Can I butt weld 1 1/8 .035 4130 tubing or do I need a "fishmouth" sleeve? 2

[booshambo]

Hello.
I am familiar with the FAA AC-43-13 which has guidance for repair via splice of 4130 tubing. There is mentioned there the
use of an external sleeve in no surprising fashion.

On the other hand there are these guys,

https://people.se.cmich.edu/yelam1k...rs/2015_asee_ncs_conference_submission_28.pdf

who claim a butt weld (tig) on 4130 3/4 .035 tubing is roughly 1% worse than one piece tubing in fatigue and tension.

Are there places in light aircraft control systems where a 4130 (say) tube is just butt welded to another tubular part? I think I've seen that in
control sticks and I've certainly seen it (meaning just a direct weldment) in attaching lever arms that connect to rod end bearings.

Question then is, does a control system get a pass on the external sleeve thing as long as you trust the weld? My thinking is that the fishmouth sleeve
is for primary structure repair and the control system, while critical is much more lightly loaded.

Thanks,
b

 

[rb1957]

a control system is not something I'd give a "pass" on.

It's less a question of strength and more an issue of reliability. The huge advantage a sleeve gives you is if one of the welds lets go, the sleeve will hold the rod pieces in alignment (obviously not under tension, but then maybe the displacement of the rod is less than the length of the sleeve ?).

Going against the guidance of AC43-13 is a bold move indeed. I'm not saying they are right, but you need really good info to support your position. More than "I read this on the internet".

Of course you'd 100% your weld. If you lose this control rod, do you lose the plane ? Then it's a flight critical weld and should probably have routine inspection. Yes, a pain for part 23. Is that pain worth saving a little on the repair ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[berkshire]

Most of the German welded airframes use straight buttwelded joints, sometimes with an internal sleeve sometimes without.
In the States when we repair them, we use a rosette welded internal sleeve, or a fish mouthed external sleeve.
B.E.

You are judged not by what you know, but by what you can do.

 

[Sparweb]

Are there places in light aircraft control systems where a 4130 (say) tube is just butt welded to another tubular part?

Nothing structural, if that's what you mean. Unless a serious mistake has been made.
I have seen that blunder made on ultralights / light-sports / homebuilt but not in the Part 23 world.

...I think I've seen that in control sticks and ...

Nope.
There will be internal tubes, as Berkshire says, or overlapping shoulders where the tube enters the fitting that you can't see once it's welded together.

...I've certainly seen it (meaning just a direct weldment) in attaching lever arms that connect to rod end bearings.

No again. Another case where the female threaded fitting that accepts the rod-end threads are a turned sleeve that enters the tube while the weld only keeps the fitting in place.

Got a photo of a questionable joint? I'd like to see something that you're looking at.

 

[booshambo]

Quote (booshambo)
...I've certainly seen it (meaning just a direct weldment) in attaching lever arms that connect to rod end bearings.

No again. Another case where the female threaded fitting that accepts the rod-end threads are a turned sleeve that enters the tube while the weld only keeps the fitting in place.

Got a photo of a questionable joint? I'd like to see something that you're looking at.

Sorry to be unclear. The weldment I was failing to describe is to connect the roll control torque tube (which also reacts human powered pitch control inputs via the silver collar in photo) to the
aileron
pushrods which are loaded in tension and compression only. They are the common (Aurora in this case) rod end bearing providing the threaded connection and AN bolts to
fix the bearing to one end of a lever that is made of 4130 with a bolt hole at one end and a 1-1/8 hole in the other, through which the roll control shaft passes and is
fillet welded on both sides ("lever plate" to torque tube). Photo below I hope.

In order to mount MS27646-42 torque tube bearings on this roll control torque tube (which I want to do) I have to cut the tube, install the bearings with appropriate spacers, and reconnect the
tube. My own inclination is to consider one of three options for putting this thing back together.
1. Turn a sleeve that is close fit but not interference and of .035 wall, and weld around the tubing in two places.
2. Follow the AC 43-13 with a fish head sleeve with tolerance much closer than the required 1/16 max oversize.
3. Butt weld the tubing since that paper mentioned in the original note says that is pretty close to full strength.

Weight and size of finished item are my main concerns after meeting strength requirements, i.e. as good as before cut.

This is the motivation for my question and since a control torque tube is so different from the context of AC 43-13 where structural members are being repaired and have no easy
bench access, I thought I'd ask. This is only one approach to the problem but it is high on the list.

In the photo the lever is fillet welded on both sides, the tube is 4130 .035. Thrust loads are reacted by two collars like the silver one against bearing standoffs, and
there is more stuff welded to the other end that prevents sliding the bearings on from that end.

Finally, thanks very much for taking the time to comment on this situation. Sorry to not be clear about the control rod end thing. I was certainly never suggesting there was anything
problematical about the way the thing currently connects to the pushrods.

Regards, b

 

[WKTaylor]

The only completely reliable tube butt weld-joints, that I'm familiar with, are 'flash' or 'friction' welds of normalized alloy steel tubes.

