SPM BDM-40 - Alarm Levels and Speed Correlation

5
Installation of Bearing Display Module BDM
RPM
RPM
Measuring range 0 to 80 dBsv, alarm level at 35 dBn
Measuring range 20 to 100 dBsv, alarm level at 35 dBn
Shaft
diameter
mm
Shaft
diameter
mm
Alarm levels in relation to bearing seize and speed
The initial shock pulse level (dBi = decibel initial) of rolling bearings in good condition depends on bearing
size and rotational speed. The beginning of the red ”bad condition” zone is normally dBi + 35 dB (35 dBn on
the normalized scale). In the diagrams, this level is shown in dBsv (unnormalized shock value) and in mA.
Example:
A bearing with shaft diameter d = 20 mm and rotational speed = 1100 rpm has a dBi of 10. 35 dBn thus
corresponds to 45 dBsv, that is 13 mA for measuring range 0 to 80 dBsv, and 9 mA for measuring range 20 to
100 dBsv.
As signal strength also depends on the quality of the measuring point, it may be necessary to adjust the alarm
level to the individual bearing, normally downwards.
Whenever possible, the bearing should be measured with a SPM Tester before setting alarm levels. This
instrument will also show the exact initial value dBi of a bearing when shaft diameter and rpm are input.