Over-Greasing Kills More Bearings Than Under-Greasing

Ask a maintenance technician what happens if a bearing does not get enough grease and you will get an immediate, correct answer. Ask what happens if it gets too much and the answer is often a shrug. That asymmetry is why we open so many failed bearings packed solid with hardened, blackened grease.

What too much grease actually does

A rolling element bearing is not supposed to be full. The rolling elements need to move through a space that is mostly empty, carrying a thin film of lubricant into each contact. When the free space is packed, the elements have to plough through grease instead of rolling through air.

That ploughing is called churning, and it starts a chain that is entirely self-inflicted:

1. Churning generates heat. The bearing is now doing mechanical work against a viscous medium, continuously.
2. Heat degrades the grease. Base oil oxidises and the thickener breaks down. Every 10 °C rise roughly halves grease life — a rule of thumb, but a well-supported one.
3. Degraded grease separates and hardens. Oil bleeds out and the residue turns to a dry, varnish-like solid.
4. The hardened residue blocks fresh grease from reaching the raceways.
5. The bearing starves.

The end state of over-greasing is a lubrication-starved bearing. That is the part people find counter-intuitive, and it is why "it failed dry, so we should grease it more often" is such a durable and expensive mistake.

The collateral damage

Over-greasing does not stop at the bearing:

• Seals fail. Pumping grease into a full housing raises pressure until something yields, and the seal is usually the weakest part. Once the seal lip is pushed out, contaminants get in.
• Electric motors lose windings. Excess grease from a drive-end bearing can migrate into the windings, where it degrades insulation. A motor rewind costs many times what the grease saved.
• Housekeeping suffers. Purged grease attracts dust and grit, which turns into an abrasive paste at exactly the point where you least want one.

How much grease a bearing actually needs

There is a widely used formula for relubrication quantity:

Take a 6312 deep groove ball bearing: outside diameter 130 mm, width 31 mm.

G = 0.005 × 130 × 31 = 20 grams.

Twenty grams. That is roughly two to three strokes of a typical lever grease gun — and it is dramatically less than most bearings receive in practice, because the common instruction is "grease it until you see it purge", which is not a quantity at all.

For initial fill on a new bearing the guidance is different: fill the bearing itself completely, but fill only about 30–50% of the free space in the housing. At high speeds, less. The empty space is not wasted — it is where displaced grease goes and where heat escapes.

Calibrate your grease gun

The formula is useless if you do not know what your gun delivers. Grease gun output per stroke varies substantially between makes, and manufacturers' stated figures are often optimistic once a gun has been in service.

How often

Quantity is the easy half. Interval is harder, because it depends on speed, load, temperature, bearing type, orientation and environment.

The dominant factor is the speed factor: shaft speed in rpm multiplied by the bearing's mean diameter in millimetres. Manufacturers publish relubrication interval charts against this factor, and the intervals shorten sharply as it rises. They then apply correction factors — for high temperature, vertical mounting, heavy load, contamination or vibration, all of which shorten the interval, sometimes by half or more.

Use your bearing manufacturer's chart for the specific bearing. What matters more than the precise number is that the interval is derived rather than inherited. "Monthly" is not a lubrication strategy; it is a habit that happens to have a calendar attached.

Signs you are over-greasing

• Grease visibly purging from seals, or a persistent ring of expelled grease around housings
• Bearing running temperature that rises after greasing and stays up — a brief rise is normal, a sustained one is not
• Hardened, dark, dry grease found on strip-down, particularly in the housing cavities
• Repeated seal failures on the same machine
• Grease contamination inside motor terminal boxes or on winding heads
• A lubrication route defined only by frequency, with no recorded quantity

Getting it right

1. Calculate the quantity for each greased point and record it on the route. Not "grease bearing" — "20 g".
2. Calibrate every grease gun and mark its output on the body.
3. Derive intervals from the manufacturer's chart and the real operating conditions, not from tradition.
4. Clean the fitting before connecting. A dirty nipple injects abrasive directly into the load zone. Wiping it takes two seconds.
5. Grease while running where it is safe to do so, so fresh grease distributes and excess can find the relief path.
6. Open the relief plug on motors that have one, and leave it open long enough for purged grease to escape rather than migrate inward.
7. Never mix incompatible greases. Mixing thickener types can produce a mixture that performs worse than either alone. Label machines with the grease they take.
8. Consider automated single-point lubricators for critical or hard-to-reach bearings. A device delivering a small, correct dose continuously beats a large, guessed dose monthly.

The point

Lubrication is the cheapest reliability intervention available to any plant, and the one most often done by feel. The instruments are a set of scales and a calculator. The technical content is one formula and a manufacturer's chart.

If your route sheets say "grease monthly" with no quantity against any point, that is the highest-return thing on your maintenance improvement list — and it can be fixed with an afternoon and a paint pen.