In this case study we show that by measuring the correct vibration parameters you can resolve defects in harsh applications, even an inner raceway defect on a vibrating screen. Hopefully, you are finding them of use and helpful.
Here is the third of the five case studies brought to you with the Reliability Training Institute. In this case study we show how vibration analysis detected a hidden failure in a variable frequency drive. Hopefully, you are finding them of use and helpful.
As promised and to follow up on my previous post on bearing failures here is the first of five technical detailed case studies presentation. This is my first voice over presentation so please excuse the English Farmer accent mixed in with some Australian twang.
We have managed to secure discounted postage with the printing company direct, this update has the new reduced postage costs.
This technical reference book comprises of over 20 years’ experience in the fields of Vibration Analysis, Condition Monitoring and Reliability Engineering. It is written with the technical tradesperson in mind, interpreting Vibration principles into layman’s terms. It has taken 7 years to fine tune the book and I have been though three demo versions. This is the first published version.
All data is from real-life situations with over 20 case studies throughout the book. This is to be used as material to help support knowledge sharing, practical training and mentoring to enhance System Reliability though Vibration technology
publication comes as an A4 300 page Paper Back printed in full colour on 120gsm
Part 1: Introduction to Condition Based Maintenance
Chapter 1 – Condition Monitoring
Chapter 2 – Mechanics of Failure
Chapter 3 – Condition Monitoring Technologies
Part 2: Condition Monitoring Management Processes
Chapter 4 – Setting up and Reporting
Chapter 5 – Practical Application
Chapter 6 – ISO Standards
Part 3: Vibration Analysis Condition Monitoring Techniques
A profitable plant is reliable, safe and a cost-effectively maintained plant.
The Seasoned Analyst
CBM Conference 2019 UK
I presented at my first Conference, and it was the first CBM Conference in the UK. It was well worth attending as I got to discuss various challenges in the condition monitoring and reliability sector, and the most common discussion point was buying from senior management and how to continually highlight the benefits of a condition monitoring program.
In addition there were vendors there demonstrating the latest advancements in condition motioning technology from vibration online and walk around, thermal imaging safety, ultrasound new high sampling rates time waveform analysis to motor electrical condition monitoring.
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.
This blog is to remind everyone that ‘this is the way we have allays done it’ doesn’t wash and also how important using the correct lubrication and lubricant cleanliness is!
This share has two questions;
What the highest Acceleration levels you have recorded on a fan rotating at around 1498 RPM?
Which bearing do you put as the fixed bearing on a fan shaft?
I appreciate the Accelerometer was only technically good for 50g’s but we had a reading of 116.28g’s Peak to Peak. Can you beat that?
requested to inspect a fan due to repeat failures of the fan bearings.
This fan process cold air, it is direct driven at 1498RPM and has two 22222 straight bore double row spherical roller bearings.
The fan NDE (Non-drive end / fan end) bearing was set as the fixed located bearing. The DE bearing at the coupling was set as the float bearing.
This fan had
been in operation for 17 days.
Data was collected with a 100mV/g Accelerometer with a flat rare earth magnet.
The vibration data indicated that the motor to fan shaft alignment was good and there were no issues with the Velocity imbalance levels.
There was however extremely high Acceleration levels indicating excessive damage to the outer raceway together with an indication of poor lubricant condition.
Vibration Acceleration Data:
Figure 1 is the Acceleration Time Waveform from the Fan NDE (Fan end bearing). This shows the very high impacting levels with a -52.59g’s peak to +63.69g’s peak.
Figure 2 is the Autocorrelation of the Acceleration Time Waveform. This shows that all this activity is being generated mostly from the bearing outer raceway.
Figure 3 is the Acceleration Spectrum. This again shows that all this activity is being generated from the bearing outer raceway.
Vibration PeakVue Data:
Figure 4 is the PeakVue Acceleration Time waveform. This shows very high general impacting up to 34.9g’s Peak.
Figure 5 is
the Autocorrelation of the PeakVue Time Waveform. This shows that all this activity
is being generated from the bearing outer raceway.
Figure 6 is
the PeakVue Spectrum. This shows that all this activity is being generated from
the bearing outer raceway.
Figure 7 is the Velocity Spectrum. This confirms that this is a late stage defect and that this energy is from the bearing outer raceway.
On visual inspection it was found as expected the grease looked oxidised in a poor state and there was a high area of damage to the bearing outer raceway – noticeably on one side of the rollers. Damage to this side of the raceway would have been caused by axial thrust from the fan shaft and motor.
Image 8: This
is on removal of the bearing caps. This shows the oxidised poor condition lubricant.
Images 9 to
12: These are further images of the grease condition.
Image 13: This shows the two tracks for the rolling elements on the outer raceway, and that it was highly loaded to one side.
and 15: These are close-ups of the defected area.
Summary and Questions:
the lubricant used we know this is not suitable for this application and that
it will displace/separate and then oxidise.
But the question is what caused the high thrusting to the one side of the raceway, is it related to what is the fixed and free bearing? Is it true to say that due to the NDE (fan end) being the fixed bearing that expansion from the motor/ fan shaft would load up one side of the raceway?
A profitable plant is reliable, safe and a cost-effectively maintained plant
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.