Most bearing failures trace back to lubrication: too little, too much, wrong type, or wrong interval. The time-based relubrication schedules posted on many plant equipment ("grease every 30 days") are rarely based on any calculation. They are habits carried over from decades ago, and they are wrong more often than they are right. A bearing running at 3,600 RPM in a clean, cool environment needs regreasing far less often than the same bearing at 1,800 RPM in a hot, dusty one. The interval depends on speed, temperature, load, contamination, and the grease itself.
The SKF relubrication method provides a systematic way to calculate the interval based on these factors. It starts with a base interval determined by bearing type and speed factor, then applies correction factors for temperature and contamination. The result is a calculated interval in operating hours that can be converted to calendar days based on the machine's duty cycle. This guide covers the SKF method, grease quantity calculations, grease selection criteria, and the over-greasing problem that causes as many failures as under-greasing.
The SKF Relubrication Interval Method
The SKF method starts with a base relubrication interval (t_f) determined by the bearing speed factor and the bearing type. The speed factor is: n × dm, where n is the rotational speed in RPM and dm is the mean bearing diameter in mm: dm = (d + D) / 2. This is the same ndm factor used for speed limit calculations.
For deep groove ball bearings, the base interval at a speed factor of 200,000 is approximately 10,000 hours. At 400,000, it drops to about 4,000 hours. The relationship is roughly logarithmic: doubling the speed factor cuts the base interval by about 60%. Cylindrical and spherical roller bearings have shorter base intervals than ball bearings at the same speed factor because of their higher friction characteristics.
SKF publishes the base interval as a graph (or equation) for each bearing type. For a quick estimate for deep groove ball bearings:
- ndm < 100,000: base interval approximately 20,000 to 30,000 hours
- ndm = 200,000: approximately 10,000 hours
- ndm = 300,000: approximately 5,000 to 6,000 hours
- ndm = 400,000: approximately 3,000 to 4,000 hours
- ndm > 500,000: approximately 1,000 to 2,000 hours (grease is reaching its speed limit)
This base interval assumes a bearing temperature below 70°C (158°F), light to normal load (P/C < 0.1), a clean environment, and good quality lithium-based grease. Every deviation from these assumptions requires a correction factor that reduces the interval.
Temperature and Contamination Correction Factors
Temperature is the single biggest modifier of grease life. Grease degrades through oxidation and base oil evaporation, both of which accelerate exponentially with temperature. The general rule is that grease life halves for every 15°C (27°F) increase above 70°C (158°F). The SKF temperature correction factor (f_T) is:
- Below 70°C (158°F): f_T = 1.0 (no correction)
- At 85°C (185°F): f_T = 0.5 (half the base interval)
- At 100°C (212°F): f_T = 0.25 (quarter the base interval)
- At 120°C (248°F): f_T = 0.06 to 0.10 (drastic reduction, consider oil lubrication)
The contamination correction factor (f_C) accounts for the environment:
- Clean environment, good seals: f_C = 1.0
- Light dust, indoor, adequate seals: f_C = 0.7 to 0.9
- Moderate contamination (cement, grain, sawdust): f_C = 0.4 to 0.7
- Heavy contamination (quarry, steel mill, outdoor marine): f_C = 0.1 to 0.4
- Water ingress or process fluid exposure: f_C = 0.1 to 0.2
The adjusted relubrication interval is: t_adj = t_f × f_T × f_C. For a bearing with a base interval of 8,000 hours, running at 90°C in a moderately dusty environment: t_adj = 8,000 × 0.4 × 0.5 = 1,600 hours. At 8,000 operating hours per year (continuous operation), that is a relubrication interval of about 73 days, not 30 days and not 180 days. The calculated value replaces the guesswork.
Grease Quantity: How Much Is Right
The regreasing quantity is as important as the interval. Too little grease does not displace contaminated grease from the contact zone. Too much grease causes churning, heat generation, and elevated temperatures that degrade the grease faster. The correct quantity is just enough to purge the old grease from the bearing and replenish the contact surfaces.
