How Much Grease to Put in a Bearing

01Two Chambers, Not One

The first space sits inside the bearing, between the rings, the balls, and the cage. The second is the free space in the housing around it, and on most machines it is much larger than the first.

Grease inside the bearing cavity does the lubricating. Grease in the housing acts as a reservoir that feeds the rolling contact slowly as the machine runs. Treating the two as one volume is where overfilling starts.

This is true across the family. Deep groove ball bearings in pumps and conveyors share the same layout, and so do the larger rings in a gearbox. The proportions change with size. On a compact part like the 6205 bearing the internal cavity holds very little, so nearly all of what you pump in goes into the housing. The principle does not.

02Myth One. Fill It Until It Squeezes Out

The habit comes from watching someone pack a truck wheel hub, where the cavity is small, the speed is low, and the seal is a lip that tolerates a little pressure.

A machine housing is a different situation. Grease with nowhere to go gets pushed against the seal from the inside until the lip lifts, and from that moment the position is losing lubricant on the side that matters.

There is a second version of the same habit. Someone fills the housing completely, then runs the machine and lets the excess work its way out past the seal as a form of self-levelling. What actually leaves is the lightest part of the grease, and what stays behind is the thickener.

03Myth Two. More Grease Means Longer Life

Extra grease does not extend life once the rolling contact is fully separated. Past that point it adds drag and nothing else.

Churning is the mechanism. When the balls have to push a solid mass of grease aside on every revolution, that work becomes heat. Heat ages grease faster than a small shortage ever would, so the generous fill can shorten the interval it was meant to extend.

Small bearings feel this first. A 6203 bearing on a high-speed shaft has little room and little thermal mass, and a heavy fill shows up as a warm housing within an hour of the service.

04Myth Three. The Seal Will Hold Whatever You Put In

Seals keep contamination out. That is the job they were designed for. A contact seal holds a lip against a running surface with light spring pressure and a thin film of oil under it.

Push grease past that lip and two things follow. The lip loses contact pressure and begins to weep, and the same path lets dust in on the next stroke of the machine. Neither shows up until the bearing is already wearing.

The seal also has a speed limit. A lip running against a surface at speed generates its own heat, and that heat is designed for with a thin film of oil under the lip, not a wall of grease behind it.

05Myth Four. Quiet Means Correctly Filled

A freshly overfilled bearing often runs quietly for the first hours, because there is lubricant everywhere and nothing is starved.

Temperature is the better signal. A housing that runs warmer after a service than it did before is telling you the fill is heavy, the grease is the wrong consistency, or both. Check it by hand at the same point on the casting, at the same load, an hour apart.

Check Temperature, Not Sound

Put a hand on the same spot on the housing before and after the service, at the same load. A difference you can feel is worth acting on. A difference you need an instrument to find is usually not.

06Myth Five. Topping Up Is the Same as Relubricating

Adding fresh grease to a housing full of old grease mixes two materials of two different ages. The new grease does not push the old grease out. It finds the easiest path and stays there.

If the old grease has separated, hardened, or collected dust, the result is a paste that neither lubricates well nor flows back to the contact. At that point the position needs a clean and a measured refill, not another shot from the gun.

When a Clean Is the Only Option

If the old grease has gone hard in the housing, no amount of fresh material will restore the film. The position needs the housing opened, the old grease removed, and a measured refill. It is an hour of work that saves a bearing.

07What Overfilling Does Inside the Housing

Churning and Heat

Every ball has to displace grease as it rolls. A light film is swept aside easily. A full cavity means the cage drags a solid plug around the raceway, and the energy that costs leaves as heat in the housing and in the grease itself.

Pressure at the Seal

Grease expands as it warms. In a sealed housing with no free volume, that expansion has to go somewhere, and the weakest point is the seal lip. Once it lifts, the position leaks and the same gap admits water and grit.

Where the Old Grease Goes

In a housing with free volume, old grease is pushed to the sides and stays out of the rolling path. In a packed housing it has nowhere to go, so it is dragged back through the contact on every revolution and worked until it separates.

The choice of lubricant matters here as well, though less than the amount. Our note on grease versus oil covers when a different lubricant type is the right answer instead of a different fill.

08A Fill Routine That Works

A routine beats a rule of thumb, because it adapts to the housing in front of you instead of the one in a manual.

• Clean the fitting and the housing around it before anything opens, so grit cannot travel in.

• Add in small amounts and let the shaft turn between them, so the grease spreads instead of stacking against one wall.

• Watch the housing temperature rather than the clock, and stop when it stops falling between shots.

• Run the machine for a shift, then recheck temperature and the seal line before you sign the job off.

• Write down what went in, on which date, and what the housing read afterwards.

On larger positions the same steps apply with more patience. A 6304 bearing in a hot housing needs the temperature check more than the small ones, because a heavy fill there is harder to notice and slower to settle.

09Four Checks Before You Add Grease

10Questions About Grease Volume