have you ever seen ?

[machinedude]

Well it at least seals so far. i have 2 air zones by design and i am glad i went that way because the more you have the harder the rotation gets when you add pressure. i would be scared to try and cut the glands any deeper than where i have them. once put together the weight of the inner shaft will pull itself through the outer body on its own. right now, it seems pretty tight to spin but when the air flow is controlled i will only have one zone under pressure and i hope that makes things spin easier. i will have to wait and see if the solenoid valve helps with that. i have to wait until i get some better testing done to see where i am at on this.

[Deezl Smoke]

o-ring friction?

[machinedude]

probably something like that. i would imagine when it seals under pressure you get some compression effect taking a guess.

[machinedude]

i figured out a way to isolate each zone without the valve and it got a little bit better but is still tight spinning. So i think i will try to polish things down a very small amount and try again. i think while i have it apart i will try to improve air volume while i am at it. i think when things are together with Oring grease i don't have enough space for enough air volume to flow very fast. These unions are pretty finicky with clearance and working well and still sealing no doubt.if the gland is not deep enough you end up with to much compression and it gets tight as a result. when you add pressure, you get some compression and tightness, so the million-dollar question is how much compression do you get with pressure? very small adjustments i guess until i get it or the seal fails

[Deezl Smoke]

The larger the diameter, the more oring surface contact, obviously. So a mirror finish will help a bunch, but a tight tolerance machined brass ring, and allow a small volume of air to flow rather than an air tight seal, might be an option to think about.

[machinedude]

from the material i have seen the glands should be a 32 finish and the shaft should be a 16 or better. and the size of things is a mater like you said the thinner the ring the faster it can spin but as the diameter grows the width of the Oring goes up in size. i'm really close i think so a few tents here and there might get me there. if it ends up being snug i will have to attach the outer ring of the union to a fixed point for strain relief.

[machinedude]

yeah did some more messing around and checked the stretch over my shaft and polished things far beyond and still no improvement on rotation under pressure. So the 4th time reworking the glands might be the charm i need that will have to wait until the weekend so i can set all that back up again. i did test the air cylinders and they work just fine with no leaking yet they have no trouble lifting me off the ground so they should have plenty of force to do what is needed if i can get this union to work right not very scientific but it work lifting 190lbs i think they should work at around 260lbs of force at 90 psi.

[05Duramax]

Very cool topic, enjoyed reading this and following your build

[machinedude]

I started building my Z axis before i came up with this rotary idea. If i can get it to work i noticed i will have to extend my travel on the Z axis but here is a screen grab of the design on my floating head Z. if i get this union to work i will add some longer round rails and swap my screw out with a longer square tube to house it all before i do anything else on the z axis.

Plasma Z axis.png

[tcaudle]

The only thing I see is hold the torch with a single loop and a strap may give you some wobble problems on moves. If the work is moving instead of the XY then its fine. A two ring torch holder with metal; ring clamps works goof for a conventional table. It will have no collision detection or breakaway to save the torch

[machinedude]

actually, i will have a magnetic breakaway on this but forgot to mention it. i don't use much other than aluminum on the stuff i make with the exception of steel on the liner rails. basically, everything is custom machined parts so i don't get too much movement . the only thing i might need to add is a set of springs if the weight of the torch is not enough to keep from bouncing around with the floating head. if i see something causing me grief it will get fixed as i go

[machinedude]

i had bought a plasma cutter a few years back with the idea of putting it on a cnc table at some point. this past thanksgiving i came across a straight torch deal on black Friday that was hard to pass up. $80 for a straight torch was a pretty good deal and that is what got me moving on this project. But the plasma cutter is a blowback type and has the cnc ports needed built in so it should be a simple build for plasma.

machine torch.jpg

[05Duramax]

That is a killer deal for $80 and even more so if that's what sparked your interest to begin the build. Looking forward to seeing more.

[cutnweld]

for the rotary seals, I would check out slip rings like powershift transmissions use. They take 3-400 psi under rotation for 1000s of hours. Id guess you would have to have them custom made for something this big. https://herculesus.com/index.php?width=800&height=600 makes any sort of custom seal you can imagine. these rings always bypass a bit, but will turn a whole lot easier than an oring under pressure

[machinedude]

i think i have ruled out the glands being the issue at this point. i reworked and reworked and i did not see much improvement. i tested pressure and this thing seals with as little as 15psi still. so i don't think it is the Oring extruding between the inner and outer body of the union. So about the only thing left is the stretch over the shaft at this point. I'm beginning to think the glands are not as critical as i thought they were from what i am seeing. i think they still seal along the outer wall of the gland when you add pressure honestly.