The joined tube Assy is rough-machined, ID/OD, to clean-off the weld-flash-out inside/outside; then MPI-NDI'ed for any detectable defects; then the Assy is solution HT/quenched and re-tempered; and then the Assy is finish-machined all-over; Then the Assy is final MPI-NDI'ed for any detectable defects... lots of work.

There are several books and documents for aircraft tube structure design/Assy that are traditional... but very effective and safe for structural/mechanical Assys.

CAUTION: there are a few things You have NOT mentioned... that are exceedingly important, thus...

What are your 4130 tubing [and sheet attachments] specification(s) and what is Your starting temper... IE; 4130, AMS63xx [MIL-T-6736, ASTMxxx] [etc] tubing and AMS63xx [MIL-S-xxx, ASTM xxxx etc] sheet plate; and what is the starting HT, IE: Normalized [HT95KSI], HT125-KSI, etc.

What are Your welding and weld-filler alloy specifications/processes?

Are You intending to fully HT [SHT/Q/Temper] or simply stress-relieve HT... after welding? What HT spec are You referring-to?

What is Your protective finish, IE: grit/bead blast to bare-white-blasted-clean steel + primer suitable for steel [high zinc], etc...??

IF my memory isn't too shot... I thought a similar-to question was asked a few years-ago RE a 2-piece tube Assy that was to be assembled/joined in a confined/pass-thru area??

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[booshambo]

Hey - reply to Will

What are your 4130 tubing [and sheet attachments] specification(s) and what is Your starting temper... IE; 4130, AMS63xx [MIL-T-6736, ASTMxxx] [etc] tubing and AMS63xx [MIL-S-xxx, ASTM xxxx etc] sheet plate; and what is the starting HT, IE: Normalized [HT95KSI], HT125-KSI, etc.

--> Whatever you get from Aircraft Spruce - 4130 N .035 wall.

What are Your welding and weld-filler alloy specifications/processes?

--> Tig ER80s-D2 or OA 32CMS = R65. Hand welded by the only user (single seat) of the final part.

Are You intending to fully HT [SHT/Q/Temper] or simply stress-relieve HT... after welding? What HT spec are You referring-to?

--> Stress relieve as necessary (Some claim none if tig...) and no heat treat - bog standard EAA stuff.

What is Your protective finish, IE: grit/bead blast to bare-white-blasted-clean steel + primer suitable for steel [high zinc], etc...??
--> The substance paint - current part is powder coated. Part is fairly easy to inspect.


As for test and QA of process, I am kinda stuck with making coupons and destructive test. I find that approach comforting actually.


As to another similar thread, yes indeed, well spotted. To my shame I'm still not finished with this thing. The other thread was focused (I hoped) on some other approaches, specifically
bearings to be used and how to avoid cut. One very nice (in my view) solution came out of that as well as much very helpful commentary including some from Will.

Now it is getting near time (I fervently hope) to actually put this thing together and another approach, viz cut and weld (on a bench, not in the aircraft), is being
seriously considered (by me).

That approach also came
from a source I trust, so I'm trying to add verify to that option as well. As part of that I thought my reference to some guys from University claiming butt weld seemed to be close to
full strength was interesting but without corroboration from real industry not compelling, hence this thread.

The only "official" doc I have is the AC 43-13 but the only reweld tube stuff there is for primary structure. It would be nice to know what best practice is for this torque tube case
which seems to me to be a different thing compared to a repair of a truss structure. Torque and tension/compression limited to human one hand if you ignore tail slides, (which
you can't).


Thanks for the comments.

Regards,
b

 

[keyepitts]

I would suggest looking at a re-design of this tube assembly if you desire to install a bearing on the tube- I don't think installing it and butt-welding the tube back together is a good idea. At some point that bearing may need to be replaced, even if it isn't damaged from welding heat and spatter. I would suggest welding the control arm to a hub made of larger diameter tube, say 1 1/4-.065, and then cross-bolting the new control arm hub assembly to the control torque tube. Another option would be a redesign to use offset bearing cages, above or below the torque tube (see Pitts model 12 control column for example). Or stick with the tried and true original design of split pillow block bearings. Again, I don't like the idea of butt-welding critical structural tubes.

 

[moon161]

Regarding the paper linked in the OP, it seems like they are looking at bicycle construction. IIRC, the move from brazed lug (socket connectors) construction which extended back to approximately forever on 'quality' steel frames started in the mid 80's when Cannondale mass produced TIG welded 6061 aluminum bike frames and the appearance of a butt welded joint, with tubes of odd sizes and shapes was acceptable and or cool, and was done by the mid 90's. Prior to that tube sizes and joint angles were pretty standard in a couple ranges for racing and touring bikes, more or less, TIG allowed use of steel, titanium and aluminum as long as you could fish mouth the tubes so they fit up.

This is wikipedia on the method.

https://en.wikipedia.org/wiki/Lugged_steel_frame_construction

TL;DR At least on bicycles, of any material and quality, lugged construction is only done these days for nostalgia, looks, or because that's what the custom builder is familiar with. There has been no rash of broken bicycles that I'm familiar with.

 

[rb1957]

yes, but let's suppose that airplanes are less tolerant of failure (and should be of higher quality) than bicycles ...

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.