we had faced high bearing temperature on the pinion shaft of gear box driven by GT and driving water injection pump. There is no increase in vibration levels(both shaft and casing). There is no abnormality in Oil analysis reports. Then we decided to strip the GB for bearing inspection. During inspection we noticed minor marks on bearings and there is no significant damage. However we found a kind of white paste(looks like white grease) all over on the no load area of the pinion shaft. This is observed on both inboard and out board bearings. The bull gear shaft is clean!! can anyone identify the reason for this? is this could be the reason for high temperature?
Pinion speed is 11,000rpm and bull gear 3600 rpm. Gear box is lufkin make. Gear box Oil is Shell Turbo T32. After the bearing replacement temperatures are now well within limits.

Pinion_bearing.doc (4,562 Kb, 58 downloads) Pinion Image

quote:

Originally posted by AKHTAR:
White grease indicates water ingress and emulsification.
Prevent this or it will result in gear or bearing failure.
Regards.
Irshad Akhtar

I can understand. but how can i find emusified water only on pinion bearing? If water is in emuslsified form, it should appear in oil analysis result. T32 grade oil can hardly allow water!! even 0.1% by vol water can be easily be identified in simple crackle test. I can rule out this possibility.

Gearbox actual configuration might have something to do do with why the emulsion appeared only on the pinion, to the exclusion of the gear. For one thing, the pinion at 11000 RPM is likely to have much smaller bearing clearances than the gear. Thus the clearances on the gear might support a bit more of a "flushing" action which would help decrease or eliminate the emulsion in the gear bearings. Also, not enough information is given to establish what the load angles might be and what effect that might have on the bearings. As an example, looking from driver to driven, say the pinion is on the left of the gear and it is a downmesh box. Also, lets make the input to the gear be CCW; then the pinion would rotate CW using the convention of viewing from driver to driven. In this instance the pinion would probably operate at an eccentricity ratio of about 0.8. Separating force, tangential force, and rotor weight all act to push the pinion down and to the left. If the pinion bearings have about 10 mils clearance then the minimum oil film thickness would be about 2 mils. The gear, in this instance, depending on rotor weight, might have an eccentricity ratio of about 0.6. Less eccentricity ratio because in this instance separating force and tangential force act to push the gear up and to the right, while the rotor weight acts down. If the gear support bearings had a clearance of say 15 mils, then the minimum oil film thickness in the gear bearings would be on the order of 4 mils, a space twice as big and less likely to form the emulsion.

As you can see lots of variables to look at and much of the needed information only available at the hands of the gear designer.

John from PA