Sign up Calendar Latest Topics Donate
 
 
 


Reply
  Author   Comment  
fredyarevalo

Member
Registered:
Posts: 30
Reply with quote  #1 
HI everyone, 
is it possible to diagnose bent shaft condition on a double suction Goulds water pump using cross phase analysis with csi 2130.
i have notice radial vibration increase on both inboard and outboard bearings. i took phase readings across coupling and is at 178 degrees which is indication of misalignment, and
i took phase reading on inboard and outboard bearings on the pump and it is also at 179 degrees. (sensors on same direction when reading taken)
is it normal? bent shaft? or just a misalignment with motor.
this pump is driven by a 1780rpm motor. in the past to my knowledge we had to broken two shafts.



thanks for your attention

regards 
fredy. 
Vibe-Rater

Avatar / Picture

Sr. Member / Moderator / Supporter
Registered:
Posts: 1,398
Reply with quote  #2 
Hi Fredy, I've never found a bent shaft with vibration but as I understand it if you measure the phase difference on the bearing housings in the axial direction.  Take to either mount the accelerometers in the same direction of to compensate if they are not.

Hope this helps.

https://www.mobiusinstitute.com/site2/item.asp?LinkID=8024&iVibe=1&sTitle=Bent%20shaft

fredyarevalo

Member
Registered:
Posts: 30
Reply with quote  #3 
thank you!
RustyCas

Avatar / Picture

Admin
Registered:
Posts: 1,810
Reply with quote  #4 
Yes, for a center-hung machine (rotor between bearings) - which a double-suction pump would be, a shaft bent between the bearings will produce axial readings 180 out of phase. I’ve seen this exactly once in 30 years. It was a large fan (1200 hp, 10,000 lb rotor) with a thermal bow. There was a fire n the ductwork and they shut it down while hot and the rotor sagged, effectively bending it. The sufficient solution was to balance it. I told them it might “relax” with time and the vibration might return to pre-balance levels. It did, and I had them cut off the balance weight and it was again, balanced.

__________________
"The trend is your friend"
Ralph Stewart

Avatar / Picture

Sr. Member
Registered:
Posts: 235
Reply with quote  #5 
Not sure what series Goulds pump you have, but a good example of one is the model 3410 seen here.
http://ifcpump.com/files/product/3410.pdf

You may already be familiar with the following, but:

On a double suction center hung pump, the bending of the shaft between the main bearings is very difficult to do, IMO.

They are usually designed with what are called "wear rings", usually one on each side of the impeller which act sort of like bearings, in addition to other duties they perform. This makes the distance from the center of the pump to the wear rings much less than the distance from the center of the pump to the main bearings.

One might would think the shaft would bend quite easily, based on the distance between the DE and NDE bearings,which possibly would be the correct thing to think, if it were not for the wear rings.

Another thing that helps support the shaft from bending is the positions of the packing glands.

A broken shaft on a center hung pump happens at a very high % rate (on relatively small pumps) when the pump is shut down, and the check valve (if one is present) does not hold the fluid from feeding backwards into the pump, which causes the impeller and shaft to spin backwards and the torque of the motor on restart can nap the shaft, usually next to one side or the other of the impeller.

The answers given so far on the phase comparisons of near 180 degrees for a bent shaft is basically correct, in theory.

Just my opinion and I could be totally wrong.

Thanks and Have a Great Day,
Ralph

__________________
Ralph Stewart
AlertAnalytical.com
Previous Topic | Next Topic
Print
Reply

Quick Navigation:

Easily create a Forum Website with Website Toolbox.