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.
I have been privileged and blessed to have experienced varied fields of Vibration Analysis, Condition Monitoring and Reliability, and had the opportunity to study under some of the great mentors and trainers in these discipline. I feel it is always good to share knowledge and learnings to help others who want to progress and to promote our discipline.
Often people discuss what makes a good vibration analyst? – electrical or mechanical background – degree or apprenticeship level, certification or experience……….. Then when we find an issue we always get asked “How long will it last?”, and our answer to this question, I feel, greatly depends on our experience and training.
In the discussions I have had with many other people, we have all spoken ‘Pearls of Wisdom’. The 14 statements below I feel are very important in the way we operate in our discipline.
1) The most important part of any program is the person performing the data collection and analysis.
2) The second most important part of any program is the training and mentoring given to the person selected.
3) 5 years of experience is not the same as 1 year of experience 5 times.
4) The most important question you can ever ask is “why”.
5) It is important to understand the values of the numbers you are using.
6) Physics of the machine is really important.
7) You can’t analyse what you don’t know or understand.
8) A person may not be stupid, they may just not understand what you are saying.
9) 1 times RPM is not always unbalance.
10) There are no universal vibration severity limits.
11) Absolute amplitude in the frequency domain is relatively useless. Don’t forget the time domain & phase.
12) There are no ghost frequencies or unknown frequencies but only frequencies not analysed enough.
13) Don’t ignore the potential benefits of chit chat in the crib/break room with the operators and maintenance teams. They know their machines!
14) When all else fails, leave the air conditioning, and go examine the operating equipment. Go look, touch, feel, smell and listen to the machinery.
Please share and if you have anymore ‘Pearls of Wisdom’ let everyone know.
Slow speed bearing defect detected though vibration analysis (Case Study of a ≈ 20RPM Bearing Defect)
What is Condition Monitoring?
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.
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
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
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.
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
data that clearly indicated a defect was the PeakVue Time Waveform.
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.
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.
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.
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
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
initiated fatigue basically comes from damage to the rolling contact surface
asperities. This is generally caused by inadequate lubrication.
Damage to Retainers
damage to retainers can be due to Poor lubrication, Excessive heat (plastic
retainer in particular) and Excessive moment load.
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.
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.
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
MIRCE Science is a theory for predicting expected functionability performance for a functionable system type. Accuracy of the predictions is governed by the degree of the scientific understanding of the physical mechanisms, and human rules, that govern the motion of functionable system types though MIRCE Space. The main objective of this paper is to address vibration monitoring as one of the possible mechanisms that governs motion of a gearbox through functionability states, which are contained in MIRCE Space. In general, and to illustrate this process through a case study related to heavy gearbox used in Plastics Manufacturing industry.
The author wishes to acknowledge the support received from Dr Knezevic, MIRCE Akademy, Exeter, UK, while preparing this paper. As the “father” of MIRCE Science, Dr Knezevic, has inspired me to view how every day Condition Based Monitoring can have a significant impact on functionability performance of the whole system. Consequently, I can now understand how many companies are performing Condition Based Monitoring but are not linking this to the business performance of the whole organisation/company. MIRCE Science is the body of knowledge that bring together these two very different but related disciplines, for the ultimate benefit of the user.
This month’s blog is slightly different from the usual ones we post. This month is more of an opinion regarding data dogs. We are seeing more equipment suppliers selling VA equipment that they promote as “anyone can use” and you know need to know or have experience to use. That the software will diagnose for you. Or even, just collect the data upload to the cloud and we will tell you if you have any issues.
I feel there are places for this type of program but one thing I dislike is companies sending “data dogs” to collect the data. These are cheap labour sent to press a button and collect the vibration data as fast as they can. This type of VA often gives this service a bad name as they miss diagnose, miss defects or the person in the office performing the analysis just gives the ‘wall chart analysis’ of its either misalignment, imbalance, looseness or resonance.
So much can be gained by a competent engineer or technician attending the asset to collect the vibration data. Most of your analysis should be performed at the machine, not in the air conditioned office!
We also find that there are many facilities/companies that are on the start of their reliability journey that require a person on site to promote and ensure the job is done and followed though correctly.
The images below back up this point. A great friend of mine, James Pearce, was performing a quarry motor VA survey and while at a motor he sensed an abnormal noise, he tracked it down to the GTU take up conveyor pulley. The GTU is not on the vibration program but when you have an experienced engineer or technician collecting the data walking the plant they also use their other senses to ensure plant reliability.
James reported this to site that had a controlled shut down of the quarry immediately to replace the pulley bearings. Site confirmed that they would have not inspected this pulley and it would have catastrophically failed causing a lot of additional hard work. This controlled shutdown cost 3 hours of production. But this saved replacing the pulley shaft as there was no damage to the shaft. If this was left to totally fail this would have cost 9-11 hours production downtime at 2,000 Tons per hour. There is also the possibility that the pulley could have failed in a way that caused damage to the conveyor belt incurring more down time and a lot more costs.
And here is the video!
You can see the bearing there – this should not be glowing red. This bearing had totally failed!
So remember that 5 years of experience is not the same as 1 years of experience 5 times and you can’t analyse what you don’t know or understand.