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Home✈️Aircraft🚁Helicoptere-AlignT-REX 500 › TREX 500XT Umbrella Gear Failure - Bearings
05-20-2018 07:01 PM  26 months ago
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hhart

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San Jose, CA

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TREX 500XT Umbrella Gear Failure - Bearings
Hi All,

Just put together the Align 500XT combo kit. Maidened without any problem and on 15th flight, the flybarless was setup pretty well. On flight #28th, the tail went to pirouette un-controllably while I was inverted, and the XT was crashed in Hold mode.

Upon close inspection post crash, I saw the umbrella head snapped off. When disassembled, it is obvious that the bearings that holding the umbrella gear seized up and the umbrella gear basically heated up and snapped.

Below are the pictures, let me know what you guys think since not much news on the 500 size.

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05-20-2018 08:38 PM  26 months ago
rcflyerheli

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Granbury, TX USA

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If the bearings don't fit exactly into their slotted recession, there will be axial preload on them, and running with the preload will make them get hot.

If they fit correctly, you should not feel any roughness when rotating the umbrella gear and its mating drive gear by hand. As tough as the bearings are to get installed onto the umbrella gear, it is hard to get them perfectly aligned with the slotted area in the case. I am lucky that I was able to get a tool for installing and removing them before the guy who made them passed away. Even using the tool, I still have to fiddle for several tries to get them exact.
Logo 700, Specter 700, Goblin 700, Trex 700DFC, Gaui X7, Logo 690SX, Logo 600SX; Trex 470 Trex 500
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05-20-2018 10:42 PM  26 months ago
hhart

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San Jose, CA

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The whole assembly come from the kit pre-installed. The bearings fit perfectly on the slotted recess. When I rotate the umbrella gear by hand on the tail case, it is perfectly fine without any pre-load. The gear mesh is also perfect.
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07-30-2020 03:19 AM  8 days ago
gnichola

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Santa Rosa, CA. USA

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I know this is an old thread, but apparently they haven't fixed any fit issues they may have. I just had a crash due to tail failure. The ENTIRE tail was a goner, except for the blades. It damaged the front bevel gears as well and when I took that end apart it looked just like the pictures posted above, the backend was worse, it looks like it melted the gear and then the end sort of twisted off. As stated in the earlier post, both of these parts are factory assembled.

Weak sauce!

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07-30-2020 11:19 AM  8 days ago
Rojoalfa

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Punta de Mata, Monagas-Venezuela

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/
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There is nothing to fix in the design.

That umbrella gear is working like a fuse. So, the behavior is OK and it is doing its job.

It is always more cheaper to replace this little thing that replace spur or something else.

So, remember... When you have a crash, the mechanism needs a "fuse part" for safety reason.

/
Saludos cordiales,
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07-30-2020 12:42 PM  8 days ago
Rojoalfa

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Punta de Mata, Monagas-Venezuela

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So, base on that fact, I recommend to have this little fellows in your tool box.

Color white!!! There are better mechanical fuse that black ones!

Color black are more resistant but when you have a crash, they usually help to break other more important things...

/
Saludos cordiales,
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07-30-2020 02:13 PM  7 days ago
gnichola

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Santa Rosa, CA. USA

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I think you may be misunderstanding my post. I was doing a tornado with the tail maybe 6 or 8 feet off the ground when the failed bearing caused the umbrella gear to fuse which locked up the tail. The loss of the tail caused obviously caused a violent spin. I was too low to do anything about it and it hit the ground. The "fuse" caused the crash.
Secondarily, I don't think the bearings are slipping and causing heat, they are falling apart. Here is a shot of the one from the front end the tail.

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07-31-2020 12:14 AM  7 days ago
Rojoalfa

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Punta de Mata, Monagas-Venezuela

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Ok.. Ok... I got it!

Your heli didn't hit the ground to cause the gear umbrella to break...

Your were flying and suddenly, you lost tail control near the ground.

This is a complete different analysis.

According to your pics, the two bearing broke, got hotter, and then stuck.

This two bearing are low profile and sometimes, it is difficult to install.
This bearing are designed for radial forces.
If you hit that bearing to put it in place, you can damaged because that bearings can't handle axial forces.

So, IMO, the two bearings failed because someone hit it to installed on the umbrella gear. With time, the bearings begun to make sounds, get hotter and finally got stuck.

And then, we know the rest of the tale...

My recommendation is that when you install the new umbrella gear and the two bearing, be careful.
The bearings must slip softly.
If it is hard to install, use sand paper to shape the umbrella gear.

PD :

Use liquid grease ( for electric motor bearings) for the two bearing before instalation.

There is a chance that the old bearings were not greased enough. It happens...

/
Saludos cordiales,
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