[machinedude]

time of death 12:36 pm 2/22/25

the best results i got were at low pressure so back to the drawing board if this idea is ever to come to life.

[adbuch]

I still never figured out exactly what your were doing - the details that is. An exploded 3D solid model would be helpful for showing all of the details.
David

[machinedude]

The best i can do is tell you to Google pneumatic rotary union and take that concept and factor a hollow body design into the mix. you need the large center bore for material to pass through. the tricky part is doing it on a shoestring budget. since it is a hollow body design you now need large seals large bearings and are fighting more friction trying to spin a seal around a large circumference. Orings might very well work for a seal of this type but i have proven to myself that they have some size restrictions.

i did spend a little bit of time looking at different rotary seals and when you look for low friction seals the search leads to PTFE type seals but those tend to be higher end seals with a price tag beyond what i would want you experiment with. Plus from what i could tell from the limited time i spent looking is those types of seal appear to be end seals on a shaft. but i did not look that deep into it since there are so many variations for different applications so they have something i bet i just did not see it yet.

the next step up from an Oring in price point would be rotary shaft oil seal but i have no clue if those would seal air but they probably have some kind of pressure rating. or maybe one of those on each end and only one Oring to create an air circuit might reduce the friction down to something that might work?

bottom line is i lost round one and to get to round two i would need a ground up redesign of a union because the other types of seals take up more space i don't have with what i started.

i don't know when i will get back to this unfortunately. yesterday was just a bad day. i found out last night we had a death in the family. So i'm in a bit of a funk at the moment. so when i need to distract myself i might mess with this to keep my mind off things but i don't expect my best productive thinking to be around.

[adbuch]

Thanks for the tip. I did google it and here is one example.
https://www.senring.com/hollow-shaft-ro ... 02006.html

hollow shaft rotary union.jpg

[adbuch]

I would be interested to see what yours looks like taken apart. What sort of bearings are you using? From your photos, I am still not clear on your actual design. I understand it conceptually. I'm more of a "pictures" guy.

David

[adbuch]

Sorry for your loss.

David

[machinedude]

if you want an off the shelf bearing solution then you are looking at something like these. and you would need two of them and that would only get you around a 5 inch bore. so you are at almost $4500 just for 4 bearings to make two unions.

[adbuch]

So what were you doing for bearings? Or was is just a slip fit of one cylinder inside another. Since I have not seen the details of your design, I really don't know exactly how you are attempting to accomplish this.
David

[machinedude]

OK my distraction this week was a deep dive into the seals on this project. i probably had a good union built until i tried to reduce the friction and went too far until i ended up with a leak. i found a manufacturer who makes these and size matters, the torque required to spin one this size is around 3.5 Nm and on something small like a 20mm bore it's around .5 Nm so the bigger they get the more torque they require.
i think i could do better on a home-built union with a different type of seal but finding them this size is a bit hard to do. I think the trick to getting this right is keeping the sealing lip isolated from the housing in a dynamic fashion. the only way i see doing this is an end bore type of seal done in two stages for each air zone on the body portion of the union. to me 3.5 Nm of torque seems like quite a bit of force to deal with for this application. any kind of energized seal adds friction, so any loaded type is counterproductive, so the search goes on i guess.
Or maybe i'm at a fork in the road moment? maybe i can figure out a way to gauge 3.5 Nm to compare to what i had. i need to get my hands on it and feel it before i drop that kind of money and just buy one well actually two of them if it seems to be a good option?

[adbuch]

Perhaps share a drawing showing exactly what you are attempting to do. It's hard for me to provide anything constructive suggestions or advise if I don't understand exactly what your geometric configuration is. I've done a lot of mechanical design related to flight parts for missions to Mars and Saturn and would be happy to try to share some of my expertise as a mechanical engineer.

On the other hand - if this is something "proprietary" and you don't wish to share your design - then I completely understand.

David