Due to positive feedback from our previous 5 case studies series this is our bonus case study brought to you with the Reliability Training Institute. If you have missed any of the previous case studies, they can be found on my LinkedIn profile, Seasoned Analyst blog and on the RMS Website.
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This case study is a lot longer at over 34 minutes. Here we cover a 2-year project period from initial root cause diagnosis to designing, trialling, and implementing an innovative cost-effective solution.
A profitable plant is reliable, safe and a cost-effectively maintained plant.
I often am lucky enough to use different vibration technologies and this post is a great example of how a vibration analysis program can protect the business using the SPM HD Enveloping technique. This post is with thanks to assistance from Dean Whiteside.
As part of a routine vibration data collection program a change in condition was noted at the fan motor vibration levels. Vibration monitoring frequencies were increased to daily as the defect deteriorated. This enabled planning and a controlled change out of the motor.
On inspection of the vibration data, bearing outer raceway (BPFO) damage was diagnosed at the drive end motor bearing. This was clearly evident in the SPM HD Enveloping.
Figure 1 shows the Acceleration RMS trend from the motor drive end (DE) bearing location. This shows the steep increase in the impacting levels with an exponential increase in the final days of monitoring.
SPM HD Enveloping Data:
Figure 2 shows the SPM HD Envelope spectrum from the motor drive end bearing. This technique shows a clear impact at 3.09 Orders that matches the defect frequency for the bearing fitted. There are many harmonics indicating a very impactive signal.
Summary of vibration:
There is a clear distinct defect in bearing outer raceway, at these levels this would confirm a spalling to the raceway.
Due to the risk of failure, a new motor was sourced and placed on-site encase of instant catastrophic failure. The risks was discussed with production and was deemed too high to the process and a plan was put in place for a controlled stop. But prior to this date there was an unexpected line stop, and as the motor was all prepared on site, the motor was changed during this downtime.
Vibration data after controlled change-out:
Figure 3 are the SPM HD Enveloping spectra from before and after motor change out. The top plot showed the clear bearing damage and now with the replacement motor there are no bearing defect signals present.
Figure 4 is the Acceleration trend from the motor drive end bearing location. This trend shows the increasing and then the lowest record level with the new motor installed.
Bearing Inspection: After sectioning and cleaning
inspection, it was found as expected, a large visible defect in the loadzone of
the bearing outer raceway. Motor Drive End Bearing FAG 6316-C3
Image 1 is the drive end bearing sectioned.
Image 2 is the defect located in the loadzone of the outer raceway.
Notice the flat bottom of the spalled area and the “neat” cracks around it. These are cracks that have come to the surface and in time, more material will break away.
Image 3 is the defect located in the loadzone of the outer raceway.
Particle over roll as the bearing comes out of the load zone.
ISO 15243:2004: 5.1.2 Subsurface initiated fatigue. Primary causes of Subsurface initiated fatigue are repeated stress changes and material structural changes. This leads to microcracks under the surface, crack propagation and then spalling.
The bearing is damaged as soon as spalling occurs. Spalling gradually increases and gives rise to noise and vibration levels in the machine. This machine was stopped and repaired before the bearing collapsed. The period from initial spalling to failure depends on the type of machine and its operating conditions.
What is sub surface fatigue? In a rotating bearing, cyclic stress changes occur beneath the contact surfaces of the raceways and rolling elements. Consider the rotating inner ring of a radial bearing with a radial load acting on it. As the ring rotates, one particular point on the raceway enters the load zone and continues through an area to reach a maximum load (stress) before it exits the load zone.
During each revolution, as that one point on the raceway enters and exits the load zone, compressive and shear stresses occur. Depending on the load, temperature and the number of stress cycles over a period of time, there is a build-up of residual stresses that cause the material to change from a randomly oriented grain structure to fracture planes.
In these planes, so-called subsurface microcracks develop beneath the surface at the weakest location, around the zone of maximum shear stress, typically at a depth of 0,1 to 0,5 mm. The depth depends on the load, material, cleanliness, temperature and the microstructure of the steel. The crack finally propagates to the surface and spalling occurs.
This is another example of how vibration technology
and knowing system health and risk of failure enables data driven decisions to
benefit the business. The motor was replaced when the line was down due to an
unplanned shutdown, with no additional downtime occurred.
