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2 Signal from a damaged bearing
The pattern shown is typical for damaged bearing surfaces:
a dBm above 35 dB, a large gap between dBm and dBc,
and a
random
pattern of strong pulses. The strength of the
maximum value dBm indicates the degree of damage:
35 – 40 dB
N
Slight damage
40 – 45 dB
N
Severe damage
> 45 dB
N
High breakdown risk.
First signs of damage
dBm values between 20 and 35 dB (in the yellow zone)
and a moderate increase of the carpet value are a sign of
stress in the bearing surfaces or minor damage. Note that
the gap between dBm and dBc gets larger.
Bearings with dBm values in the yellow zone should be
measured more frequently, to determine if their condition
is stable or deteriorating.
Note
: a similar pattern is caused by contaminations in the
lubricant (metal or dirt). The particles either originate from
parts of the bearing itself, for instance from a damaged
cage, or they are transported by the lubricant into the
(undamaged) bearing. Test bearing and lubricant accord-
ing to the description “Confirming bearing damage” in
this manual.
Cracked inner ring
A clean crack in the inner ring of a bearing is difficult to
detect, especially at a low rpm. You may get low readings
through most of the bearing’s rotation, then one or two
peaks while the crack is in the load zone. Signal strength
can differ considerably as the crack opens or closes de-
pending on bearing temperature. In time, the surface
tends to spall along the crack, leaving sharp edges and
metal particles which cause high shock values until they
are rolled out.
dBm
dBc
dBm
dBc
Irregular measuring results
Large variations between consecutive readings are a dan-
ger sign. Damaged bearings do not improve with time,
although their shock values may temporarily drop.
Make sure the measuring interval is established according
the variations in production load (e.g. air compressors).
Always measure under the same production conditions.
Wide variations in the readings taken at different times
can occur on heavily loaded roller bearings with surface
damage. The high readings are caused by metal particles
breaking off the surfaces and by the sharp edges of new
spallings. When particles and edges are rolled out, the
readings will drop again.