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VibraMac

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Reply with quote  #1 
Been sent these photos taken on a gearbox inspection.  New gearset installed October '16.  Bullgear (driver) /pinion (driven) arrangement with damage to the pinion.  Tooth contact looks good; however, the damage may suggest tooth clearance errors although I'm sure this was within spec.  In Oct '16 problem were experienced with a new 'soft start' control for the synchronous motor (many start/stop sequences) which maybe the real culprit.

Hopefully the gearbox boffins can intrepret these photo's better then myself 

Attached Images
jpeg 20170404_152348 (3).jpg (157.21 KB, 35 views)
jpeg 20170404_152443 (3).jpg (343.40 KB, 34 views)

Danny Harvey

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Reply with quote  #2 
Is it the same teeth in both photos?  Is there uniform damage on that side of all the teeth or just those three?  Any vibration data?

How many starts and stops?  I'm wondering if this is a lubrication issue where the oil is not in the mesh at start so you momentarily get metal to metal instead of riding on an oil film.

You are confidant that the clearance was good but was it also properly aligned?
John from PA

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Reply with quote  #3 
I've taken the liberty of rotating one image and putting them side by side.  Take note of the areas I've marked in the red ellipse.  Is that a "step" in the flank of the tooth or just some form of discoloration.

teeth.jpg 

VibraMac

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Reply with quote  #4 
Thanks Gents for your responses.

Danny,  Photos are of the pinion (only gear to be damaged) and apparently on all teeth with varying degrees of damage and towards one end only.  Told there were +- 12 start/stops to optimise the soft start sequence.

John,  Told there are no steps on the contact areas.

After some reading, pitting maybe the problem and the following remarks made:

Pitting:This is a major cause of gear failure accounting for nearly 60% of the gear failures. Pitting is the formation of craters on the gear tooth surface. These craters are formed due to the high amount of compressive contact stresses in the gear surface occurring during transmission of the torque or in simple terms due to compressive fatigue on the gear tooth surface. There are two types of Pitting. They are

a) Micro Pitting: These are basically formed due to

 §Inherent Errors in the gears

§Presence of water in the lubricant that is lubricating the gears

§Wrong viscosity selection of the lubricant used.

Believe the first two have been confirmed on site.




 
John from PA

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Reply with quote  #5 
Your reference source (like many) may have led you astray.  Doing "forensics" on a gearbox to determine root cause of failure, can be difficult at best, but it is important to determine root cause to be certain the failure is not repeated.

I would put forth that the likely root cause is a bearing or bearings that were not set properly when the gearset was replaced in October 2016.  A poorly set bearing allows the two gears to run "out of parallel" and thus will end load one or both of the gears.  I note your comment "Photos are of the pinion (only gear to be damaged) and apparently on all teeth with varying degrees of damage and towards one end only."  

The unfortunate thing is on disassembly of the gearbox all the information relative to bearing settings has likely been lost.  A good gear shop when diassembling a gearbox will take extra care to document how the bearing are set because that can be a large factor in premature wear of a gearset.
 
VibraMac

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Reply with quote  #6 
John,Thanks for your input.  Very much appreciated knowing your knowledge of gearboxes.  I have a difficult question for you to answer should you do so:  Based on the photos (apparently the worst damage) do you think they can continue to operated the gear set until another set can be manufactured (+- 6 months). I will apppreciate your response either way knowing this type of question is like asking 'how long is a piece of string'.  My limited experience on similar gearboxes was to find a broken tooth/teeth that soon lets you know there is a problem (vibration , noise, etc).  Again from your experience if they continue to operate this gearset as is, could it experience a similar scenario (i.e broken tooth/teeth).  Have you experienced a gearbox that has disintregrated/exploded damaging both the driver and driven units?
   
John from PA

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Reply with quote  #7 
Yes, running 6 more months, hard question to answer!  Maybe! 

Some questions to address would be can you identify a root cause.  I don't want to rule out metallurgy, but as I've pointed out I would lean toward a bearing setting issue, especially with the evidence at load being toward one end and dominantly on the pinion.  Was this a new gearbox or a rebuild?  If a rebuild, were the current gears supplied by a reputable shop.  Ultimately, should you chose to rebuild and use the current set, I would pay very close attention to the bearing settings, then blue and check tooth contact after running.  If you can get good contact (more central as opposed to end loading) you might be able to get six more months.  Is the service such that you can reduce load, even by 10%?  

As to your question "Have you experienced a gearbox that has disintregrated/exploded damaging both the driver and driven units?" the answer is "yes" and on multiple occasions.  Tell me more about this particular gearbox and its type of service.  Any cross sectional drawings?

 
VibraMac

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Reply with quote  #8 
John thanks for your immediate response.  Latest update:  They have started the machine (very critical situation) and apparently all is OK (i.e. vibration and noise). I have been promised  the relevant drawings.  Regarding the gearbox, it was a complete new unit.  Been told that there was no supporting documentation which should have shown the tooth contact pattern  before dispatch to ensure a similar pattern is achieved on site once erected.  The erector was therefore not prepared to alter anything, taking the assumption that tooth contact would be correct providing the gearbox was set 100% level and coupling alignment was correct.
Erector believes the NDE pinion side is damaged due to the no load compressor state  immediately after start up allowing the pinion NDE side to 'rattle' (I thought the noise was more associated with the gears) before the compressor guide vanes are opened.
Apparently water was found in the oil, how much I do not know.
Once I have more info regarding the box I will forward.  I appreciate your support.
   
Walt Strong

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Reply with quote  #9 
"Apparently water was found in the oil, how much I do not know."
Did Erector store gearbox onsite for significant time before installation and startup?

Walt
John from PA

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Reply with quote  #10 
Quote:
Originally Posted by Walt Strong
"Apparently water was found in the oil, how much I do not know."
Did Erector store gearbox onsite for significant time before installation and startup?

Walt


Water is not the cause in this instance but may be an aggravating influence.  If we trust the statement "Photos are of the pinion (only gear to be damaged) and apparently on all teeth with varying degrees of damage and towards one end only." that likely summarizes the root cause to bearing settings (if REB) or a distorted housing (soft foot) shifting contact.   
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