BPFI – Ball pass frequency Inner

The spectrum shows several harmonic peaks of the inner ring failure frequency (typically between 8 and 10 harmonic multiples of BPFI) modulated by sidebands at 1x revolutions per minute (RPM).

E.g. for the SKF 22228 bearing, the frequencies are calculated for 1500 revolutions:

So in the spectrum we would see 1x BPFI peak at 269.7Hz and harmonics 2x, 3x, … and then side bands of speed around them, in our case 1500 RPM / 60 = 25Hz, i.e. 1xBPFI = 269.7 +25Hz -25Hz

Examples of inner ring damage:

Real-world example, time, spectrum and envelope, inner ring damage: