The spectrum of the vibration measured on a machine containing a faulty bearing will contain one or more of the bearing frequencies. Most often, and especially when a fault starts developing, the vibrations caused by a bearing fault will be obscured by the vibrations from other rotating parts like shafts, gears, etc., and the bearing frequencies cannot be seen in either the time history or the spectrum of the vibration. The vibration from the healthy rotating parts will show up in the lower frequency range of the spectrum, i.e., a few harmonics of shaft speeds, tooth mesh frequencies, etc., but fortunately, the bearing fault vibration will manifest itself throughout the spectrum. This is due to the fact that a repetitive impulse, with repetition time = T, has a corresponding line spectrum consisting of all the harmonics of the repetition frequency = 1/T. The radial load in the bearing determines the strength of the impact from rolling over a fault. A fault in a stationary bearing race will be subjected to the same force at each roll and consequently all the pulses in the pulse train will be of equal strength/height. On the other hand, a fault in a rotating race will be subjected to a varying force, the variation repeating itself with the RPM of the race. This means that the pulse train will be amplitude-modulated with the RPM of the race, and in turn all the harmonics in the line spectrum, BPFO or BPFI (whichever correspond to the rotating race), will appear amplitude-modulated by the RPM of the race. Likewise, the BFF caused by a ball/roller fault, will be amplitude-modulated by the FTF, the RPM of the cage. If there is more than one fault of a kind, the line spectrum will still contain the harmonics of the bearing frequency. Only the 'shape' of the spectrum will change depending on the relative positions of the faults.
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