Key Takeaways
- NLGI grade is a worked-penetration classification of stiffness, not a measure of load capacity.
- Base oil viscosity and thickener type determine lubrication performance; NLGI grade determines handling and pumpability.
- Centralised systems and cold environments often need a softer grade than hand-packed bearings.
NLGI grade describes how stiff a grease is, not how well it lubricates. Confusing consistency with base oil viscosity is one of the most common causes of the wrong grease being specified.
What the NLGI number measures
NLGI consistency is defined by the worked penetration of a grease sample, measured in tenths of a millimetre under a standard cone. NLGI 2, the most common industrial grade, corresponds to a penetration range of 265 to 295. NLGI 1 is softer, NLGI 3 is stiffer, and grades below 0 behave more like semi-fluid products.
Because the test measures stiffness only, two NLGI 2 greases can behave very differently in service if their base oil viscosity or thickener differs.
Consistency versus base oil viscosity
The thickener holds the oil in place; the oil does the lubricating. A soft NLGI 1 grease with a high-viscosity base oil can outperform a stiff NLGI 3 grease with a light oil in a slow, heavily loaded bearing. When specifying, state both the NLGI grade and the base oil viscosity, otherwise the specification is incomplete.
Choosing a grade for the application
Hand-packed horizontal bearings commonly use NLGI 2. Vertical shaft bearings and high-speed applications often favour NLGI 1 or 2 to avoid channelling. Centralised lubrication systems usually require NLGI 1 or softer, and the pump manufacturer's minimum pumping temperature should be checked against the grade.
- Low ambient temperatures: softer grade and lower base oil viscosity improve cold feed.
- High ambient temperatures: stiffer grade and higher viscosity resist softening and leakage.
- Large bearings with slow rotation: confirm the grease will be distributed rather than channelling.
Over-greasing is a real failure mode
Adding grease until resistance is felt can overfill a bearing housing, raising temperature and pushing seals out. Where a relief port or drain plug is provided, use it. Where the bearing is sealed for life, follow the manufacturer's replacement interval instead of adding grease.
Writing a usable grease specification
A specification that survives procurement and maintenance should state the thickener type, base oil viscosity, NLGI grade, operating temperature range, water resistance requirement and any certification such as NSF H1. Adding the application and the lubrication method removes most of the ambiguity that leads to the wrong product being ordered.
Published October 31, 2026 by Metalub Application Engineering Team, Lubricant Application Engineering. Reviewed and updated October 31, 2026.
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