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gguevara

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Reply with quote  #16 

Thanks Ralph, what you tell me makes a lot of sense, this welding operation was really our first thought when we were told that there was a current problem - but then we were told by the same specialists that the type of failure was related with current passage while the machine was turning, and that in case the damage had been done in a static state the damage would have been quite different..  and that's why we concentrated only in the drive.., but we may have been wrong, at least we have to go on investigating this...  

I will try to get the inner race photos soon. 

Just to explain a  little more about the welding that takes place:

The machine is loaded with 6 spools of wire. Each wire travels along the tube, which is turning at 450 rpms and is supported by the bearings that we are talking about. The 6 wires, plus a 7th that goes outside the machine get together at the end of the tube and become a 7-wire strand.

Every time the machine is loaded the operator has to weld the tip of the new wire spool to the tip of the old wire spool, to do that he uncoils some wire from the new spool, and takes both tips to a butt welding machine (8 to 12 V, 1kA, 50 hz) that is not more than 2 o 3 meters from the machine. 

Each bearing housing is grounded, and the tube itself is grounded through slip rings.. ground resistance were carefully measured...but, you never know with stray currents... 

grounding.png 


electricpete

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Reply with quote  #17 
Just some thoughts on the possible electrical sources covered:

If the welding occurs while the machine is secured, then associated markings would include families of roller-spaced marks (similar to false brinneling).  Each time it occurs, you'd get a new family of roller shaped marks at a random angle from the first. Similar to false brinneling where you have multiple families of roller-spaced marks with random spacing between families. You photo doesn't look anything like that to me.

From the motor drive? I think the belt drive rules that out.

Static electricity might be another electrical-related phenomenon to think about, but I have no idea what it would look like (I'd think brushes would take care of it).


It's a head-scratcher.
Ralph Stewart

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Reply with quote  #18 
Thanks gguevara,

Looking at your drawing of the ground wire, it looks quite suitable for the task.

You mentioned that the load zone of the outer race of the bearing is approx 120 degrees. And it appears that is the only place on the surface of the outer race in your picture that has these marks, correct? [confused]

If there are 36 rollers in the 360 degree outer race surface, that would mean there is 10 degrees per roller, or 12 rollers at any one time in the "120 degrees" load zone.

Looking at the picture, there looks like there may be 75 to 100 roller "marks" or more overlapping each preceding mark within this 120 degree load zone, which goes without saying, it never stops in the same place when shutdown which could, IMO, point to the welding possibly as being the culprit. OR NOT.

One other thing, the pattern of the marks seem to show the rollers are not running "true to the center" of the race as seen from the oil hole on the right, to the upper left corner of the picture, as the pattern seems to drift off center toward the edge of the race. Sort of like the outer race is not aligned to the shaft. It may just be that this is a self aligning bearing and can run out this way in an effort to align itself.

Only my opinion and I could be totally wrong on the "marks" number count in the load zone.
Clipboard04.jpg 

Thanks and Have a Great Day,
Ralph


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Ralph Stewart
AlertAnalytical.com
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