FTF – Fundamental Train Frequency – cage defect
Cage defects are characterized by the presence of the cage failure frequency (FTF) and its harmonic peaks in the spectrum. Generally, a defect in the cage is accompanied by defects in the ring tracks, and the FTF usually modulates one of these track failure frequencies, resulting in sum and/or difference frequencies.
E.g. for the SKF 22228 bearing, the frequencies are calculated for 1500 revolutions:
So in the spectrum we would see 1x TFT peak at 10.8Hz and multiples of 2x, 3x, …
A real case of diagnosing a damaged cage:
Examples of cage damage: