Loose Nose Strut

Bob Axsom · #1 03-08-2006, 06:40 PM

During the annual I jacked the front of the plane up to check the breakout force and it was fine (I guess it settles in after you tighten it three or four times) but I did notice some play in the strut itself. I traced it back to the bolt in the engine mount/nosegear mount weldment. It was not detectable with the nosegear on the ground.

Bob Axsom

osxuser · #2 03-08-2006, 07:37 PM

I think it wanted to be moved to the other end...

DeltaRomeo · #3 03-08-2006, 08:08 PM

Bob, I've heard that is quite common. However, can you somehow measure the amount of play - maybe fore/aft distance at the axle nut? We're talking hundredths of an inch, right? Maybe 1/64th?

Best,
Doug

Bob Axsom · #4 03-08-2006, 08:43 PM

It was not a great deal of movement but it was quite noticable down at the end of that long arm when I was pulling on it with the fish scale. There is probably some settling over the years since I installed it and maybe it didn't like all the times the breakout force got low and it shook so violently. Once I tightened it down again there was no noticable play. Just another thing to check at the annual.

Bob Axsom

jonbakerok · #5 03-09-2006, 07:26 AM

The nut specified in the kit is a nylon locknut that's not even heat rated. And it sits, what, about 8" above the exhaust stacks?

I struggled with bad nose gear shimmy for a long time before it finally dawned on me to replace that nut with an all-metal lock nut. It used to loosen about every 30 hours. I could tell it was loose when the shimmy reappeared. But on my last annual it was just starting to get some movement, about 140 hours after replacing the nut. I was disappointed that even the metal nut had loosened, but at least it's better than before.

To answer Doug's question -- it depends on how long you let it go. It eventually wallows out the hole. Mine probably moves 1/32 if the nut isn't tight -- which translates to a big movement at the bottom of the leg (1/4" or more). I've thought about reaming it to the next larger size.

One other thing -- I suspect it would help a lot if I was better at holding the nose off on landings. Any castoring nose wheel is going to shimmy a little if it's moving too fast, and any shimmy helps loosen that nut. Unfortuanately, my home field is a (narrow) 9-27, so I'm usually touching down in a 10-knot crosswind. It's hard to keep the nose off when you need the brakes to stay on the runway.

Bob Axsom · #6 03-09-2006, 07:40 AM

I do have exactly the condition you describe. Thank you very much for sharing this information. I just got the cowling back on after replacing the alternator field connector yesterday evening. I think I will add a split type lock washer the next time I have the cowl off. You know, a cross bolt through the mount and the strut might help the problem as well. Big job!

Bob Axsom

AlexPeterson · #7 03-09-2006, 08:13 AM

The problem of the nose gear strut working back and forth is more significant than it may seem. After about 200 hours and around that many landings, I wanted to inspect my nose gear strut. It took about 3 or 4 hours to remove it, owing to this problem. The gear leg had worked back and forth enough to raise a burr on the gear leg and on the bolt itself, which made removing the bolt almost impossible. The gear leg was also very difficult to remove once the bolt was out.

At initial assembly, I didn't like this play and replaced the standard AN bolt with a close tolerance bolt. In my particular assembly, it was not possible to tighten this bolt enough to prevent the play (IIRC, it took several times the rated torque of the bolt to even begin getting friction).

I filed a small fillet in the nose gear bolt hole, to minimize the effect of a sharp edge dinging the bolt, and this has fixed the problem. I took the nose gear strut out again at about 650 hours with no problems.

I highly recommend removing all gear legs every couple years, as failure to do so may result in not being able to remove them at all. There is some galling that happens on the pressure points (against the sockets) on the main gear legs. And yes, this is with copious amounts of thick grease applied.

Alex Peterson
N66AP RV6-A 722 hours

Bob Axsom · #8 03-09-2006, 08:40 AM

I assume the bolt hole you relieved the edges on was in the strut and not the mount or did you do both. [I realized that you might have actually added a fillet on the outside of the mount - could you clarify that for me?] From your description I assume the close tolerence bolt was installed after the 200 hour observation. Do you happen to have the part numbers of your replacement hardware? I have the uncomfortable feeling this is going to become a significant maintenance issue over time.

Bob Axsom

AlexPeterson · #9 03-09-2006, 03:11 PM

Bob,

I added the fillet on the outside of the gear leg. A fillet on the inside of the mount would be good also, but tough to do. A fillet on the outside of the mount would simply make the airplane a little lighter.

I don't have the hardware callout here, but look for the same basic size/length of AN bolt and get a few of the close tolerance bolts. A say to get a few, but there will be a size distribution and you might pick the best fit. It isn't the close tolerance per se, but the fact that the close tolerance bolts are slightly larger than the standard AN bolt. I had the close tolerance from the beginning, as when I was working on building the plane, I would often use the motor mount as a pull to get up from the floor. When I did this, I often would hear the click as the strut went from one position to the other.

Alex Peterson
N66AP RV6-A 722 hours

Captain Avgas · #10 03-09-2006, 04:14 PM

Quote:

Originally Posted by Bob Axsom

During the annual I jacked the front of the plane up to check the breakout force and it was fine (I guess it settles in after you tighten it three or four times) but I did notice some play in the strut itself. I traced it back to the bolt in the engine mount/nosegear mount weldment. It was not detectable with the nosegear on the ground.

Bob Axsom

There are a number of places on an RV where you can expect extreme dynamic forces including of course the interface of the gear legs and their mountings. I noted on my RV7A that Vans specified a .311 drill (or alternatively a 5/16 drill) to size the hole in the gear leg and associated mount. Well it is absolutely positive that a standard AN bolt through a drilled 5/16 hole in that location will not be "snug". And in this location, as well as the nose gear (and also the rear wing spar to fuselage connection) you really want "snug" bolts.

If a bolt is not "snug" in these locations then it will eventually wear and deform the surrounding metal resulting in an even looser fit...and so on.

The best way to get a truly snug fit in these locations is to drill the holes undersize and bring them up to snug in steps with an adjustable reamer.

Producing a less than snug fit in these locations cannot be compensated for by simpling torquing up the bolt to remove movement by friction. This only works until the first landing. And the problem is that the smallest amount of looseness at the bolt results in quite a lot of movement at the wheel. I personally believe that much of the wheel shimmy that builders report (on the mains and on the nose) starts at the loose bolt at the gear leg to mount interface.

If the bolt in these locations is not "truly snug" I recommend the hole be drilled out with an adustable reamer and an .016" oversize NAS bolt be installed. The best way to do this is to bring up the hole in steps and test fit the bolt at every step (because there is always some dimensional variation in every diameter bolt). In the end the bolt should be a "tap in" snug fit just like the bolts that attach the wing spars to the 704 bulkhead. That's the fit that you want on the gear legs as well.