Why topping up with the wrong grease can wreck a bearing

A bearing fails a few months after a routine relubrication. The grease that came out was the right NLGI grade, the gun was clean, the interval was correct. What changed was the drum — a different brand, same specification on paper. This is one of the most under-recognised causes of bearing failure in industrial plant, and it is entirely avoidable.

A grease is not an oil, and the difference is the point

Grease is base oil held in a thickener. The thickener is a matrix — think of a sponge — that holds the oil in place and releases it slowly into the contact as the bearing runs and warms. The bearing is lubricated by the base oil. The thickener's job is to stay where it is put and meter the oil out.

That structure is what makes grease useful in a bearing that would throw an oil straight out. It is also what makes grease vulnerable in a way oils are not: the matrix can be destroyed by chemistry, and once it is, the grease can neither stay in place nor release oil correctly.

What incompatibility actually does

When two greases with incompatible thickeners are mixed, the mixture usually fails in one of two directions, and both end at the same place.

• It softens, sometimes dramatically. The mixture loses its structure, runs out of the bearing housing, and leaves the contact dry.
• It hardens. The matrix stiffens, stops releasing base oil into the contact, and the bearing runs on what is left.

Either way the bearing loses its oil supply, and it does so some time after the relubrication — which is why the connection is so often missed. The maintenance record shows a correct relubrication; the failure looks like a bearing that simply did not last.

Which thickeners cause trouble

Compatibility is a property of the specific pair, and published compatibility charts disagree at the edges. A few patterns are consistent enough to work from:

In our own range that has a direct consequence. The general purpose and EP greases are lithium-thickened — Grease EP2 uses lithium hydroxystearate, Grease MP and Grease EAL WR are lithium. Grease WR2 is thickened with water-free calcium, which is precisely what gives it its water resistance. WR2 and the lithium greases are not interchangeable on a top-up, even though all of them are NLGI 2.

Base oil viscosity is the other half of the specification

Because the bearing is lubricated by the released base oil, the viscosity of that oil should be selected the same way you would select an oil — for the speed, load and temperature of the bearing. A grease with the correct thickener and the wrong base oil viscosity is still the wrong grease.

The spread within a single NLGI 2 range is wide. From our published data, base oil viscosity at 40 °C runs from 110 mm²/s on the stern bearing grease, through 150 on the general purpose grades and 220 on the EAL, up to 800 mm²/s on Grease WR2. That last figure is not a rounding error — it is a grease built for slow, heavily loaded, water-exposed service, and it would be entirely wrong in a high-speed bearing.

Changing grease without gambling

If a change is necessary, the mixture is the thing to avoid — not the new grease.

• Where the bearing can be dismantled, clean it out completely and repack. This is the only method with no residual risk.
• Where it cannot, purge: relubricate repeatedly while the bearing runs until only the new grease emerges from the relief. Expect to use several times the normal charge.
• Label the point with the grease now in it. An unlabelled grease point is the reason this happens in the first place.
• Where compatibility is genuinely unknown and purging is impractical, treat the pair as incompatible.

And while you are there: more grease is not better

Over-greasing is a failure mode in its own right and it is more common than under-greasing in plants with a conscientious maintenance culture. A bearing housing packed full of grease has no space for the grease to move aside, so it churns. Churning generates heat, the heat degrades the grease and drives off the base oil, and the bearing ends up running in a hardened residue.

On electric motors there is a second consequence: excess grease forced past the inboard bearing has nowhere to go except into the windings. On sealed bearings, the pressure blows the seal and opens the bearing to the very contamination the seal existed to exclude.

Relubrication quantity and interval come from the bearing manufacturer's calculation, based on bearing size, speed and temperature. Both are calculable. Neither is a matter of judgement at the grease gun.

Full published data for the grease range — NLGI grade, thickener, penetration, drop point and base oil viscosity, with the test method beside each value — is on the technical data page.

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