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
(Case Study Electrical Defect detected thought CBM)
This is a case history brought to you with data from James Pearce – another great find! This shows how utilising multiple CBM technologies, with a certified and experience technician, can help prevent unplanned failure to assets.
Using vibration analysis and thermal imaging condition based monitoring techniques a change in condition was found and a diagnosis of electrical issue with the VFD was given. From this the variable speed drive history parameters were interrogated. This confirmed it was indeed an electrical issue. Further analysis carried out by the site electrical supervisor pinpointed the IVI card as the issue. The IVI card controls a lot of optic connections controlling the IGBT’s. This was replaced and the vibration, temperature and current reverted back to normal.
We have been monitoring assets at the production facility utilising vibration analysis and infrared thermography. On a routine survey a change in condition was noted and investigated.
The motor in this case study is a 4 Pole 50Hz AC motor on a Siemens Variable Speed Drive. This asset has 2 of the same motors both driving a roller each to crush and grind product.
On-Site CBM Recommendations:
Motor: It was reported on the day that the windings temperature has been higher in the warm weather and is 10oC warmer than the comparable motor. This survey there has been an increase in the electrical activity across the motor. Please note we can only detect indications of an electrical anomaly. Recommended actions to investigate the electrical drive.
Vibration Analysis Data:
The dominant change in condition in the vibration data was an appearance of running speed electrical frequency in the PeakVue data and the increase in the high frequency electrical data.
Figure 1 compares the last four PeakVue acceleration spectra taken from the motor non-drive end. This displays the normal 2xLF activity and then the appearance LF activity this survey.
Figure 2 compares the last two Velocity spectra’s. This shows the increase in the high frequency electrical activity. The top plot is the normal activity and the bottom plot is with the defect.
Data with electrical defect
Thermal Imaging Data:
The thermal data below compares the suspect motor and the comparison motor. These motors are on the same asset performing the same duty at the same time.
This data confirms that the windings are indeed warmer on the suspect motor.
Electrical Supervisors Investigation:
Below trace shows the current varying.
The below trace is the Phase 1 Current under load conditions, only reading positive part of cycle.
This compares Phases 1 and 3 motor current under load conditions. Phase 1 only reading positive part of cycle.
On start-up temperatures all came back to normal.The IVI card in the inverter was replaced. The below plot is Phases 1 and 3 motor current equal after changing IVI card, under no load conditions.
An Insulated Gate Bipolar Transistor (IGBT) is a key component in what makes up a VFD (Variable Frequency Drive). An IGBT is the inverter element in a VFD, pulsing voltage.
IGBTs have become highly reliable devices that can handle high voltage devices and are able to switch in less than a nanosecond.
The IGBT acts as the switch used to create Pulse-Width Modulation (PWM). An IGBT will switch the current on and off so rapidly that less voltage will be channelled to the motor, helping to create the PWM wave. This PWM wave is key to a VFDs operation because it is the variable voltage and frequency created by the PWM wave that will allow a VFD to control the speed of the motor. Therefore, without the IGBT switching the current on and off so rapidly a PWM wave—and the speed control that comes with it— could not be created.
The IVI card in the drive controls a lot of optic connections controlling the IGBT’s
Failure mode ISO 15243: 5.4.2 Subsurface initiated fatigue
Is this normal Fatigue Failure, how many of you get to see a bearing actually fail from normal fatigue? Usually we come across bearing failures/damage due to secondary factors such as misalignment, over or under lubrication, imbalance, resonance and poor installation……
This is also a great example of how important knowing the asset you are monitoring is as to know when to remove the asset from service, ensuring that the client has got the maximum life out of the asset for the associated risks.
# You can’t analyse what you don’t know or understand #
This Case Study Application:
This is a DC motor that is direct coupled driving a gearbox.
We have been monitoring this motor since 2006 and in May 2017 a subtle change in the PeakVue level was noticed, closer monitoring was initiated and a bearing inner raceway frequency was found. Next in June 2018 there was a further step change that prompted the decision to remove from service as we felt the risk of failure was too high.The motor was overhauled at the next opportunity, this was in July 2018.
Figure 1 is the Velocity spectrum, there are no indications of any defect in this data.
Figure 2 is the Peak Acceleration 10 KHz FMax trend from October 2017 until change out in July 2018, this displays an increasing trend.
