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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 Bryan Walstrom on
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VERY interesting read, and the links associated with it. Thank you very much for sharing this.
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Permalink Reply by Tim Juhl on
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I understand the bungee was a bit overage but how many landings did you make with it before it broke?
Tim
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Permalink Reply by John Austin on
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I don't know, Tim ... I've never kept a cumulative total, but of course early-on there were many landings per hour and most flights are one hour or less - but sometimes those involve multiple touch-and-goes. So with 280+ hrs on the Hobbs, ? 750-1000 ? Just a wild guess.
However, there are probably a lot of variables in longevity. Age of the bungee, number of landings, cowl exposure to heat (I think a Rotax heats that area worse than a Jabiru), and exposure to dirt and dust (my cowl stays relatively clean).
John
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Permalink Reply by James R Depfer Sr. on
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John,
A couple questions please.
Do the helicoils come in different lengths ? If yes, what length did you use.
Did you get a complete install kit, or what tools can you use ?
Thanks,
Jim
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I "think" there is only one length in the 1/4-28 thread and it's about 3/8"long. The threads on that particular bolt are very little longer than that, so I just tried to place the Helicoil in the hole to match where the threads would be max engaged. You can, however "stack" Helicoils by screwing one in on top of another, but I really don't think that's necessary if accurately place the first one in this particular situation.
I bought a kit through Amazon for about $35 (cheaper elsewhere but I've got Prime shipping and got them in 2 days) - includes 12 coils, the tool to screw them in, the tap, and a plastic handle that I could never figure out what it was supposed to fit! Only other thing needed is a 17/64" bit to drill out the previous threads. Since the hole is parallel and close to the nose gear leg, there's no room for a conventional tap handle, so you can just use very small wrenches to turn the tap (it cuts the nylon easily) and the helicoil tool can be turned with fingers or a very tiny wrench if necessary.
John
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Question: Does the outer diameter of the Helicoils have a more course thread (i.e. 20 thread) compared to the inner diameter which is the 28 thread? Otherwise won't it repeat the situation at a later date? Thanks.
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No Bryan, the "threads" are the same. I say "threads" because the Helicoil is actually just a precision-wound coil - looks like a spring - so whatever the inside thread is, the outside is, too. However, and very significantly, the coil is ever-so-slightly oversized in diameter relative to the hole. As you screw the coil into the hole, you'll see it slightly compress and reduce in diameter. So, once it is installed, the metal threads are exerting a slight bit of lateral pressure. I'm guessing that screwing the bolt inside the coil increases that lateral pressure and helps imbed it into the oversize threads you've cut into the nylon with the kit's tap. Also, since you're now threading your bolt into metal threads, there's no danger (with reasonable care) of the bolt cutting the threads then or in the future. I'm sure you could deliberately strip the coil out of the hole, but I guarantee it'll take a lot more effort than stripping the bare nylon threads! Arthur C Clarke once said that future civilizations will have technology so advanced that it would appear to be magic to us ... maybe this is "magic?" ;>)
John
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Permalink Reply by Randy Boykin on
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Why not install 1/4 20 helicons? I have used helicons before and I think the 1/4 28 are way better than just the bolts alone in the nylon because of the coil being in the nylon and the bolt in the coil. As the bolt pulls on the threads the coil expands slightly which tightens it in the hole. I think it would be very hard to pull the coil out of the nylon. Having said all that I think I will use the bolt through and JB weld method when I fix mine.
(Still working on my landings ;0)
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Why not install 1/4 20 helicons?
Randy,
To be honest, it never occurred to me at the moment I ordered the 1/4- 28 Helicoils! I had the perfectly good drilled AN4 bolts in my hand and just thought I might as well use them again! I felt vindicated when I installed the bolts into the Helicoil-repaired holes - they really positively and firmly tightened-up ... none of the "mushy" feeling like the bare nylon threads!
The next time the bungee and nose gear comes off, I may well do the bolt-through and JB weld fix, too. However, If the Helicoil repaired holes still seem firm and tight, I might not! This time, it was certainly a plus to be able to repair the holes with the nose gear, bungee, and bearing block installed ... took 15 minutes, tops!
John
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Permalink Reply by Mark Maltais on
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This bungee system (and front gear for that matter) has bugged me since day one. John have you thought about the viking replacement with the spring? It seems like a good replacement idea! I really liked the helicoil idea. I didn't like the fine thread system either and I debated on going with a coarser thread on my own, but didn't do it in the long run... Now I'm thinking I should have! :( Great write up though!
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Mark,
As I said above, I didn't give much consideration to any other repair or modification such as installing a steel bungee since I already had the nose gear, bungee, and bearing block re-installed when I detected the stripped bolts when I started to re-insert them. The Helicoils allowed a repair without disturbing (or destroying!) the new bungee and you don't even have to remove the bearing block or nose gear..
Although I love to do modifications where it makes sense, IMHO the steel bungee is still in its infancy and before some service history has developed, there could always be unintended consequences/service difficulties down the road (down the runway? Ha!) . Also, the steel bungee is not an inexpensive mod and the bungee nose gear as designed by Chris Heintz is the ultimate in simplicity, low cost, and light weight. After going to a lithium battery on the firewall and chucking 12 lbs of weight off the nose, I'm slow to want to add 4 lbs of weight back with the steel bungee!
I really had dreaded the eventuality of changing the bungee, but when I had to, I discovered like many others that it really wasn't that difficult at all - probably a 2 hr job once every 5 years and $28.50 for a new bungee ... that's not too bad! Guess it is easier for me to say that since my rudder pedals are silky smooth with no stiffness or "notchiness." I've deliberately tried to feel for the centering notch and I simply cannot detect it! If I had rudder stiffness problems, the steel bungee (with time and a proven track record) would certainly be a consideration.
By the way (to creep my own thread!), I noticed SBC industries rates the 1080HD bungee for 900 lbs. I wonder if those who do have stiff rudder pedals/notch problems AND one of the lighter engines such as the Rotax or Jabiru might benefit by going to the 1080 bungee, rated for 750 lbs? Just thinking out loud.
John
P.S.: (To further creep my own thread) I found that there are other metal inserts other than the Helicoils that look like good solutions to the nylon thread problem, but I "think" most of them would require removing the bearing block to insert as they have a flanged head on them that wouldn't fit through the aluminum plate under the nylon block ... maybe someone in the construction stage might want to take a look at those, too.)
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Here's some additional useful information:
This link to SBC Industries' website has a chart to decipher the production date code of the colored threads woven into the bungee covers: http://www.sbcindustries.com/products.htm
Aircraft Spruce advertises "fresh" bungees and indeed my new one was "fresh." It had three yellow stripes so that means it was manufactured in the 4th quarter of 2014.
The chart also lists the various bungees' dimensions and load ratings.
John
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