(Dumb?) Linear servo question

[vital.spark]

Don't fool yourself about the little skipjack being a no brainer. I can tell you that it gave me pleanty of headaches as much if not more than the Seawolf, Type VII and the Gato that I had already finished!!

[He Who Shall Not Be Named]

The voice of experience speaks. Listen. Put the other toys aside and concentrate on that little SKIPJACK. When you come up with real problems, air 'em out here. Between me, Myron, and the other bums at this site, we'll get your through it.

M

[SSBN659]

David, For some reason I'm not able to bring up the links to the pix you posted. Is anyone else having this problem?

SSBN659

[trout]

I have had that happen on several occasions where the pictures are displayed as links.. Usually when I take a long time making a post. For me, I edit my post and replace the pictures with the same one, just do it quicker.

[Subculture]

Sounds like the servos you have on order are using pager motors. These only tend to poke out about half a watt, and you will need a minimum of three times that to actuate a piston diameter of 1" at a reasonable speed.

To give you come idea of the forces involved, if we choose to spec the tank to empty at a maximum depth of 10 feet (a reasonable depth for such a small boat, and correlates to the deep end of a swimming pool) the force on the piston is getting on for about 3.5 pounds or around 1.5kg. Don't need to be an Einstein to figure that the you need a bit more grunt than 227 grams.

In fact you don't have to look very far for the right stuff- a standard sized servo fits the bill. Most modellers have an old servo or two with stripped out gear trains, but where the motor and electronics are still functional. Dismantle the servo recover the motor, control/amplifier board and feedback pot- hey presto, you now have the motor and control system for your tank. However you now need to fit that to a tank, so you need to machine a small jack screw, and make a small gearbox, or you can purchase a small 3 volt motor and gearbox (cheap on ebay), which would be a lot easier, and wire that in place of the original motor, then mate that up to the jack screw with a simple single stage gearbox.

One last thing, servos have no limit switches, which are required in a piston tank. You have a couple of choices, you can wire in a pair of micro switches and have them cut the throat to the servo amplifiers drive transistors, which will be difficult if you're a bit challenged with electronics, or you could machine in some mechanical limit switches e.g. so the thread runs off to a spring-loaded stop at each extreme of the tank. The feedback pot needs to be connected to the piston via a small lever (length of which is 0.7 times the stroke of tank e.g. 50mm stroke equals 35mm lever length)

Apologies if all this sounds a bit complicated, piston tanks are a bit more involved than other ballast systems, but do offer unrivalled levels of precision.

[Albion]

I don't want to take away from your excellent post, but when filling the tank at the surface, the piston will compress the air in the cylinder. this will help push the water back out of the tank. My experience was that as well as the compression force the seal ring friction was a big factor.

[Subculture]

That's true, it does depend on how big the volume is inside the sub/wtc etc. For most boats I guess it should raise pressure by about 1-2psi assuming the tank is being used as a main tank. Naturally the motor has to have sufficient grunt to compress the air too.

Agreed about seal friction too. You can get airzet (made by Freudenberg/Simrit) rings which reduce friction quite a bit over a conventional o-ring, but they do cost quite a bit, and aren't that easy to source, plus I believe the benefits are marginal, especially on smaller tanks. Silicone o-rings tend to be a bit softer than nitrile, and really one o-ring should be sufficient provided the inside bore is of decent tolerance. You should also aim to minimise squish, about 10% of the o-ring thickness is right, and use a thin o-ring.

I would avoid polycarbonate tube, as like most extruded tubing the high tolerance is concentrated on the external diameter, and the internal bore tends to be considerably more vague.

Much better choices are larger syringes, with a home made piston (the seals on syringes tend to be a bit on the tight side for our application) spent silicone caulking tubes, toothpaste tubes (the ones with a pluger), toner fuser rollers from laser printers/photocopiers.

