![background image](http://html.mh-extra.com/html/spm/bc100/bc100_user-manual_1349098013.webp)
9
Normalized and unnormalized readings
The Bearing Checker measures impact velocity over a large dynamic range. In order to simplify readout
and evaluation, a logarithmic measuring unit is used: decibel shock value (dBsv).
dBsv is the general measuring unit for shock pulses. By measuring the shock pulses from a bearing in dBsv
a value for their magnitude is obtained, for instance 42 dBsv. However, this value is only part of the infor-
mation needed to judge the operating condition of the bearing. We also need a standard of comparison,
i.e. a norm value for identical or similar bearings.
Such norm values have been obtained empirically, by measuring the shock pulses from a large number of
new, perfect ball and roller bearings. They are called “initial values”
dBi
(decibel initial). The dBi value can
be set manually or calculated by the instrument after input of rpm and shaft diameter (see chapter “Input
data”). The highest dBi value that can be entered is +60, the lowest -9. Any attempt to enter values below
this will result in dBi “- -” and an unnormalized shock pulse reading (see below).
By subtracting the dBi from the dBsv value we obtain the “normalized” shock pulse value or dBn (decibel
normalized) of the bearing, for example: 42 dBsv–10 dBi = 32 dBn”. The normalized shock pulse value dBn
is the measuring unit for the operating condition of bearings. A maximum value of 32 dBn means “32 dB
above normal”, which implies “reduced operating condition” for the measured bearing. By programming
the Bearing Checker with the dBi before taking a reading, the bearing condition will be indicated directly
on the condition display in green-yellow-red for “good”, “reduced” or “bad” operating condition for the
measured bearing. “Bad operating condition” can be synonymous with “bearing damage”, but the term
also includes a number of other “bearing faults” which can be detected by shock pulse measurement. The
initial value dBi of a bearing is directly related to its rotational speed and shaft diameter.
The absolute shock pulse level of a bearing, measured in dBsv (decibel shock value), is both a function of
rolling velocity and of bearing condition. The dBi value of the bearing must be entered in order to neutral-
ize the effect of rolling velocity on the measured value.
The Bearing Checker takes a sample count of the shock pulses occurring over a period of time and dis-
plays:
•
the
maximum value dBm
for the small number of strong shock pulses.
•
the
carpet value dBc
for the large number of weaker shock pulses.
•
a lit-up LED on the condition scale (for normalized readings only): green for dBn up to 20 dBn =
good condition, yellow for 21-34 dBn = caution, red for 35 dBn and more = bad condition.
The maximum value dBm defines the bearing’s position on the condition scale. The difference between
dBm and dBc is used for a finer analysis of the causes for reduced or bad condition.
Unnormalized readings
For unnormalized readings, set the dBi value to “- -” (see chapter “Input data”). You will then measure in
dBsv (absolute shock values) and get
no condition indication
, as the condition scale is graded in normal-
ized shock values, dBn. This method is used for comparative readings on different bearings and/or other
shock pulse sources.