Bearing Lubrication Intervals: SKF Source Boundaries & Grease Selection

The SKF Relubrication Source Boundary

SKF handbook guidance defines the speed factor as n × dm, where n is rotational speed in RPM and dm is mean bearing diameter in mm: dm = 0.5 × (d + D). The handbook obtains the relubrication interval from a diagram using speed factor A, bearing factor bf, and load ratio C/P. That diagram/source step matters: a simple closed-form equation in a web tool should be treated as a local approximation unless it has been digitized and validated against the current manufacturer source.

For a maintenance review, the useful workflow is to collect actual bearing series, bore d, outside diameter D, width B, speed, temperature, load ratio, grease product, orientation, contamination, seals, housing, and duty cycle. Then compare any screening result against SKF/Schaeffler/OEM data, product grease data sheets, and site condition-monitoring history before scheduling work.

When a guide or calculator gives a single interval from bore and RPM alone, treat it as a planning screen. It cannot know the exact bearing factor, catalog dimensions, cage, clearance, seal, grease, purge path, or operating condition changes that the manufacturer method expects the reviewer to consider.

Temperature and Contamination Review Factors

Temperature is one of the strongest modifiers of grease life. SKF handbook guidance reduces relubrication interval above 70°C and limits how far colder operation can extend the interval. Product grease limits, bearing design, housing airflow, and measured bearing temperature still have to be checked.

• Above 70°C (158°F): review the SKF temperature reduction and the grease product temperature range.
• Every 15°C increase above 70°C is commonly treated as a halving check in SKF-style guidance.
• Below 70°C: do not extend intervals without checking low-temperature pumpability, grease separation, condensation, and manufacturer limits.
• High-temperature applications may need different grease, oil lubrication, bearing clearance review, or root-cause work to reduce heat.

Contamination also needs review rather than a blind multiplier:

• Clean environment, good seals: screening intervals may stay near the base review value.
• Dust, moisture, washdown, or process exposure: shorten the review interval and inspect seals, breathers, purge path, and housing design.
• Heavy or abrasive contamination: review whether improved sealing, labyrinths, purging, or bearing/housing changes are needed instead of simply adding more grease.
• Water or chemical ingress: check grease compatibility, corrosion protection, relubrication path, and failure history.

Use correction factors as visible assumptions. The calculated value should start a reliability review; it should not replace the OEM procedure, condition data, or qualified judgment.

Grease Quantity: How Much Is Right

The relubrication quantity is as important as the interval. Too little grease may not replenish the contact surfaces or purge old grease. Too much grease can churn, raise temperature, overload seals, and leave degraded thickener in the housing.

SKF handbook guidance gives different replenishment factors for different paths. Relubrication through features in the bearing ring uses Gp = 0.002 × D × B. Side relubrication uses Gp = 0.005 × D × B. In both cases Gp is grams, D is bearing outside diameter in mm, and B is bearing width in mm. For a 6310 bearing with D = 110 mm and B = 27 mm, the side-relubrication quantity is 0.005 × 110 × 27 = 14.9 grams.

The factor is not the whole procedure. Confirm the actual bearing catalog dimensions, the grease path, the housing drain or relief path, the grease-gun delivery per stroke, seal condition, and whether the OEM procedure uses a different amount or method. If a housing cannot purge excess grease, adding the calculated quantity without a relief path can still create a problem.

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.

Over-Greasing and Review Controls

Over-greasing is a common failure contributor because excess grease in the bearing cavity can be churned by rolling elements, generating heat. The temperature rises, the grease oxidizes faster, and separated or hardened thickener can block fresh grease from reaching contact surfaces.

Over-greasing can also damage seals when the housing cannot relieve pressure. A leak after relubrication may be a sign of excess quantity, a blocked purge path, an unsuitable grease path, or a seal/housing problem rather than proof that the route needs more strokes.

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 starts with measured gun output, actual D/B dimensions, the right SKF/OEM replenishment path, relief fitting condition, and a route that records what was added. A metered or calibrated gun reduces guesswork, but it still has to follow the approved procedure.