This is a design I made over the past week for a telescoping arm made from carbon fiber. I was never very happy with the weight of my last telescoping arm, so I tried to reduce the weight as much as possible. The new one weighs 11.6lb compared to the old one’s 25, so I’m pretty happy with that success. I also included a section view in the renders above, so you guys can get a better idea of how the whole system works.
I split the tube caps into separate parts to make them possible to fabricate using 2.5d milling. The dowel pin slots can be cut out by slotting with a 1/8" ball nose endmill. It would require two setups to do both the top and bottom sides, so you’d likely end up making a jig for locating the second setup.
You certainly could run the energy chain on the outside, but the benefit of running on the inside is that you don’t have a giant spearable energy chain outside your frame perimeter, which is more of an issue with a telescoping arm than an ordinary elevator.
All the bearing blocks are currently designed to be made from 2 pieces of aluminum, screwed together and located by dowel pins. There’s no reason they can’t be 3D printed, though.
The ropes manage the rotation of the tubes. There are a handful of locating grooves for some of the rope runs to prevent them from walking off of the pulleys. Additionally, I’m not concerned about the minor amounts of rotation from a non-rigid solution here- 100 did this for their 2020 climber, which was a 4-stage telescoping arm with only 1 rope per side, to great success.
While square tubing makes a lot of the challenges of a telescoping arm easier, the problem is that it’s much harder to wrap your own square tubing, compared to round tube. The intent of this design was to save on cost by fabricating the tubes in-house, which is pretty straightforward when you get down to it.
Thanks for the clarification - I call it the arm hub. In this design, I would probably 3D print it out of nylon.
If one wanted to make something similar out of aluminum, the ideal way (in my eyes) would be to machine the hub and the clamp as separate parts. then weld them together. This would save you a lot of time on the CNC machine.
What I’m picturing is wrapping a continuous sheet of carbon fiber about a round tube, adding a layer of epoxy between every revolution, and going for a few revolutions. This way you just have to use any ordinary pipe as your form. The sleeve option also works, and would certainly make more complex profiles easier.
What are the ODs of the three tubes? Also will you need to do anything special during the manufacturing process to keep the tubes at consistent diameters along their lengths? I would think this would need to be pretty precise to not have slop running against those small bearings. Are tubes of these sizes available to purchase?
I currently have them set to be 2", 3", and 4" OD. Realistically, I should have made the more nominal dimensions to be the IDs, since the tubes would be fabricated by wrapping a sheet or sleeve around a pipe.
The primary factor would be the cylindricity of your mandrel when you form the tube. You do need to be exacting on the tolerances of any form when working with CF, since the errors in the form always show up in the final part.
Note: If one were to actually build this, I would recommend fabricating the tubes, then reading off their true dimensions with calipers and updating the CAD for all the end caps, and then fabricating the end caps.
Yeah I suppose if your mandrel was very consistent that should take care of the inside. I don’t have any experience with CF, but I have done a bit with fiberglass on a round form with a large OD, and it seemed like it was almost impossible to keep out lumps and droops on the OD. I’m sure it could be done more carefully than I did it though. For the OD, you could maybe put it in a lathe and take out any imperfections, I don’t know anything about the machining properties of CF though. Anyway, neat project, looks really cool.
I will caution against machining CF unless you have to, since the stuff is just nasty to work with from both a machining and safety perspective. However, if all you need to do is remove imperfections, the ideal way is to sand it. If you’re manic about it, you can get them to be super smooth after many hours of sanding.
If wrapped as you suggest there will be a lump in OD of the tube wherever the fabric happens to wind up wrapping more than in the rest of the tube section.
More practically speaking - if you lay up a wrap OVER a straight tube (which you seem to be implying by design and speech) you’ll be in for a really rough go of it. Resin shrinks during curing, thus the tube will clamp down on whatever tube you’ve wrapped it over, making removal immensely difficult. When CF is formed with a positive mold like this the mold is either sacrificial or drafted to be removable.
I doubt this very much. Standard types of tubes are may by a process called pultrusion. This process is almost entirely automated. Itrequires specialized equipment and the materials are still quite expensive. This is also why they have reasonable control on ID and OD dimensions. I opine that no company producing standard cross sections like this is laying them up by hand in a batch process.
I’m not trying to give you too hard of a time here. You just keep saying things that do not jive at all with my experience in composites.
I’ll offer my experience from college rocketry - we do this by coating the mandrel with wax and baby oil to make it come off easier. You’re right in that it’s very difficult to remove, but it’s certainly possible - we did this with a 6" ID tube that was 8 feet long. We didn’t have significant lump issues; this may be due to the fact that it was wrapped to 8 revolutions.
Thanks for the info, though. Much appreciated. I was likely in error on the labor comment.