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Author Topic: Piston Tank advice  (Read 13430 times)

mermod

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Piston Tank advice
« on: June 01, 2014, 09:00:04 AM »

I need a bit of advice from some submarine experts, I'm having a go at making my own using gearing from an RC car, so far it works quite well but I have noticed after 20 or so cycles that the threaded rod seems to be getting sharper on the thread even though the action is smooth, also the motor seems to be working harder than I would normally like, the current motor is a 550 with a 19tooth pinion but I am considering using 2 low drain 540 motors and maybe dropping down in pinion size, thoughts?
My other option is to lose the piston tanks and just go for the mini aircompressor type of system although I do like the thought of a tank either end for balance instead of one big one in the centre



Here was my original setup but it was sooooo slow, took a minute and a half to fill but had plenty of power thanks to the gearhead.
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derekwarner

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Re: Piston Tank advice
« Reply #1 on: June 01, 2014, 09:11:52 AM »

mermod....I know zilch about submarines :D ....but ask

1. what is the threaded rod diameter sizing [M10...M12] & material?.....
2. what is the corresponding nut material?
3. what is the ratio of nut length to diameter?
4. if it is Grade 304 on Grade 304........both components are scrap ....anti galling paste is too late for these items
5. a bronze nut with a length ratio of x 3 on the diameter with a new threaded rod will assist with a light lithium based lubricant

Derek
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mermod

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Re: Piston Tank advice
« Reply #2 on: June 01, 2014, 09:46:57 AM »

Hi Derek, I will do my best but I'm just a guy mucking about in the shed.
1. the threaded rod is stainless 12mm unknown grade
2. the nut is about 45mm long and just steel but it has that bronzey look about it
3. nut length to diameter sounds a bit personal  {-)
4. not sure what 304 is and have never heard of anti galling but it sounds painful
5. would lithium grease help with the current materials and is the sharpening effect a normal thing?

I really appreciate your help as I'm feeling a bit out of my depth here (pun intended)

Phill
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Subculture

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Re: Piston Tank advice
« Reply #3 on: June 01, 2014, 11:04:49 AM »

The gearing looks a little tall to me. The previous version had too much gearing by the sounds of it.

The motor you're using looks more than large enough for the application, although the RPM may be higher than optimum. You could perhaps get away with the taller gearing if you used a lower revving motor.

What Derek mentioned is in relation to moving parts using stainless on stainless causing unacceptable wear, and can in fact cause cold welding of parts under extreme cases. You need a sacrificial part, either the nut or the threaded rod, so that one wears whilst the other does not (or wears very slowly). Some of the German modellers recommend using a brass threaded rod, as it's easier to replace than the nut- which usually has to be custom machined.

Anti-galling paste is usually a copper based grease, which effectively puts a coating of softer metal between the surfaces. Of course that only lasts a while as it wears off, so it's probably not a lasting solution , but more the kind of thing for assembly of parts.

Mild steel studding will wear much better than stainless BTW.

mermod

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Re: Piston Tank advice
« Reply #4 on: June 01, 2014, 11:42:46 AM »

Thanks for that Subculture, I think I can get hold of brass threaded rod, might give that a shot and make that the weak point (don't want to re machine that nut) as far as the gearing I might get hold of some smaller pinions and try that as trying to source some decent motor/gearboxes is proving difficult.
What would your thoughts be on scrapping the pistons and going with the tyre inflator type compressor system with a solid wall tank like the old Eden subs? seems pretty bullet proof, keeping in mind my sub is a 1.8 metre typhoon, really shouldn't have started watching  my traplet rc sub DVD should I:)

Phill
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U-33

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Re: Piston Tank advice
« Reply #5 on: June 01, 2014, 12:10:48 PM »

Just a thought...but have you considered using a screenwash pump, filling/emptying a sealed tank? Nothing technical about it, use a servo tripping two microswitches..one to fill the tank, one to empty it.




Rich
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mermod

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Re: Piston Tank advice
« Reply #6 on: June 01, 2014, 12:31:35 PM »

I quite like the idea of a single reversible pump but apparently I need something called a hydraulic valve and I don't know why or where to get one, OTW submarines have a system like that.

Phill
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Re: Piston Tank advice
« Reply #7 on: June 01, 2014, 12:59:52 PM »

I would persevere with the piston tanks, they give the best control of ballast, and no exposed water surface to reduce stability. Looks like you're doing a good job, just needs a little refining here and there.

MFA do a good range of geared motors, and ebay is also a good place to find cheap motor/gearbox combinations, although the latter aren't always as well documented, so you need to know what you're buying.

