The motor is a 500Kw 3574 rpm with sleeve bearings fitted with bently probes on both de & nde(motor recieved back after overhaul).The drive end casing vib (horizontal)was around 6 mm/sec and both bently X & Y around 4mils(above trip setpoint).Trim balanced using housing sensors and brought down vib to 1.7 mm/sec but bently readings remained nearly the same with a prominent 2X all this was done with motor uncoupled.

After coupling up to the screw compressor at full load the drive end casing vib was around 3.5 mm/sec and the bently reading the same but 2X decreased significantly and was a pure 1X spectrum.Checked bently sytem nothing seems to be wrong and the twf from them is a nice smooth one without any kinks.

I am aware that the bentlys are reading the relative disp of the actual shaft movement,also the bearing housing is not so robust so i expect a fair amount of vib transmitted to the housing as well but the change on housing vib does not seem to affect the bently vibs.

i cant think of anything else can someone throw some light on this (any option other than faulty bently sys) attaching the plots

plots.doc (76 Kb, 62 downloads)

It looks to me that what appears to be as 2x is in fact 2xLF which stayed about same with amplitude of 1.2 mm/sec on the casing throughout the unbalanced-balanced-coupled-uncoupled procedure. Likely, it also shows up in prox readings as well, since bearing is moving at 2xLF.

The fact that prox readings at 1x came back after motor has been balanced but coupled, apparently means that you may have shaft alignment problems as well.

David,
The 2X frequency changed from uncoupled 120.44 to coupled 119.83 on motor horizontal DE which appeared to be almost exactly two times running speed in both cases. Would that not suggest that it is 2X running speed and not 2X LF. Strange how the amplitudes stayed relatively unchange though.
Viki,
Didn't someone post earlier this morning about possible runout on shaft under prox probe? I guess they deleted it. David pointed out how your 2X amplitude stayed about the same on the casing but they sure didn't on the prox probes. 2X went from a little over 60 microns to about 10 microns from uncoupled to coupled. That is strange isn't it. Was any of this going on before overhaul? What kind of work was done during overhaul? Any bearing repairs made? Possible internal alignment in bearings due to overhaul? Just some thoughts.

Thanks,
Ronnie

Ronnie
The motor was dismantled at the workshop inspected cleaned etc all old parts nothing was changed. Rotor was balanced at 850 rpm (strangely in one plane on the external cooling fan, it wasnt right i had to redo the balance at rated speed and brought the casing vib down). The 2X seems to be the running component but my 2X coastdown plot in uncoupled shows it comes down drastically below 3450 rpm.Another point worth mentioning is the proxies are mounted on the bearing outboard at around 2.5 inches from the bearing.
I am in a quandry wheter i can certify the motor vib ok based on casing readings cause if there is a runout on bently cant be resolved at site

Thanks
Viki

Maybe a shaft rider measurement could help resolve whether there is a false high reading from the prox probes.

Also do you have access to the Bently data (orbtis, average shaft position etc)

Thanks electricpete
I dont have the adre with me now for those measurements am using a 2120-2 where can i find some info using shaft riders never used them before.How accurate are these shaft rider measurements

Viki

quote:

The 2X seems to be the running component but my 2X coastdown plot in uncoupled shows it comes down drastically below 3450 rpm.

Viki,
The 2X drops but not the 1X? This is casing readings? Is it possible there is some resonance at 2X? Is everythng tightened properly as in casing bolts? Did they document the the clearances, journal to bearing, bearing to housing? If this was not documented, you can check the bearing to housing clearance with plastigauge. If you are not familiar with this, it is a soft string like plastic that can be placed on the bearing where the housings seats to it. The housing is bolted back tightly and then opened back up. The outside of the packaging is then compared to how much the plastigauge compressed telling you the clearance. This can be done by removing just the top of the housing without further disassembly. If your repair facility is not familiar with this, check with bearing suppliers.
Something I missed earlier, your Bently uncoupled is MOH and coupled MIH. What happened to the 2X MOH coupled?

Thanks,
Ronnie

Viki,
Do a right click with your mouse just above and to the right of the suspected 2x in your plot. Drag the box all the way down to zero fmax and see if that separates the 2x from 2xlf. What is the shaft diameter at the bearings?

viki,
I have some info on shaft riders. If you would like I can e-mail it to you. Let me know at
roy dot gariepy at dorlastan dot com

Shaft dia at brgs is about 3inches. Moh & Mih mistaklingly interchanged cause these spectrums were accquired on existing old route point confirm all plots are at Mih thats why i added a note at the end.Will recheck clearances and tightness of all fasteners