I have just had a look inside the drive end bearing of a 3000 kw 32 pole mitsubishi mill motor. This motor direct drives through an air clutch to a pinion between two bearings. The reason for looking was to replace a failed temperature sensor (rtd)

There was considerable fretting corrosion between the bearing and the outer housing. This had been going on for a very long time, perhaps forty years, and at least eight. The oil had been changed days earlier and was already severly contaminated.

Bearing clearance was flexigauged at 0.016" for an 11" shaft, and the clearance between the bearing and housing gauged at around 0.020".

The journal had a dull finish to it, except in the areas where the oil rings run, which was nice and shiny. The non drive bearing journal was a normal finish. (we had that off to change the sliprings) The babitt is in reasonable condition, given the crap in the oil.

Vibration monitoring is being done on a monthly basis at each bearing and appears to consist of a single horizontal measurement, and is being reported as a trend graph with limits based on iso standard levels. Satisfactory seems to be the favorite word.

Some grout has come loose from the outside of the base frame, however the grout on the inside of the base frame seems sound, on a quick inspection.

Questions
Dull journal finish what does this mean? (shaft currents seem unlikely as other end is fine)
Fretting Is this long time wear starting from a loose fit or something else like the air clutch exerting uneven force.
Does anyone have any experience setting up prox probes on plain bearings, as a temporary system, to do trouble shooting?
I am due to collect some vibration data from the machine this week when they restart, and will update then.

Thanks in advance for your thoughts

Simon

Those are tough questions. I really don't have a clue about this one, but just some wandering thoughts...

You have normal clearance at the shaft but extremely large clearances at the housing. I would think if the cause were abnormal loading or alignment over a long period of time you could have gotten abnormal wear patten evident on the babbit an considering bad oil also increased clearances between bearing and the shaft. So my swag is that most likely cause of the housing clearance is that it started out too loose and got worse from vibration.

With those very large clearances at the housing I am surprised they didn't see some abnormal vibration. Since you say you're setting up temporary prox probes I assume they measure housing vibration. I'm sure someone will point out that many people prefer to monitor sleeve bearing vibration by prox probes. For the specific case of loose housing, I'd almost think housing measurements might be more sensitive to impacting between bearing and housing.

The dull finish of the journal. A common setup for large motors is to insulate the outboard bearing only. Current induced by magnetic assymetery wants to flow in a loop through two set of bearings with the motor in between, so that setup prevents this flowpath involving either the 2 motor bearings or outboard motor bearing and a bearing from the driven machine. But if you have a vfd or some source of static charge, then that flowpath is not the flowpath of concern (high frequency components of vfd create their own capacitive return path and don't require a loop between two bearings). If VFD or static charge were the source of current flow then an insulated outboard bearing (common practice) would explain why you don't see similar pattern at the outboard bearing.

The lack of dullness at the oil ring location is an interesting thing that maybe provides some clue (or maybe not). Once scenario maybe the oil rings polished away whatever damage from some other source... doesn’t seem likely to me. Another scenario maybe it has something to do with the fact that there is a groove in the top half of the bearing at the location of the oil ring. Maybe the contamination includes very large particles from an outside source. The large particles go into the distribution groove and flow out axially... too big to make it down inside the smaller clearances of the lower half of the bearing. But they also get splashed around up at the top where the clearances is much larger. Maybe large particles within the large clearances at top of the bearing caused the problem and of course could not create any damage at the location of the groove because of the oil ring groove in the top bearing half. Or even further out (I almost don't want to mention it), maybe some very strange transient banging around of the loose housing causes contact at the top of the bearing but again not at the oil ring groove.

All my comments above are just a lot of speculation.

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

Hi electripete

Thanks for you thoughts. We have not done the prox probe thing yet, it is an idea we are looking at, and we would look at the shaft initially. The machine is very slow, 32 poles, and I suspect the motion of the bearing in the housing would be more of a rocking one, rather than an impact. That may explain why it has not been picked up.

The dc supply is mg set, and the environmnet is covered in iron ore dust, forty years worth, so static is unlikely . We just cleaned 10" of crud out of the pit under this machine, with a crowbar.

My thinking is that the housing was initially a tad loose, and the alignmnet has moved over time. The resulting forces have generated the wear observed. An agravating factor may be uneven force from the air clutch pushing the shaft to one side, and the prox probes would help prove that theory.

I will be looking at the history of the machine tomorrow and will update if anything interesting shows up.