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Mikado Logo 800 Xxtreme Mikado Logo 800 Xxtreme Helicopters Discussion


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Old 05-10-2012, 09:37 PM   #1
HeliRush
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Default How much play in your tail

I'm not sure how to explain my question but here goes:

With my XXtreme on the bench and no power applied to the heli, if I grab the tail hub and twist it clockwise and then counter clockwise I have what I consider excessive play. I would say at the blade tips there is as much as 4mm to 5mm of total movement, ie2.5mm in each direction from center line.

I'm used to a little bit of play and I realize that the XXtreme is much larger than even the Logo 600SE but it still seems like alot of movement. I have quite a bit of vibration in my tail fin (I am quite OCD when it comes to having a smooth operating machine) and I feel like I may have something either assembled wrong or unbalanced causing the vibration and the excessive tail play.

Is anyone else seeing this much play in the tail of their XXtreme?

BTW have I mentioned I love this heli??
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Old 05-10-2012, 10:48 PM   #2
rcmarty
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I had a bit much play in my tail also.

I shimmed the tail shaft gear over a touch to tighten up the mesh.

Before i shimmed it the teeth would engage on about the top 1/3 of the gear teeth.
After shimming i had well over 50% of the teeth engaging.


The amount of slop that your refering to was drasticly reduced also.


Martin
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Old 05-11-2012, 04:28 AM   #3
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The technical name for what you describe is backlash. Some backlash is essential to accommodate thermal expansion of the gears as they run. Too little and the gears bind and melt, too much and there is insufficient tooth engagement and they strip.

Getting it right is a bit of an art and depends on circumstances such as gear material and profile. I aim for about 0.3mm which is judged more by feel, sound and running smoothness rather than measurement!
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Old 05-11-2012, 06:17 AM   #4
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This is the shim I used

http://www.rmqproducts.com/ProductDe...XXtreme%2DShim

Works perfectly...

Rob
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Old 05-11-2012, 09:37 AM   #5
dahld
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"...grab the tail hub and twist it clockwise and then counter clockwise I have what I consider excessive play..."

This "test" doesn't really tell you much. Why?

Starting from the main shaft, there is some play between the front drive belt pulley and the steel pin that drives it off the main shaft (actually, the play is between the steel pin and the hole in the main shaft). Then there is the backlash between the front tail drive bevel gears, then there is play in the TT splines the slide into the bevel gears on both ends, and then there is the backlash between the rear bevel gears.

When all these tolerances are added together, the movement at the tail's hub can be alarming...but, with no ONE tolerance being too much.

There's nothing you can do with the play that develops in the front pulley/pin. Although I used to wonder about it, the system is constantly loaded during flight, and has NEVER caused us an issue. So, I've long since quit worrying about it.

By holding the front pulley stationary and wiggling the rear bevel gear as it exits the tail boom, you can feel the combination of the front drive bevel gears and TT splines. Again, this can develop some play over time, and I used to worry about it. But...if the front bevel gears were properly meshed during the build, they seem to be quite resilient.
We've NEVER stripped the front bevel gears...yet do NO maintenance on them. Just build it properly, and forget it.

By holding the bevel gear as it exits the tail boom and wiggling the tail shaft, you can of course check the play in those two bevel gears. This is where I pay my attention.

During the prototyping of the model, we allowed these gears to continue to wear to the point that they did strip...so we know they can and will if neglected long enough. It took awhile, but these gears do wear more than the front bevel gears. I believe it is due to the rear gears being subject to more vibration from the tail disc as it does its work.

So, shim the rear gears if they get too loose (I want to see at least 50% of the teeth meshed...more is better for really hard flying with lots of hard piro reversals), and then eventually replace them if they get to far gone.

Hope this helps,

(-: Dave
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Old 05-11-2012, 02:58 PM   #6
rcmarty
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Thanks dave . good reading there.

Ausant was super nice and sent me a few shims (linked above)

I fitted the shim into the tail assembly.

Now i wondering how close should i go ?

I don't want to have issues with being to tight and generating heat and melting the gears nor be too loose.

Granted mesh is a feel thing but i'll give it a try.


I have the TT gear fully seated into housing (push and it pops/clicks ) into place

Then i removed material several times to get it where the gears are smooth all the way around except for 1-2 teeth (slight high spot).
If you spin the assembled tail box by the grips you can slightly feel the 2 tighter teeth go by.

I left it this way figuring the first few flights would wear them in.

I also took a few tries to get the tailbox bearings not to be side loaded. roll smooth

Martin
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Old 05-11-2012, 09:25 PM   #7
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Dave, thank you again for some really good information and for putting it into terms a 4 year old (that would be me) can comprehend.

I did as you suggested and held the gear coming out of the tail hub and found minimal backlash when turning the tail rotor.

Next I removed the tail assembly from the boon and noticed a little more movement by turning the TT back and forth, I could feel two stages to the movement which I believe is the TT tolerance where the splines go inside the front bevel gear and then the backlash of two gears themselves.

I reinstalled the tail assembly then very lightly turned the tail rotor and could feel the bevel gear turning ever so slightly before making contact with the TT splines. I can see where adding up all of the different areas of tolerances (sounds better than slop) would give me the movement that I am seeing when I move the tail rotor so I feel better about it.

@ Martin

I was one of the lucky ones, my gears seem to have seated properly. I can push and pull on the gear and it has no real movement at all and the mesh is perfect and smooth.


Jeff
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