age, the K(t) decreases. During operation the K(t) value may also increase at a low
rate. In this way the condition of the rolling bearing can be classified:
K(t)
Rolling Bearing Condition
> 1
Improvement
1 .. 0.5
Good condition
0.5 .. 0.2
Damage progressive influences
0.2 .. 0.02
Advanced damage process
< 0.02
Damaged
Multiplication of the RMS and peak value indicates the acceleration over the course
of time as well as changes caused by shock, local damage (pitting), a general in-
crease in mechanical vibration due to distributed damage (worn race rings, corro-
sion, wear of rolling elements) or also insufficient lubrication.
The K(t) value only provides a statement about the condition of the bearing when
the trends are viewed. One single measurement does not enable a diagnostic state-
ment to be made.
The K(t) value is strongly dependent on the rotational speed. It is therefore impor-
tant to ensure that the rotational speed is the same, when viewing trends.
3.6. Rotational Speed
Alongside vibration the VM25 measures the rotational speed by means of an inbuilt
contactless optical sensor (Figure 34 on page 25). To enable this, the instrument
needs to be aligned with the rotating part, for which a laser pointer can be used to
help with the positioning.
Notice: Please do not cover the sensor apertures on the back of the instrument with
your fingers during measurement.
3.7. Temperature
The VM25 has an inbuilt infrared thermometer for taking contactless temperature mea-
surements (Figure 34 on page 25).
4. The Batteries
4.1. Inserting the Batteries
The VM25 is supplied by three alkaline standard cell type AAA (LR03) batteries.
NiMH batteries (HR03) may also be used. The minimum energy requirement of the
VM25 enables maximum utilization of the batteries.
Attention: Please switch the instrument off before changing the batteries. When
switched off, the contents of the memory are stored for a few minutes, without need
of the batteries, and the internal clock continues to run. If the batteries are removed
when the instrument is switched on or if they remain in the instrument until the bat-
teries have been completely discharged, the date and time will need to be reset. Fur-
ther settings as well as the saved measurands, are stored without need of the batter-
ies.
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