If this motor had failed without any planning this would have lost product and reduced profit. In addition these actions have protected the customers, supply chain and brand our reputation.
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 one I recently finished and thought it would be a great one to share so people know what can be achieved.
We had three pump sets suffering from elevated vibration levels when operated in different combinations. Conventional vibration analysis was performed and this indicated a structural resonant condition.
The pump motors are mounted on a false floor:
and the pump barrels are below the floor:
The pump with the worst motion was on pump 3, the one far away from the edge of the drop. Also this pump has the least structural support under the floor. When ran in certain combinations pump 3 would be excited very badly.
The cost effective solution.
I designed a vibration dynamic absorber.
Dynamic Absorbers are often overlooked and not used, they can be seen as a band aid or a last option for some vibration problems. Whereas in some cases they can be the only cost efficient option, and they are very effective.
The Vibration Dynamic Absorber is a unique bespoke item, maintenance free, that is designed to absorb unwanted energy. It is tuned to have the same resonant frequency as the structure to set up an out of phase signal reducing the signal generated by the structure.
How did I design these?
For this one it was more of a ‘gut feel’. I looked at the motor and then drew out a design that wouldn’t look out of place when mounted, and that had some adjustment to it when fitted as theory doesn’t always pan out in real life. Then from this I worked backwards to get the correct material dimensions/configuration so it was resonant at the target frequency. I also made some weight configurations so I could cover my target range.
I will be going back in 6 months to see how it fairs. I did consider a round bar and weight but thought that with the rectangular bar you have more control on what way it will be resonant. As once you have performed phase analysis on the motor you then know what way it is moving and can mount the absorber accordingly.
Image of Pump 2 Vibration Dynamic Absorber:
Image of Pump 3 Vibration Dynamic Absorber:
Pump 2 Live Motion Video
Pump 3 Live Motion Video
Pump 3 Slow Motion Video
What am I covering?
On pump 3 I am covering the one problem frequency, 1 Order, but the two arms are of different lengths in terms of the length from the point of pivot (clamping) to the mass. Also the arms are of different dimensions with different mass at the end so they could be tuned to the same frequency.
I also did find that the sweet spot was not necessary the point of higher deflection of the absorber and that the three motors all reacted differently.
Final Review of actual vs theory:
I have had time to review the final theoretical tuning of the three pumps to actual results. They are all different and no one motor is the same, they all have their own personalities dynamically wise.
Pump 3 had the highest overall vibration, one dominant frequency at 1 Order on pump 3 and this was successfully reduced.
Pump 1 and pump 2 had two frequencies in the data. And both of the vibration dynamic absorbers were tuned to the lower frequency not the one order.
Table of final overall levels:
Motor NDE (Top)
Motor DE (Coupling end)
Motor NDE (Top)
Motor DE (Coupling end)
Motor NDE (Top)
Motor DE (Coupling end)
Pump 1 actually showed the text book results. The theoretical calculations for the tuned damper was for the lower frequency not the running speed (1520 CPM yes they are on soft start VFD). It actually split the frequency – text book……….beauty!!
I have more questions and theories now, this is pretty exciting stuff. Hopefully I can keep this going on other pumps.
Ultrasound trending and Vibration Analysis working together.
This is a good example of how condition monitoring technologies work well as integrated technologies.
Through routine in house overall ultrasonic dB trending a change in condition was noted from one of the motor bearings and this was an increasing trend. I was called to verify the asset condition through vibration analysis.
Removal of the motor on condition of the bearing enabled a control change-out and a more cost efficient repair rather than running to failure.
The cause of the elevated Ultrasonic levels and the vibration defect frequencies was false Brinelling to the drive end bearing.
In addition there appears to be grease compatibility problems result from either mixing incompatible greases, or from ingress of other contaminate, Dry powers absorb the oil causing the grease to thicken.
From inspection the primary failure mode as per ISO 15243:2004 is 18.104.22.168 False Brinelling, there is also a secondary failure mode as per ISO 15243:2004 of 5.2.2 Abrasive Wear due to inadequate lubrication.
False Brinelling occurs in the contact area due to micromovements and/or resilience of the elastic contact under cyclic vibrations. Depending on the intensity of the vibrations, lubrication conditions and load, a combination of corrosion and wear can occur, forming shallow depressions in the raceway. In the case of a stationary bearing, the depressions appear at rolling element pitch.
