How to choose grease by NLGI grade and viscosity
The right grease choice decides the lifespan and reliability of bearings and guides. This guide explains the NLGI consistency grade per DIN 51818, base oil viscosity and temperature range, and how to pick the right thickener for maintenance work.
View lubricantsWhat does the NLGI consistency grade mean?
The NLGI grade describes the consistency - the stiffness - of a grease. It is derived from worked penetration per DIN ISO 2137: a standard cone sinks into the grease at 25 °C, and the depth is measured in tenths of a millimetre. The deeper the cone sinks, the softer the grease and the lower the NLGI grade.
The scale runs from NLGI 000 (almost fluid, penetration 445‑475) to NLGI 6 (block-hard, 85‑115). Each grade spans a 30-unit band. For around 80 percent of all rolling bearing applications, NLGI 2 is the right choice because it stays in the bearing yet still bleeds oil to the contact.
Why is base oil viscosity so important?
Actual lubrication is done not by the thickener but by the base oil it releases like a sponge. The base oil viscosity at 40 °C (given in mm²/s per DIN 51562) must match the bearing speed and load. Oil that is too thin breaks the film; oil that is too thick raises friction and temperature.
- High speeds, small bearings: low viscosity around 15‑100 mm²/s.
- Medium speeds, general machinery: around 100‑220 mm²/s.
- Low speeds, high load, shock: high viscosity 460 mm²/s and above.
- Use the speed factor n x dm (speed times mean bearing diameter) as a rule of thumb.
- For sustained high temperatures choose synthetic base oils (PAO, ester).
Plan lubrication intervals and equipment cleaning systematically.
Read the guideHow do you pick temperature range and thickener?
The thickener, together with the base oil, sets the usable temperature range and the resistance to water and media. A key figure is the dropping point per DIN ISO 2176: the temperature at which the grease turns liquid. The continuous service temperature is usually around 30‑50 °C below it.
Under DIN 51825, greases are coded systematically, for example KP2K-30: K for rolling and plain bearing grease, P for EP additives, 2 for the NLGI grade, K for an upper service temperature up to 120 °C, and -30 as the lower limit temperature. This short code allows a quick comparison between products.
Frequently asked questions
Which NLGI grade for normal rolling bearings?
For most rolling bearings NLGI 2 is the standard choice. It stays well in the bearing, bleeds oil sufficiently and covers a broad range of general machinery applications.
What matters more, NLGI grade or base oil viscosity?
Both. The NLGI grade controls how stiff the grease is and whether it stays at the lubrication point. The base oil viscosity governs the actual film and must match speed and load.
Can you mix two greases?
Only if thickener and base oil compatibility is confirmed. Incompatible thickeners such as lithium and polyurea can soften or harden. If in doubt, clean the bearing and re-grease completely.
What does the short code KP2K-30 mean?
Per DIN 51825: K = bearing grease, P = EP additives, 2 = NLGI grade 2, K = service temperature up to 120 °C, -30 = lower limit temperature in degrees Celsius.
Looking for the right lubricant?
We supply greases and oils for maintenance and servicing - sorted by NLGI grade, viscosity and temperature range, with data sheets.
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Greases classified to DIN 51818 and DIN 51825.
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Viscosity, dropping point and range documented.
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From NLGI 000 to 3 for every application.
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