I have normally used greasing as a way to confirm that hi bearing noise is from a fault and not just lack of grease. I have at least once in late stage of disaster got a increase of bearing values when greasing in all other cases it normally drops up to 50%. Now I have a tissue machine where the DC motors have reoccurring bearing fails, 1-2 years. They have carbon brushes grounding the shaft but that only made some longer time and concentrated the failing to the free end bearing where the brush sit. Now I did the greasing test and on 3 motors of varying disaster there was no or minor change. When I cut apart the bearings they have all signs of classic DC motor current disease. I have measured the brushes all way I can AC/DC, volt/current and resistance and to me it looks as expected. So I am confused, more than normally. Greasing is by a small 5 cm extender steel tube and I saw the grease meter tick away when greasing... Anybody else had something similar? Olov

OLI,
Since you are talking about DC motors on a paper mill, I am assuming these are large motors (200 hp and up).
DC motors of this size normally require insulating the NDE bearing in the motor housing. That being said, caution must be taken to ensure that the addition of a tach, bearing RTD, lubrication tube, etc. does not reconnect the ground path.
When you discuss DC motor disease, I am assuming you mean fluting or EDM.

The brushes can also become coated with dust and grease especially in the winders.

SKF Insocoat brg have insulated outer ring. No need for expensive retrofit.
Best regard, Marcel

quote:

Originally posted by OLI:
When I cut apart the bearings they have all signs of classic DC motor current disease.

Olov,

Has vibration data indicated this failure as noise or bearing fault frequencies?

David

Ron, it´s not so large machine so the largest motor is the Yankee drive motor.
Yes bearings from one of the motors of last year have classic fluting, black stripes.
Danny, Yes but I did check during operation, no voltage, no current, no resistance with a DVM across carbon brush.
Marcel, Good suggestion, haven´t needed that before but worth a try, at least we make some change.
David, yes there are bearing freq´s starting at hi freq´s as they should and getting worse with time. In cases up to 7g in demod due to stop planning and such when normal new is as lo as 0.05g at times. Yankee motor NDE is a good example that was out for a total refurb a couple of years ago and now started off again and have a real increase since last colection in may 2007.
Thank´s for your suggestions.
Anyone seen gear resonance excited from gbx overload? Or is it always a tooth damage? I have one case where it was a crushed key/keyway that was/ wasn´t holding the gear.
It´s the same yankee drive but in the drive end of gbx I get approx 700Hz tune that does not fit bearing or gear frequency and it seems very load/speed dependent and you hear it loud across all other noise. Olov

Have you try fractional gear mesh frequencies? 1/3, 2/3, 3/3, 4/3, 5/3, etc, Could it be a natural frequency from another part of the gearbox as input shaft, brg housing, etc. I am not very familiar with paper mill but I did worked with a lot of gears an never encounter gear resonance, I did see very weird phenomenon with gear but they mostly 99% were gear’s factors. Could it be transient even that shows in the spectrum? What about long time waveform? Sorry, I only had a lot of questions and not too many answer!
Best regard, Marcel