Grease is the most common bearing lubricant and the most commonly misapplied one. The typical maintenance approach is to pump grease until it comes out the seal, but that approach creates as many problems as it solves. Over-greasing generates heat, damages seals, and wastes money. Under-greasing starves the contact zone and causes premature wear.
Choosing the right grease and applying the right amount at the right interval requires understanding what grease actually is, how it works inside a bearing, and what happens when conditions change.
What Grease Actually Is
Grease is a base oil held in a thickener matrix, like a sponge holding water. The thickener is a soap or non-soap structure that gives grease its consistency and holds it in place. The base oil is the actual lubricant. When the bearing rolls, mechanical working releases base oil from the thickener, and this oil forms the thin film that separates the rolling elements from the raceways.
The three properties that matter most are base oil viscosity, thickener type, and NLGI consistency grade. Base oil viscosity determines the oil film thickness at operating temperature. Thickener type determines compatibility with other greases, temperature limits, and water resistance. NLGI grade determines how firmly the grease stays in the bearing cavity.
A common mistake is selecting grease by brand name or color rather than by specification. A green grease and a red grease may be fundamentally different formulations, or they may be the same chemistry from different manufacturers. Always select by specification, not by appearance.
• Lithium complex: General purpose, good to 150°C, most common industrial grease
• Polyurea: Long life, good for sealed-for-life bearings, not always compatible with lithium
• Calcium sulfonate: Excellent water resistance, good for food and wet environments
• Aluminum complex: Good water resistance, tacky, good for open gears and pins
Bearing Grease Speed Factor Guide
Check grease suitability for bearing speed and temperature. Get regreasing interval estimates based on ndm speed factor, NLGI grade, base oil viscosity, and operating environment.
Base Oil Viscosity: The Most Important Choice
Base oil viscosity must match the bearing speed and operating temperature. The viscosity ratio kappa compares the actual viscosity at operating temperature to the minimum viscosity required for the bearing size and speed. A kappa above 1.0 means the oil film is adequate. A kappa above 4.0 means the film is more than adequate and a lower-viscosity grease would reduce friction and heat.
For most general industrial bearings running at moderate speeds and temperatures between 40 and 80°C, a base oil viscosity of ISO VG 100 to 220 is appropriate. Higher viscosity provides a thicker film but generates more churning friction at high speed. Lower viscosity reduces friction but may not form an adequate film for the load.
High-speed bearings (ndm above 300,000) need lower-viscosity grease, typically ISO VG 22 to 68 base oil. The thin oil generates less churning heat and can flow into the contact zone fast enough to keep up with the rolling speed. Using heavy grease on a high-speed bearing is one of the most common lubrication mistakes in maintenance.
Low speed, heavy load → ISO VG 220 to 460
Moderate speed, normal load → ISO VG 100 to 220
High speed, moderate load → ISO VG 22 to 68
Very high speed (spindles) → ISO VG 10 to 22
When and How Much to Regrease
The regreasing interval depends on bearing size, speed, temperature, and contamination level. SKF, Timken, and NSK all publish regreasing interval charts that give a base interval for clean, moderate-temperature conditions, then apply correction factors for temperature, contamination, and vertical shaft orientation.
The base interval decreases with speed and increases with size. A small bearing at high speed may need regreasing every 500 hours. A large bearing at low speed may go 10,000 hours between applications. Temperature above 70°C cuts the interval roughly in half for every 15°C increase. Contaminated environments (dust, moisture, process chemicals) cut the interval by 50 to 75 percent.
The correct amount is typically 1 to 2 grams per regrease event for small bearings and proportionally more for large bearings. A common formula is: volume (grams) = 0.005 × D × B, where D is the bearing outer diameter in mm and B is the total bearing width in mm. This is a regrease volume, not a fill volume. Fresh grease displaces the oldest grease at the edges.
Bearing Grease Speed Factor Guide
Check grease suitability for bearing speed and temperature. Get regreasing interval estimates based on ndm speed factor, NLGI grade, base oil viscosity, and operating environment.
Why You Cannot Mix Greases
Mixing greases with different thickener types can cause the mixture to soften, harden, or completely lose its structure. Lithium complex and polyurea are the most common incompatible pair in industrial maintenance. A bearing greased with lithium complex that receives a shot of polyurea can soften to the consistency of motor oil. The grease runs out of the bearing, the contact zone starves, and the bearing fails.
Even greases with the same thickener type from different manufacturers may not be fully compatible due to differences in additive packages. The safest practice is to completely purge the old grease before switching. Run the bearing with the new grease and continue pumping until only new grease exits.
If you must standardize across a plant, pick one grease that covers the majority of your applications and use it everywhere possible. Document the specification, not the brand, in the CMMS. This prevents the common problem of a vendor changing a product formulation without the maintenance team realizing it.