In many cases, it is possible to discern rust at the bottom of the depressions. This is caused by oxidation of the detached particles, which have a large area in relation to their volume, as a result of their exposure to air.
Key Points are:
rolling element / raceway contact areas
micromovements / elastic deformation
corrosion/wear shiny or reddish depressions
when stationary: at rolling element pitch
when rotating: parallel “flutes”
Abrasive wear. Most of the time, real abrasive wear occurs due to inadequate lubrication or the ingress of solid contaminants. Abrasive wear is generally characterised by dull surfaces. Abrasive wear is a degenerative process that eventually destroys the microgeometry of a bearing because wear particles further reduce the lubricant’s effectiveness. Abrasive particles can quickly wear down the raceways of rings and rolling elements, as well as cage pockets. Under poor lubrication conditions, the cage may be the first component to wear.
Bearing Inspection: Motor Drive End Bearing – FAG X-life NU324-E-TVP2-C3
Image 1 is of the poor grease condition from the bearing.
Image 2 is an image of the false Brinelling indetention on the inner raceway.
Image 3 is a microscopic image of a false Brinelling depression on the inner raceway. Rust at the bottom of the depressions. This is caused by oxidation of the detached particles
Image 4 is a microscopic image of the inner raceway showing the over roll of particles.
Image 5 is an image of the outer raceway in the load zone showing the false Brinelling. This is only present in the load zone.
Image 6 is a microscopic image of a false Brinelling depression on the outer raceway.
Image 7 is a microscopic image of a rolling element. Here you can see the flat spot from the false Brinelling. In addition the ring that is around the inner and outer raceway is due to over roll of particles and poor lubrication condition. Flat spot from the false Brinelling Ring of over roll of particles
The comparison below show Fan 1 (in blue) and Fan 2 (in green). This highlights the very high destructive levels of the drive end bearing and that it was close to failure.
The PeakVue spectrum plot below confirmed that it was a bearing defect and highest at the outer raceway.
This was a first for me in 17 years! It was one of those with the right experienced people at the right place at the right time beauties!
We were called to a site when the operators reported an unusual sound from a 4 Pole 1500RPM fan motor. We attended and through vibration analysis and temperature measurements recommended stopping the motor and checking the connections in the motor terminal connection box. This enabled the site electrical team to quickly pin point the defect.
The vibration data indicated a dominant 100Hz electrical noise, the motor felt like it was hunting or pulsing and in addition the whole motor was hot with the motor DE housing was over 100oC.
It would be interesting if anyone else has ever found a defect like this?
The overall velocity Increased from a normal level of 1mm/s RMS to 3.9mm/s RMS and then in two days increased to 8.88mm/s RMS.
The velocity spectrum below displayed a new peak at 100Hz:
This is a single frequency 100Hz Trend, and shows a marked increase at 100Hz:
The PeakVue spectrum displayed dominant 100Hz and harmonics.
The acceleration 10 KHz spectrum displayed two mounds of activity with 100Hz sidebands:
We then decided to look at the auto correlation data and found the following.
10 KHz Acceleration time waveform circle plot with four main peaks and inner lower peaks:
PeakVue Acceleration Time waveform circle plot showing 4 clear peaks:
And this is what was found. (Note cable broke when it was lifted for photo.)
We were surprised the motor was still running (very badly with high vibration) in this condition !!!!!
I have come across many bearings with bearing fluting (EDM) with the damage on the inner or outer bearing raceway. But this is the first bearing where I have seen it on the outer diameter of the bearing outer raceway. Has anyone else come across this?
This was on a AC soft start.
Images of the outer diameter.
This was the inner raceway.
Update: 8th August 2017.
Site Inspection: This motor drives a process blower and is isolated from the ground via rubber isolation mounts, there is no earth bonding on the motor or blower. The blower pipework is also isolated by expansion joints.
The motor supply cables are three seperate SWA with a seperate earth cable that has 28.5 Amps on it, probably due to back EMF.
After onsite shaft current discharge tests also revealing no current discharge when direct on line this looks to be Static generated Electrical discharge damage.
Update: 13th August 2017.
Responding to some comments here are some more photos: