Sources for small hydraulic actuators

Lewin A.R.W. Edwards wrote:


I think most hull piercings (as electrical connections) get called penitrators.
The cheap and nasty (but probably good enough for you) have sealed flange fitting to suit a cut-out on your hull. The flange carries a pipe thru' which you run your electical connections all the space is filled with a sealant. problems come from excessive depth may push water up the cable, this is where moulded penitrators come in... but that big bucks business.
pressure compensated oil systems have been mensioned else where... they work a treat
regards, Colin
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Have you considered simply potting a few wires with epoxy? It's not as easy as it sounds, because you have to strip a section of each wire so that the epoxy can reach the metal (if you don't some water will leak up the insulation jacket, depending on your application that may or may not be a problem), and you have to be able to rely on the epoxy as insulation for that half inch (sorry, you're from UK, for that centimeter) or so. So you have to ensure adequate separation of the wires while the epoxy is curing. It's not the greatest sealing system (its pretty good), but it's cheap! It's main problem is that it takes careful work. If you don't get a good fill, you're done.

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I build water-tight boxes for combat warships, to protect equipment that doesn't like to be submerged. The nature of the hobby requires that ships get sunk every now & then so it's important to be ready for it.
I make many of my penetrations through my (polycarbonate; non-conductive) boxes by drilling & tapping holes, then installing screws into the holes. The screws act as terminals with wires attached to both inside & out with lugs. A dab of silicone is placed on the screws before insertion, just to be sure.
For servo connections, I drill a grid of small holes, 3 x (whatever), with 0.10" spacing, Headers are instlled into the holes & sealed with silicone. The solid headers prevent water from being wicked in through stranded wire.
Using the above methods, all penetrations through the box are solid. Ships so equipped have been sunk multiple times in water up to 10' deep with no water penetration. I usually try to recover ships ASAP when they sink, but sometimes it's not possible for various reasons. Sometimes, they stay down for 30 minutes or more, usually when they are deeper & more difficult to recover.
JM
wrote:

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John Mianowski wrote:

That's a great idea. However, how do you deal with the external terminals being shorted through the water?
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wrote:

Never been a problem. Either:
- voltages are low enough, terminals spaced far enough, & water not conductive enough - by that time it's sunk anyway
JM
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John Mianowski wrote:

Interesting. On a similar note, do you have any problems with servos? Do you have to specially prepare servo casings to prevent seepage?
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wrote:

Some yes, others no. I've found a mini Hitec metal-geared servo that survives multiple dunkings with no problem at all, with no modification. They're fairly expensive, though, & I only use them for barrel elevation where there's a lot of torque load when the guns fire (hence the metal gears). OTOH, most standard cheapo servos will start to jitter & dance if they get wet. The cases as built are usually fine for keeping out the water that gets into the ship pre-sink, but once fully submerged most will leak. Sail winch servos seem to be the 1st to get affected - probably has something to do with the fact that they're expensive & Murphy's Law is always in effect. Some people dip the entire cases in tooldip or Scotchkote & put an O-ring around the output shaft to try to keep water out. Others just coat the control boards. General consensus is that the component that fails most often is the feedback potentiometer, which you really can't waterproof.
One could "build" their own waterproof servo pretty easily from standard servo components - Scotchkote the control board & replace the pot. with a sealed one (which probably won't fit inside the case, so it'll have to be connected externally & coupled to the output shaft).
Then again, many people (myself included) use mostly cheap standard servos & treat them as a disposable commodity. At $7-8 apiece, it's often not worth the effort of salvaging them. Most times, when they do get wet, they can be dried out & cleaned. When I get a wet one, I disassemble it, dunk the board in alcohol for awhile to displace the water, then allow it to dry thoroughly. This works maybe 80-90% of the time, & is about all the effort that I'm willing to put into them.
JM
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    --Have you looked at Robart hardware? They make mini pneumatic stuff intended for R/C model airplane retractable landing gears. It's fairly cheap and eminently hack-able...
--
"Steamboat Ed" Haas : Just another fart in
Hacking the Trailing Edge! : the Elevator of Life...
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