Figure 3 is the PeakVue Max Peak Acceleration trend from October 2017 until change out, this also shows the increasing trend.
Figure 4 is the PeakVue spectrum. This shows the running speed activity and a beautiful text book bearing ball pass inner raceway defect frequency with harmonics and sidebands at 1 Order.
Figure 5 is the PeakVue time waveform, this shows a distinct periodic impactive activity.
Figure 6 is the Auto correlation of the PeakVue time waveform. Auto correlation is great tool for distinguishing periodic activity within a time signal. This data shows us that there is a defect that is modulating by 1 Order. Therefore a component on the motor shaft, rotating with the motor shaft has a defect.
Figure 7 is a zoom in on the Auto correlation of the PeakVue time waveform. From this we can see that the 1 Order activity is side banded by the inner raceway defect frequency.
Images of the bearing defect
Image 1 is of the bearing inner raceway. This shows the track of the rolling element in the race way, due to the DC drive, also within the arrows there is the defect.
Image 2 is a microscopic image of the defect. Has anyone else pulled a bearing with this type of defect?
Suspected failure mode is ISO 15243: 5.4.2 Subsurface initiated fatigue,
The images show that this bearing had reached its end of life, the cyclic stress changes occurring beneath the contact surfaces had initiated subsurface micro cracks, and 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.
A special thanks to James Pearce for the data and working with me on the analysis.
Electrical defect found with Velocity data – Case Study
Has anyone found many electrical defects though vibration analysis? We know that VA will show the indications of electrical activity but not necessary the severity. This case study shows that the Velocity vibration data can indicate what the cause of the vibration problem is, this will enable the engineer to target the investigation.
Thanks to James Pearce for the data. linkedin.com/in/james-pearcevibrationanalysis
A routine client called after the operators noticed an increase in noise and vibration from a main plant drive motor. This is a DC motor and usually operates around 400-500 RPM. This is a rather old motor and drive system.
Initial Vibration Survey:
On attending site vibration data was collected, analysed and before leaving site recommendations were given.
Figure 1 is the Velocity Spectrum collected from the motor. This showed a 1 Order amplitude of 0.07mm/s RMS, with a dominant peak at 49.95Hz with an amplitude of 3.2 mm/s RMS with many harmonics. The motor was operating at 384 RPM during data collection.
Figure 2 is the PeakVue Spectrum. This displayed a dominant peak at 149.86Hz, 3xLf. This was also sidebanded by running speed.
The recommendations was to check all supply cable connections and inspect the variable speed drive components for condition.
The site electrical engineer was dispatched to inspect the drive for this variable speed motor. Upon inspection 2 Thyristors were replaced and all electrical connections checked for security.
The operator then reported that the vibration magically disappeared.
Post Maintenance Vibration Survey:
Vibration data was then collected after maintenance. The motor was running at a higher speed of 456 RPM on the follow up survey.
Figure 3 is the Velocity overall trend from the initial survey and post maintenance survey. This trend shows the reduction on motion from 4.301 mm/s RMS to 1.162 mm/s RMS.
Figure 4 compares the before and after maintenance Velocity Spectra’s. From this you can see the dominant 49.55Hz and harmonics have disappeared. The only activity left is a peak at 299.74Hz again sidebanded by 1 Order.
This again shows the benefits of sending a certified, experienced and correctly mentored Vibration Engineer and not a data dog to investigate vibration issues. James quickly pinpointed the cause of the excess vibration that enabled the client to efficiently target the area of concern and quickly rectify the issue saving time and money.
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.
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.
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;
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%
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.
Figure 3 is the Velocity spectrum and this show activity that is dominated by the bearing outer raceway defect frequency.
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
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.
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).
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.
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.
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.
On inspection the damage due to the bearing after running one week after the water incident was highly evident.
The new bearing was then installed using the hydraulic nut drive up method.
Fluid film bearings are mainly monitored with proximity probes. It is often stated that “you can’t detect early defects in fluid film bearings with normal vibration techniques (Velocity, Acceleration or a bearing condition unit)”. But in fact you can detect the effects of a fluid film bearing deteriorating with a normal accelerometer.
Under abnormal circumstances metal to metal contact might occur, leading to occasional high-frequency noise that can be detected with normal vibration equipment. The following case study is a great example of this and also using lubrication analysis as part of a maintenance program.
