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About Voltage Gap

01/09/2010 10:40 PM

Hi all, I am new in this engineering world and wonder what does voltage gap represent? In the factory I work at, there is a blower which has one of its voltage gap tends to increase. Last week it's only around -8 vdc, now it's -12,7. The vibration seems constant. Can anyone explain to me what does that mean? Thanks

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#1

Re: About Voltage Gap

01/10/2010 12:03 AM

Gap voltage represents the average distance between the tip of the probe and the shaft surface. The scale factor is typically 200 mV/mil.

-10 VDc is the center of the range and should be about 50 mil (thousandth of an inch) clearance between the probe tip and the shaft surface. The gap voltage gets more negative as the distance between the probe tip and the surface increases.

In your case the voltage went from -8 to -12.7 a change of 4.7 volts.

4.7 V times 1 mil / 0.2 V means the the shaft moved away from the probe by 23.5 mils (which is about 600 micron). That is a large shift and could well be due to either a loose bearing, a loose probe, or a journal bearing that is damaged.

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#2
In reply to #1

Re: About Voltage Gap

01/10/2010 4:04 AM

Thanks Steve for the immediate answer. It confirms what I've thought, thanks for explanation about the calculation. To continue my question about voltage gap of a blower, the voltage gap of probe A is always around -8,3 to -8,4 , but the gap of probe B is greatly increasing in 7 days (from -8,0 to -12,7). This is on Journal Bearing Position. Is that mean the journal bearing damaged only below the A side (so the shaft moved in diagonally down to probe A side)? FYI the probes are 45 degrees to the left and right from vertical position.

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#3
In reply to #2

Re: About Voltage Gap

01/10/2010 5:45 PM

If one probe showed a significant shift, and the other probe showed no shift at all, it is most likely a problem with the probe that shifted, and not journal damage.

The probe may be loose, the connection may be bad, it may be side viewed (field obstructed by the mounting) or it may be damaged.

Typically if there is damage to the bearing, it will be in the bottom, which would cause both probes to show a change in voltage, given the 45-45 mounting.

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#5
In reply to #3

see below

01/10/2010 10:44 PM

see below

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#14
In reply to #3

Re: About Voltage Gap

09/15/2015 6:43 AM

"it may be side viewed (field obstructed by the mounting)"

can you specify indetail, because we have an issue where only one probe gives very high readings 75microns other gives only 10 microns from steam turbine journal bearing. But machinery point of view, we observed, bearing and other condition are OK. But no idea, why that probe gives high reading

Thnaks!

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#4

Re: About Voltage Gap

01/10/2010 6:32 PM

Hi,

If vibration value has being constant there is a good chance the probe is loose if that's the case the good thing is (if there is one) that it's moving away from the shaft but if it keeps moving it's going to eventually get out of the proximitor's voltage range and that's going to give you some problems.

thanks

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#6

Re: About Voltage Gap

01/10/2010 10:45 PM

Thanks for both of you, Mr Steve and Mr Wrenchpuller

My senior said that the probe was set tightly so it unlikely moves, and it was possible that the shaft moved in a resultant vector of both voltage gaps, so it moved diagonally. Is it possible that the bearing damaged in one side (the scratch deposited on the other side)?

Another question, does the distance between the tip of the probe and shaft surface (voltage gap) affect the accuracy of the probe to measure vibration? is there a limitation of voltage gap (a range in which the probe work well)?

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#7
In reply to #6

Re: About Voltage Gap

01/11/2010 1:00 AM

Hello sir anrava,

The probe is just a solenoid coil whose inductance varies in response to the gap between that probe tip and the shaft it is monitoring. An oscillator (Proximator in Bently Nevada terminology) drives the probe with an AC signal so that the change in probe gap can be monitored and recorded. As Steve stated, the nominal probe output is 200mv/mil and this holds over a range large enough for the service life of most modern machine bearings. If this were a thrust position probe, large movements, of several mils, due to thermal growth etc. may occur. Vibration will still be indicated properly as the shaft surface moves relative to the probe location throughout each revolution of the shaft.

Luther M

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#8
In reply to #6

Re: About Voltage Gap

01/11/2010 1:33 AM

Possible, but very very unlikely. I am usually very suspicious of any signal characteristic that is not seen on both probes. Again, if you are only seeing the gap shift on one of the probes, it is most likely a probe problem and not a machine problem.

If the probe is not loose, that does not mean that there is not a probe problem.

Another issue is to check for oil or water in the conduit. That will sometimes cause a gap shift. Also check to see if there is water in the junction box where the proximitors are housed.

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#9

Re: About Voltage Gap

01/11/2010 10:29 AM

Thanks Mr Luther and Mr Steve for the responses

So there're no limitation of gap that affects the accuracy of vibration detection? B'coz my senior said that there are limitation around -12.3 vdc, after that the probe wont detect both vibration and gap accurately. Is that right? fyi the blower uses bently nevada 3300 system.
and OK I'll try to clarify the problem when the machine stopped and disassembled.

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#10
In reply to #9

Re: About Voltage Gap

01/11/2010 5:49 PM

Your senior is correct that there is a voltage range limitation, he is incorrect as to that limit.

Assuming that this is a BNC 7200 or 3300 probe with a -24 VDc supply then the ends of the linear range for the probe will be at about -2 VDc and at about -20 Vdc. Every probe is a bit different and you have to run a calibration curve to determine the limits for sure, but -12.3 should be well within the linear range for a Bently probe. Please go to the Bently website and get the probe data sheets and review them.

http://www.ge-energy.com/prod_serv/products/oc/en/bently_nevada.htm

Please do confirm that there was no voltage shift at all on the second probe. If there was some, and it was just smaller than on the probe with the 4 volt shift then it could well be bearing damage.

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#11

Re: About Voltage Gap

01/14/2010 12:40 PM

oic, thanks for the response and information.
The bearing of the turbine has been disassembled and we found that the journal bearing was in a heavy damage.
The babbit layer has been "eaten" so that the metal part damaged too.
In this case, I think the voltage gap indicated that the shaft became lower and lower because the bearing was damaged.

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#12
In reply to #11

Re: About Voltage Gap

01/14/2010 6:17 PM

Great!, but then you need to find out why there was not a significant shift in voltage on the second probe! Or was there?

The second probe may have been sideviewed, ie gapped on something that is not the shaft.

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#13

Re: About Voltage Gap

06/20/2010 7:16 AM

thanks Mr Steve. Sorry I havent logged in for long time.

After we replace the bearing, the machine runs well until now. The voltage gap is both good.

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#15
In reply to #13

Re: About Voltage Gap

11/07/2016 5:14 PM

Dear Sir Anrava,

After bearing replacement in 2010 have you faced again the problem of increased gap Voltage on 1 probe till to date (2010 to 2016)

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