pic: Medium load linear actuator with almost all COTS parts



Inspired by asid61’s LIN-101, I made my own flavor.

With the goal of reducing base length, I used telescoping tubes and a 3/8-12 acme nut with 1/2" hex outside instead of a longer coupler thing. The base length is around 2.5" (with pivot), with a maximum stroke of 5.5" (limited by https://www.mcmaster.com/#precision-bushing-stock/=17u8r81, whose tolerance guarantees nice sliding).

LIN-101 is way better for longer arms and higher loads, like the huge arms from 2016. Meanwhile, this is made to handle slightly smaller stuff, like moving the 2017 gear sideways and manipulating game elements within the robot.

Every single part is COTS except for the 3D printed blue pivot part, which is optionally anyways.

You know chak I used to respect you. But you really botched this thing up. Look at that vp. Where are the lines between the stages. And… No bolts come on man. Smh. What’s next shiny cim motors?

In actuality good design. Although it seems like the tubes would suffer a lot from friction rubbing against each other and if you could I’d try to get some hpde in there between the tubes to act as a bushing of sorts. Also, while I do appreciate the use of the vp cots plates I feel as if a custom milled plate is a much better option as the plates just plain don’t look beefy enough.

Ditto on the VP- not even V2, really? You’re losing your touch.

Really interesting, I like the use of plastic tubes to create a basic thrust bearing. I also really like the use of the plate bearing gussets! I might steal that for my own…
I think that should be strong enough because you’re using two plates anyway, and this is low-load.
I think sourcing a 3/8"-12 x 1/2" acme nut would be difficult. Do you know where I would be able to find that? On a similar tangent, the plastic bushing stock looks like it can get pretty pricey. Is it possible to use an aluminum tube for the inner or outer tube?
How is the outer tube secured to the plate?
Honestly this is super cool. All-COTS plus pivot is just amazing.

The rods are already made out of some plastic. The outer tube right now is https://www.mcmaster.com/#1988t13/=17ufdzw.

Here ya go. You do have to chop it to length, which you tell in the picture cause I didn’t do it in CAD.

You would have to source undersized aluminum tube. The only undersized tubes I could find on mcmasters are for making bushing/shims that fit into each other, and are only 6" long, limiting the stroke of this thing.

Or if you buy the metal in real life instead of internet you can test the fit yourself at the store. Or you could buy some blind and hope that it works/be ready to sand off a layer.

All of these are compromises though, and there will still be some wiggle between the 2 tubes. Perhaps the answer is to not do telescoping- but it will probably suffice for lower loads.

Haha… I would have to line up the tube with the vex bearing plate (probably with a flange bearing whose flange will fit in the 1.25" ID tube) and drill and tap the 4 holes on the smaller bolt circle for #8. The wall thickness is a bit jank, but I’m somewhat confident that it will work.

Holy cow, that coupling nut is exactly what I’ve been looking for! I skipped right over those when I was looking for ACME nuts…

From my understanding you’ve got a plastic outer sleeve & metal inner sleeve…What’s driving that? Seems like it would be a lot easier to knock a couple thousandths off the outer diameter of a plastic interior shell, fit into an off-the-shelf metal tube, and get a lot of length range back.
(though it would violate the “almost all COTS” rule to use a lathe to get the sanding done, so there’s that.)

I thought plastic outside + metal inside would be stronger than metal outside + plastic inside, since the weaker material has the larger tube.

But your method is still a very valid tradeoff. The length range is probably worth more than whatever extraneous strength it gains.

I don’t know how annoying sanding would be without a lathe though, since this is designed assuming you have no fancy tools.

Probably gonna design another one assuming infinite tool resources.:smiley:

Fair point :slight_smile: I’d rather have my extendable/replaceable tube break than the tube that’s fastened to my gearbox. Frankly, both are going to be pretty strong until its at the very edge of the extension, when I’d expect it to fail unexpectedly in buckling…

I don’t know how annoying sanding would be without a lathe though, since this is designed assuming you have no fancy tools.
You’re assuming a lathe has to be fancy. I’ve done some pretty ugly things with power drills, bench vises, sandpaper, and TLC that end up looking a lot like a lathe operation…