Root Cause Analysis -Out Board Vibration
Root Cause Analysis -Out Board Vibration
(OP)
I have a pump which had been experiencing increased Outboard vibration, prior to shaft failure.
What do you think could have been the cause of increasing OB vibration on the pump, could it have been due to defective bearings? The pump operates at about 70% efficiency, so I do not think that the shaft was experiencing any un due forces? I did however find out that the shaft material was changed from AISI -4140 to SAE 4340-
Maybe it could be the shaft material?
Any suggestions would be helpful.
John
What do you think could have been the cause of increasing OB vibration on the pump, could it have been due to defective bearings? The pump operates at about 70% efficiency, so I do not think that the shaft was experiencing any un due forces? I did however find out that the shaft material was changed from AISI -4140 to SAE 4340-
Maybe it could be the shaft material?
Any suggestions would be helpful.
John





RE: Root Cause Analysis -Out Board Vibration
Others are more qualified to address the difference in shaft material, but, it would not seem to be the cause of the problem you describe.
BK
RE: Root Cause Analysis -Out Board Vibration
RE: Root Cause Analysis -Out Board Vibration
What is shaft diameters?
At what point on the shaft is it failing?
As stated by Carburize;
Heat treated correctly the 4340 should perform better.
RE: Root Cause Analysis -Out Board Vibration
RE: Root Cause Analysis -Out Board Vibration
Do you think that misalignment could cause a 4340 shaft to fail, this pump operates at 70% efficiency, so the loading on the shaft would not have been unusual.
Finally a BHN of 302 corresdponds to what hardness on the Rockwell C scale?
FYI - We are pumping slurry.
Thanks,
John
RE: Root Cause Analysis -Out Board Vibration
With sufficient number of cycles of load reversal all materials will eventually fail - in tougher materials the fatigue crack will grow a little further before final failure but there will not be a significant effect on total life. In fatigue cracking situations about 80% of the life is involved in crack inititiation and then 20% in crack growth and final failure.
RE: Root Cause Analysis -Out Board Vibration
This shaft had been in for only 18 days...that is a short time, for a pump which operates very near BEP.
RE: Root Cause Analysis -Out Board Vibration
At 4-inch diameter you don't really need 4340 to make the hardness 4140 or 4145 would have been OK but the material is probably not the cause of the failure.
Where did the fatigue failure initiate? I suspect is was from a change of section with a fairly sharp radius.
RE: Root Cause Analysis -Out Board Vibration
You are correct, in that the diameter at the previous section is 3.5" Probably resulting in concentrated stress build up at that point(section change) - resulting in gradual crack propogation, then eventual failure...
but, the material may be of a lower grade , than specified, which would have affected the crack propogation characteristics.
I'll know further after the shaft returns from analysis.
RE: Root Cause Analysis -Out Board Vibration
RE: Root Cause Analysis -Out Board Vibration
What type drive does the pump have?
Have there been more than a normal number of starts and stops.
RE: Root Cause Analysis -Out Board Vibration
RE: Root Cause Analysis -Out Board Vibration
As you stated the vibration frequency and phase indicate coupling misalignment. Misalignment can put tremendous loads on a shaft and bearings no matter the "load" rating of the pump (amount of work being done). If there is a problem with improper radiusing, as others have previously mentioned, the increased load on the shaft due to misalignment would have made matters worse.
Skip Hartman
http://www.machinerywatch.com