April - Library

AMMJ
Cage noise is not uncommon especially
in grease lubricated bearings and is often
symptomatic of the running-in process
as the grease is worked or “milled”
and disperses itself within the bearing.
Similarly, the presence of vibration at the
BPFO does not necessarily indicate a
problem and may be a result of variable
compliance (see “Variable compliance”
section of Part 1).
3 - Vertical Impact Crusher
A vibration assessment was made on a
vertical impact crusher prior to undergoing
field trials. The main aim was to verify
that the new bearing arrangement,
comprising a cylindrical roller bearing
(type NU2230E) and duplex bearing
(type QJ326) at the DE and a cylindrical
roller bearing (type NU2230E) at the
NDE, was operating satisfactorily.
The shaft rotational speed was 1750 rpm
and it was driven by a pair of bevel gears
with a ratio of 1:1(36 teeth), giving a gear
mesh frequency of 1050Hz.
Vibration acceleration was measured
radially on the rotor gear drive housing,
Figure 8.
Vibration at shaft rotational frequency
(29.2Hz) is evident along with a number
of harmonics. Vibration is also present
at 237Hz, which corresponds to the
BPFO of the cylindrical roller bearing,
along with harmonics at 474Hz and
711Hz which are just evident on the
linear amplitude scale.
The predominant vibration is at the gear
mesh frequency, f gm, of 1048Hz, along
with a number of sidebands at the shaft
rotational frequency, f s. The presence
of sidebands at rotational frequency is
not unusual, especially in the case of
sidebands at f gm ± f s.
As more sidebands appear at higher
amplitude, however, this is normally
an indication of gear eccentricity or
backlash. It was therefore decided
to remove the drive shaft, inspect the
bearings and adjust the gear backlash.
All the bearings appeared in generally
good condition, although it should be
emphasised that because the bearings
were not removed from the housing it
was not possible to inspect the outer
ring raceways, especially those of the
cylindrical roller bearings where vibration
at f b/o had been detected albeit at a
relatively low amplitude.
Role of Vibration Monitoring in Predictive Maintenance
Figure 9 Radial vibration acceleration spectrum on the housing
of a vertical impact crusher after adjustment of gear backlash
Figure 10 Comparison of sidebands around gear mesh frequency
(a) Before adjustment of gear backlash
(b) After adjustment of gear backlash
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Vol 24 No 2