The SKF formula for regreasing quantity is: G = 0.005 × D × B, where G is the grease quantity in grams, D is the bearing outer diameter in mm, and B is the bearing width in mm. For a 6310 bearing (110mm OD, 27mm width): G = 0.005 × 110 × 27 = 14.9 grams, or about 0.5 ounces. This is roughly one shot from a standard grease gun (most guns deliver about 1 to 1.5 grams per pump stroke, so 10 to 15 strokes).
This quantity assumes the bearing housing has a drain or relief path for excess grease. If the housing is sealed with no drain, the old grease has nowhere to go, and adding new grease builds internal pressure that pushes grease past the seals, overloads the bearing cavity, and raises the operating temperature. For sealed housings, use half the calculated quantity or install grease relief fittings.
For initial fill (new installation), the total grease volume in the bearing and housing cavity should be 30% to 50% of the free space. Filling to 100% causes immediate overheating during the run-in period. SKF recommends running the bearing at reduced speed for 15 to 30 minutes after a full fill to allow excess grease to be displaced from the rolling element path. Some procedures call for running the bearing for an hour, then removing the drain plug to let excess grease purge before capping the drain.
Grease Selection: Base Oil, Thickener, and Compatibility
Grease is not just thick oil. It is a three-component system: base oil (provides the lubrication film), thickener (holds the oil in place), and additives (anti-wear, anti-oxidant, corrosion inhibitor). Choosing the right grease means matching all three components to the application.
Base oil viscosity is the most important selection criterion. The oil film must be thick enough to separate the rolling elements from the raceways. The required viscosity depends on the bearing speed and the operating temperature. SKF provides a minimum required viscosity (nu_1) chart based on bearing mean diameter and speed. If the actual base oil viscosity at operating temperature is below nu_1, the lubrication film is too thin and wear accelerates.
Common thickener types and their characteristics:
- Lithium (Li): General purpose, good water resistance, usable to 120°C (248°F). The most widely used thickener in industrial applications.
- Lithium complex (LiX): Higher temperature capability (to 150°C / 302°F), better mechanical stability. The preferred upgrade from simple lithium for demanding applications.
- Polyurea (PU): Excellent high-temperature performance (to 160°C / 320°F), long life, commonly used in electric motor bearings. Not compatible with lithium greases.
- Calcium sulfonate complex: Outstanding water resistance, excellent EP properties, moderate temperature range. Ideal for wet environments, steel mills, and marine applications.
- Aluminum complex: Good water resistance and adhesion. Common in food-grade applications (with appropriate base oils).
Grease compatibility is critical. Mixing incompatible greases causes the combined product to soften, harden, or separate. The most dangerous incompatibility is between polyurea and lithium: mixing them produces a runny, ineffective lubricant that drains out of the bearing. If you change grease types, purge the old grease completely before introducing the new one. Apply three to five times the normal quantity to flush the housing, run the machine for 30 minutes, drain the excess, then apply the normal quantity.
The Over-Greasing Problem Nobody Talks About
In most plants, over-greasing causes more bearing failures than under-greasing. The mechanism is straightforward: excess grease in the bearing cavity gets churned by the rolling elements, generating friction heat. The temperature rises, the grease oxidizes faster, and the base oil separates from the thickener. The hardened thickener forms a crust that blocks fresh grease from reaching the contact surfaces. The bearing starves for oil in a housing full of degraded grease.
Over-greasing also blows out seals. When a grease gun pumps 150+ psi into a sealed bearing housing, the pressure has to go somewhere. It pushes past lip seals and labyrinth seals, creating a grease leak that looks like a seal failure but is actually an overfill problem. Once the seal is compromised, contamination enters, and the real failure starts.
Signs of over-greasing:
- Bearing housing temperature rises after regreasing and stays elevated for hours
- Grease leaking from seals shortly after relubrication
- Dark, oxidized grease found during bearing inspections despite frequent relubrication
- Grease accumulation around the seal area or on the shaft
Prevention: use the calculated quantity, not "a few pumps from the grease gun." Calibrate your grease gun by weighing the output per stroke. Most standard lever-type grease guns deliver 1.0 to 1.5 grams per stroke, but this varies with grease consistency and gun condition. Know the delivery rate of your specific gun. Better yet, use a metered grease gun that measures the volume dispensed. The $150 investment in a metered gun prevents thousands of dollars in over-greasing failures.