Grease vs oil: lubricating a bearing

A rolling bearing does not run metal-on-metal — it floats the rolling elements on a film of lubricant microns thick. Get that film right and the bearing lasts; get it wrong and it fails long before anything wears out. The first choice is grease or oil, and they suit different jobs.

Why lubrication decides bearing life

The lubricant separates the rolling elements from the raceways, carries heat away, and helps keep contamination out. Most bearings that fail early do not wear out — they are starved of lubricant, given the wrong one, or let grit into the film. Lubrication and cleanliness matter more to how long a bearing lasts than almost anything else.

Grease — stays put, low maintenance

Grease is oil held in a soap-like thickener: it stays where you put it, helps seal the bearing against dirt, and needs little attention. It suits the great majority of applications — motors, pumps, fans, general machinery — and is what fills sealed and shielded bearings from the factory. Its limits are speed and heat: grease does not circulate to carry heat away, so very high speeds or hot environments can outrun it.

Two numbers describe a grease. The NLGI grade is its stiffness (NLGI 2 is the common general-purpose consistency); the base-oil viscosity is the oil inside doing the actual lubricating, chosen for the speed and temperature. A grease that is too stiff or too thin for the duty lubricates poorly even if it is "the right amount".

Oil — cooling and high speed

Oil circulates, so it carries heat away and reaches into a bearing at speeds grease cannot follow. That is why gearboxes and high-speed spindles run in oil — a bath the bearing dips into, or a circulating/mist system. The cost is containment: oil needs a sealed sump and seals to keep it in and dirt out, which is more plumbing than a packed grease bearing. This is also why oil-bath housings use open bearings — an internal seal would only block the oil.

Which bearings can be relubricated

• Sealed (2RS): greased for life — you cannot relubricate them. When the grease is done, so is the bearing.
• Shielded (ZZ) and open: can be relubricated, through the shield gap or a grease line, on a schedule.
• Open in an oil system: lubricated continuously by the bath or circulation.

So the closure choice and the lubrication plan are linked — see 2RS vs ZZ. A machine with a grease line and a relubrication schedule is not designed around a sealed-for-life bearing.

The over-greasing trap

More grease is not better. Packing a bearing and its housing completely full leaves the rolling elements churning through grease that cannot escape — that churning generates heat, and the heat can do more damage than slightly too little grease. The usual guidance is to fill the bearing itself and roughly a third of the free space in the housing, leaving room for the grease to move. On high-speed bearings the fill is smaller still.

Practical takeaways

• Grease for most things; oil for high speed or where cooling is needed.
• Match the grease to the duty — NLGI grade and base-oil viscosity, not just "bearing grease".
• Don't wash the grease out of a sealed or shielded bearing.
• Don't over-pack — a third of the housing space, not full.
• Keep it clean: contamination in the film is a leading cause of early failure.

General engineering reference only. Correct lubricant type, quantity and relubrication interval depend on the bearing, speed, temperature and environment — follow the equipment or lubricant manufacturer's recommendations.