MPPalmer(at)aol.com
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 Posted: Wed Apr 30, 2008 10:20 am Post subject: FT Nose Gear Stop fix |
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We have had a subtle "shimmy" in the nose gear for about a year or two now. Unlike typical shimmy that happens at speed when the nose gear pivot friction isn't set correctly or when the rubber shock mounts sag and move the nose gear out of the correct geometry (there was a Service Bulletin or a Service Note about that), this shimmy only occurred at the end of a stop.
When I pulled the nose gear fork off the pivot arm for routine maintenance, I discovered, quite by accident, that the friction pad was loose and able to rotate just a tad. Almost like it could cause that slow, end of run, back and forth rotation of the nose wheel. Both the Philips head screw and the hex head stop bolt had backed out a quarter turn, despite Loctite 13 years ago.
Since they were loose, I decided to pull the friction pad assembly to do some deep cleaning. When I did that, I discovered, again by accident, that 1) the top threads in the nose gear fork for the hex head bolt had crumbled and 2) the top of the hole was elongated. I suspect both were caused when you back the plane into a parking space without a tow bar and the nose gear pivots wildly and slams into the stop bolt. (I took a photo and will plan, someday, to post it on a website.)
There wasn't much I could do about the elongated hole. I tried making a steel bushing to capture the top of the bolt in the elongated hole, but without a lathe, I was unable. But fine threads in aluminum didn't seem right. A little googling on the web and the consensus is that you should use coarse thread in aluminum. Indeed, this is how our exhaust studs for the engine cylinders are done.
So I drilled out the hole and inserted a 5/16" - 18 Heli-coil insert and bought a new Grade 8 coarse thread hex head bolt. (BTW, Ace Hardware seems to have the lowest prices for Heli-coil inserts. NAPA is next lowest.) I didn't change the threads on the Philips head for now.... not sure if I can get a 37 degree head in a coarse thread. But those threads looked okay so I simply reassembled with fresh Loctite.
This post may be premature, as I haven't flown yet since doing this fix. I don't know if this fix fixed the problem. But I'm passing it along as something to check for your next inspection. If you're building and Glasair is still using the fine thread bolt, you might want to change over now, unless you will ALWAYS use your towbar when you move the plane backwards.
Hope this helps,
Mike Palmer <><
**************
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