It's fairly easy to calculate motor wattage and RPM needed for a given filling/emptying time provided you know the piston diameter and pitch of thread, plus the depth you wish to be able to empty your tanks at.

To give an idea of power engel tanks, which use a reasonably sized 70mm piston only need about 12 watts to empty and fill, and that only increases to about 25 watts at about one atmosphere compression. This compares favourably with pump systems in terms of efficiency.

Typical filling time is about 15-20 seconds and they have sufficient reserve to compress to  two atmospheres, although at that pressure they do become very inefficient as you are getting close to stalling the motors.

david48

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Re: Piston Tank advice
« Reply #8 on: June 01, 2014, 01:18:10 PM »

The thread that is for something that has a load to move is usually an acme or butters thread ,have a look at G cramps or screw type car jacks this type of thread won't feather due to the cut angle of the thread.Finding a size to suit your use might be something else.
David.
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derekwarner

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Re: Piston Tank advice
« Reply #9 on: June 01, 2014, 02:07:04 PM »

mermod .... stainless steel blends are many...

1. the family termed as the Grade 300 Series are austenitic  ;) which means non magnetic, generally......this also suggests non rusting
2. the lower number in the Grade 300 family also determines by content <*< being a more easily machineable material
3. you certainly will not find a Grade 300 series length of 12 mm diameter pre machined acme or buttress thread profiles .....

David......these threadform profiles have little to do with load moving or feathering but with compressive strength or load tightening O0

4. 12 mm diameter pre-machined [rolled thread] brass rod is readily available
5. I would prefer not to ask for any further references to your nut length  {-), however suffice to suggest a ratio of 3.75:1 is more than adequate......OK?  O0
6. OK ....one last request........ :embarrassed: place a magnet on your nut  ;) if the nut is not attracted to the magnet....the nut is machined from Grade 30X? material

Derek
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Re: Piston Tank advice
« Reply #10 on: June 01, 2014, 06:39:22 PM »

V-thread stock is higher in friction than square or Acme threads, but it's inexpensive and readily available as studding. All piston tanks I've seen are made using normal v-thread bar. If they wore out with quickly I think their owners would soon be looking for alternatives.

In a nutshell if the thread is wearing out quickly it's either a combination of materials and/or a misalignment in the mechanism resulting in mechanical binding.

Davy1

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Re: Piston Tank advice
« Reply #11 on: June 02, 2014, 10:23:13 AM »

Hi Mermod,

Sometimes Forums can give you too much advice  and I would stick with the approach you are using with the piston tanks. You are going to get there and once you have piston tanks you will not want to go back to any other dive system.

You have already solved some of the most significant problems - the piston and seal and the drive system. (BTW I can now see on your photos that you are using microswitches as limit switches - fine.)

Adding to what we have already  talked about on the AMS Forum, I have been using two home made piston tanks, very similar to your own, for the last 2 years on my subs (last used at Barrow yesterday). Polycarbonate piston, O ring seal, aluminium cylinder. I would recommend a stainless steel shaft but with a brass driver. (All these are pretty much as used by German modellers - the home of piston tanks, really.) I use MFA geared motors on both tanks - they are good.

I would tend to stick with a high gear ratio - a slow piston tank is no huge problem and it makes any later proportional control simpler. That is the only extra refinement I have fitted compared with yours - proportional control, but lots of people use piston tanks without this.

Keep on at it - you are nearly there!

David
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Re: Piston Tank advice
« Reply #12 on: June 02, 2014, 04:44:02 PM »

One thing to watch for is the o-ring compression. Too much increases friction a lot. The general advice is about 10% of the o-ring thickness, and a maximum of 15%. I done some tests of my own and found o-rings are quite a bit higher in friction (more than double) than the pneumatic seals used by Engel and Airzet rings sold by Freudenberg. The snag with the special seals is limited availability and cost. The Engel seals are also rather fat, which increases the length of the tank. Not a major issue in a large boat but could be a problem in smaller vessels.

Seal friction is relatively minor when compared with water pressure however, and it is much alleviated things get wet (behave yourself!).

