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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.

 

 

Summary:

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.

 

 

Vibration Data:

Figure 1 is the Velocity spectrum, there are no indications of any defect in this data.

Fig 1:

 

Figure 2 is the Peak Acceleration 10 KHz FMax trend from October 2017 until change out in July 2018, this displays an increasing trend.

Fig 2:

 

Figure 3 is the PeakVue Max Peak Acceleration trend from October 2017 until change out, this also shows the increasing trend.

Fig 3:

 

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.

Fig 4:

 

Figure 5 is the PeakVue time waveform, this shows a distinct periodic impactive activity.

Fig 5:

 

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.

Fig 6:

 

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.

Fig 7:

 

 

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 1:

 

Image 2 is a microscopic image of the defect. Has anyone else pulled a bearing with this type of defect?

Image 2:

 

 

Failure Mode:

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.

 

 

Final Note:

A special thanks to James Pearce for the data and working with me on the analysis.

 

 

A reliable plant is a safe plant

…..an environmentally sound plant

….. a profitable plant

……a cost-effective plant

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 linkedin.com/in/mathew-plant-46000456 .

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.

 

A reliable plant is a safe plant

…..an environmentally sound plant

….. a profitable plant

……a cost-effective plant

Hello all

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.

 

An Example:

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.

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.

 

Executive Summary:

  • 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.

 

Failure Mode:

From inspection the primary failure mode as per ISO 15243:2004 is 5.3.3.3 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
  • vibrations
  • 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 1:

 

Image 2 is an image of the false Brinelling indetention on the inner raceway.

Image 2:

 

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 3:

 

Image 4 is a microscopic image of the inner raceway showing the over roll of particles.

Image 4:

 

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 5:

 

Image 6 is a microscopic image of a false Brinelling depression on the outer raceway.

Image 6:

 

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

Image 7:


 

Vibration Data: 

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.

Hi All, this is the last post for 2017 – Enjoy

Background:
We were called to inspect a gearbox as the client had reported an abnormal sound. This was a very large old extruder high torque gearbox with a single input and dual output shafts.

Executive Summary:
Through onsite vibration analysis we were able to pinpoint the shaft that was generating the abnormal noises, this enabled the bearings for the shaft to be pre-ordered so they arrived at the repair shop the same time as the gearbox. This ensured a quick turnaround was completed with minimal production loss.

On Site Initial Assessment:
The gearbox vibrational levels as measured under full load conditions were >20mm/s RMS. This is considered “Vibration Causing Damage” as per ISO 10816-3. The Acceleration Peak to Peak impactions at Gearmesh #1 was excessive at 162G’s. There was also indications of misalignment on the 1st intermediate shaft and considerable looseness present. The 1st intermediate shaft ‘binds’ for 1/4 to 1/3 of a revolution when turned by hand.

Vibration Data:
The Input shaft high frequency Acceleration spectra clearly shows a high 2x gearmesh frequency for the gearmesh 1. This indicates there is misalignment within the gearing setup. The sidebanding at 19.20Hz indicates that it is relative to the 1st intermediate shaft.

The plot above is the Acceleration Spectrum from the Gearbox NDE Horizontal.

The Peak to Peak measurements on the Acceleration Time Waveform below indicates the Acceleration forces are within the 1st Intermediate shafting. The total reading of 162G’s is highly destructive and is impacting at 19.2Hz, the 1st intermediate shaft speed.

The Velocity spectrum taken from the NDE of the 1st intermediate shaft shows a considerable amount of run speed harmonics attributed to the shaft speed. This is an indication of looseness.

Cause of Failure:

On inspection the tab washer on the first intermediate shaft outer bearing had failed. In addition the suspected gear on the 1st intermediate shaft was extremely loose. It was found that this shaft had been previously repaired with metal spray and this had failed. On closer inspection the stress raiser appears to be around the keyway, as there was no strengthening welds around the keyway to support the metal spray.

Strip Down Images:

This is an image of the gearbox internal layout.

Images of the failed tab washer found in the bearing cap from the 1st Intermediate shaft.

Image of the key that supported the 1st intermediate gear that was loose.

Metals spray coating that was under the 1st Intermediate shaft gear. This failed initially at the metal spray coating at the keyway.

 

 

G,day all, here is another interesting job I got called to

 

Background:

This pump and motor had a history of reliability failures from bearings, shaft shearing and pipework flanges leaking. This was a pair of pumps on separate base frames but secured to the same concrete floor with a pipework common outlet.

I performed vibration analysis with phase analysis and diagnosed a foundation and structural problems as the root cause.

 

Vibration Data:

Pump Vibration Data:

Figure 1 shows the overall Velocity vibration trend from our first visit and second visit. This is gathered at the motor DE.

From this trend you can see a marked increase in the velocity vibration levels from 6.304 mm/s RMS and 8.388 mm/s RMS.

Fig 1:

 

Fig 2 compares the acceleration time waveforms from the motor drive end bearings

From this comparison you can see the lower levels of pump A (Blue Plot) and the very high impacting from pump B (Green Plot)

Fig 2:

 

Fig 3 is the vibration data from the motor drive end bearing

There is high impacting form the motor bearing and damage to the inner and outer raceway

Fig 3:

 

Pump B – Motor Bearing Inspection

Failure Mode:

From inspection the failure mode as per ISO 15243:2004 is 5.3.3.3 False Brinelling.

