NLGI Grease Grades are a globally accepted classification system developed by the National Lubricating Grease Institute (NLGI) to define the consistency (hardness or softness) of lubricating greases.
Unlike oil viscosity grades, which describe flow behavior, NLGI grades describe how stiff or fluid a grease is at standard test conditions. The grades range from NLGI 000 (semi-fluid) to NLGI 6 (block-like solid).
NLGI grading helps users select the correct grease based on bearing type, speed and load, operating temperature, lubrication method, and leakage or sealing requirements.
How NLGI Grades Are Determined
Cone Penetration Test (ASTM D217 / ASTM D1403)
A standard cone is allowed to sink into a worked grease sample at 25°C. The depth of penetration (in tenths of a millimeter) determines the NLGI grade.
➡ Higher penetration = softer grease
➡ Lower penetration = harder grease
NLGI Grade Classification Table
| NLGI Grade | Worked Penetration (0.1 mm) | Consistency | Typical Applications |
|---|---|---|---|
| 000 | 445 – 475 | Very fluid | Centralized systems, enclosed gearboxes |
| 00 | 400 – 430 | Semi-fluid | Gear lubrication, pumps |
| 0 | 355 – 385 | Very soft | Cold climates, centralized lubrication |
| 1 | 310 – 340 | Soft | Light bearings, low temperatures |
| 2 | 265 – 295 | Medium (general purpose) | Most bearings & machinery |
| 3 | 220 – 250 | Firm | Vertical shafts, high temperature |
| 4 | 175 – 205 | Very firm | Heavy loads, leakage control |
| 5 | 130 – 160 | Hard | Special industrial uses |
| 6 | 85 – 115 | Block-like solid | Extreme niche applications |
Key Performance Characteristics Explained
Consistency / Hardness
Determines pumpability, leakage tendency, and ability to stay in place. Softer greases flow better, while harder greases resist leakage.
Temperature Performance
Lower grades (000–1) remain pumpable in cold temperatures. Higher grades (2–3) resist softening at elevated temperatures. NLGI grade alone does not define maximum temperature capability.
Load-Bearing Capability
Heavier loads generally require NLGI 2 or 3 greases combined with EP additives to resist squeeze-out.
Mechanical & Shear Stability
High-quality greases retain their NLGI grade after prolonged mechanical working, avoiding leakage or lubrication starvation.
Refining & Manufacturing Considerations
Base Oil Quality
Mineral or synthetic base oils refined via solvent refining, hydrocracking, or synthetic synthesis control oxidation stability, oil bleed, and film strength.
Thickener System
The thickener creates grease structure and directly influences NLGI grade.
| Thickener Type | Influence |
|---|---|
| Lithium | Balanced, widely used |
| Lithium Complex | Higher temperature capability |
| Calcium | Excellent water resistance |
| Aluminum Complex | Strong adhesion |
| Polyurea | Long life, high temperature |
| Barium | Extreme corrosion resistance |
Additives
Anti-wear, EP, rust inhibitors, and antioxidants enhance performance but do not change NLGI grade.
Selection Guide
Choose Softer Grades (000–1) When:
✔ Centralized lubrication systems
✔ Very low temperatures
✔ High-speed, low-load components
Choose Medium Grade (NLGI 2) When:
✔ General-purpose bearings
✔ Automotive & industrial machinery
✔ Moderate speed and load
Choose Firmer Grades (3–6) When:
✔ Vertical or overhead bearings
✔ High temperatures
✔ Heavy shock loads
✔ Leakage must be minimized
Common Myths About NLGI Grades
❌ Higher NLGI means better grease — False
❌ NLGI grade defines temperature limit — False
❌ All NLGI 2 greases are interchangeable — False
Always evaluate NLGI grade together with base oil, thickener type, and additive system.
Advantages of NLGI Classification
✔ Simple and universal standard
✔ Easy comparison across brands
✔ Reduces lubrication errors
✔ Improves equipment reliability and life
Limitations
NLGI grades do not define oil viscosity, EP performance, water resistance, or oxidation stability. Correct grease selection must consider the full formulation.