Bearing Relubrication Intervals: How Often to Grease Bearings

Bearing Relubrication Intervals: How Often to Grease Bearings decides whether a rolling-element bearing reaches its calculated fatigue life or fails early from starvation or over-greasing. Grease is not a "top it up when you remember" consumable. Its working life depends on speed, temperature, bearing type and environment, and that life is calculable, not guessable.

Why Relubrication Intervals Matter

Grease degrades through oxidation, mechanical shearing of the thickener, and bleed of the base oil that lubricates the rolling contacts. Once bleed rate drops and the thickener hardens, the bearing fails weeks or months later from a starvation wear mode that looks, on inspection, like normal fatigue. Getting the interval right is a direct lever on achieved L10 bearing life, not just housekeeping.

The Speed Factor Method (n x dm)

The standard starting point for a regrease interval is the speed factor: rotational speed in rpm multiplied by bearing mean diameter dm in millimetres (dm = (bore + outside diameter) / 2). Manufacturers publish nomograms converting it to a baseline interval in operating hours: below roughly 200,000 (lower for rollers), a year or more; 200,000 to 400,000, roughly 5 to 12 months; above 400,000, weeks, with grease sometimes giving way to oil lubrication or oil mist. That baseline assumes a clean environment, light to normal load, and bearing temperature at or below about 70 C, so treat it as a starting point and apply the derating factors below.

Bearing Type and Load Effects

Deep-groove ball bearings are the most grease-tolerant and give the longest intervals for a given speed factor. Cylindrical roller bearings run shorter due to higher contact stress and heat; spherical and tapered rollers, with more sliding contact, run shorter still. Heavier loads (load ratio C/P below about 10) accelerate grease shear and oxidation, cutting the interval by 50% or more versus a lightly loaded reference case.

Temperature and Environmental Derating

Temperature is the most aggressive accelerant of grease ageing. As a rule of thumb, grease life roughly halves for every 10 to 15 C rise above a normal reference point (commonly 70 C at the outer ring), an Arrhenius-type effect also seen in general oxidation chemistry, though the real curve steepens further at higher temperatures.

A bearing at 100 C on a machine speced for 70 C needs relubrication roughly four times more often than baseline. Tracking temperature alongside vibration severity levels gives an early read on whether the interval still holds. Contamination shortens intervals independently: dust cuts it 20 to 50%, humidity or washdown duty 30 to 50%, nearby vibration or misalignment further still. Where factors stack, multiply them together rather than applying only the worst one; ignoring that is a common reason "correctly scheduled" bearings still fail early.

The Grease Quantity Formula

Interval and quantity are separate calculations: too little starves the contact, too much raises churning temperature past the grease's thermal limit. The widely used SKF formula for a single relube shot is G = 0.005 x D x B, where G is grams of grease, D is bearing outside diameter and B is total width, both in millimetres. An 85 mm OD, 19 mm wide bearing takes about 8 grams per shot, the top-up mass at each interval, not the initial fill (typically 30 to 50% of free internal space, less at high speed).

Over-Greasing vs Under-Greasing Damage

Both look similar on a maintenance record (bearing replaced, cause "unknown") but the mechanisms differ. Under-greasing leaves contacts starved, running in boundary or mixed lubrication regimes rather than full film, causing adhesive wear and accelerated spalling; temperature trends up slowly, then vibration rises sharply as damage starts. Over-greasing lets excess grease churn in the housing, generating heat that softens the thickener and causes bleed-out or channel collapse; a temperature spike right after relubrication is the classic signature, usually from a grease gun with no metered dose. Metered shots, or automatic lubricators sized to the formula above, remove the guesswork.

Relube vs Replacement Decision

Relubrication assumes the bearing itself is sound. Replace rather than relube when the bearing has run past its calculated L10 life, vibration shows defect frequencies rather than a lubrication signature, purged grease contains metal particles or discolouration, or housing seals are compromised. Relubricating a bearing with surface-initiated fatigue only delays a failure vibration data would otherwise flag early.

Tracking speed factor, temperature trend and relube history per asset in a CMMS turns this into an auditable schedule. In Fabrico, relube intervals attach to the asset record and adjust automatically when a temperature or vibration alarm flags a derating condition, so the next work order reflects actual severity, not a calendar date. Book a Fabrico demo to see interval-based and condition-based lubrication tasks run side by side.

Frequently Asked Questions

How do I calculate the grease relubrication interval for a bearing?

Start from the baseline interval for your bearing type and speed factor (n x dm), then derate for operating temperature (roughly halving per 10 to 15 C above 70 C), load severity, and contamination, multiplying the factors together rather than using only the worst one.

How much grease should I add at each relube?

Use G = 0.005 x D x B, where G is grams of grease, D is outside diameter and B is width, both in millimetres. This is the per-relube dose, not the initial fill.

What happens if a bearing is over-greased?

Excess grease churns and generates heat in the housing, softening the thickener, causing oil bleed-out, and producing a temperature spike shortly after relubrication, a failure mode as damaging as starvation.

When should I replace a bearing instead of relubricating it?

Replace when the bearing has exceeded its calculated fatigue life, vibration shows defect-frequency signatures rather than a lubrication signature, purged grease shows contamination, or seal integrity is compromised.