This case history covers a Production facility Extraction Fan which has been monitored as part of a site wide Condition Based Monitoring program. The drive of this program is to integrate condition monitoring techniques and to drive the maintenance program.
This fan unit had a motor that is a direct drive to a fan shaft, the fan shaft has a white metal fluid film bearing. The fan is a standard overhung centrifugal of about 2.5 meters diameter.
This data was collected over an extended period by myself and Ian Graham.
Ian Graham flagged this reliability risk very early in the program for having a considerable 1 Order impact present in the PeakVue™ data (see Figure 1).
Figure 1 shows the initial Vibration data collected on the fan bearing with the 1 Order event present.
Vibration Trend Analysis:
Figure 2 is the PeakVue™ Trend of the bearing as it was nursed through until maintenance could be conducted.
This showed the initial level, reduction in levels after an oil flush, then a period of monitoring until another oil change and a bearing inspection.
As the initial fan data had a dominant 1 order event and was being monitored on a monthly basis, we needed to determine whether the event was consistent or deteriorating, and what possible causes were.
Oil samples were then taken (see image 1), on visual inspection the oil was in a very poor contaminated condition. The lab report (see image 2) stated a serious concern with particulate matter contamination and high levels of Sodium, Iron, and Tin.
The indications of Tin suggested probable sleeve (Babbitt) wear of the bearing.
Image 1 is the Fan DE bearing oil sample:
Image 2 shows the initial oil sample report and diagnosis with emphasis on the high Tin levels:
This fan was a critical component of the facilities production process, however with a plant upgrade planned for the very near future, the decision was made to closely monitor the deterioration of the assembly rather than to rectify this potentially expensive piece of equipment.
The temporary measure of an oil flush and change was conducted immediately, with a visual inspection planned for the next shutdown.
During the shutdown the bearing housing was split and the bearing shells separated. The damage to the bearing was very extensive with ‘scalloping’ of the Babbitt material evident in the direction of rotation forming a build-up at the end of the lower shell. The cause of this is most likely the failure of the lubricants ability to sustain an adequate oil wedge between the shaft and the bearing.
Image 3 shows the Fan DE white metal bearing upon inspection:
Image 4 shows the Lower half shell of the white metal bearing with a piece of ‘free floating’ Babbitt that was found in the sump:
In conclusion, with the company’s utilisation of all the available Condition Monitoring technologies and tools, they were able to monitor and be consistently and accurately informed of the state of deterioration of the bearing. This allowed them to implement a rolling program of temporary measures to stave off what was essentially an unserviceable critical machine until the Factory upgrade was conducted.
What happens when recommendations are not followed – “when things are left to burn”.
How often have you performed a reliability survey and issued a report of findings and recommendations to reduce the risk of unplanned system failure… and the client does not follow the recommendations.
This is one example of an infrared thermal imaging survey that highlights the importance of following the recommendations and also that a thermal survey should be performed by an experienced/qualified reliability technician who does not just rely on the thermal camera to rush round the site but also uses the human senses and experienced to assess system condition.
One panel unfortunately had Perspex in the way of the cable terminations, so this could not be surveyed with thermal imaging. Through the perspex cover it was noticed that the cable sheath has split, probably due to excess heat and exposing the copper cable.
This was reported on the day to the site supervisor, and in writing in the report. Site confirmed that they were going to schedule in repair at the soonest opportunity due to the high unknown probable risk.
This is the thermal image of the panel, note no readings as infrared energy doesn’t pass through Perspex.
This is the digital image of the panel showing the Perspex cover and damaged cables.
This was not inspected/repaired and the panel caught on fire. This caused shutdown of the plant and a huge costs to the company in downtime and reputation due to unfilled orders to their customers.
Images of the failed component.
Image of the repair. Here you can see the burn fire marks on the back panel.
Sometimes we try our best to ensure our clients do the right thing for reliability on their plant. Unfortunately they don’t always action what we recommend, not matter how much we try to convince them. In this instance all we can do is keep spreading the word of how important it is to know the condition of your system and then to actually action any risks. This in turn will reduce the risk of unplanned failure.
A special thanks to James Pearce for sharing his experience.
Recently I saw a post from Terrence OHanlon of Reliabilityweb.com, that I feel summed up Reliability.