ADASH Ltd.
Adash 4900 – Vibrio III
6
Machine and bearing condition diagnostics
Basic defects which we’d like to find out are
- unbalance (heavy spot on the rotor causes vibrations),
- misalignment (machinery parts are not in alignment ),
- looseness (machine is not properly connected with its base - soft foot),
- bearing defect (wear of bearing, bad assembly, bad lubrication or overload).
First three defects influence the whole machine (e.g. the vibration caused by unbalance we can take
from any point on the machine). We use the vibration velocity [mm/s] measurements for that.
Roller (ball) bearing condition we can detect only on the nearest point. This is the local failure. We
measure always vibration acceleration only [g].
Methods for ball bearing condition diagnostics
We can find out several methods for bearing condition detection in literature. We should repeat again:
we have to measure vibration acceleration in [g]
for correct data acquisition. All methods must satisfy the conditions.
We can choose different procedures for evaluation of the acceleration signal. Measured signal can be
imagined as a level of the river. It flows with appropriate speed and there are little or big waves. If we
want to measure the stream we can measure the flow per hour or prompt wave's level. The value of
the flow will be stable and it will change slowly. But the wave's levels are unstable because
measurements have significant variety of values.
The similar effects occur for bearing condition measurements. You can measure RMS value (the total
energy in signal) or PEAK value (the highest peak in signal). We can use both types for evaluation,
just have to realize of advantages and disadvantages.
RMS measurement
- advantages
-
stable and well repeatable, time trends are well readable
- disadvantages
-
if wear increases the response is slower then PEAK, but sufficient for
maintenance.
PEAK measurement
- advantages
-
fast response for any condition change.
- disadvantages
- not stable and well repeatable (extremely sensitive), time trends are not well
readable.
From these two basic measurements further measurements are derived:
- g
ENV
-
envelope
signal modulation. Advantages and disadvantages are in the middle
of RMS and PEAK measurements.
-
g
SE
BCU, SEE, SPM -
measurements are performed usually on the sensor resonance frequency.
These methods have the same advantages and disadvantages like the PEAK measurement.
The relation between gears and ball bearings
For gearboxes diagnose it is also necessary to measure acceleration signal like for ball bearings.
When the balls are rolling over the damage tracks (pitting), the shock pulses occur in signal.
Unfortunately similar shocks are also in signal from on worn or damaged gears. So if we measure
gearbox with roller bearings then higher vibration values can be caused by both sources.
More information for this kind of analysis you can find in chapter Vibrations in frequency range -
gearboxes/bearings.
