Torque Tube Vibrations

[120ccpm]

Hi guys,
a discussion regarding on-board camera mounts and ways to reduce a noticeable jello effect in my videos lead me to question whether there was something mechanically wrong with my Pro.

I took the main & tail blades off, gave power (around 90% throttle) and checked for vibrations or anomalies: everything seems true and nothing wobbles, but there is a fair amount of vibrations coming from the torque tube department. It's not enough to cause any visible shaking, but enough so that if you hold the boom for a minute, your hand will feel a little weird afterwards.

Having zero experience with this, could someone give me an idea of how much is too much? I took off the tail assembly (leaving the TT inside the boom) and spun the main rotor by hand: the TT does wobble a bit, let's say 1mm around its axis... should it be perfectly true?

I also remember someone suggesting to file the TT ends to make them a little loose inside the umbrella gears (which apparently are often out of round), and/or to use two bearings inside the boom... what's your opinion about these mods?

[BaconRaygun]

I run dual bearings and will never go back... Also, if ultimate balance is what you are after, get a scale accurate to .01g and balance every opposing part that spins (grips, links, washout, EVERYTHING!)... you will have a bird that is scary smooth.

As for filing TT ends, IMO there is no need... finless had this covered, just back the boom out 1mm or so.

[flerpie]

Originally Posted by Vicovaludemero

I run dual bearings and will never go back... Also, if ultimate balance is what you are after, get a scale accurate to .01g and balance every opposing part that spins (grips, links, washout, EVERYTHING!)... you will have a bird that is scary smooth.

Vic - I already balance tail blades...hub...and shaft...it works well.
I can't visualize how you would balance links and washout.
How do you do that?

[120ccpm]

Right, now I recall, it was you who posted instructions on where to position the two bearings on the TT.... I'll dig out that thread, and give it a shot: it's an easy mod, worth a try.

Talking about the TT bearing (inside the boom), do you glue it to the TT itself? I see quite a bit of play between the TT and the bearing, and I'm afraid that if I glue it, I might end up with the bearing at a slight angle and/or offset...

[BaconRaygun]

Originally Posted by 120ccpm

Right, now I recall, it was you who posted instructions on where to position the two bearings on the TT.... I'll dig out that thread, and give it a shot: it's an easy mod, worth a try.

Talking about the TT bearing (inside the boom), do you glue it to the TT itself? I see quite a bit of play between the TT and the bearing, and I'm afraid that if I glue it, I might end up with the bearing at a slight angle and/or offset...

Yep that was me... And yes sir, you are right, its a PITA! I just use CA and get it as good as I can, however there are additional steps you can do.

-a higher ABEC bearing will have tighter tolerances, particularly in the inner and outer races, but they are more expensive.
-use epoxy instead of CA, its thick so you can probably get it almost perfect... But regardless how you glue it, align it on a hard flat surface to ensure both hearings are nice and level.
-you can try to slip some heat shrink tubing to the tube then glue the bearing to the tube, not sure if heatshrink is thin enough though.

I was able to get it good enough with thick CA and 2 ABEC #1 ceramic hybrids from boca.

[120ccpm]

Thick CA or epoxy seems a good option, I'll do that. Will report on vibrations level after the dual-bearing mod...
Thanks!

[BaconRaygun]

Originally Posted by flerpie

Vic - I already balance tail blades...hub...and shaft...it works well.
I can't visualize how you would balance links and washout.
How do you do that?

The washout, well yeh I guess I was pushing it there lol.

Links, balls, grips, paddles and most any head parts that come in two's can be balanced with a scale and a file. They are usually really close as it is but can be problematic if you happen to assemble your heli and the heavier link and grip are on the same side of the head. I don't really do this but some FAI/F3C guys do.

You simply remove non critical material from the heavier part till you have them exact.
On my pro for example, one of my grips weight around .05g more then the other... I balanced it by weighing my bearings and sticking the 2 heavier ones (they are usually +-.02g) on the lighter side.

If I didn't find that discrepancy with the bearings then I would have filed some material off in a non critical area, like the thickest part of the outside of the grip, but on the bottom side where you can't see.

You don't need to do any of this though, its already close enough. Sometimes just swapping 2 links makes a difference.

[flerpie]

Thx for the tips on balancing - hadn't considered filing grips.
I'll check that out on my next build.

[744drv]

I too run 2 TT bearings. Getting the bearings square appears to be more of a problem with the black TT .... mine was a little thinner than the silver ones I had used earlier. To try to sort this I tried to slide the bearings onto a small section of TT with a sticky tape cover. I managed this with one but couldn't do the second ..... so I then resorted to epoxy. I managed to get some of this onto the bearing shield .... but don't try to remove it until after it is dry. Using a small screwdiver it came off cleanly and the bearing is smooth. I think that if you try to wipe off wet adhesive it might get pushed into the bearing. For spacing I measured them out so that all the gaps were about 5 mm different 115/120/125mm if I recall correctly. Unfortunately I made a number of other upgrades at the same time so I cannot say that 2 bearings is worth it ..... but my Pro is silky smooth at the moment.

I must try my camera on it again .... good idea!!!

[chrismechanic]

i could never get the bearing 90 degrees to the tt...allways slightly off so it causes more vibes...so in the end i found the answer...dont glue it at all.
just use some wd40 to lube up the inside of the boom and push the bearing/s up to where you want them then leave it half hour for the wd40 to evaporate...slide the tt through the bearings and its all good...they wont move.
i have had my pro like this for months with no problems and much less vibes...makes changing tt and boom much quicker and simpler...
chris

[744drv]

Chris, that's how I did mine at first. Unfortunately I didn't get to fly it like that for more than a dozen flights . However, on rebuild I noticed some wear where the TT had been turning within the inner races. I was worried that this may cause thinning of the TT .... more movement ..... leading to worse vibes. May go back to unglued if you are having no long term problems. Are you using black or silver TT?

[120ccpm]

A quick update: added a second bearing to the TT and vibrations drastically dropped, although they didn't completely disappear. Not sure if this will have any noticeable effect in flight, in any case it's a very simple mod, and less vibrations can only be a good thing (I'm sure my gyro will be happier).

As for securing the bearings to the TT, I ended up using normal CA, doing it carefully and trying to keep the bearings perpendicular to the TT.... final result was good enough, so I'll leave them alone.

On the subject, some time ago I purchased a "V2" bearing holder from AC-RC (http://www.ac-rc.com/product_info.php?products_id=1105) as it seemed a good upgrade. A couple comments about it:
- it does hold the bearing much better than the rubber one, perfectly centered and squared in the boom, BUT since the coupling bearing/TT is quite approximate, I think the stock rubber holder is a better option, as it allows the bearing to work slightly angled, should it be glued this way on the TT
- should I one day decide to install the TT without gluing the bearings, then I would consider using this holder, as it would guarantee that the bearings are perfectly lined up inside the boom.
- bearing doesn't seem to be replaceable... I tried to open the holder but there was no way to separate the two halves.

[janol]

at what distances do you put the 2 bearings?

[120ccpm]

Originally Posted by janol

at what distances do you put the 2 bearings?

I followed Vico's instructions (https://www.helifreak.com/showthread.php?t=281935) which can be summarized this way:
- mark the midpoint of the TT, then again the midpoints of the two halves, so you end up with 3 marks equally spaced
- get the first and third mark and put the bearings about 5mm from them, toward the same end of the TT

Since the hollow portion of the TT (the black part, if you have that type) is 347mm, first mark would be at 87mm, third at 260mm: shift them both toward the front of the heli, and you have first bearing at 80mm, second bearing at 255mm (measuring from the start of the hollow section, ie black).