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Couplings
I recently learned more about these couplings at the CPA seminar.
There are two styles. The "new" style is a one-piece assembly and appears to be the type that people are referring to in this thread. The "old" style is more like a miniature version of the magneto couplings (a collar consisting or two rubber bushings around a cylindrical shaft).
Don't quote me on this, but I believe the "new" style was introduced because there was some risk that the driven gear would fall back into the engine if the old style coupling sheared and the operator didn't notice this fact on a timely basis.
However, in reality, I believe that the new style coupling has turned out to be more problematic than the old style coupling. As mentioned, the bond of the "rubber" in the cup will apparently break down if it stays oil soaked. And when you have to replace one of these, it hurts! They are around $1,500 new, or around $700 on an overhaul/echange basis. The best that I have been able to figure out, Lord Corporation is actually the only place that these things can be overhauled. However, I don't think you can go directly to them. You can do an overhaul/exchange through Aircraft Spruce, or through any Kelly Aerospace dealer.
The old style coupling, on the other hand, can be repaired for less than $50 by simply replacing the wear parts. The trick with the old style coupling is to just keep an eye on it, and fix it if it goes bad.
There is a Service Bulliten for testing these couplings (both styles).
This is TCM SB 95-3A available on the teledyne continental site. Either style coupling requires inspection/testing every 500 hours. This is a test your A&P can do with no problem. For the new style coupling, you have to see if it will withstand 100 in-lbs of torque without slipping. With the old style, you have to inspect the coupling and replace the wear parts.
In the space of two months, I had opportunity to test both my couplings and found them BOTH to be bad. And I don't mean a little bad. I could turn them by hand!
The first one, I replaced with an overhauled "new" style unit.
When I found the second one was also bad, I became pretty dissolusioned with the new style couplings, and just replaced it with an old style coupling (which costs around $500 if you build one up from scratch).
After making these fixes, a number of problems that I have been experiencing with my electrical system went away. Be advised that a slipping coupling can produce some fairly subtle problems because the alternator may "put out" until it becomes warm, or heavily current-loaded, then the coupling starts to slip. In my case, the alternator would still generate enough power so that the "alternator not charging" light would NOT light and you would measure bus voltage in the range of battery voltage. However, there was insufficient current to charge the battery. It is easy to misdiagnose this as a bad alternator.
In any event, there is a "quick and dirty" check that you can do. Whenever you have the cowlings off, reach in and turn the alternator fan blades with your finger. Use as much force as you can apply by hand (which isn't much). If you can turn the fan more than 1/8" (new style) or more than 3/8" (old style) then you should pull the alternator and do the full test. This is not a sure thing, but it is one way that you might spot the problem early if you do this periodically and develop the "feel" of what the coupling should feel like.
Hope this helps!
Kyle Townsend
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