Sealing a Rotating Threaded Rod

With the size of your vessel, why not use compressed air? Using an old small LPG cylinder like one from a benzomatic torch, you could use it to drive solenoids to purge and flood the ballast tanks.

Using a car air compressor (like the one you plug into a cigarette lighter) you could refill the tanks whilst on the surface(or just under via a snorkel).

Having compressed air onboard allows for other functions like firing torpedoes.

You could modulate the ballast tanks for attitude control, similar to how the vessels you serve on operate.

The solenoids have only 1 operating part where as your motor/syringe will have several. you'll be better able to control the ballasting and other functions utilising the processor to drive the solenoids.

Wish I could join you in the challenge looks like fun.

With the size of your vessel, why not use compressed air?

You are on the right track, Why not use Propane- Butane gas Mix?In a light weight container in liquid form. A 50-50% mix would give you a vapor pressure mix of 50Psi at 60 Deg F and more than 15Psi in very cold water.

Your mix will have an expansion factor of approximately 200 times depending on temperature and operating depth so you will be able to move a lot of water.

Distribution through solenoid valves with very small apertures would give you good control. You would save a lot weight and bulk of mechanical components.

No I just meant for the OP to use the cylinder as a compressed air receiver, The op wants to use his idea for sucking or pumping water into a cylinder.

Using LPG in this model would be problematic, think boom!

Would need to use I.S. relays and motors and stuff. Very $$$$$$$!

WRONG!!

Such gases are LESS likely to cause an explosion as here is no oxygen around!!!

The model will contain atmospheric air as it is put together. I estimate that it will contain at least 20 litres of air, of which 21% will be oxygen.

The model will contain atmospheric air as it is put together. I estimate that it will contain at least 20 litres of air, of which 21% will be oxygen.

You could flush it with butane to remove all air.

In any case the system is operating at such low pressures you would be a poor engineer if you had any leaks .

Indeed, you could. Or, as post #20 suggested, you use compressed air in the first place.

Or even CO2 cylinders, which are easily purchased and use.

The big ones for pellet guns are the best.

Buttress thread has some hydraulic sealing properties and the make thread seals.

I have nothing to technically add here, but just wanted to complement you on your chosen path. A tip of the hat, a hardy shake of your hand, and a pat on the back for service to your country.

Best of luck in your endeavors

Tobugrynbak, I've thought of compressed air, and I will be using an air bank to serve as my emergency blow function, and assist in part of my vertical launch system. As for torpedoes, I may end up using it as well, though I had thought of using a small magnetic reed switch in the torpedo to control power to the motor, and when opening the torpedo tube muzzle door, a magnet inside the ship( though outside of the tube) would allow the reed switch to shut, and the torpedo would launch

As for compressed air in the ballast, there are two reasons that I haven't considered it more, and I'm not even sure at this point if they will be problems: 1, space. A 6" tube fills up quickly, and the controllable ballast tanks' size will be completely dependent on what my buoyancy calculations come out to be in the end. So I may end up having space. 2 the amount of water displaced at depth will be different near the surface where i would need yo have pretty good control in order to launch VLS. With a syringe I'd get the same amount of water for a given movement. What I'm not sure of is if that difference in water weight would make so much of a difference that compressed air shouldn't be a serious consideration.

Ozzb: I've found thread seals, but I cannot find any that allow movement, they all seem to be for a static seal

averagejoe: thank you for support. It means a lot to us all to know that we are here for something and/or someone.

A stuffing box and Teflon packing.

I'd like to place the drive screw stationary, and move the cylinder back and forth as the screw rotates.

This appears to be contradictory. Please elaborate. Nice project. I remember seeing a model of a submarine that was built as a recruiting promotion for the Navy. It was at the Alameda Naval Air Station. The boat was about 16' long and held one operator. I believe it could submerge, but I never saw it in action.

