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.
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