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Bearing Speed Guide — dn Value Check
Multiply bore by speed to get the dn value, then compare it with the indicative range for the bearing type, lubrication and sealing you plan to use.
dn value (mm × rpm)
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Indicative dn guidance
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Utilisation of guidance
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Shaft speed limit (m/s at bore)
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What dn means
The dn value is bore in millimetres multiplied by speed in rpm. It measures how fast the contact between rolling element and raceway travels, which is what limits a bearing: heat generation, lubricant film and cage loading all scale with it rather than with rpm alone. A 25 mm bore at 3,000 rpm and a 50 mm bore at 1,500 rpm both give dn = 75,000.
The figures used here are the ranges commonly quoted for typical catalogue products - about 300,000 for a grease-lubricated ball bearing, 500,000 with oil, higher for precision angular contact designs and lower for thrust and spherical roller types.
What changes the limit
Lubrication: oil removes heat as well as lubricating, so it roughly doubles the range.
Seals and shields: a contact seal adds friction and heat; derate roughly 25 %.
Cage: machined brass or polyamide cages support higher speeds than a plain pressed steel cage.
Precision and internal design: precision classes and optimised internal geometry raise the limit further.
Load: the quoted limits assume light load; heavy load lowers the attainable speed.
Reference speed versus limiting speed
Catalogues publish two numbers. The thermal reference speed is where a defined reference temperature is reached under reference conditions, and it is the realistic ceiling for continuous running. The limiting speed is a mechanical limit for short runs or light duty only. See thermal reference speed and speed limits and dn for the detail.
Frequently asked questions
Is dn the same as the catalogue limiting speed?
No. dn guidance is a screening aid. The catalogue figures for the exact part - thermal reference speed and limiting speed - account for the cage, internal design, seal type and grease, and they are what you should quote in a specification.
Why does a contact seal reduce the speed limit?
Because the rubber lip rubs the counterface continuously, generating friction torque and heat at the seal. The energy has to leave through the same housing as the bearing heat, so the attainable speed falls - roughly 25 % in typical industrial sizes.
How do I raise the speed limit?
Move to oil or oil-air lubrication, reduce the load, choose a machined cage, use a precision class with better geometry, or switch to hybrid ceramic balls - which also cut heat generation and electrical erosion. Each step trades cost and complexity for speed.
Can a bearing run above the limiting speed?
Not continuously. The limiting speed is a short-term mechanical ceiling; running there and beyond quickly destroys the lubricant and then the cage. If the duty demands more, change the internal design or the lubrication system.
Need the exact bearing for this duty?
Send the load, speed and environment - we will suggest the type, clearance and lubricant.