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To All the Bently Nevada folks out there,

I have a shop within my old organization that has documentation from BN that it is perfectly acceptable to chrome plate the reading surface of a journal.  That sounds counter to everything I know about the system.  How can you burnish a chromed surface?  Once you've chromed it, that's it, no?  Also, how does this affect the useful linear range, calibration, etc.

Thanks for your input in advance,

Ron Brook

Tags: surface, proximity, Bently Nevada

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Ron, chrome is an acceptable method of repair with some caveats.

When journals are in need of repair it is common practice to turn them down to some undersize dimension, plate the area with chrome or perhaps tungsten to an oversize dimension, and then grind the area back to the proper dimension.  Using proximity probes in the repaired area commonly results in indications of excessive vibration that are not related to the actual dynamics of the rotor.

Understanding the problem is relatively simple.  In the attached graphic assume that the red circle represents the diameter that results when the area has been turned down to an undersize dimension.  Care is generally not taken in getting this diameter to run β€œtrue” to the rotating axis of the rotor since the machinist is aware that the journal will ultimately be plated to an oversize dimension and ground back to proper dimension and concentricity.  The result could be as shown; a 5 mil thickness of chrome at one location on the journal and approximately 180Β° away a thickness of 15 mils.  When a proximity probe observes the target area, there is a variable output as the shaft rotates due to the variable scale factor.  The field at the tip of a proximity probe penetrates that shaft surface to varying degrees depending on material.  Chrome and steel are very similar however and the penetration depth is about 15 mils.  It can be seen then that at the 5-mil thickness location the scale factor will be that defined by the base material plus 5 mils of chrome, whereas at the 15 mil thick location, the scale factor is essentially that of chrome.  The result of this common scenario is a sinusoidal output from the proximity probe when the shaft rotates.  Even at low RPM, the typical signal approximates a sine wave, often quite large in comparison to applicable industry acceptance criteria.  The characteristics of the signal at low speed will typically be a large 1X component of constant phase lag angle.  If two orthogonal transducers are utilized the direct orbit will be relatively clean and circular.

So the important thing is to have uniform thickness of about 15 mils throughout the entire periphery and "yes" you will need  a proximitor calibrated for chrome.  As you likely know, the proximity probe actually "sees" below the target area surface.



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