A good empircal test for your tank is to block off the outlet and run the tank to half full. If it can't get there relatively effortlessly, then it's underpowered or under geared.

mermod

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Re: Piston Tank advice
« Reply #13 on: June 03, 2014, 12:02:51 AM »

I must admit, I cheated when it came to the piston and seal, I had a friend that worked at a hydraulic ram manufacturer a few years back, he took it to their machinist who was ex navy and told him it was for a 1.8m long model military sub, he was more than happy to help :)
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Re: Piston Tank advice
« Reply #14 on: June 03, 2014, 09:45:39 AM »

If your machinist friend sized the groove for hydraulic applications, then it's likely to have too much squeeze on the ring, as hydraulic pressures are typically much higher than the kind of duty our models will require.

mermod

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Re: Piston Tank advice
« Reply #15 on: June 03, 2014, 11:53:02 AM »

I made sure that the guy was aware that I was only using it in water, I had a play with the existing setup today and it seemed to run a lot smoother than the other day, I might get hold of some smaller pinions and try that first before I revert to the slower motor/gearbox setup again.
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Re: Piston Tank advice
« Reply #16 on: June 03, 2014, 12:20:37 PM »

What are the dimensions of the tank e.g. diameter and spindle length?

mermod

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Re: Piston Tank advice
« Reply #17 on: June 04, 2014, 05:09:46 AM »

Derek, I placed a magnet on my nuts today :o and they stick,
Subculture, the note that came with my O rings says BS148 70 DURO, the maximum length of travel, so face of the piston to the back of the end cap is 315mm, the ID of my pipe is 73.74mm and the piston itself is 73.6.
I ordered a couple of sets of pinion gears to play with today so that will be interesting.

Phill
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derekwarner

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Re: Piston Tank advice
« Reply #18 on: June 04, 2014, 05:37:11 AM »

Mermod......

1. the geometry for BS2-148 sounds the best law of fit
2. 2.148 = 69.52 ID with a nominal wall of 2.62 = 74.14 so a crush of 0.2 per side is OK here
3. 70 duro in your application is preferable to the other option of 90 duro  :-))
4. you may well find it advantageous to smear the o-ring with clear Vaseline for improved lubricity
5. at the pressures & speeds you will be using....it will not was off

Derek
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mermod

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Re: Piston Tank advice
« Reply #19 on: June 04, 2014, 07:26:36 AM »

I have been using a product called rubber grease, is that ok for this situation?

Phill
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derekwarner

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Re: Piston Tank advice
« Reply #20 on: June 04, 2014, 08:34:26 AM »

Yes mermod..... :-))
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Re: Piston Tank advice
« Reply #21 on: June 04, 2014, 10:15:14 AM »

Plugged a few numbers into a spreadsheet I made up. This uses the formulas in Norbert Bruggens book.

This is what it spat out. Based on an M12 thread with 1.75mm pitch for the tank to fill in about 18 seconds (about equivalent to engel tanks) you require a motor that can make about 45 watts and which is geared to produce a spindle speed of 600 RPM under load. Such a combination should also enable the tank to empty at a pressure of 0.8 bar or about 12 psi.

The motor shown looks like a Traxxas Titan 550 buggy motor, and I would hazard a guess that it's the 21 turn model. This will make 45 watts easily but will be high revving, so I think you'll be undergeared with any practically sized single stage gearbox.

Looking at your pictures it looks like the present gearing is about 4 or 5:1? I'd say you need more along the lines of 15:1 or possibly higher.

mermod

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Re: Piston Tank advice
« Reply #22 on: June 04, 2014, 11:20:15 AM »

Thanks for that, and I must say, I'm impressed that you deciphered Model Submarine Technology, I gave up and just look at the pretty pictures :)

Phill
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Re: Piston Tank advice
« Reply #23 on: June 04, 2014, 11:39:16 AM »

I understand- I know that book gives people a lot of headaches, but is a valuable reference tool.

The thing that gave me most problems when that book first came out, was knowing where to get stuff.

What's a simmerring I thought, and where do I get one. Internet wasn't much cop back in the mid-nineties. No google then!

The equations were less of a problem, really just Newtonian mechanics, although there are one or two errors in the book which doesn't help.

Once you sort them out however they do make it easier to select appropriate components.

If you're a Facebook member the spreadsheet is available to download on the Dive-in FB page along with lots of other interesting articles and information.

Feedback indicated that it's still too complicated. Oh well, don't think I can't make it any simpler.

mermod

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Re: Piston Tank advice
« Reply #24 on: June 04, 2014, 11:55:46 AM »

I can relate to having trouble sourcing stuff, try living on an island off the coast of Australia, I mostly have to rely on local engineering supplies and just hope they don't ask what its for as I usually get some stupid comment and a snigger, I'm lucky in a way in that I used to run a hobby shop so I know where to get most standard items (hence why I use buggy motors and gearing) but  other than that its the net and I still don't like using credit cards on line (old fashioned I know)
As for equations, I'm a cabinetmaker so I can build you a traditional solid timber roll top desk, do your shopfitting or fit out your kitchen or fishing trawler but don't ask me to do equations.
Oh, and I will be finding the Dive-in FB page asap
Thanks again

Phill
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