Older catalogues gave every bearing one limiting speed. Modern catalogues give two: a thermal reference speed and a limiting speed, and buyers who read only the first number, or only the second, can seriously misjudge a machine. The pair exists because "how fast can this bearing run" has two honest answers - how fast before it overheats in a defined thermal environment, and how fast before its mechanics (cage dynamics, seal contact, precision) run out of margin. Understanding which number applies to your duty is a ten-minute lesson that prevents overheated gearboxes and over-conservative designs alike. Two bearings with the same 45 mm bore can differ by a factor of three between their two ratings, so reading only one of them is a systematic error, not a rounding one.
This guide unpacks the heat balance behind the reference number, explains what the mechanical ceiling actually protects, and walks through the corrections that turn a catalogue figure into an estimate for your machine. The companion speed limits and dn guide covers the wider speed budget; here the focus is on reading and using the second number correctly.
One question, two honest answers
Ask a bearing how fast it can run and physics gives two answers because two different things fail first depending on the duty. In a cool, lightly loaded fan the grease or the housing temperature gives out long before any cage problem appears - that is the thermal side. In a precision spindle with generous cooling, the heat is removed easily but the cage, the seals or the running precision reach their validated envelope - that is the mechanical side. Neither rating replaces the other, and neither is a safety factor for the other. Catalogues publish both so the engineer can check both.
The heat balance behind the reference speed
The thermal reference speed answers a thermodynamic question: at what rotational speed does the bearing's frictional heat generation equal the heat that a standardised reference environment can remove? The reference conditions are defined in ISO 15312: a light mineral oil, a defined load (typically 3 to 5 percent of the static load rating), a specified ambient and housing heat dissipation, run without external cooling. At that speed the bearing reaches thermal equilibrium at a permissible temperature, commonly 70 °C. Run slower and it stays cooler than the limit; run faster and heat accumulates until something gives - the grease, the clearance or the material temper.
Because it is a heat balance, the reference speed is not a property of the bearing alone. It shifts with viscosity: a thinner oil generates less churning friction, so the equilibrium moves to higher speed. It shifts with load: heavier load means more contact friction and more heat. It shifts with cooling: forced air, oil circulation or a massive housing all raise the sustainable speed above the reference value. The published number is a calibrated starting point for an engineering estimate, not a wall.
The mechanical ceiling of the limiting speed
The limiting speed marks where the bearing's mechanical systems leave their validated envelope: cage design and material (stamped steel cages centrifuge outward, machined bronze and polyamide cages each have their own regimes), seal friction and lip wear, the centrifugal loading of balls or rollers on the raceways, and the precision of the running surfaces. Exceeding it does not necessarily produce instant failure; it produces operation outside the conditions under which the bearing was tested and rated. For sealed bearings the limiting speed is usually set by the seal; for high-speed open bearings it is set by the cage, as the cage design guide explains. Roller bearings feel this ceiling sooner than ball bearings of the same envelope: a cylindrical roller cage carries heavier element inertia, and a spherical roller adds the drag of two tilted rows, so the gap between a ball and a roller rating at the same bore is often 30 to 50 percent.
Which rating governs which duty
| Situation | Which rating governs | Why |
|---|---|---|
| Standard grease, moderate load, natural cooling | thermal reference (with corrections) | grease and the heat balance limit first |
| Oil jet or circulating oil, precision mounting | limiting speed | heat is removed; mechanics govern |
| Sealed-for-life bearing | limiting speed (usually seal-limited) | seal lip friction and wear |
| Heavy load, slow duty | neither - life calculation governs | load, not speed, consumes life |
The last row is worth dwelling on. Below roughly a dn of 100,000 for most ball bearings, speed is rarely the design driver at all: the ISO 281 L10 life calculation against the applied load decides everything, and the L10 life calculation guide shows the method. Speed ratings earn their keep between the low-speed plateau and the mechanical ceiling.
Correcting the reference number
Catalogues and standards provide adjustment factors to convert the reference speed into an estimate for real conditions. Three corrections dominate, and they act in different directions.
Viscosity correction
The reference conditions assume a light oil of defined viscosity. If your lubricant is more viscous at operating temperature, churning losses rise and the sustainable speed falls; if it is thinner, the speed rises but the film margin falls. The corrections are not trivial: replacing a reference VG 32 oil with a VG 68 gear oil can pull the sustainable speed down by a quarter, while a VG 10 spindle oil can push it up by a similar margin. Check the base-oil viscosity at the real operating temperature, not the room-temperature grade, using the grease selection guide.
Load correction
The reference load is a small fraction of the static rating. As the actual load ratio rises, contact friction grows and the thermal equilibrium moves to lower speed. Corrections for load ratios up to about 0.15 of the dynamic rating are modest; beyond that, life rather than speed usually becomes the governing question anyway.
Cooling correction
Forced airflow over the housing, a cooling jacket, circulating oil or simply a larger casting all raise the heat-removal side of the balance. A pump on a light spindle oil with a cooling jacket can legitimately run well above the tabulated reference; a gearbox oil with a viscous EP additive in a compact unventilated housing may need to run below it. Forced air alone over a typical motor end-shield recovers 10 to 20 percent of the speed headroom; circulating oil can recover far more, because it removes heat at the source. The corrections are arithmetic, but they must be applied to the right baseline - which is why copying a speed rating between catalogues without reading the stated conditions is a quiet way to build a hot machine.
