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I'm fairly illiterate when it comes to Engineering, so please understand I have pretty much 0 idea what I'm talking about.

I'm having to build a seawater distillation device for school, and we decided to go with reverse osmosis, to try and achieve this. (Reverse osmosis being the filtering of salt from water using massive applied pressure)

What I'm looking for, is a part that's a "Screw Piston" (I have no idea what it's actually called). So it's like a hydraulic piston, except the piston rod is threaded like a screw, so you as you tighten the screw, it applies a downward pressure. Here's a crudely made diagram of what I'm talking about enter image description here

I'm almost certain that something like this exists, (I mean if not, than I guess I should probably hire a patent lawyer and get to work) but since I have no idea what it's actually called, being mechanically illiterate, I have no idea how to search it up or where I could find something like this. I'd really appreciate help telling me what it's called and where I could possibly get one.

So my TL;DR is, what the heck is this thing called, and where do you think I could buy one? A giant thank you in advance to anyone who bothers to read this and answers!

(btw the reason I need a screw for the piston is that I need the mechanical advantage because reverse osmosis requires like 8000 pounds for force to filter the water.)

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What is it called? Your name of "screw piston" sounds good to me. Where are you going to get one? That is harder. If that is the way you want to go, you are going to have to have it made yourself.

The Wikipedia article on Reverse Osmosis says you will need between 600 and 1200 PSI for sea water. At 1200 PSI, 6000 pounds means a piston 3 inches in diameter. I would suggest a smaller piston. I don't know where you are getting your membrane from. That might be putting constraints on your design.

If I were presented with an assignment to do ANY RO on some water, I would probably tap a 3/4 inch hole in one of the aluminum blocks in my basement and go to the hardware store for a 3/4 inch threaded rod. I would then bolt the membrane across the bottom with another piece that had a hole for the filtered water to leave. If I then filled the threaded hole half way with salt water, put food grade grease (I presume you will test the result by drinking it) on the threads of the rod and cranked it down, hopefully I would get clean water out the bottom. A rod 3/4 inch in diameter has an area of 0.44 square inches. For 1200 PSI, you are going to need 530 pounds force. You can do that with threaded rod and aluminum, no problem.

Good luck.

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