Rolling Element skidding Lubrication issues Rolling Element skidding

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

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

Hay stacks around 4000 Hz

Autocorrelation Autocorrelation indicates random noise

Overall acceleration trend is erratic

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

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

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.

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

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

Broadband non synchronous energy

PeakVue shows fault frequency’s

Autocorrelation shows a BPFO

PA Fan 6A

Autocorrelation shows FTF and BPFO

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.

Adding grease can increase the overall trend

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

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.

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.

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.

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

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

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

Useful Web Resources Emerson CSi Technical Notes http://www2.emersonprocess.com/en-US/brands/csitechnologies/productsupport/Pages/TechnicalNotes.aspx Svenska Kullagerfabriken (SKF) calculators http://www.skf.com/skf/productcatalogue/jsp/calculation/calculationIndex.jsp?&maincatalogue=1&lang=en

References: Drayton M, case studies viewed 28th September 2010 at http://www.lubrication.com.au/cs_bearing_skidding.htm Goodman M, 2006, Using PdM technologies to determine, optimize lube condition, viewed 1 October 2010 at http://www.reliableplant.com/Read/23990/pdm-technologies-optimize-lube 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 www.reliabilityweb.com/art07/hall_lube_doc.pdf 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 http://www.bearings.machinedesign.com/guiEdits/Content/BDE_6_4/bdemech6_30.aspx