Lubrication
Deficiency
Case History Number 4
Rolling element bearings are among the
most important components in the vast majority of machines and exacting demands are made
upon their carrying capacity and reliability. The continued research and development of
rolling bearing technology has enabled engineers to calculate the life of a bearing with
some considerable accuracy, thus enabling bearing life and machine service life to be
accurately matched.
Unfortunately it sometimes happens that a bearing does not attain its calculated rating
life. There are many reasons for this - heavier loading than had been anticipated, Inadequate
or unsuitable lubrication, careless handling, ineffective sealing or fits that
are too tight with resultant insufficient internal bearing clearances. Each of these
factors produces its own particular type of damage and leaves its own special imprint on
the bearing.
This case history serves to demonstrates the possibility to detect in some cases the
effect of inadequate lubrication within a rolling element bearing. The machine used for
this demonstration is a 2 speed 1475/990 rpm/90 hp cooling tower fan motor. during testing
of the motor it was observed that an intermittent high pitched noise was being omitted
from what was suspected as the NDE bearing. Overhaul velocity rms. readings throughout the
motor exhibited values below 1 mm/sec rms and were considered low and acceptable.
To determine the cause of this noise, a set of 5000Hz acceleration readings were obtained
from both the NDE and DE motor bearings in the vertical, horizontal and axial directions.
On examination of the spectra from both DE and NDE bearings in the vertical direction a
'haystack' effect was evident in the region of 2000 to 3500 Hz see Figure 1
Figure 1 - Vibration spectrum
before lubrication
This type of spectrum, based
on experience, is normally generated by a reduction in the lubrication effect resulting in
some degree of metal to metal contact within the rolling element bearing. If this
condition is not remedied then accelerated bearing wear will occur leading to an increase
in operating temperature and consequent bearing failure.
Once the condition had been determined, it was decided to apply a given amount of
lubricating grease to this bearing. With the vibration data collector set to analyser we
were able to monitor the immediate effect the lubricating grease had on the vibration.
After grease was added and distributed within the bearing the live spectra indicated a
slight recurrence of the original condition, it was then decided to apply a further few
shots of grease. The effect was immediately noticeable with a considerable reduction in
vibration activity exhibited by the analyser. The same procedure was carried out at the
motor DE bearing, see Figure 2
Figure 2 - Vibration spectrum
taken after lubrication.
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MIET FIDiagE
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