Bearing grease selection and relubrication intervals: a practical guide

Updated 2026-10-05

The four decisions in grease selection

Bearing grease choice comes down to four properties, in this order of practical importance:

1. Base-oil viscosity at operating temperature — the oil does the lubricating; the thickener only holds it. ISO VG 100–220 mineral oil covers most general industrial duties at 40–100 °C. High speed or cold start points to lower viscosity (VG 68 or below, or synthetic PAO); high load and low speed point to higher viscosity (VG 320+) or EP additives.
2. NLGI consistency grade — how stiff the grease is. NLGI 2 is the default; see the FAQ for when to deviate.
3. Thickener type — determines water resistance, mechanical stability and compatibility:
   • Lithium-complex: the general-purpose standard, good all-round balance.
   • Polyurea: long life at high temperature, common in sealed-for-life motor bearings — do not mix with lithium.
   • Calcium-sulfonate: excellent water and corrosion resistance, good for washdown and marine.
   • Aluminum-complex: food-grade (H1) and good water resistance.
4. Temperature range and additives — check the grease’s operating range against the real bearing temperature (not ambient), and use EP (extreme-pressure) additives for heavily loaded roller bearings, oscillating motion or shock loads.

Factory fill vs field grease

A 6205-2RS leaves the factory roughly a quarter to a third filled with a long-life grease matched to its size. That fill is calculated to last the grease’s service life under normal conditions — which is why the maintenance decision for a sealed bearing is replacement timing, not relubrication. Open bearings like 6308 in a relubricatable housing, or spherical roller bearings like 22210 on conveyors, depend entirely on your grease program.

Relubrication interval — the working method

The widely used starting point is the standard relubrication diagram: from the bearing’s speed factor (bearing bore × running speed, adjusted by a bearing-type factor) read a base interval in operating hours, then correct it:

• Temperature: halve the interval for roughly every 15 °C above about 70 °C bearing temperature. Heat from running near the bearing’s thermal speed limit accelerates grease oxidation much faster — see reference speed vs limiting speed for the distinction.
• Contamination/moisture: dusty or wet environments can cut the interval to a fraction of the base value.
• Vertical shaft: typically halve the interval.
• Vibration: severe vibration shortens intervals and may call for a stiffer or more mechanically stable grease.

Treat the result as a starting point and adjust from inspection: grease pushed out dark, dry or contaminated means the interval was too long; grease purged continuously past the seals means it was too short or the quantity too large.

Grease quantity per relubrication

A practical field formula for the replenishment quantity is:

G (grams) ≈ 0.005 × bearing OD (mm) × bearing width (mm)

For a 6308 (90 mm OD × 23 mm wide) that gives roughly 10 grams per relubrication shot. Pump slowly while the shaft rotates so grease distributes through the rolling elements, and let excess escape through the purge path — if the housing has no escape route, use much smaller quantities and longer observation.

Five mistakes that cause most grease failures

1. Overgreasing — the number one cause of hot-running bearings after maintenance; churning heat then oxidizes the grease faster, a self-reinforcing failure.
2. Mixing incompatible thickeners — always identify what is in the bearing before switching.
3. Greasing a 2RS bearing — destroys the seal; see 6205 vs 6205-2RS for the open/sealed trade-off.
4. Ignoring temperature — grease life halves roughly every 15 °C; the datasheet interval assumes ~70 °C.
5. Using the bearing catalogue rating as a lubrication spec — speed and load ratings assume correct lubrication; the grease program is what makes them achievable.

If a failed bearing shows hardened, dark or absent grease, that pattern maps to lubrication failure — see the bearing failure modes guide to read the evidence before replacing like-for-like.