Grease life couples speed to maintenance
Grease-lubricated bearings carry a coupled constraint: speed shortens grease life, because higher shear and temperature age the thickener. A bearing thermally capable of high speed may still have impractical relubrication intervals on grease; the catalogue's grease-life curves against speed and temperature are the practical referee, and the interval guide gives the halving rule of thumb. Oil lubrication decouples the two - oil can be cooled, filtered and replaced continuously - which is why every genuinely high-speed spindle runs on oil-air, oil jet or mist systems rather than grease. The lubrication decision and the speed decision are therefore one decision, taken together.
Closures and their speed penalties
The same basic bearing ships with meaningfully different speed ceilings depending on its closure, so the closure belongs in the speed budget from the first draft.
| Closure type | Contact with rings | Speed penalty | Grease retention |
|---|---|---|---|
| Open | None | None | None - relubrication required |
| Metal shields (ZZ) | None | Very small | Good for fine particles, not for grease loss |
| Non-contact rubber labyrinth | None | Small | Good, the usual motor compromise |
| Contact seals (2RS) | Lip on inner ring | Largest, often 20 to 30 percent | Excellent - enables sealed-for-life |
When a design sits close to the thermal reference speed, choosing shields or non-contact seals over contact seals can be the difference between a cool bearing and a chronically warm one. The suffix codes for each closure are covered in the seals and shields guide.
A checking sequence in five steps
- Record the duty. Actual speed, load ratio, lubricant and its viscosity at operating temperature, cooling available, and the exact closure variant.
- Correct the reference speed. Apply the standard viscosity, load and cooling factors for the closest honest thermal estimate.
- Compare with the limiting speed of the exact variant - open, shielded, sealed, cage type - and respect the lower of the two numbers.
- Sanity-check grease life if grease-lubricated, and the precision and mounting system if high speed.
- Verify in service. Log the housing temperature after warm-up and compare it with the assumption; treat a rise as a correction to be made, not a nuisance.
When duty exceeds both ratings, the paths are known: better lubrication, hybrid ceramic balls, higher precision classes, or a larger bore running slower - each a standard lever, with candidate parts filterable in the bearing selector and the angular contact range and deep groove range, where most high-speed candidates live. Brand top-end figures differ; the FAG and SKF catalogues are worth comparing before a redesign, because the same 6208 envelope can carry limiting speeds 15 to 25 percent apart.
One last habit worth adopting: when a bearing runs warm in service, log its actual speed, grease type and housing temperature alongside the catalogue numbers before anyone orders a "faster" bearing. In most of those investigations the reference-speed arithmetic, done for the first time on real data, explains the temperature exactly - and the fix turns out to be lubricant viscosity, cooling or relubrication practice rather than a new part number at all. For machines where temperature trends matter, the technical library keeps the monitoring checklists.
- A heat balance, not a wall. The thermal reference speed is where friction heat equals heat removed in a standardised environment; viscosity, load and cooling move it.
- The limiting speed protects the mechanics. Cage, seals and precision define it, and it applies even when cooling is abundant.
- Check both numbers for the exact variant. Open, shielded and sealed versions of the same bearing can differ by a quarter of the speed range.
- Grease couples speed to maintenance. High speed on grease usually means short intervals; high speed usually means oil.
- Correct, then verify. Apply the factors, then let the measured housing temperature be the final judge.
Frequently asked questions
What does the thermal reference speed of a bearing actually measure?
It measures the speed at which friction heat generated equals the heat a standardised reference environment can remove, defined in ISO 15312 with a light oil, a small standard load and no external cooling, reaching about 70 degrees C. It is a thermal equilibrium figure, not a mechanical one.
Which speed rating governs a sealed bearing?
Usually the limiting speed, because the contact seal lip sets the mechanical and thermal envelope: lip friction adds heat and wears with speed. Sealed-for-life bearings therefore sit well below the open variant's figures, often by 20 to 30 percent.
Is it legitimate to run above the published thermal reference speed?
Yes, if the corrections justify it. Better cooling, thinner oil at operating temperature or a light load all move the heat balance upward, and circulating-oil designs routinely exceed the tabulated reference. What is not legitimate is exceeding the mechanical limiting speed, which no cooling can rescue.
Why does grease life fall as bearing speed rises?
Speed raises shear rate and temperature in the grease chamber. Grease ages by oxidation and oil separation, and its usable life roughly halves for every 10 to 15 degrees C above its base rating. A thermally capable bearing may still have impractical relubrication intervals, which is why high-speed duty usually switches to oil.
Do metal shields limit a bearing's speed?
Barely. Shields (ZZ) do not touch the rings, so their speed penalty is very small, and they keep fine particles out. What they do not do is retain grease against leakage, and they offer no barrier to grease loss at high temperature, so they suit clean, relubricatable duty rather than sealed-for-life designs.
How do I correct a catalogue reference speed for my real conditions?
Apply the catalogue's factors for lubricant viscosity at operating temperature, the actual load ratio, and the cooling arrangement, then compare with the limiting speed of the exact variant and respect the lower figure. Finally verify against the measured housing temperature after warm-up.