Instead of a syringe, how about an air cylinder? plenty of different diameter and stroke length to choose from. if you use a geared motor you could divine the stroke/time/effect for your calculations. you would drive the cylinder piston with a crank rather than a screw setup, would make for a more compact assembly.

The thing about purging the water out of the ballast tanks is the need to overcome the latent pressure at depth which gets back to using compressed air, its all about control. you would have more than one ballast tank a side, if you used several compartments per section i.e. 4 aft 6 mid and 4 bow per side and monitored the depth at the 3 stations (waterproof depth sensors are not that expensive) it would provide you with the control you need to maintain depth by flooding or purging paired sets of ballast compartments. Having the 3 depth sensor will tell you if your aft or bow heavy relative to the mid section. The length of you model makes this very do able.

Yes you could even automate this, using a PLC or Rasberry Pi/Aruino microcontroller.
As I see it, this all comes down to maths. I have been developing a product over the past 12 months which involves water depth so I am familiar with reading depth in distance/water pressure, there is a lot of stuff on the interweb including divers tables which will help you here.

You know the internal volume of your model which I'm assuming you intend to keep dry so it will have at least 1atm @ sea level air pressure which will afford a degree of latent buoyancy so the trick will be to be able to overcome that buoyancy with ballast. As your running this in seawater it will behave slightly differently to just being in a fresh water pool/lake. Your ballast tanks need to accommodate enough water to sink the boat(in a controlled manner# to at least say 5 metres#er 15ft#. you mentioned 20 ps1. that is roughly 1.360 atm or near enough to be 3 metres depth so I'm presuming that is the depth you want to operate at. if you maintain a slightly higher internal air pressure than external water pressure you will find it easier to keep the water out plus you'll have an air bubble trail to follow.

Flooding and purging the individual compartments should afford you the necessary control to ensure proper trim. Knowing the volume of the individual compartment and the inlet/outlet orifices should allow you to calculate the time taken to flood and purge#at depth# that will help in your programming as well for automated recovery control.

For depth sensors think Motorola(freescale) they have a useful product calculator. Consider using Burr Brown INA125 instrument amp.
Oops run out of doodling time.....

Sorry I should've proofread haha. So what I'd like to do is have the screw sit stationary axially, centered inside the cylinder and do nothing more than rotate. Since it won't move back and forth and only rotate, this will allow the plunger to move back and forth, depending on the direction of rotation, and create a syringe kind of effect.

I'm liking the Teflon packing idea. We use graphite packing all the time, and I had planne on using something like the graphitic packing for the shaft, but I didn't even know the Teflon existed.

sorry I'm not sure I'm picturing what you mean with the air cylinder and geared motor and crank. I'm picturing a old locomotive drive wheel piston kind of thing, but the piston/cylinder is set out 90* to turn the crank, attached to the threaded rod from my syringe idea...

I'm at work so I haven't time to doodle out a sketch, however if you are thinking old school loco in reverse where the wheel drives the piston via a conrod (think Bell crank) . The electric motor drives the "wheel" via a cog on the periphery of the wheel.

Think large diameter Acetal/nylon gear wheel with a pin (bolt)just inside the rim to act as the big end crank pin for the conrod. The other end of the conrod is connected to the piston shaft of the air cylinder. The stroke of the air cylinder will determin the necessary diameter and pin placement of the acetal/nylon gear wheel.

Alternatively you could use a stepper motor out of an old school floppy drive or cd/dvd (it does limits the stroke length) and using the screw, drive the piston shaft in and out. Yes there'll be a bit of nifty fab work involved. The average stepper drive screw length is around 50mm/2".

Note you could use the stepper motor to drive other functions rudders, dive planes etc.