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
  • vibrations
  • corrosion/wear & shiny or reddish depressions
  • when stationary: at rolling element pitch
  • when rotating: parallel “flutes”

 

Findings:

  1. Depressions appearing at rolling element pitch indicating damage while the pump was in standby stationary bearing (Image 1)
  2. Indications of oxidation of the detached particles, which have a large area in relation to their volume, as a result of their exposure to air.

 

Bearing Inspection: Motor Drive End Bearing – FAG X-lite NU319E.TVP2

Image 1 is the outer raceway, and displays depressions appearing at rolling element pitch which indicates damage to the bearing when the motor was stationary

Image 1:

 

Image 2 is a close up of the depression at rolling element pitch on the outer raceway, this is from the load side of the bearings and also shows the roll over.

Image 2:

 

Image 3 is a microscopic image of a depression on the outer raceway.

Image 3:

 

Image 4 is an image from the inner raceway, this also displays the depressions at the pitch of the rolling elements.

Image 4:

 

Image 5 is a microscopic image of a rolling element.

Image 5:

 

 

Motion Amplification

Even with this vibration data the client was not convinced so I had to use another technology to show the client how the structural and base was causing them their reliability headache.

 

This first video shows how the pipe work was moving, this was the cause of the stress and strain to the flange joints that led to the leaks

 

This second video is of the base plate, this showed the true motion of the pump and how these failures were being induced.

Hi all,

Here is an interesting one. History is a worm wheel conveyor drive (Radicon Type). This operates in a rough environment and it ran for 8 months after maintenance and started to make a lot of audible noise.

It has a MJT 2 ¾ and a LJT 3 ½ on the input shaft. Coupling was the rubber pin type.

On inspection, there looks to be poor worm/wheel contact with some hair line surface cracks in the wheel gear. The gearbox input worm shaft NDE bearing has very bad damage.

We are thinking three possibilities for the root cause: Impact during mounting resulting in spalling at ball pitch and/or Spalls (Hertzian Fatigue) due to excessive thrust loading due to assembly or alignment errors. Also the possibility of transportation/storage damage. 

Anyone got any input?

Vibration Trends:

 

Acceleration trend showing increase after maintenance and the sharp increase.

 

 

PeakVue Trend showing a similar increase as the Acceleration trend.

 

 

Velocity Spectrum with high BPFI matches.

 

On Inspection Gear:

Incorrect gear meshing indications.

Visual surface hair line cracks.

 

Visual Inspection Outer Raceway:

Top left of image looks to be a hole in the raceway, there were two at 180 degrees opposite.

 

Microscope image of the outer raceway defect.

 

Visual Inspection of the Inner Raceway:

Images from around the inner raceway.

 

Microscope image of the inner raceway defects.

 

Visual Inspection of the Rolling Elements:

Two of the rolling elements, they all have various level of similar damage.

Microscope images of the rolling elements defects.

Hi all,

How do you monitor vibrating screen exciter gearboxes for deterioration and reliability risks? Do any of you monitor vibrating screens exciters for bearing defects using routine vibration analysis, how do you cope with the harsh environment? Or do you just use oil analysis?

The screens in question are the in line type with two gearboxes with weights at the ends of the gearbox shafts, direct driven by a motor via carden shaft. These screens vibrate around 298mm/s RMS in the highest direction of motion.

I found a beauty of a defect when I was conducting a vibrating screen structure survey, I decided to collect the usual routine vibration data from the exciter gearboxes via a flat magnet mount and found an inner race defect! Site actually pulled it 2 days later due to increase in noise and temperature. This gearbox was only installed two weeks prior.

The unit was removed from service with the bearing in the early stages of failure, prior to catastrophic failure and secondary damage. I would be interested in others thinking for the root cause of this infant failure?

 

VA Data:

No historical data as this was a one off survey. The top plot is the PeakVue spectrum and this displays one order and harmonics together with a match for the bearing inner raceway defect frequency (BPFI).

The bottom plot is the PeakVue acceleration time waveform, and this displays dominant one order activity. In PeakVue this means that something is modulating at 1 Order i.e. Inner Race defect.

 

Bearing Images:

The long and short shaft fixed bearing had an inner race localised spalled area at the inner ring centre shoulder, on one side of the raceway. Failure due to flaking of the inner raceway. I think the most likely cause is ISO 15243:2004 – 5.1.2 subsurface initiated fatigue due to overloading (Axial shock load).

The above is the short shaft fixed bearing.

The above is the long shaft fixed bearing.

I believe these images show rolling fatigue flaking that may be caused early by over-load, excessive load due to improper handling, poor shaft or housing accuracy, installation error, ingress of foreign objects, rusting, etc.

Root Cause:

As for the cause of subsurface initiated fatigue is, among other things, caused by surface distress. Under the influence of loads in rolling contacts, described by the Hertzian Theory, structural changes will occur and micro-cracks will be initiated at a certain depth under the surface i.e. subsurface.

There are another two major causes of bearing flaking; (1) Fatigue Life and (2) Improper Handling. (1) Fatigue Life: This is discounted as the cause as the bearing has only been running for two weeks before being removed from service. (2) Improper Handling: There are no signs of any ‘True Brinelling’ with marks on the inner raceway equal to the distances between the rolling elements.

So what is your opinion of the root cause?

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.

Thoughts?

 

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:

Image of the motor and blower.

DE Shaft.

DE Rolling element.

DE Zoom into rolling element.

DE Bearing Outside diameter.

 

 

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