Short answer
Generally fill 1/3 to 1/2 of the bearing's free space; 1/3 for high-speed, 1/2 to 2/3 for slow heavy-load. Too much is worse than too little.
3 harms of over-greasing
- Temperature rise: excess grease is churned and heats up, accelerating degradation;
- Churning losses: higher motor starting current and noise;
- Leakage/throw-off: grease squeezes past seals, contaminates equipment, wastes grease.
Filling guide by application
| Application | Fill ratio |
|---|---|
| High-speed (>3000 rpm) | 1/4 to 1/3 |
| Medium (1000–3000 rpm) | 1/3 to 1/2 |
| Low-speed heavy-load (<1000 rpm) | 1/2 to 2/3 |
| Sealed bearings | Pre-filled, do not add |
| Gearbox | Per chart by tooth width × count |
How to estimate quantity
Rule of thumb (ball bearings):
Grease (g) ≈ bore (mm) × 0.005 × width (mm) / fill ratio
Practical:
- Medium-small (6204): ~5–8 g;
- Larger (6309): ~30–50 g.
What about too little?
Also bad. Under-greasing causes:
- Insufficient lubrication → faster wear;
- Poor heat dissipation → bearing overheating;
- 30–50% life reduction.
Conclusion
More grease is not better. Generally 1/3–1/2 is right. Too much hurts more than too little; too little shortens life. Just right is best.