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Ralph Stewart

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Reply with quote  #1 
I have not been to any vibration analysis classes in the past few years, but one guy who has attended one, asked me a question about a "theory" he heard and I have never heard anything related to it. Of course I may have not caught it when it was passed out that day. [smile]

The question or statement is on prox probe reading's of sleeve bearings' overall trend value of the "Horizontal" vs the "Vertical" overall trend value when compared or plotted together.

The "statement" was, "when the overall trend value of the vertical amplitude overlaps (or has a higher value than) the overall trend value of the horizontal amplitude on any given date, this is a sign the bearing has "wiped"."

The question was asked because of our noticing such a trend plot today on a motor.

I have never heard or read of this theory before. Have any of you guys?

One is never too old or too smart to learn something new, everyday. If I do not learn at least one new thing every day, I feel I have basically wasted that day as far as the gaining of knowledge goes. So I am all ears today! [smile]

Thanks and Have a Great Day,
Ralph

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Ralph Stewart
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Reply with quote  #2 
Would that be seismic/absolute or eddy probe data or either? If seismic I believe it when I see it.
Didn't consider the subject, so in that case which is which H/V, X/Y? Or is  it only valid for H/V sensors?
I still think I doubt it. 

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John from PA

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Reply with quote  #3 
Quote:
Originally Posted by Ralph Stewart
The question or statement is on prox probe reading's of sleeve bearings' overall trend value of the "Horizontal" vs the "Vertical" overall trend value when compared or plotted together.

The "statement" was, "when the overall trend value of the vertical amplitude overlaps (or has a higher value than) the overall trend value of the horizontal amplitude on any given date, this is a sign the bearing has "wiped"."

You can't make that type of statement as a generalization unless you know the type of rotor, the type of bearing, the forces on the rotor, and the probe mounting angles.

Let's take a rotor, clockwise rotation and no forces acting except for gravity (see attached graphic).  Perhaps something like a motor rotor.  Under rotation, the rotor will take a position slightly up off the bottom of the bearing and slightly left of the bearing centerline.  You can see at the bottom left of the graphic, about the 7:30 position, is the point of minimum clearance.  That is sometimes called the "hard side".  Roughly at 90 deg to that is what is sometimes called the "soft side".  Regardless of what you call it, the axis of the hard side represents an axis of high stiffness as compared to the soft side axis.  Ideally we would mount a probe pair in line with the hard side and soft side, but this often can't be accomplished because of external features of the machine, like piping.  So we tend to mount probes at 45 deg left and 45 deg right of top dead center.  For the example 45 deg right is very nearly the hard side and 45 deg left is very nearly aligned with the soft side.

Now, with respect to vibration, what does the hard side stiffness being higher mean when compared to the soft side?  With any given source of vibration, say 1X due to unbalance, then the rotor will respond more in the soft side orientation as opposed to the hard side orientation.

OK, one more step, take the graphic and view it as looking from driver to driven, the standard BN convention.  The probe at 45 deg right of top center is often called the horizontal or "X" probe; the probe at 45 deg left of top center is called the vertical or "Y" probe.  By the way the "X" and "Y" terminology is a throwbacks to oscilloscope days (remember them).  You plugged the "X" into the X or horizontal input of the o'scope and the "Y" into the "Y" or vertical channel.  But the probe at 45 deg left, the vertical is nearly aligned with the soft side (lower stiffness), and even in a healthy machine will vibrate more than the probe at 45 deg right, which is aligned with the hard side (higher stiffness).

For a single casing transducer the preferred orientation is as near possible in line with the hard side.  This location, due to its being in close proximity to the minimum oil film thickness, affords the highest transmissibility of vibration related energy directly to the transducer.

As always, it may or may not be practical to obtain the “preferred location.”



soft sde hard side CW.jpg  


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