Rolling Element Bearing: Forcing Frequencies

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Rolling element bearings are used in all sorts of rotating applications. Unfortunately, because of poor machine setup and maintenance, fewer than 10% of bearings reach their intended design life; this is because even marginal increases above the maximum allowable load on a bearing can dramatically shorten its life span. For example, if the load is increased by 20%, the bearing life decreases by almost half.

Learning to recognize bearing wear is useful for maintenance practices, and can prevent damage to machinery if a catastrophic failure occurs. Forcing frequencies due to bearing faults and/or defects are usually easy to detect because they are typically non-synchronous. Non-synchronousfrequencies are usually a good indicator that a bearing problem may exist and other sources of non-synchronous frequencies should be ruled out first to confirm this diagnosis. 

Each component inside a rolling element bearing will produce its own, unique, forcing frequency if it is damaged or defective. Bearing forcing frequencies include:

  • Ball pass frequency inner race (BPFI) – caused by an inner race defect
  • Ball pass frequency outer race (BPFO) – caused by an outer race defect
  • Cage frequency or fundamental train frequency (FTF) – caused by a defective cage
  • Ball spin frequency (BSF) – caused by defects to the balls

The forcing frequencies for bearings are sometimes provided by their manufacturers, but they can also be calculated if you know the following information:

  • The number of balls in the bearing
  • The diameter of the balls
  • The pitch diameter of the bearing
  • The contact angle between the bearing and the inner and outer races

The bearing forcing frequencies can also be estimated for those bearings that have between 8 to 12 balls or roller elements, as follows:

  • BPFI     = 0.6 x n x N
  • BPFO    = 0.4 x n x N
  • FTF    = 0.4 x N

where N is the shaft speed, and n is the number of balls