosing grease check our grease products by NLGI grade to match specifications efficiently.
How NLGI Grade Affects Grease Performance in Practice
The NLGI grade has a number of practical tolerances on the behavioral characteristics of greases in equipment and it is commonly used to control lubrication achievement or failure in reality.
The lower numbers are softer, which enhances pumpability, but leaks through seals, whereas the higher number ones are hard; therefore, they last longer, yet can channel or block flow in cold-start.
| Performance Factor | Impact of Lower NLGI Grades (000–1) | Impact of Higher NLGI Grades (3–6) |
| Pumpability | Excellent flow in centralized systems | Limited; may require heated lines |
| Retention | Higher risk of leakage under centrifugal force | Better resistance to washout and vibration |
| Cold start | Easier distribution at low temperatures | Potential channeling or starvation |
| Re-lubrication | Shorter intervals needed due to possible bleed | Longer intervals possible with good retention |
Practically, the lack of compatibility between the conformity results in a preponderance of leakage by overindulgence or insufficiency of film as a result of unsatisfactory distribution.
Choosing the Right NLGI Grade for Your Application
The appropriate NLGI grade is contingent on the design of equipment, the environment in which the equipment operates, and the lubrication method and not a universal rule.
Key factors include:
Operating Temperature
softer grades work well in temperatures below 0 and firmer grades are in structure at high temperatures.
Equipment Speed and Load
Balanced flow and retention Whenever needed, a high-speed bearing should use NLGI 2; however, a slow and heavily loaded plain bearing may demand NLGI 3.
Lubrication Method
NLGI 2-3 is frequently satisfactory to use in manual greasing, and centralized systems require NLGI 000-1 to depend on in a pump.
Component Orientation
Vertical shafts or overhead work have an advantage of firmer grade to stop drip or migration.
Always ensure used recommendations of OEM, and then ensure they match operating conditions.
Common Misunderstandings About NLGI Grades
A number of myths regarding grease may be sustained leading to the wrong choice of greases:
- Greater NLGI grade is more protective: Only additives are necessary to raise the firmness; the property of extreme pressure and anti-wear is not of a constant grade but rather a characteristic of additives.
- A single NLGI grade can be used with all equipment: Because of varied components on the same machine, different grades may be needed.
- NLGI grade marks the quality of the grease in its entirety: Quality is determined by base oil, thickener, and additives not the penetration value.
- NLGI grade does not require any field trials: The compatibility is still to be verified by trial.
Conclusion — NLGI Grade Is About Consistency, Not Performance
NLGI grade is used to categorize the firmness and flow characteristics of the greases used to provide correct application and retention in the respective working environment. The right consistency guarantees that grease is deposited on bearing surfaces, that it is not displaced, and helps in ensuring the equipment is reliable in terms of its life.
Knowledge of NLGI grade grease (penetration testing to practicable matching) enables maintenance departments to prevent the frequent lubrication failures and customize re-lubrication options in a wide variety of industrial environments.