Also a company called Beiserdorf make packaging for aerosol products. They are aluminium cans with extremely precise inner tolerance, and they are coated inside in a very hard wearing resin, so you don't have to concern yourself with corrosion. The cylinders are available in various diameters. My shaving foam, which is part of the 'Nivea for men' brand (I have sensitive skin) is one company that uses this packaging. The tubing is very thin, so you can dent it if you bang things about, but mounted in a tech rack it should have a long life.

[crazygary]

Great information and tips, Subculture. I will definitely file this for future reference!
You have saved me an immeasureable amount of researching time! Much appreciated.

O.K., M!! Work on my Skippy is in progress! Am at the point of cleaning up the internal surfaces of the two hull halves, and still trying to get them to line up a bit better than right out of the box. Seems the upper half is a bit wider than the lower. Am working with hot water and masking tape to correct the deficiency! Coming along pretty well.

A bunch of thanks to all of you who have chimed in on this thread! Much information to be had from all who have
"been there, done that"!

And, yes, SSBN659! Am also having no luck with Davids attachments!Will look for "fixes" when M has time!

[crazygary]

Sorry, Albion! For got to answer you!! My bad!!

I'm thinking that a well machined piston, with minimal clearance to the i.d. of the tube, along with double o-rings and some really smooth and slippery silicone grease will do the trick!!??

Appreciate your input!!

[Subculture]

Grease doesn't tend to hang around very long, or gets squeezed out by the piston, and thus ends up all in one spot in the cylinder. Lightly abrading the surface with something like 1200 grit wet and dry helps the cylinder walls retain some lubricant, much in the way honed cylinders on an internal combustion engine help the oil cling to the wall.

Another method I've read about, but not tried is to use one of the dry PTFE lubricants. These cost a packet, but are reportedly waterproof, and because they impart a dry slippery film onto the surface they're sprayed on, should not wash off or compact up with operation of the tank. The water itself also acts a pretty good lubricant.

[Albion]

Sorry, Albion! For got to answer you!! My bad!!

I'm thinking that a well machined piston, with minimal clearance to the i.d. of the tube, along with double o-rings and some really smooth and slippery silicone grease will do the trick!!??

Appreciate your input!!

What material do you plan to use? Machining to piston quality is not so easy. the cans mentioned by Andy sound a good idea.
I tired with a moncoque cylinder which i machined out a delrin like material and simply couldnt get a good enough finish. Before abandoning the idea (i may go back to it), it tired incorporating a lexan tube. Even then the axial torques was still quite high. Im using a high torque motor driving a jack screw via a pulley (o ring). The strain on the motor was quite noticeable even with an empty tank.

[redboat219]

What about using a winch servo http://puremtc.com/info_faq/ballast_system/

Wonder why this site folded?

[Scott T]

You would have to bring that up. (winch servo) Now I have to sit here and think about building one of these.
Which I won't get past the thinking stage.

I wonder if a glass syringe would make a good piston tank. Probably pricey though. The ones used for oil
sampling are ground to very close tolerances. Glass surface to glass surface. Hold your finger over the
inlet and the plunger hangs there. Release your finger and the plunger starts to slide very smooth, but don't let it
slide out and bust.

Scott

[crazygary]

A glass syringe sound like the ticket, but am wondering about weight?

[Subculture]

I'm sure it would work, but glass would probably be one of my last choices of material, as it's rather fragile and would be difficult to trim to the right length.

Weight wise, glass is about the same density as aluminium, but would need to be quite a bit thicker for adequate handling strength.

My first port of call for a small piston tank would be large syringes. I have some Baxa 60ml syringes, they have a 27mm internal bore, just a little over the specified 1" you wanted. The material is perfectly cylindrical inside, and made of a lightweight plastic- most likely polypropylene, as its one of the few unexpanded plastics that is lighter than water. You could use the incorporated seal and piston too, however they are a little stiff for our purposes- I think you could achieve greater efficiency if you machined your own piston with an o-ring seal and minimise the compression.