DN Factor


  • This factor is used to set the limit for ball bearing (precision bearing) speed.
  • It limits the use of precision bearings (ball bearings) if the bearing bore (in millimeters) x rpm is 300,000 or greater (shaft diameter, in inches, x rpm = 12,000 or greater)
  • The real limit of bearing speed, however, is the temperature of the bearing lubricant.
    • Both oil and grease have a useful life of about thirty years at 30°C (86°F).
    • This life is cut in half for each 10°C (18°F.) rise in temperature.

I received the following very practical information from: Heinz P. Bloch, P.E., Process Machinery Consulting 5459 Ponderosa Drive, West Des Moines, IA 50266-2843

  • Oil rings become unstable at certain linear speeds. They are very sensitive to shaft horizontality; after all, not even laser optic alignment tools ensure horizontality. And aren’t we putting shims under part of the assembly? What does that do to horizontality? Why should we care?
  • Out of horizontality, oil rings run downhill and touch a stationary interior part of the bearing housing. Then they slow down and become unstable. Worse yet, they will now abrade—you can see loss of chamfer after a few months of operation. The wear product contaminates the lube oil, which causes bearings to fail very rapidly (see reams of FAG, NTN and SKF data on the subject).
  • Oil rings are “immersion-sensitive”—-more oil, more drag. They are “viscosity-sensitive,” meaning thicker = oil more drag. They are awfully sensitive to both out-of-roundness and wrong RMS surface finish.

Combine all of the above and you’ll probably end up with a “safe” DN-limit (inches of shaft diameter times RPM) of somewhere around 6,000. That’s my experience-based limit; it was also used in Exxon Company’s lube marketing internal training courses.

Many U.S. pump manufacturers violate this limit, thinking perhaps of an installation at plant “X” where they got away with a DN-limit in excess of 6,000. Sure it worked at “X”—perhaps because certain of the various sensitive factors were not there. But beware when a few of them combine. You’ll be in deep trouble and won’t even suspect why!