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Homebrew hydraulics? Answered

I've been working on a project that, if I manage to build it as designed, would require four actuators or some kind no more than 1" in diameter. Hydraulics seem to be a good fit for this application (I wouldn't need more than 30 PSI, single acting only) since compressed air would be too "spongy." The actuators also need a total travel of about 24." That's where I run into trouble: I can't find hydraulic cylinders that meet the bore and stroke requirements. I have an idea to build my own which I'd like some feedback on before I dive in, or perhaps someone has an alternate suggestion. I have some thin walled, 1" chromed shower rod that makes a perfect outer cylinder (and the chrome finish would look snazzy!). Add in some 1/2" PVC pipe for the inside and I think I'm pretty close. First attempt was to make molded silicone pistons on the PVC. I got some pretty good results but it's not entirely leak-proof, so I lose pressure. My next idea is to use a rolling-sock seal made of 1" lay-flat PVC discharge hose. Bind the hose to the PVC pipe with a good seal, double it up over the pipe and seal the other end before stuffing it into the metal tube. Anyone think this is a lost cause, or have another suggestion?



9 years ago

G'Day Smidge, Have you thought of wandering by a wreckers or even a suspension shop,(they sometimes also replace/fit gas struts for hatchbacks), and grabbing some struts heading for the bin? Yeah, small dia. but you could multi up to suit your needs , plus using collars could use struts as part of structure, mabey? You would need to check that seals are compatible with your working fluid. Whatever you use, if you are using propriety parts, (read best scrounged ), they will all meet ISO 9001 min. standards which means you can buy anywhere. You are right on the mark using hydraulics if you want fine control at low to med. speed & manual control. Hydraulics & Pnumatics are great by themselves , are very complimentry and allow use to do neat stuff.Can you cut & weld?

Someone else mentioned those gas springs. I didn't really consider it because they only push with no control over them. You do make an interesting suggestion to modify them... also something I didn't consider since they're under pressure and drilling into one is generally not my idea of a wise move. But if the cylinder is 'dead' then it would be marginally safer. I was planning on using water as a fluid, since the application is only ~30PSI and very light duty, water is also cheap, plentiful and easy to clean up should there be a spill/leak! I can do basic stick welding and brazing that holds pressure. What would you suggest is the best way to go about it? Lop off the entire bottom of the cylinder and weld on a new base? Drill a hole in the side and weld on a nipple? Maybe cut and drill the bottom and tap it for a threaded connection? Gonna try the PVC hose thing first, since I already ordered it and have all the materials. If I don't have something working by the end of the month I'm definitely coming back to this idea. Thanks!

If it only has to push once, you can go with a steel rod, cotton balls, half in. PVC pipe and some way of opening a value to let air in. I'm curious, what is the application?

It needs to be a repeatable action. The application is lifting a small platform, and the mechanism must fit below: 22"x22" by about 4" high at most. It's actually sort of a double platform - imagine a box with a lid and another platform inside, so the "lid" and the platform lift up together. The platform itself may have a variable weight on it. I already tried a mechanical hoist arrangement, but depending on how much weight I put on it I can't reliably control the speed the platform descends. I also considered a screw drive but it would be incredibly slow and difficult to keep even (chains and belts and stuff) Kinda running out of options! I figured hydraulics would be the hardest build but best probability of succeeding.

. Use an O-ring between the piston and cylinder.

O-rings require fairly accurate machining and close tolerances. I don't have the equipment to produce a piston with adequate specs. This is why I tried molding a piston out of silicone, using a short piece of the outer pipe as a mold. It worked but after four attempts I could not get something that really held a seal (spec: hydrostatic test; hold 30PSI for 1 hour), plus lubrication is a problem I didn't even get far enough to address.

Can you not use some kind of lever to get the travel you need from a shorter ram?


Unfortunately, no. The application is a linear actuator in a pretty tight space, so there's just no room. Fortunately it only has to push, which is why the rolling seal will work provided it can actually be constructed.

Hmm, I'm thinking of those gas-lift cylinders you get on e.g. tailgates that warn you not to attempt to dismantle them... No, I'm stuck - good luck. L