This was to be the final of the five case studies brought to you with the Reliability Training Institute. But, following on from feedback we will have one more case study next week, this will be an extra long project case study!
In this case study we demonstrate vibration analysis of a slow rotational 4 Point Contact Bearing, with a 23RPM Defect. This is to remind us that correct database set up, and Time Waveform Analysis is so important in slow rotational bearings.
Slow speed bearing defect detected though vibration analysis (Case Study of a ≈ 20RPM Bearing Defect)
What is Condition Monitoring?
In general,
Condition Monitoring techniques use instrumentation to take regular or
continuous measurements of condition parameters, in order to determine the
physical state of an item or system without disturbing its normal operation.
Condition Monitoring is basically applicable to components whose condition deteriorates with time. The objective of the Condition Monitoring technique is therefore to provide information with respect to the actual condition of the system and any change in that condition.
This information is required to schedule
conditional maintenance tasks, on an as needed basis instead of relying on
predetermined times. The selection of the Condition Monitoring technique(s)
usually depend on the behaviour of the failures, type of equipment used and
finally on economic and safety consequences.
This case study shows that when you collect the correct data parameters, vibration analysis can be invaluable in early detected of slow rotating bearings to enable a controlled change out prior to disruption to production.
Benefits
of Reliability
The main benefits of applying an effective condition based maintenance programme are that repairs can be scheduled during non-peak times, machine productivity and service life are enhanced, and repair costs due to a loss of production time are eliminated. Safety is improved – Maintenance costs managed – Reliability reduces Maintenance costs
Case History Background
We were
asked if we could offer a solution to detect when a rolling element bearing was
failing prior to catastrophic failure. The clients concerns was not the cost of
the bearing but the cost of the disruption to the production schedule if the
bearing failed during a production run. The client was unsure what would detect
the bearing issues as the bearing only rotates at around 20 RPM and it is in a
harsh environment.
Application:
This is a
slurry pot in a dusty foundry environment, the slurry pot is approximately 1.5meters
in diameter and 2 meters in height. The bearing installed is an INA U250433 four
point contact bearing. The outer raceway is stationary and the slurry pot is
connected to the inner raceway that rotates.
The image above is the four-point contact ball bearing, these are radial single row angular contact ball bearings with raceways that are designed to support axial loads in both directions.
Trial:
We set up
various sampling rates, various number of sample and utilised different filters.
Data was collected using a magnetically mounted 100mV/g accelerometer. Velocity,
Acceleration and PeakVue data was stored for analysis in the frequency and time
domain.
Trial
Summary:
The vibration
data that clearly indicated a defect was the PeakVue Time Waveform.
Trial data – Defective bearing PeakVue TWFTrial data – Good bearing PeakVue TWF
The PeakVue time waveforms above are from the initial trial, and this compares the suspect failed bearing and a bearing that is expected to be good.
Trial data – Plot of an inner raceway defect
The above PeakVue
spectrum is from the suspect bearing on the trial data. This data shows a mound
of activity at 24.50 orders, and this activity is sidebanded by 1 orders. The theoretical
overrolloing defect frequency for the rotating inner race way is 24.47. This indicates
that we have an inner raceway defect.
We selected the slurry pot with the damaged bearing and requested the bearing to be change out and removed for inspection.
Comparisons of the original and new bearing
The above
PeakVue time waveform comparisons show the before (in red) and the after with
the new bearing fitted (in blue). This data confirms the new bearing has been
fitted correctly and has no early defects. This also confirms that the bearing
indeed had a defect.
Bearing
Inspection:
On inspection
the bearing cage elements had fatigued and failed, there is also a lot of
spalling to the inner and outer raceway most probably due to subsurface and
surface initiated fatigue.
ISO 15243: 5.4.2 Subsurface initiated fatigue
This shows
that this bearing had reached its end of life, the cyclic stress changes
occurring beneath the contact surfaces had initiated subsurface micro cracks
this would have been in part of the bearing at the maximum shear stress. We are
at the point where the crack has propagated to the surface and spalling has
started to occur.
ISO 15243: 5.1.3 Surface Initiated Fatigue
Surface
initiated fatigue basically comes from damage to the rolling contact surface
asperities. This is generally caused by inadequate lubrication.
