What is this thing?

I use these at work. That is a sleeve style clamp used to seal off leaks

That's a "shackle" when you put it on a limb. I'm no plumber, but if that exists in plumbing, my guess would be coupling or bracket.

This sounds like it would accomplish what you need. Look up two piece shaft collar, or shaft coupler.

https://www.mcmaster.com/search?q=hinged%20shaft%20collars

Use McMaster to find what you need. You should be able to find a cheaper supplier using McMaster's terminology.

We used something similar in this job I once had. We usually referred to them as "collar" or "clamp". In our procedures it was refered to as "clamping collar" to make it as little ambiguous as possible.

It was nicknamed "Dachs armor" as it was about the same length as the belly of a dachs hound, and roughly about the same diameter.

Clamshell clamps or Truss Clamps aren't exactly what youre looking for, but they'll work. Clamshell clamps are used in automotive and plumbing applications, truss clamps are used for mounting lights and speakers to stage trusses. You can get them cheap on amazon, and stacking several of them up works great for spacing shafts.

"Stroke Limiting Shaft Collars" are exactly what you're looking for, since they're designed to do exactly what you're attempting here but for industrial pneumatic or hydraulic equipment. Unfortunately since they're for industrial equipment, they can be stupid expensive and a royal PITA to find. But they do have a very silly name, even for hydrualic equipment (which is a field whose jargon is entirely made up of double entendre), which is a point in their favor. Also, many of them work very well as bearing surfaces!

PLEASE be careful messing with the cylinders in office chairs though. It doesnt sound like youre planning to, but just a blanket warning. Much like garage door springs, those cylinders store WAY more energy than you would believe, and fucking up trying to take apart a damaged cylinder can send the piston an impressive distance through your chest.

A pipe shackle is probably what you are looking for. If you get the wrong size and overtighten, it could deform the pipe and increase the likelihood of the pipe getting a kink and failing.

Sleeve.

Shaft collars probably the closest to what you're looking for that's easy to search for

"Manacle"?

I got some relevant results for "plastic clamps." Uline has a similar looking product listed as "plastic shelf supports for wire shelving units."

Cable collar maybe?

It looks like a shackle.

Kozyrev Mirror, you sit inside it and hallucinate other dimensions or something like that

Not quite the same, but a hose clamp would probably work

What about those metal bands that can be tightened with a screw? ‘Hose clamps’ I think.

So I found products like you've shown on Amazon while looking to do the same for my chair but the cylinder on my chair is bigger than I guess "normal" ones, so what I ended up doing instead was using something apparently called a pipe nipple, like this one, that fits over the cylinder. I got the idea/process from this video except substituted a metal nipple like I linked above instead of a plastic coupling like they show. Anyway, it's been working fine for 9 months so far, just make sure you know how much height you want between the base and the seat for the length, and that the inner diameter (ID) of the nipple fits the outer diameter (OD) of your cylinder.

I made my own. My piston kept sinking on my chair, so I got some 40mm waste pipe I had lying around, cut it to length, then slit it once lengthways.

I put it over the piston (pulled the pipe wide enough to slide over the piston, not easy, but doable) and wrapped it with duct tape to stop it pinging off.

It is still there 5 years later.

split muff coupling

Where'd you get that image? Did you draw it yourself?

Scaffolding sleeve clamp or Scaffolding sleeve coupler

Eh, make your own: short bit of pipe cut to length, split down the length, then held on the chair column with hose clamps.

I have used something like this on a chair with a leaky piston, there are ones specifically made for this purpose. They are plastic, and useless.

I suspect that using a metal one would last longer, but still fail, either by breaking itself, or by embedding it into the plastic of the chair on the top or bottom. I bet you would be more successful replacing the entire piston assembly with a stout metal pipe, but even that could break the piston seats at the top or bottom.

Penis-breastplate.