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Hi! We have five induced draft cooling fans installed at our site. Recently we experienced high vibration on one of our fan gearbox having accelerometer installed in horizontal direction to gearbox casing. The max vibration was around 17 mm/sec and spectrum revealed GMFs with modulation where 2 x GMFs dominated for first set gears which are spiral bevel gears.

Any idea what could be the probable issue?? Is it due to misaligned gears??

If yes then what factors could contribute to this issue?


Fahad Qureshi


Original Post

In my experience 2xGMF is indicative of a serious lubrication problem and/or severe wear of the tooth profile.  I would be shutting down the suspect unit and examining it through the inspection openings.  Almost all cooling tower drives have the inspection openings.  Sometimes it is a steel bolted on cover, perhaps 10 cm x 15 cm.  I have also seen cooling towers with relatively large pipe plugs that when removed enable you to see the mesh.  If you post a drawing of one of the drives I can perhaps mark it up and show you were the openings might be.

Dear John,

Thanks for your quick response.

Yes it has a inspection window. We actually removed that gearbox and found gears and their profile were intact. Even contact patters seemed to be fine. Backlash was within acceptable range.

I will post pictures of the gear soon as well.

Can a bad machine foundation contribute to misaligned gears.



OK, sounds like you’ve taken care of the obvious.

With respect to lubrication if you can do so through the inspection covers make sure there isn’t any plugged oil spray nozzle.  I would also try to get a quick test on the lubricant to make sure it is in the range of the correct viscosity and the moisture content is low.

Reading contact patterns is not the best indicator unless you clean and re-blue the teeth.  Otherwise, you can’t tell the pattern from today, from one generated a year ago.

Yes, a bad foundation can affect alignment because it can affect casing distortion.  Is this a fabricated steel or cast housing?



Thanks John. Yes it looks corrosion.

What are your thoughts on casing as attached in pictures?

Also, Would appreciate if you can provide your input on my below post,


If indeed that is corrosion, then you need to be looking for a mechanism for water to be entering the unit and breaking down the lubricant.  

As far as the casing, my one comment is why is there an abundance of oil stains on the outside in the area of the nameplate?  Is it just spillage from the disassembly operation, a possible crack????

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