Case Study #5 – 23RPM Defect on a 4 Point Contact Bearing

Hello Everyone,

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.

These case studies are to support my book ‘Enhanced System Reliability Through Vibration Technology’ and my new role as an RMS Trainer with the RMS Reliability Training Institute.

Many thanks to Dr Jezdimir Knezevic from MIRCE Science for his enlightening discussion (and MIRCE Science) and to Dean and Stuart at RMS for all their support.

This case study and more can also be viewed on the RMS Blog.

Look out for the extra-long bonus case study project coming next Monday

Slow speed bearing defect detected though vibration analysis

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.


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.


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 TWF
Trial 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 failure
Image 4: Bearing cage pocket failure
Image 5: Cracked cage pocket
Image 6: Cage pockets in various stages of failure
Image 7: Inner raceway

Inner raceway, on the load side, has various stages of spalling all the way around with one area of heavy spalling.

Image 8: Inner raceway ‘Cracking and spalling’
Image 9: Inner raceway Spalling
Image 10: Inner raceway overrolling
Image 11: Outer raceway

The outer raceway has less of spalling but again there is one area of higher spalling.

Image 12: Outer raceway Spalling
Image 13: Outer raceway Splaing
Image 14: Outer raceway Cracking and spalling
Image 15: Rolling element damage

The rolling elements display damage from over-roll of the spalled inner and outer raceways


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

A reliable plant is a safe plant

… environmentally sound plant

….. a profitable plant

……a cost-effective plant

The Power of Water – Case Study

The Power of Water

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.

This case history comes from a great friend of mine Matthew Plant .

Matt collected the vibration data and performed the analysis and recommendations on his findings to his client.



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.



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.


A reliable plant is a safe plant

… environmentally sound plant

….. a profitable plant

……a cost-effective plant