The Bearing That Was Greased to Death
A maintenance department had a rule: “grease every bearing every month.” It sounded diligent. On a 75 kW fan motor, the technician pumped grease until it oozed out the seals. Six months later, the bearing failed — the seals had blown out from over-greasing, the grease had churned into a hot paste, and the bearing ran dry. The motor had to be rewound. The failure was not a lack of maintenance. It was too much of the wrong kind.
Relubricating bearings is one of those tasks that looks simple and punishes overconfidence. Do it wrong and you either starve the bearing (under-lube) or cook it (over-lube). This is how to get it right.
When a Bearing Needs Grease
Grease inside a bearing does two jobs: it separates the rolling elements from the races (boundary lubrication) and it carries away a little heat. The grease degrades over time — oil bleeds out, the thickener breaks down, contamination enters. The replacement interval depends on speed, load, temperature, and bearing type. The manufacturer’s table is the starting point; never invent a number.
As a rough guide, a sealed bearing (ZZ or 2RS) is factory-greased for life — do not regrease it. A shielded or open bearing (with a grease fitting) is the one you service. If you cannot tell the difference, look for a zerk fitting: if it has one, it wants grease; if not, leave it alone.
How Much Grease: The Volume Mistake
The most common error is pumping until grease comes out the seal. That is wrong. The bearing cavity is only about one-third full of grease — the rest is air space, which allows the grease to circulate and prevents churning. Overfill causes churning heat, seal blowout, and early failure. The right amount is roughly:
Volume (cm³) ≈ 0.005 × Bore (mm) × Outer diameter (mm)
For a 6208 bearing (40 mm bore, 80 mm OD), that is about 16 cm³ — a few pumps, not the whole tube. If grease squeezes out the seal, you have overdone it.
The golden rule: Pump in small bursts (one or two pumps), rotate the shaft (by hand or jog), and let it run. Watch the temperature — if the bearing runs hotter after greasing, you over-filled. The fan motor that failed had grease squeezed out both seals.
How Often: Interval by Speed
The relubrication interval is dominated by speed. A slow bearing (under 100 rpm) may go years between greases. A high-speed bearing (3000 rpm) needs grease every 3–6 months. The bearing manufacturer’s speed factor (dn = bore × rpm) gives the exact interval. If you have no data, a practical starting point:
| Speed | Interval |
|---|---|
| <100 rpm | 12–24 months |
| 100–1000 rpm | 6–12 months |
| 1000–3000 rpm | 3–6 months |
| >3000 rpm | 1–3 months (or consult factory) |
Temperature halves the interval. If the bearing runs above 70°C, halve whatever the table says. Contamination (washdown, dust) also shortens it.
Clean the Fitting Before You Pump
Before attaching the grease gun, wipe the zerk fitting. Dirt on the fitting gets pushed into the bearing with the grease. A bearing that fails early often has contamination introduced during the greasing itself. This 10-second step saves more bearings than the grease does.
The Right Grease: Do Not Mix
Greases are not interchangeable. Mixing incompatible thickeners (lithium with polyurea, for example) can cause the grease to soften and bleed. Use the same brand and type the bearing was packed with. If you must switch, flush the bearing with solvent and repack — do not top up a different grease.
Signs of a Bearing in Trouble
Even on schedule, a bearing tells you when it is dying. Listen for a rattle (rolling element damage), feel for heat (over 80°C is a warning), and watch for discoloration (blue, brown races). A weekly touch (with the back of the hand) and a stethoscope save more bearings than any schedule.
A Relubrication Checklist
- Is this bearing open/shielded (grease fitting) or sealed (no fitting)?
- Do you have the manufacturer’s interval table?
- What is the actual running speed (rpm)?
- Is the temperature high (halve the interval)?
- Is the environment dirty (halve again)?
- Wipe the zerk before attaching the gun.
- Pump in small bursts — a few, not the whole tube.
- Run the bearing; watch the temperature for 30 minutes.
- Use the same grease type — do not mix thickeners.
- Log the date and grease type on the bearing.
The Bottom Line
Bearing greasing is not a “more is better” task. The fan motor that failed had too much grease, not too little. Use the right volume (a few pumps), the right interval (by speed and temperature), the same grease type, and a clean fitting. The bearing that lasted was greased on schedule — not on impulse.
