Zenith Aircraft Builders and Flyers
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Bungee and Nose Gear Bearing Block Repair
Recently, I was taxiing back to the ramp at my hangar and heard a loud "POP" from up front and suddenly the nose wheel steering was very heavy. I immediately knew the bungee had broken! I shut down on the ramp and sure enough, the nose gear bearing block was resting on the welded ring. I was relieved to see that I still had 9+" prop tip/ground clearance, so no harm there!
I had noted the bungee's cover had appeared slightly frayed around contact areas during the last couple of oil changes and intended to change it this summer at the condition inspection. It was the original bungee that came with the kit - I found the original label from it and it was dated "July 2008," so it was 6+ years old ... definitely too old! When it broke, the bungee had completely separated and I noted those "slightly frayed" areas were much worse on the opposite side of the bungee where they contacted the cross pin and the retaining protrusions. I contacted SBC Industries, the bungee manufacturer, to see what their replacement interval recommendation was, and was told they didn't have any recommendation - just go by whatever the aircraft manufacturer recommends. Since I had 6+ yrs on this one, I think my criteria for a replacement will be 5 yrs or immediately at signs of any fraying, whichever comes first. I got a new 1080HD from Spruce and the change-out was much easier than I anticipated. I won't go through details here as it has been covered elsewhere except to say I used the "ratchet" trick to stretch the new bungee (put ratchet with socket on protruding tube with loop of bungee over handle, swing handle up and it's on!) - no complicated hoists or Rube Goldberg mechanisms - took about 15 seconds and easy as pie!
The real point of this post is that when re-installing the nylon bearing block, I found the front right bolt that is screwed directly into the nylon was stripped, and the front left was doubtful as well. I don't know if they had been stripped all along as they are safety-wired in place and mostly act in shear and not tension, or if the force of dropping onto the welded ring when the bungee broke may have put tension on them and stripped them. There have been some ingenious and elegant modifications to this area to avoid tapping the nylon. The thread is here. The basic problem is that you're tapping a 1/4-28 thread into nylon. Nylon really doesn't do well with fine threads and it would be better to have been a 1/4-20 coarse thread. (Ironically, my brother ran a manufacturing plant and they had the exact same problem many years ago - they were stripping 1/4-28 bolts out of nylon left and right until they changed to a 1/4-20 thread!)
Since I already had the new bungee installed, I didn't see any way to get the bungee off and remove the gear without sacrificing the new bungee by cutting it. I decided that a Helicoil repair was the best route as I could do that leaving everything in place. I drilled-out the stripped holes with a 17/64 bit, tapped the hole with the provided kit tap, and inserted the 1/4-28 Helicoils. Worked like a charm and only took about 15 minutes! I then found I could confidently torque-up the bolts without that queasy feeling you get without Helicoils as you slowly torque the bolts and wonder whether they'll tighten-up or strip! Some new safety wire and I was good to go!
I highly recommend the Helicoil repair if you have stripped one of these bolt holes in the nylon bearing block. In the future, at a bungee repair when the bearing blocks are removed, I'll likely modify the blocks along the lines of Brian Manlove's suggestion. Brian, you can FedEx me some crow and I'll eat it! ;>) (I gave Brian a hard time about "fixing something that ain't broke," and truly my bearing block hadn't broken ... yet!)
John
N750A
Replies to This Discussion
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Permalink Reply by Walt Snyder on
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Great info, thanks John, and the helicoil sounds like a minimal amount of work for an extra level of security for those rough backcountry areas where you don't want to hear "uh oh"…...
Walt
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Permalink Reply by Bryan Walstrom on
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Question #2
For us just building... would it make sense to order/specify this part from Zenith "non-drilled" and simply drill/tap ourselves to a more course thread or Helicoil option?
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Someone else could confirm this, but I think it comes non drilled/tapped…..at least on Ed 1.
Walt
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Permalink Reply by Mark Maltais on
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Mine wasn't drilled or tapped either, and one of my tapped holes makes me nervous. It doesn't seem to grab like I would prefer. I was thinking of going one size bigger but the bolt and lock nut idea is great and I will probably go that route. It will also make me sleep better!
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OK. Thanks Walt and Mark. I am actually building a Cruzer but I assume it is the same parts. I'll just have to make a note about that for when I order my finish kit later. It would maybe be a good idea for all of us to bring this up to Zenith now. It is possible they could put this into their router/mill and carve out a hex head that is countersunk in at the top?
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Permalink Reply by John Austin on
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It is possible they could put this into their router/mill and carve out a hex head that is countersunk in at the top?
Maybe .... that would mean the bolt holes would have to align with the hex recess. If I remember, the holes are match-drilled through the aluminum plate below. There possibly could be some fore-and-aft alignment issues that might make for a difficult fit if the holes are pre-drilled, but I'm not certain! Still worth pursuing with Zenith!
John
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Same here - the blocks I received were not drilled.
John
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A follow-up observation ...
There has been much activity on the forums about stiff and "notchy" rudder pedal problems and alternative solutions such as milling the nylon bearing blocks flatter or installing the "steel bungee," etc. My personal experience has been that I have had absolutely no problems with the nylon bearing blocks/bungee system as designed by Zenith! For a while, I wondered if it was just something that I was ignoring because I had accommodated or gotten-used to it, but not long before the bearing block thread repair, I deliberately tried to critique my rudder efforts and just couldn't identify any problems ... they were silky-smooth and for the life of me, I could not detect the "notch" in the bearing block ramps as I swung the rudder back and forth.
It also occurred to me that perhaps since my bungee was obviously worn (it did break, after all!), maybe it was "more relaxed" or exerted less pressure and that's why the pedals moved so easily. Today, I flew for the first time with the brand-new, factory-fresh (produced 4th quarter '14) 1080HD bungee. I expected it to be "a little stiff" or need "breaking-in," but I'm glad to say I can report that I can detect absolutely no degradation in smoothness, rudder effort, nor can I "feel the notch!" So, as they say on TV, "I guess that myth is busted!"
The only other variable that I think could remotely be related is that I have a 600 series straight-rib tubeless tire on the nose wheel rather than the 800 series. It weighs 2.8 lbs less, so in-flight, there would be that much less force on the nylon bearing block. Doesn't seem like that would be significant, but who knows? To be honest, I really don't remember much difference when the heavier 800 series tire was installed. Guess I shouldn't think so much about a problem I don't have! ;>)
John
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Finalizing a bunch of stuff and started lock wiring things. I checked my bolts in this block and sure enough, one for sure is stripped and another one is questionable! :( What a piece of junk!!! Thanks to John's success I ordered a helicoil kit and will be doing that fix for now and see how it holds up. I'm putting amphibs on, hopefully sooner than later, but if it's later I might try and machine this thing out of solid aluminum. I always like trying new things! :D
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