Rolling element bearing problem

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.

Note:
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)

Comments are closed.