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Tail rotor drive shaft coupling failure

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Hello, iam analysing failure of coupling shaft. Iam exactly not able to make out whether it is a failure due to overheating, loss of lubricant or when my tail rotor hit the ground resultantly its impact load caused it. However, i didnot think it is due to sudden impact load. One more thing if OEM is recommending G355 grease in that coupling and iam using G353, Does it affects or not. Picture of failed coupling is attached. This is the other end. Here you can see teeths of coupling are damaged/wear in a specific regular pattern. Only area of approx 5mm is damaged in this coupling and on the other end splines teeths are of 5mm approx length. This drive shaft has allowed play of 5mm in axial direction




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From the pictures:

1- This gear coupling has very high teeth wear, far beyond allowable. This is named a gradual deterioration failure mode.

2- Such gear couplings should be manufactured with gear tooth surface hardness of no less than Rockwell C-45, i advise making a hardness confirmation test using a local lab.Even a university mechanical engineering dept can assist you.take hardness looking axial at top of tooth in a non worn location.

3- The high wear at center of tooth length definitely shows high misalignment of drive shaft coming from main rotor drive, to the tail rotor. Misalignment throws high loads causing fretting wear. Contact a local machinery alignment company, and request them to perform laser Alignment for you..This is the most superior technique.

4- Be careful of grease types selection; For this application, there are special shaft coupling greases on the market, specially made for gear couplings; just search them on Google: "Polyurea shaft coupling grease".

5- Very Important: Contact the helicopter  manufacturer and give them the part identification name and even part number and ask them to provide all aviation bulletins associated with the tail rotor coupling that were published in past 20 years.Ask them to include your E-mail in all future service bulletins.

6- Check all shafts connected to coupling for cracks using Dye check. This coupling was basically stuck and was not acting in a flexible manner, so any shafts coupled to it were subjected to bending fatigue, not only torsion.

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I agree with Abdulrahman-1....there are a couple of other observations though....

From the images (which are not overly clear where I would like to look...), there appears to be a LOT of iron filings in the female gear..although it may be the way the light is reflecting...  They SHOULD NOT BE THERE and erosion of the gears should have been picked up during routine maintenance.

Also, there appears to have been some sort of fluid ingress to the gears...neither should have rusted, and both show evidence. The grease should have kept the fluid out, and at worst allowed "black" rust due to lack of oxygen, not red rust, which shows  the environment is oxygen rich!.

One of the main inspection criteria during maintenance is looking for wear marks in gearing and evidence of iron filings...IMHO, this should NEVER have gotten his far!

I would be seriously concerned about this level of wear on a 100 year old pushbike...but on a "whirly-gig", this is mindblowing! I would have thought both gears should have been replaced when a set number of engine hours had been logged...check the log-book!

You are lucky to be able to submit the query!


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