Short answer
An NLGI grade describes how firm a grease is, from NLGI 000 (semi-fluid) to NLGI 6 (block-like). It is based on worked cone penetration measured under ASTM D217: how far a standard cone sinks into the grease after it has been worked 60 times at 25 °C. The grade says nothing about thickener type, base oil viscosity or additives.
The penetration ranges
Penetration is reported in tenths of a millimetre. The NLGI ranges are 445 to 475 for NLGI 000, 400 to 430 for 00, 355 to 385 for 0, 310 to 340 for 1, 265 to 295 for 2, 220 to 250 for 3, 175 to 205 for 4, 130 to 160 for 5 and 85 to 115 for 6. A deeper penetration means a softer grease, which is why the grade numbers run in the opposite direction from the penetration numbers.
There are deliberate gaps between the ranges, so a grease measuring, say, 300 falls between NLGI 1 and NLGI 2. Manufacturers formulate to land inside a range.
Worked versus unworked penetration
Grease softens when it is sheared, so the grade uses worked penetration, measured after the sample is stroked 60 times in a standard grease worker. ASTM D217 also describes unworked and prolonged worked penetration; the prolonged test, with many thousands of strokes, shows how much a grease softens with extended shearing, which relates to mechanical stability in service.
Choosing a grade
NLGI 2 is the most common general-purpose consistency for chassis points, many rolling-element bearings and equipment pins. Softer grades such as NLGI 1, 0, 00 and 000 are used where grease must pump through long lines in centralized lubrication systems, in cold weather, or in enclosed gear cases designed for semi-fluid grease. Firmer grades such as NLGI 3 appear in some bearing applications where a stiffer grease is less likely to churn or leak.
The equipment manual or bearing maker is the authority. Choosing a grade outside the recommendation can lead to poor pumpability, leakage, or grease that does not reach the contact surfaces.
What the grade does not tell you
Two NLGI 2 greases can be completely different products: one might use a lithium complex thickener and a light base oil, another a calcium sulfonate thickener with a heavy base oil. Thickener type, base oil viscosity, dropping point, water resistance and load-carrying tests all appear separately on a data sheet.
Frequently asked questions
What does NLGI 2 mean?
NLGI 2 is a consistency grade with a worked penetration of 265 to 295 tenths of a millimetre, roughly the firmness of peanut butter. It is the most common grade for general greasing.
Is NLGI 00 grease the same as gear oil?
No. NLGI 00 is a very soft grease that still contains a thickener. Some gearboxes are designed for it, but it should not replace a fluid gear oil unless the manufacturer allows it.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].
