The vibrations caused by bearings are called bearing tones. All antifriction bearings generate more or less high levels of lagertoner. The more worn the bearing becomes the higher the levels of the bearing tones are.
An incorrect bearing generates vibrations not exact multiples of 1xRPM, i.e. asynchronous vibration components. Except these components bearing faults can also generate a broad banded noise.
There are four different types of bearing tones:
Fundamental Train Frequency (FTF) = The rotational speed of the train, usually 0.4 multiplied with the rotational speed. FTF seldom appears in vibration spectrums as the train hardly carry any load.
Ball rotation frequency (BSF) = The balls or rollers rotaional speed. A hole in a ball/roller causes BSP to appear in the frequency spectrum.
Ball passage frequency, outer race (BPFO) = The rate of which the ball/roller passes a defect in the outer race. This frequency is estimated as:
the number of rollers x rotational speed x 0.4.
Ball passage frequency, inner race (BPFI) = The rate of which a ball/roller passses a defect in the inner race. The level of BPFI is often slightly lower than BPFO as the vibration is generated further away from the transducer. The vibration has to pass through the roller and the outer rece before it is registrated. BPFI is estimated as
the number of rollers x rotational speed x 0.6.
|Source of vibration||Exciting frequency||Dominating direction||Amplitude||Spectral characteristic||Comments|
|Defect inner ring, small single defect||Bpfi, multiples on bpfi||Radial||Constant||Asynchronous peaks||Radial load|
|Defect inner ring, increased single defect||Bpfi, with multiples and side band on 1 X||Radial||Amplitude modulated||Asynchronous peaks with sideband on 1 X||Radial load increase level|
|Defect inner ring, two or three small defects||Bpfi and significant multiples||Radial||High, constant||Broadband||broadband noise, increase in level|
|Defect inner ring many small defects with damage all around the bearing race||Bpfi and significant multiples||Radial||High, constant||Asynchronous peaks||Broadband noise, increase in level|
|Defect outer ring, small single defect||Bpfo and multiples||Radial||Constant||Asynchronous peaksr||Multiples have higher level than basic frequrncy|
|Defect outer ring, increased single defect||Bpfo and multiples||Radial||Constant||Broad band||Increased broaband noise level|
|Defect ball or roller, single ball defect||Bsf and multiples||Radial||Constant||Narrow band|
|More defect balls||Bsf and multiples||Radial||High, constant||Narrow band||Increased broaband noise level|
|Damaged fundamental train, broken at one place||Broadband noise||Radial||Broad band||Low level on noise|
|Fundamental train broken in pieces||Broadband noise||Radial||Broad band||Noise due to fragments from FTF in the bearing|
|Bad pretension or oil film||High frequency noise “haystack”||Radial||Broad band||Natural bearing frequency|
|Loose bearing, rotation around shaft||1 X multiples||Radial||Narrow band||Number of multiples and the amplitude is a measure of how loose the bearing is.|
|Loose mount in bearing house||1X, 2X, 4X||Radial||Narrow band||Increased low frequency noise floor.|
|Severe looseness or bearing fault||0,5 X and multiples||Radial||4X to 8X and/or 7X to 15X|
|Cocked bearing||1X, 2X bearing frequences||Axial, radial||High constant||Narrow band||180 degrees phase difference at the axial direction at each side of the bearing house.|
X = speed (RPM)
ftf = fundamental train frequency (approx. = 0,38X to 0,42X)
bsf = ball rotation frequency (approx. = 1,5X to 3X)
bpfi = ball pass frequency, inner ring (approx. = 4X to 10X)
bpfo = ball pass frequency, outer ring (approx. = 2X to 7X)