Sealed for life means the bearing is filled with grease at the factory and is not intended to be relubricated. The claim is not infinite life: it means the grease life, calculated at the design operating temperature, exceeds the bearing's calculated fatigue life. Change the temperature or the speed and the claim no longer holds.
Grease life is estimated from the base oil viscosity, the speed factor and the operating temperature, using a standard life equation. If that figure is longer than the bearing's L10 fatigue life at the design load, the bearing is declared sealed for life. Nothing about the seal itself is involved in the claim — it is entirely about the grease.
Three ways: the housing runs hotter than the design assumption (very common — motors with blocked cooling, conveyors in summer), the speed is higher than assumed, or the seal is damaged at mounting and water enters. All three shorten grease life without changing the label. When a sealed bearing fails at 30% of its rated hours, the cause is almost always temperature.
| Meaning | Factory grease fill sized for the design life |
|---|---|
| Relubrication | Not possible — seal must be destroyed |
| Fill | 25–35% of free space |
| Governing calculation | Grease life versus L10 fatigue life |
| Temperature rule | Life halves per 15 °C above 70 °C |
| Typical claim | 20,000–40,000 hours at rated temperature |
| Failure mode | Grease exhaustion, not fatigue |
| Check | Measure the housing temperature in service |
| Condition | Assumed | If exceeded |
|---|---|---|
| Temperature | 70 °C at the outer ring | Life halves per 15 °C |
| Speed | Below the grease ndm limit | Grease cannot reach the raceway |
| Load | Within the rating | Fatigue before grease life |
| Contamination | Seal intact | Water or dust enters, life collapses |
| Mounting | No seal damage | Immediate ingress |
No. It means no relubrication. The bearing still has a calculated life, after which it is replaced.
Not in practice. The seal is destroyed on removal and replacement seals are not supplied for standard bearings.
Temperature, in most cases. Measure the outer ring temperature in service and compare it with the design assumption — 15 °C is a factor of two in grease life.
Send the designation, the three dimensions and the quantity — we reply with price, brand options and lead time within 30 minutes on working days.