1. Rolling Element skidding
Lubrication issues
Rolling Element skidding

2. Pulverized Fuel Mill Laurence-Scott 985 RPM 522KW Driving a bowl mill
Continuous operation
12 Across the site

3. Details Drive End Bearing - NU322 Greased weekly with 8 ‘shots’
Audible screech from bearing on various motors
Cavity shows grease exiting grease relief, some more than others

4. Hay stacks around 4000 Hz

5. Autocorrelation
Autocorrelation indicates random noise

6. Overall acceleration trend is erratic

7. PeakVue spectrum shows no significant changes
60 Orders (990 Hz)
1600 LOR
1kHz HP

8. PeakVue waveforms show no significant impacting events, but a slight increase in amplitudes.

9. Overall PeakVue trend shows no increase in stress wave activity
Overall PeakVue trend shows no increase in stress wave activity. Downward trend could indicate that the reduction in grease quantity could be heading in the right direction.

10. Primary Air Fans Laurence-Scott 2130kW 1485 RPM
Four in continuous operation
Similar noise from Drive End bearing
NU330
Greased weekly on a ‘shot’ basis

11. Temperature spike coincides with the weekly lubrication program
Temperature spike coincides with the weekly lubrication program. In some cases up to 90 Degrees C.

12. Broadband non synchronous energy

13. PeakVue shows fault frequency’s

14. Autocorrelation shows a BPFO

15. PA Fan 6A

16. Autocorrelation shows FTF and BPFO

17. Other Symptoms
During lubrication, little or no grease will exit the grease relief
Why? As grease is worked by the rolling elements oil is squeezed out of the matrix, causing the grease to harden in service. This can block the grease relief and grease can no longer be flushed through the bearing. When lubricated the fresh grease will take the path of least resistance, on older motors this is usually the inboard motor bearing seal. This results in the grease entering the windings or armature.
Using a strobe can detect irregular movement of the rolling element
However the noise is an obvious method to detect skidding. Skidding sounds like a high pitched screech that goes away with a pump of grease but usually comes back shortly after.

18. Adding grease can increase the overall trend

19. Spectrum will show haystacks
PeakVue can show BPFO (not always)

20. Bearing skidding Causes Over lubrication
If a grease cavity is full, the rollers can pull in (churn) extra grease causing over lubrication
Blocked grease relief (causes over lubrication)
Grease too thick for the application
Light radial loading i.e. Minimum load requirements have not been met; gearbox is taking all of the load; magnetic centering of the motor has unloaded the bearing.

21. Minimum load requirement
NU 322 ECJ
Dynamic load rating 468 kN (47 tonne)
Minimum load rating 3.36 kN (340 kg’s)
Rotor weight 17% – 33% of total weight
Total weight=5080 kg
Rotor weight = 864 kg’s – 1524 kg’s
At the most each bearing is supporting 762 kg’s without taking into consideration magnetic centering, and weight supported by the gearbox.

22. Action by the Client
The charge of grease has been reduced significantly. This has resulted in less over temperature alarms
However vibration data does not suggest a significant reduction in skidding.
Why no more action?Lack of conclusive evidence. Lack of understanding of vibration analysis, no acoustic data, no grease analysis performed.
No recommendation for specific action and fear of inducing further damage by changing current practice i.e. overheating a bearing by flushing out the cavity with fresh grease.

23. Solutions Most desirable Convert to oil lubricated bearings
Purge old hard grease from the cavity
Reduce the amount of grease
Machine down the shaft and install the appropriate bearing
Replace the cylindrical roller with a deep groove roller
Least desirable
Utilise an out of round raceway
Use two preloaded hollow rollers
Not recommended
Misaligning the driver to driven component

24. Other possibilities:
The noise and non synchronous broadband energy could be from a rub e.g. A bearing seal. This would also cause a temperature increase.
There could be metal to metal contact occurring. Why? If the grease has thickened in service metal to metal contact could be occurring due to the increased base oil separation and lack of fluid film thickness.
What effect does skidding have on longevity of the bearing?
Actions:
Utilise Acoustic monitoring
Inspect the bearing
Take a grease sample
Clear the grease path/grease relief
Alter the lubrication interval – Reduce the lubrication frequency, increase the charge
Change the lubrication method – Use the appropriate technology
Change grease

25. What is missing?
More data:
Acoustic
Temperature
A structured approach to problem solving
Evidence:- Strobe the rolling elements, inner and outer race inspection

26. Useful Web Resources Emerson CSi Technical Notes
Svenska Kullagerfabriken (SKF) calculators

27. References:
Drayton M, case studies viewed 28th September 2010 at
Goodman M, 2006, Using PdM technologies to determine, optimize lube condition, viewed 1 October 2010 at
Granger M, 2006, Using Sound Analysis to Monitor Lubrication Condition in Greased Roller Element Bearings, Doctor Know application paper, Computational Systems incorporated
Hall J, 2006, Acoustic/Ultrasonic Lubrication, viewed 16th April 2010 at
Honeycutt J, et al, How To Design An Electric Motor Re-greasing Program, Tennessee Valley Authority, cited Johnson W, Electrical Power Research Institute
Jacobyansky B, et al, 2009, Changing Greasing Habits with Predictive Maintenance, Guardian Industries
Bearings Reference Centre, viewed 1 October 2010 at