Reference speed vs limiting speed: what bearing speed ratings actually mean
Two numbers, two different failure mechanisms
Open any bearing page — for example 6308 — and you find two speed ratings. They are not redundant:
Reference speed is a thermal rating. It is the speed at which, under a standardized light load and standardized lubrication, the bearing reaches a defined temperature equilibrium. Above it, heat generation outruns dissipation: grease oxidizes, oil film thins, and the bearing’s practical life collapses even though nothing mechanically breaks.
Limiting speed is a mechanical rating. It is the ceiling set by the physical strength and stability of components — primarily the cage — under centrifugal forces and component kinematics. Exceed it and the failure is mechanical: cage fracture, skidding damage, seizure.
Which one governs
| Your speed is… | Meaning | Action |
|---|---|---|
| Below both ratings | Standard duty | Any standard execution works |
| Above reference, below limiting | Thermally critical | Improve heat removal: oil lubrication, better grease, cooling, looser clearance (C3) |
| Above limiting | Mechanically unsafe | Different execution or bearing type — precision class, hybrid, smaller bearing |
Sealed bearings compress this table: 6205-2RS carries a single reduced limiting speed because the seal lip is the weakest thermal and mechanical element. That is one of the real costs in the open vs sealed trade-off.
The four factors that move the real limit
Catalogue speeds assume reference conditions. Your application shifts the practical limit through:
- Load — heavier load means more contact friction and heat. High speed combined with high load is the worst case; ratings assume light reference load.
- Lubrication — grease is the constraint in most positions; oil bath, circulating oil or oil-air raises the achievable speed substantially. This is why the grease selection guide treats temperature as the dominant variable.
- Internal clearance — at high speed the inner ring runs hot and expands, consuming clearance; insufficient clearance at speed leads to preload from thermal expansion, more heat, and a runaway loop. High-speed positions typically pair with C3 — see the clearance guide.
- Cooling and surroundings — a bearing in a sealed gearbox with hot oil has far less margin than the same bearing on a ventilated shaft.
Practical selection rules
- For continuous-duty grease-lubricated positions, keep running speed at or below roughly two-thirds of the reference speed unless you have verified temperatures in service.
- For angular contact bearings like 7205 in paired arrangements, remember the pair’s speed rating is lower than the single bearing’s — preload adds friction.
- For sealed bearings, design against the sealed limiting speed, not the open one; if the duty needs more speed, switch to shields (ZZ) or open with external sealing.
- When you are near a limit, measure temperature — bearing position temperature stabilizing 20–40 °C above ambient is normal; climbing steadily past ~90–100 °C on the housing means the thermal limit is being exceeded in practice.
The ratings on each model page are catalogue values from the respective manufacturer sources; when two brands list different speeds for the same size — visible on the model’s equivalents table, e.g. the 6205 cross references — treat the lower one as the safe planning value.
Frequently asked questions
Can a bearing run above its catalogue limiting speed?
Sometimes, with engineering measures: precision-class bearings, oil-air or circulating-oil lubrication, adapted internal clearance, and accurate seats. Running a standard catalogue bearing past its limiting speed without these measures risks cage failure and seizure — treat the limiting speed as a hard ceiling for standard executions.
Why is the 2RS limiting speed so much lower than the open version?
Contact seals add rubbing friction that scales with speed. A sealed 6205-2RS typically limits around half to two-thirds of the open bearing's limiting speed — the seal lip, not the rolling elements, becomes the heat source. Non-contact shields (ZZ) keep almost the full open-bearing speed rating.
Which rating should I design against?
Start with the reference (thermal) speed for grease-lubricated, continuously running positions — heat is what kills the grease first. Check the limiting speed as a mechanical ceiling, especially for the cage. If your speed is below both with margin, any standard execution works; between the two, you need better lubrication or cooling; above both, a different bearing design.
Does lubrication method change the speed rating?
Yes. Catalogue reference speeds assume specific reference conditions; oil lubrication removes heat better than grease and raises the achievable speed. That is why high-speed spindles use oil-air or oil-jet systems — the same bearing cannot reach those speeds on grease.
What happens if a bearing runs too slow?
Very low speeds can be as problematic as high ones: below a minimum speed the rolling contacts cannot build a full lubricant film, causing mixed friction, wear and smearing. Heavily loaded slow positions need higher-viscosity lubricant and sometimes EP additives rather than a bigger bearing.