If I keep this up I'm gonna want to build one myself

I've re read your opening thread, hmm I think I now see what you mean about syringe. I can see issues with fore/aft balance if I think I have your concept right.
essentially you're ballast tank is 1 tube (per side# with a screw running through the centre #longtudinally# with a round plate that runs #gets driven by# the screw. the plate is sealed against the tube. when the plate#piston# is run back it draws the water into the cylinder. when the plate is driven forward the water is pushed out again.
The problem is that the weight of water in the partially filled cylinder will be biasing the pitch attitude of the boat either making it bow or aft heavy #depending which end is being filled)making for poor trim control.
To get what I mean float an open (Toy# boat hull, you'll see it floats horizontal, now place some weights #large steel nuts are good for this# up forw'd. you'll see that the boat is now bow heavy. place more weights toward the aft and you'll see the boat "trim out". yes there is a limit to the amount of weight you can put in. That is how you're sub will behave. in your case with the boat half ballasted #ballast tank 1/2 full# the sub will either try to keep diving while your driving forward because its bow heavy, or porpoise because the aft is dragging.
That's why I mentioned multiple compartment ballast tanks in my previous responses.

Yes your description is correct. the threaded rod would run through the piston.What do you think about two syringes, each drawing water from each end of the ship, one forward and one aft and the two motors (one for each piston screw) would be in the center.. That'll keep the ballast controllable fore and aft right?

Kinda like this?

http://youtu.be/TcnKvH-1RCg

Where the piston would be on the end of the connecting rod, and the stepper motor would drive the gear. I like that idea.. I only wonder how much water I'd have to move, as it seems like there is limited movement.

what I really need to do is figure out what this thing is going to weigh, and then figure out ballasting. From there I can figure out how much water ill need to move. But I can't figure out what it'll weigh exactly until I figure out a way to operate this.. Catch 22... How would you guys approach this as engineers?

"How would you guys approach this as engineers?"

My approach, and it's the same as for dry-land robots, is to develop some "budgets" (mass, range, speed, load, required functions vs desired functions) (one of yours would be the mass or weight vs available displacement in the specified volume, but another would be equipment volume available vs total volume driving available space for buoyancy). One all the "budgets" are down on paper, with each one rendering a "bottom line", match up the bottom lines and figure out what can be done as desired and what has to be modified to work. Occasionally they all line up, and the machine can be built, but most times this a reiterative process, repeated until you've dropped some items, changed some methods of meeting requirements, found a new type of battery, reworked the budgets again, and finally got everything to match up. It hardly ever goes off the first try without a hitch, and most of the time, once you start building you figure out what you didn't budget right in the first place. But you also realize quickly how valuable that upfront time was, when you see what COULD have gone wrong, and didn't.

That is a bad design to run the threaded rod through the piston, primarily you have the sealing problem, but also, if ever the piston jams on the threaded rod, it will just turn with the rod and not move up or down on it. Your sub may stay submerged for ever.....

The threaded rod needs to be passed through a threaded nut, that has no contact with water, on the "dry" side so to say of the piston. No (little) possibility of debris getting into the thread.

It should be connected to the dry side of the piston using two axial (ball) bearings for both pushing and pulling. Such bearing can be cheaply purchased on ebay (done it for my CNC machine). Easy to use, easy to lubricate and the right ones accept a slight angle on the rod with no trouble, though no angle should ever be other than axial to the travel.

Two nuts, soldered in a tube at the right distance apart to match the threaded rod's thread, can have a grease nipple on the tube for good lubrication. (I do not recommend this as a sealing idea though...!)

The piston needs to be quite long (D x 2 would be a good start) to make sure that it can never jam in the bore, which will also allow an extra "O" ring for good sealing. Use a grease that does not attack "O" ring rubber, or buy rings that can handle normal grease without problems - test to make sure.

Do have an emergency float system, with a long, strong line attached, hide it in the fin. It must be released if any of the following happen, batteries empty, too long under water, too deep, remote control not being received, stuck motor (sense current or blown fuses or both).....I am sure you can think of a few other good reasons to release it too....

