What is the limiting speed? Why are the limiting speeds different for grease lubrication and oil lubrication?

What is the limiting speed?

The limiting speed (n) is a key rated value given in bearing catalogs/spec sheets: the maximum rotational speed at which a bearing can run continuously under specified conditions (typically load no more than 10% of the basic dynamic load rating, i.e. P ≤ 0.1C, with proper lubrication and cooling).

It is essentially a heat-balance boundary: friction inside the bearing generates heat while the housing dissipates it. The higher the speed, the faster heat is generated; once generation outpaces dissipation, temperature keeps rising, causing lubricant breakdown, accelerated wear, and cage damage — life drops sharply. So the catalog limiting speed is essentially the safe ceiling near the point where "heat generated = heat removed."

Why do grease and oil lubrication have different limiting speeds?

The difference comes down to two curves: friction heat generation vs heat dissipation capability (as shown in the diagram):

Grease lubrication (left)

1. High churning resistance: grease is base oil thickened into a semi-solid paste. At high speed, rolling elements and the cage have to churn through this viscous medium, creating more friction and more heat than oil.
2. Heat gets trapped: grease sits statically in the bearing cavity — it doesn't circulate, so heat escapes only slowly through the housing walls.
3. More heat + poor dissipation → the heat-balance point arrives earlier → lower limiting speed, typically only 70%–85% of the oil-lubricated value.

Oil lubrication (right)

1. Low viscosity, less friction: oil flows easily, oil-film shear resistance is small, so less heat is generated in the first place.
2. Active cooling: oil can circulate (oil bath, jet, oil-mist, etc.), and incoming cool oil carries frictional heat away — a built-in "cooling system."
3. Less heat + strong dissipation → heat balance arrives later → higher limiting speed.

summary

The limiting speed is determined by the balance between frictional heat generation and heat dissipation; grease generates more heat and dissipates poorly, while oil has lower friction and can carry heat away by circulation — so for the same bearing, the grease-lubricated limiting speed is typically well below the oil-lubricated one.

Practical tip for your bearing website: in product spec tables, the limiting speed is conventionally shown in two separate columns — "Grease" and "Oil" (e.g. deep groove ball bearing 6208: ~8,500 r/min with grease, ~10,000 r/min with oil) — that's the standard industry presentation.