INSPECTION OF BELL THOMAS COUPLINGS

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Thomas couplings are located on bell helicopters 206, 206L on the tail rotor driveshaft. The amount of couplings will vary depending on style of driveshaft (one piece driveshaft, or segmented driveshaft). They allow for misalignment of the shaft and flexing during flight.

One thing I find overlooked at the 100 hour inspections are the Thomas couplings. It is not a requirement to pull them apart and clean them unless they are showing separation of 0.003” between the discs. It seems if you wait until there is 0.003” separation usually the discs are heavily pitted and you will need to replace them. I recommend you pull them apart and clean them at the first sign of separation. This will prevent pitting and give you maximum life of the coupling. Below are a few pictures showing some things you should look for when inspecting and how to clean the Thomas couplings.

When inspecting the coupling you should look for separation of the discs and cracks. Any separation the discs should be removed and cleaned. Any cracks you should replace the coupling.

You should also look for couplings that have been assembled wrong. The discs should alternate. Below you will see a coupling assembled wrong

The hardware holding the discs together can differ in stack up of washers due to driveshaft installation tolerances. Make sure you install the same hardware back in the same location. Also make sure that washers that are beside the coupling are beveled on the one side that is against the coupling.

CLEANING THE COUPLING

To clean the coupling, first remove it from the aircraft. Make sure you do not mix the discs up or change their alignment. The coupling must go back on exactly how it came off. One way of doing this is by using a bolt or lock wire through one of the holes when the coupling is removed. This allows you to separate the discs for cleaning without losing there orientation.

Now that you have the coupling off and a bolt in one hole to prevent mixing the discs up, clean the corrosion off with soap and a piece of scotchbrite. After you have done all 3 holes move the bolt/lockwire to one of the other holes and clean the last one.

After this you should check for pitting where you have removed the corrosion. Check the manual for the max allowed. If there is no pitting or it is within tolerance, spray the coupling down with WD 40 or T9. Then wipe it off. You can now reassemble the coupling on the aircraft.



Once the coupling is reinstalled you will notice there is no gap between the plates.



After you have installed it and retorqued the coupling you should inspect it to make sure you have put the washers on correctly and that the discs are in the correct position.



By cleaning these couplings sooner than when they get 0.003” separation you should be able to prevent any pitting on the plates and maximize there life.

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