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Maintenance Minute - Hold Your Mouth Right

Posted 4 years 359 days ago ago by Admin

One item that used to perplex me was reinstalling the K-FLEX drive shaft on a Bell 407. It wasn’t too bad on the earlier models. But once the aft T/R driveshaft flywheel was “incorporated” to form the new thicker rotorbrake (R/B) disc up front, it became a little tighter getting the shaft to drop in.

However, when Bell increased the size of the transmission drive flange it became downright impossible without some persuasion. That was until a fellow wrench, Kevin H., showed me the light. This tip is directed at the latter K-FLEX installation. While it’s a bit hard to explain, it’s even harder to draw a diagram. But it works. You just need to hold your mouth right.  

First, be sure to use the correct K-FLEX compression tools. Install the forward and aft compression tools per the maintenance manual and fully compress the shaft flex-beams as noted.

Second, do not install the R/B disc on the free wheel unit (FWU) flange.

Using the diagram below as a basic guide, we’re ready to give it a try. This can be done from either side of the aircraft, but I prefer the right side. Slide the R/B disc in front of the FWU flange and place the compressed K-FLEX shaft, engine side up, on the transmission deck.

Cradle the R/B disc in one hand, lifting it up toward the engine compressor housing. The important thing is to position the top portion of the R/B disc opening (B) above the top of the FWU flange (A) with the disc top edge tilted slightly aft.

The idea is to get the R/B disc up at an angle so it sits a smidgen aft of the FWU flange allowing the shaft to clear the transmission drive flange as you lower that end.

Now grab the K-FLEX shaft in the other hand and place the top of the K-FLEX engine flange (C) into its mounting area (green portion) on the R/B disc. Position the shaft at about a 20 to 40 degree angle.  If the planets are aligned, the shaft will miraculously drop in when you lower the transmission side of the shaft on the first time. But normally it doesn’t, so don’t give up.

Jiggle the disc around while repeatedly stabbing the engine side of the shaft and lowering the transmission side. Sometimes engine oil lines, especially the #1 bearing scavenge line, will limit the vertical height you can lift the disc. Sometimes the shaft flex-beams hit the firewall. But when you hit the sweet spot you’ll know it. The shaft will settle to the deck and the disc will drop in front of the FWU. Pure magic.

It took me several different attempts to get the hang of it. After it became a sure thing I started to win a few bets. Bottom line: it works. Good luck!

About the author: After 32 years maintaining helicopters in various capacities, Scott Skola concluded a full time career with a major operator in 2014. When not pursuing future writing projects, he can still be seen around the flight line providing maintenance oversight, litigation support, and technical research services. He can be contacted at [email protected].