That way, you can pull it up again if the worst happens.....plan for that!!

wont rate you GA (?what's the point) // well there is such -- (dad worked as buildings srvicing blah, smth.) -- so in spring the snow (read ice-shelf) melted on a roof taking a layer of water below -- the folkus living on upper floor complained their ceiling getting wet -- so what'hey found was invisibly small hole in roof-foil -- with constant extra fine spray-release of the under-shelf . . . you got two such invisible throughs // i'm not in this business may be there are measures for . . . + and how about the force required to drive that syringe ? how fast your winding thread wears out . . .

The USS Texas is a Virginia class Sub 114.9m(377') long with a beam of 10.5m(34ft) with a displacement of 7900 tonnes. Your approximate model being 6.25ft long 6" beam is roughly scale of 62:1. If you up the scale to 50:1 makes your model 2.29metres long with a beam 0.21metres(210mm≠8") you will have roughly 0.318 cubic metres internal volume if you consider it to be a long cylinder.
The reason for me scaling to 50:1 is to make more room internally for doing stuff.
What is the build budget? You are not going to build this for just a fist full of dollars...
Next what are you going to make the hull from? @50:1 you could use 8" PVC pipe as a base for the hull. or you could use resin coated balsa wood with fibreglass reinforcement. You could also make it from steel tube or fashion it from alloy (Aluminium) plate. Fun costs money how hard so you want to laugh? TTB(time to build) is also a factor which relies on your ability to make it happen.
The USS Texas is a single shrouded screw vessel so you only need 1 drive motor and propeller, your choice whether you replicate the exact prop or find a suitable one from a model shop or perhaps one from one of those electric trolling outboards. Motor? maybe a fan motor from the cooling system of a car or the heater/aircon fan motor from a car wreckers
Batteries, decide what main voltage your going to run, if you go 12V or 24 you can use either "sealed" Gel batteries or you could go for LiPo4 lithium if your looking at keeping the weight down.
Control, Remote controls these days are usually either VHF or UHF neither which work well under water. so you would have to consider a trailing antenna that is suspended on the water surface. you want to be able to effect steering, diving, raising and flood/purging of the ballast tanks as well as forward and reverse. All of these need to be proportional controls.
Onboard automation, SHTF recovery system, Adruino, Rasberry Pi Microcontrollers are readily available.
Ballast tanks, we've already started this discussion. I favour several per side as it would be easier to maintain trim and the boat will behave better/realistically. I also prefer to use compressed air and solenoid control as this to me is a direct analogue for how the actual sub operates.
Think about onboard video, perhaps a couple of cameras mounted in the sail to view forward and aft and perhaps one in the bow or midships pointing down #with illumination# to view what's underneath.
Finally (well not really but I gotta hit the sack) a couple of sounders (ala fish finders) to provide information about the bottom, you will know how deep you are with the sensors I mentioned before.

May I suggest that the pistons inside your cylinders be driven by sliding racks? The pinion that drives a rack in one direction also drives a second rack and piston in the other direction, making the fore-and-aft balancing easier. Indeed, the pinion driving the arrangement on one side could share an axle with the pinion on the other side, all driven by a single motor, so making the lateral balance easier as well.
The only seals you would need would be between piston and cylinder wall.

Seems to me this could and should be simpler. Why not make it a closed system? Take the syringe idea to another level and make a large ( long ) displacement piston. Then pump fluid from one end to the other of the cylinder and as the piston moves, the light weight of the piston will change the centre of gavity of the system. It would be like moving a bubble ( the piston ) from one end to the other. no seals, no screws, fewer potential problems and a longer maintenance cycle. Your fluid would be a lubricant to maintain cylinder and piston life indefinitely, so the only maintenance would be the pump ( barring conflict induced injuries, lol ).

That would help only if the piston movement served to balance the attitude of the model, but not if you wanted to blow or fill the ballast tanks to control the depth.

Just a thought: Could you put the thread on the outside of the cylinder and drive a long nut with the motor. The piston element could be held in one position with no sealing issues. The cylinder would need to have an anti-rotation feature and a flexible connection to the water supply.

Drew