Damage to Retainers
Causes of
damage to retainers can be due to Poor lubrication, Excessive heat (plastic
retainer in particular) and Excessive moment load.
Bearing Images:
Image 1: Bearing as received collected from site prior to Sectioning Image 2: Bearing as received collected from site prior to Sectioning
Once the bearing was split the outer races were
moved to allow the rolling elements and cage pockets to be inspected as a
whole. On inspection there are many areas of bearing cage failure.
Image 3: Bearing cage pocket failureImage 4: Bearing cage pocket failureImage 5: Cracked cage pocketImage 6: Cage pockets in various stages of failureImage 7: Inner raceway
Inner raceway, on the load side, has various stages
of spalling all the way around with one area of heavy spalling.
The outer raceway has less of spalling but again
there is one area of higher spalling.
Image 12: Outer raceway SpallingImage 13: Outer raceway SplaingImage 14: Outer raceway Cracking and spallingImage 15: Rolling element damage
The rolling elements display damage from over-roll
of the spalled inner and outer raceways
Summary:
The inspection confirmed that by utilising the correct data collection parameters a slow speed bearing defect can be detected in this working environment. We were successful in determining a failed bearing prior to catastrophic failure
This is a great example that shows how powerful water can be in destroying a bearing, after only 1 week, and also highlights that when you perform vibration analysis the normal Velocity data should never be forgotten about.
Matt collected the vibration data and performed the analysis and recommendations on his findings to his client.
Asset:
The asset is an Automotive Dynamometer test system in an altitude test facility, the bearing supports the dynamometer rolls. The unladen (no vehicle) rolls shaft weighs 3 tonnes and speed is variable from 0 to 720 rpm (0-250kmh). There is a SKF 22228CCKW33 installed at both shaft ends, the bearing in question however is location end, all radial loads are within spec.
Background:
This forms part of a routine maintenance condition based monitoring program. The client reported activation of the facility water sprinkler systems and a service inspection was scheduled to ensure no asset was damaged due to the water sprinkler activation.
Vibration Survey:
All the data is the survey before the incident and the survey after the incident. The data was collected one week after the incident due to de- contamination works.
On analysis of the vibration data the following points were noted;
Velocity Data:
Figure 1 is the overall Velocity trend. The overall Velocity increased from 0.137 mm/s RMS to 0.602 mm/s RMS. Even still low this was an increase of 440%
Fig 1:
Figure 2 compares the before and after incident Velocity spectra’s. This clearing indicates a change in the bearing condition after the incident. The top green plot is after the incident on site.
Fig 2:
Figure 3 is the Velocity spectrum and this show activity that is dominated by the bearing outer raceway defect frequency.
Fig 3:
PeakVue Data:
Figure 4 is the PeakVue Max Peak trend from before the incident at 2.019g’s and after the incident at 5.623 g’s
Fig 4:
Figure 5 compares the PeakVue spectra’s from before (Blue plot) and after the incident (Green plot).
What can be seen is a 3.566 Order and harmonics. This 3.599 Order is the fundamental defect frequency for the SKF 22228CCKW33 installed. You can also note that 2XBSF is the highest frequency.
Fig 5:
Acceleration Data:
Figure 6 compares the before (Blue plot) and after (Green plot) of the raw Acceleration time waveform. This also indicated a high increase in the acceleration impactive data (note crest >5).
Fig 6:
Figure 7 compares the before (Blue plot) and after (Green plot) Acceleration spectra’s. This also shows an increase in the friction and impactive levels.
Fig 7:
Vibration Analysis Summary and Recommendations:
Due to the high increase in all vibration parameters and defect frequencies evident for the bearing outer raceway and rolling elements it was advised to replace the bearing.
What was the ‘Alarm bell’ for the analysts was the Velocity data.
Root Cause
Obviously water ingress was the instigator in the corrosion; however it was noted that the SKF SNL housings should withstand wash down. Further inspections pointed to the cap lifting eye being absent allowing water to enter the enclosure through the W33 lubrication groove.
Bearing Inspection:
On inspection the damage due to the bearing after running one week after the water incident was highly evident.
Outer Raceway.
Outer Raceway.
Inner Raceway.
Bearing Replacement:
The new bearing was then installed using the hydraulic nut drive up method.
