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Front Landing Gear Lower Bearing Block
I am very uncomfortable with the idea of mounting the plastic front lower bearing block to the fuselage mounting bracket by tapping threads into the plastic blocks and using AN4 bolts.
I looked up engineering standards for steel bolts in threaded plastic, nylon, TPFE, ABS, etc. and the smallest recommended thread pitch for a 1/4" bolt is 20 tpi. The plan calls for safety wired AN4's which are 28 tpi. Even though they are safety wired, they thread in upside down through the bracket into those plastic blocks.
With the potential beating this plastic block can take from grass strip use or bumpy taxiways, etc. - plus whatever twisting friction exists from constant rudder use in flight - the only direct metal on metal structural bolt is the one long AN3 that goes through the 2 sides of the center structural strut channel at the rear of the block. The front AN3 is only through the plastic. That strut channel is riveted to the firewall with A4 rivets. Even taking into consideration the countering tension of rudder cables - that tension is not fully there when the strut is partially compressed while on the ground. I've seen other builders' modifications like spring-loading the steering push-rods, etc.
There is constant movement being imparted to that plastic block, one way or the other. Lateral, up/down (impact), rotation, friction... and safety wire will do nothing to keep a bolt in when there are no threads left to hold the bolt... other than keeping it from hitting someone on the ground in the head.
Am I just being paranoid? Has anyone used a different process of securing this plastic bearing? I have a solution that I want to use but I thought I'd see if anyone else has done something else with theirs.
Thanks in advance!
Replies to This Discussion
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Permalink Reply by John Austin on
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Brian,
When I read your post, it struck me that I've never seen a report on this forum or elsewhere of a problem with the front bearing block as far as security of mounting or catastrophic failure, etc. I think this design is used across most, if not all the Zenith aircraft designs, so there are thousands of hours of field experience ... some in much tougher conditiions than turf such as gravel bars and other unimproved surfaces. Since you piqued my interest, I did a few site searches and turned up nothing. Frankly, I expected to find something, since the bearing block is occasionally modified, i.e., the rear corners ground back to allow full rudder travel, the "V" flattened or eliminated to decrease pedal effort, etc. About the only thing that did turn up was someone had accidentally stripped-out the threads while tapping the block.
In my case, my 750's safety-wired bolts in the bearing block mount were painted-over. I'm based on a turf airstrip and at nearly 200 hrs, the paint isn't even cracked around the bolt heads, so, apparently they're still pretty secure!
I think you're trying to "fix something that ain't broke!" ;>) However, that's not to discourage you from pursuing your alternative ... this is experimental aircraft building and you're free to experiment! Perhaps someone seeing this topic has had a problem with the bearing block's security and we'll hear from them.
John
N750A
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Permalink Reply by Brian A Manlove on
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Thanks John -
Every bit of info is good! Gotta love experimental aviation.
One of the reasons I wondered about this in the first place was because in some of Jeff Garrett's pictures - it looks like he has repaired a whole bunch of Zenith's that have had rough off-field landings, collapsed gear, etc. - I noticed bearing blocks with AN locknuts holding the block to the support - no safety wire. Maybe the original threaded bolts got stripped out because the entire nose gear collapsed and that change was part of a repair method. At any rate, I went ahead and modified my installation. I'm happy with it.
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Permalink Reply by Gil DeVault on
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That could be my plane N601LP. It made an off field landing prior to my owning, damaging the nose wheel and Jeff made the repairs. He showed me when we transported the plane from KY to MO and was reassembling the wings. I just remove all the bolts/nuts/ block when I replaced the bungee and everything with the block was OK
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Permalink Reply by Louie Lacy on
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what is the size of the aluminum block you have added to the lower bearing block? I want to do that and was wondering did you take a regular right angle piece and modify it??
loulacy@gmail.com
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another view:
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Jeff's picture that caught my attention:
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Permalink Reply by Ian McClelland on
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That looks good! Spot facing those bolts in looks like a better way of doing it. I considered heli-coils into the plastic block. I believe my Zenith design will need to be repaired soon.
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Permalink Reply by Jerome Shepard on
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I used a 4130 flat bracket on both sides bolted to the firewall channel, also all A5 rivets. My thoughts were also that it needed more
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Permalink Reply by Rick George on
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I made two "L" brackets. Attached to the plastic using the two through bolts and bolting and riveting it to the channels on the firewall. Just one of those"I"ll sleep better" improvements.
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The bearing block mod:
- Countersunk holes just deep enough to allow about 1/8" of fill between bolt head and surface of the bearing block (bolt has an AN washer also).
- Liberally coated shank & underside of bolt head with JB Weld epoxy.
- Pressed bolts in
- Filled over recessed bolt head with JB Weld.
- Sanded flush after epoxy dried.
This gave me more of a "motor mount" style of mounting the bearing block and allows use of AN lock nuts that don't need to be safety wired. Also the bolts can't possibly be pulled out from underneath. The wing brackets wrap around under the fuselage attach bracket, so now the long front AN3 bolt is not just going through plastic, but is carried by the bracket and is now mechanically attached to the air frame.
Total weight of mod: 2 aluminum brackets, about 2 oz. each = 4 oz. Additional weight for AN4-7A vs the original bolts = approx. .3 oz. Total added weight for the mod = 4.3 oz.
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Permalink Reply by David J. Beaulieu on
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Brian,
Looks good, I think that is a nice mod (even if just for peace of mind) - I plan on doing the same thing!
Thanks for posting...
Dave
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Brian,
So far, no one has chimed-in and said they have experienced any sort of problem with the Zenith-designed bearing block mount in normal use. While your design is certainly even more secure, I suspect the bearing block mount is the least of one's problems in the event of a crash or nose-gear collapse! I wonder if tying-in the bearing block to the mount with additional metal brackets and bolts with nuts might contribute to even more firewall or forward fuselage damage in the event of gear collapse? A certain amount of "break-away" ability is desirable in some components so components can progressively fail. Again, it's probably a more theoretical than real problem because the crash/gear collapse is likely to do major damage, anyway, but sometimes there are unintended consequences to our modifications. I apologize if I sound critical ... just thinking out loud here! ;>)
I've tried to get my mind around the forces and vectors that might be encountered in various situations. Looks like in a nose-down attitude contact with the ground, or hitting a solid object with the nose gear, the mount will act as a fulcrum to bend the nose gear. The force would more or less drive straight back into the firewall, so it probably matters little how the mount is secured. However, in an extremely hard landing in a level or nose-high attitude, the force will tend to pry the mount away from the firewall. Seems in that situation a more robust mount might pull the front of the lower fuselage and lower firewall forwards.
Other than a crash/gear collapse, I'd still like to hear from someone who discovered their bearing block loose or other problems with it in routine use!
John
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