Greases / 5 min
Grease Selection and NLGI Class: How to Determine the Right Grease
Grease is a semi-solid lubricant formed by trapping a liquid base oil inside a thickener (soap) structure. It's used at points (bearings, journal bearings, pins) where liquid oils can't stay in place or sealing is difficult, to prevent wear and block contamination.
Published by: Rotek Technical Content Team
Content is published based on manufacturer technical documentation and equipment requirements as the basis for final selection. This is general guidance and does not replace equipment-specific manufacturer approval.
Review our editorial guidelinesTable of Contents
- - Grease Structure: Base Oil and Thickener
- - What Is the NLGI Class and How Is It Read?
- - The Importance of NLGI in Central Lubrication Systems
- - Resistance to Temperature and Water
- - Correct Grease Selection Criteria
Grease Structure: Base Oil and Thickener
The liquid base oil inside a grease does most of the lubricating work. The thickener structure (lithium, calcium, etc.) acts like a sponge, holding the oil and releasing it slowly at the points needed. The type of thickener determines the grease's temperature and water resistance.
What Is the NLGI Class and How Is It Read?
The NLGI (National Lubricating Grease Institute) class expresses how solid or fluid a grease is. 000 is the most fluid (near-liquid), and 6 is the hardest (brick-like). The most common consistencies used in industrial bearing and journal-bearing lubrication are NLGI 2 and NLGI 1.
The Importance of NLGI in Central Lubrication Systems
Automatic central lubrication systems have to pump grease along long lines. Since pumpability is critical in these systems, softer NLGI 0, 00, or at most NLGI 1 greases are generally preferred. NLGI 2 or 3 grades can clog the lines.
Resistance to Temperature and Water
As operating temperature rises, attention must be paid to the thickener's melting risk (dropping point). Simple lithium-soap greases operate up to roughly 120°C, while lithium complex greases can withstand higher temperatures. Calcium-soap greases offer high water resistance but have lower temperature tolerance.
Correct Grease Selection Criteria
When choosing a grease, equipment speed (RPM), load, ambient temperature, water or chemical exposure and application method (central or manual) are evaluated. High-speed bearings should use greases with lower base-oil viscosity, while heavily loaded, slow-speed systems require thicker base-oil, EP-additive greases.
Frequently Asked Questions
What's the difference between NLGI 2 and NLGI 3 grease?
NLGI 2 grease is medium consistency (peanut-butter softness) and is the most common general-purpose standard; NLGI 3 is stiffer and is preferred for high temperatures or special bearings where grease leakage risk is high.
Where are NLGI 0 or 00 greases used?
NLGI 0 and 00 greases are very soft, semi-fluid consistencies. They're generally used in place of liquid oil in enclosed gearboxes where full sealing isn't possible, or in central (automatic) lubrication systems with long lines, for easy pumpability.
What is lithium complex grease, and where is it used?
Lithium complex grease is a grease type with much higher mechanical heat resistance (up to roughly 160°C) than standard lithium grease. It's used in industrial bearings and automotive wheel bearings exposed to high heat and heavy load.
Why does grease bleed oil (separate)?
Some oil separation from grease (bleeding) is normal, but excessive bleeding is generally caused by exposure to very high temperature, expired shelf life, unsuitable storage conditions, or the thickener structure breaking down after two different grease types are mixed.
Which grease should be chosen for wet environments?
In environments with high risk of water contact or washdown, water-resistant calcium sulfonate complex or calcium-soap thickener greases should be chosen. These greases retain their structure and lubricating properties even underwater.