Hi Dear all,

Currently I met a couple of motor faults causing by damaged motor NDE bearing . One motor run for one year. Another motor run just for 4 months. (but there are some same type of motor running from commissiong to now on some same type of plants, almost 3 years, And now still keep in good condition.)

The motor maker: TECO from TaiWan
The motor type:AD081001B1 AEJH- 6P- 900HP 3.3KV 50HZ
The motor bearing: DE:NSK6326 NDE:SKF 6324/C3VL207/AUSTRIA 191C
The motor is driving ROOTS blower .Coupling type: FALK 1130T10

Due to process requirement the BLOWER has relative high pulse vibraiton existing . (the reading form METRIX vibration probe is from 0.36----0.6 in/s peak-peak), this also result in relative high vibration on the motor .

During motor disassembly some findings are as following,
A. the DE bearing is in good condition. And the grease leaving in the bearing also is very clean and good. Based on the bearing ball's running trace on the outer&inner ring raceway , the trace is not located on the middle of ring raceway, some offsets toward the coupling side, Also indicated that there is some axial force existing on this bearing during motor running
B. the NDE bearing is badly damaged. the partial insulating layer of bearing outer ring is damaged and lost.Based on the bearing ball's running trace on the outer&inner ring raceway , the trace is not located on the middle of ring raceway, much more offsets toward the NDE SIDE than DE BEARING, Also indicated that there is some axial force existing on this bearing during motor running .

Some pictures attached for your reading.


Any thoughts and comments from you will be appreciated!

Regards
LUX

D190_MOTOR__BEARING.zip (1,917 KB, 105 downloads)

BEARING PIRCUTES

D190_MOTOR__BEARING-1.docm (1,181 KB, 103 downloads) PICTURE

ANOTHER BEARING PICTURES

D190_MOTOR__BEARING-2.doc (1,826 KB, 57 downloads) PICTURES

Sorry , Some pictures cannot be opened before, now repost.

D190_MOTOR__BEARING-1.doc (1,386 KB, 57 downloads) pictures

quote:

Based on the bearing ball's running trace on the outer&inner ring raceway , the trace is not located on the middle of ring raceway,

I have seen exaqmples of bearing balls runnin, not in the middle track but the track being near the oute race end. This track is also clear in drawings posted. In my case , it was a vertical pump with deep groove ball bearings on both sides. NDE side bearing would break like cucumber every 1 month or two and in each bearing the ball track was very much offset from center. Normally the towards edges of the outer race of the bearing, unable to take up thrust load. In your case also, it requires angular contact ball bearing if axial thrust is more than what can be taken by deep groove baLL bearing. The angular contact bearing may be provided on the pump side.
Regards
Irshad

Thank Irshad for sharing your experience.

To this kind of bearing fault pattern on the two motors I am thinking if the primary reason is due to wrong axial clearance preset by vendor. maybe the small axial clrearance setup block the rotor expansion during motor run with load. It result in the axial force existing on the bearing.

I donnot know what else can result in the axial force existing.

Any comments from you will be appreciated.

Thanks
LUX

Since you had thrust inboard on inboard end and outboard on outboard end, it seems to rule out thrust originating from the driven machine (*)

The designed bearing configuration may be either:
1 – inboard fixed / outboard floating
2 – cross locating.

The more common configuration is the 1st. Either way, I agree with you, it seems there was thermal expansion that was not accommodated.

Here are some wild guesses at possible reasons why thermal expansion was not accommodated :
1 – Bearing inner rings were not seated fully inward against the shaft shoulder. This can reduce the clearance available for expansion. This could be due to installation technique... I believe a warm bearing can sometimes sometimes pull back from the shoulder if not held in place while it cools. This could also be design flaw if the inside radius on the shoulder is not coordinated with the outside radius on the bearing inner ring.
2 – Binding occurred so that bearing OD did not slide within housing bore as designed. One thing to check would be bearing housing bore dimension, bearing OD dimension and also visual inspection of the bearing/housing seating surfaces for anomalies. I am not very familiar with rolling bearings with insulation on od of bearing... I wonder if that allows or restricts movement within the housing?
3 – poor design
4 – motor running much much hotter than design cause excess thermal expansion
5 – Something inserted between bearing and endbell restricted movement. Perhaps related to bearing insulation. Perhaps related to some type of problem with preload washer if present.
6-original design had shims or spacers between endbell and frame which were removed.

You can measure the clearance available for thermal expansion. For cross locating arrangement it would be the endplay. For fixed/floating arrangement, it is tougher to measure, but EASA has instructions for doing it involving loosening the endbells. Then you can compare against rough calcs for thermal expansion (thumbrule at least 0.010" per foot of shaft length between bearings. Of course it would be helpful to seek advice from OEM as well.

* One last thought very improbable – this could be due to driven machine load IF the driven machine doesn't take it's own thrust and the couplnig transmits thrust to the motor and the driven load thrust changes directions so it alternatley loads both of the motor cross locating bearings. But very improbable - usually the driven machine has it's own locating bearing to takes its own thrust and the coupling does not transmit thrust.

This message has been edited. Last edited by: electricpete,

Hi electricpete,

Thanks a lot for your wonderful&detailed explanation. I believe it is very helpful to search for the rootcasue of bearing fault.

Thanks again for your help.

Regards
Lux

Glad I could help. The EASA procedure for checking allowance for expansion is posted on their site:
http://www.easa.com/mo/TN-pdfs/TN42-1005.pdf

Pete,

Thanks for your help .

Happy new year!

Regards
lux

Hi all,

At present, We got the analysis report of motor damaged bearing from Bearing vendor SKF .Due to chinese version I willnot post the document. In the report the primary reasons are as follow for your reference.
1. The damage of the bearing was due to excessive uni-directional force from shaft acting on bearing.
2. The excessive pre-load created uneven rubbing and accelerated frictional wear, as observed on the damaged bearing.
3. The excessive pre-load was a result of the overly tight fit between NDE bearing outer race and NDE bearing housing.
4. SKF recommended the NDE bearing outer race and bearing housing fit to be H7.
5. SKF also recommended the shaft and NDE bearing inner race fit to be m5.
6. To improve lubrication, SKF recommended we use LGWA2 type grease


Thanks
LUX