Two or three components
A grease is a mixture of base oil (60–95% by mass), thickener (5–40%, usually lithium soap), and additives (0–10%)—the thickener “gels” the base oil into a plastic mass and determines the NLGI class according to DIN 51818. A lubricating oil, on the other hand, has a simpler composition: only base oil (90–100%) plus additives, without a thickener. This structural difference determines the application, speed limit, and maintenance interval.
The dn value is the deciding factor
The most important rule of thumb for the initial decision is the dn value—the product of bore diameter (d, in mm) and speed (n, in rpm): dn = d × n (Manufacturers often use the average bearing diameter d_m instead of the bore diameter for precise sizing.) It provides a rough guide to the friction-induced heat generation in the bearing. As a rule of thumb: Up to dn ≈ 300,000, NLGI-2 grease is standard; the exact limit depends on the bearing type and grease. Above this value, you should switch to oil lubrication because grease does not dissipate the generated heat effectively.
Additional criteria reinforce or qualify this basic rule: If the lubrication point is exposed, dusty, or damp, the superior sealing performance of grease makes it the better choice. If, on the other hand, continuous heat dissipation is necessary—such as in high-load gearboxes—or if automatic recirculating lubrication is already in place, oil is the better choice despite requiring more complex sealing.
Mnemonic
Worked example: dn value of two bearings
A rolling bearing with a bore diameter of d = 50 mm rotates at n = 3,000 U/min:
dn = 50 × 3,000 = 150,000 → significantly below 300,000; NLGI-2 grease is compatible without any issues.
The same bearing at n = 9,000 U/min: dn = 50 × 9,000 = 450,000 → via 300,000. This poses a risk of overheating and failure with grease lubrication—oil lubrication is required.
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