Dragon Shark - a new start

[IdefixRC]

With the first step of the end caps done and while waiting for the piston tank O-rings, some additional work done on the PC.
Servo tray and pushrod seals in place, first attempt on the piston tank with dual gear train to get a 1:22 gear ratio for the Speed380 motor I'm going to use.
I will try and 3D print the gears first and have selected larger teeth (module 1) due to that.

[IdefixRC]

Some progress on the WTC. Rear cap is basically done with the Motor, servo tray and servos installed.

[IdefixRC]

Exciting moment...... The piston tank design is ready. At least on the PC it looks like it will work but then I have no idea if the 1:16 gear ration will work out and be able to move the piston......let's see how it will turn out.
Time to make some "chips" and find out :-)

[Guest]

You don't have to guess, you can work it out with a few, fairly simple calculations. These are presented in Norbert Bruggen's Model submarine technology book, but you can also calculate it using mechanical science formulae. Norberts book makes it a lot easier though.

74mm piston is quite a big piston, I would say that 16:1 is too fast for a 380. Assuming it's a 6 or 7.2 volt model rather than a 12v 385, as they tend to wind about 16000RPM unloaded, and the tank will pull a reasonable amperage. I would personally aim for more reduction. 35:1 would give you a dive time of about 15 seconds (assuming a M8 thread with 1.25mm thread pitch and 16000RPM motor), which is fine, and will lower the stress on the motor and electronics. The lower transit speed will also help dial in the feedback on any proportional controller.

[IdefixRC]

let's see what it does. I'm struggling with higher gear ratios as this means finer gears which I can not manufacture at the moment.
If it fails I will see if I can use a larger, lower rpm motor or go with a pre-made gearbox for it.

[Guest]

let's see what it does. I'm struggling with higher gear ratios as this means finer gears which I can not manufacture at the moment.
If it fails I will see if I can use a larger, lower rpm motor or go with a pre-made gearbox for it.

The simple way to go is to get a geared motor on ebay. They're cheap and plentiful, and the gearboxes are metal, so very strong. Then all you need is a single stage of gears to transfer to the jackscrew.

A simple way to test the strength of the tank, is to cap off the end, and run it, compressing the air inside. The motor should be able to fill the tank about half way without stalling for it to have a good safety factor.

[IdefixRC]

Thanks. Will give it a try.

[Guest]

Something like these work well, and not expensive.

One snag is that most of the larger geared motors tend to require 12volt or higher. Not so good for lower voltage set-ups.

http://www.ebay.co.uk/itm/zhengk-12V-DC-300-RPM-37GB-High-Torque-Gearbox-Electric-Motor-37mm-Diameter-BF/252647898648?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D2220072%26algo%3DSIC.MBE%26ao%3D2%26asc%3D43781%26meid%3D79adee8f97d14bb594c98d99320de7cf%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D331084881408

[IdefixRC]

Thanks. I have ordered a 6V and a 12V version to give it a try.
Now for the progress on the Piston tank.......drum rolls...... it is done.




As you correctly pointed out, it is quick. Very quick
The compression test you suggested manages to move the piston to the 40% position before the gears slip (on the shaft - which is a good thing instead of stripping)
Compression is very good (did the test before I had the metal brackets in place and took quite a bit of my own weight to keep the caps from popping off).

After that tested it in water. No leaks. Works both ways.

I plan to address the speed issues with the arduino and the H-Bridge to see if I can dial the voltage down to make it less "speedy".
If that fails I will bolt the - to arrive in the future - planetary geared motor on and use that one instead.

But always important to have plan C ready: as you can see from the video below, if it does not work for the sub it can still be used as a high tech water gun......or if that fails.....to wake the dead

Short video of the test run (tune you audio down)

[David F]

There will always be an element of "trial and error" for piston tank drive, in my experience, because calculation is only very approximate.

For example, the "fit" of the O ring has a big influence on the motor amps. You are better off going with quite a high gear ratio, perhaps at the expense of diving time, in my opinion.

David

PS Just seen the video in your simultaneous post. Looks to work nicely but you could maybe make it a bit slower.

[IdefixRC]

PS Just seen the video in your simultaneous post. Looks to work nicely but you could maybe make it a bit slower.

yes it is a bit fast (and noisy )

[Guest]

As you're 3d printing the gears, how about trying some herringbone pattern types? They'll be a lot quieter and stronger too.

[IdefixRC]

Strange, somehow my posts are not showing up.
Try again: I went with ready made gears from a diy robot shop as the 3D printed ones did not hold up.
Good news is I found some other gears now from an rc car which I will try. They have 48pitch which will increase my gear ratio to 1:27 to 1:40 (depending on how i manage to install them later).

We are getting there :-)

Thinking ahead... I see that the chinese boats have a red/black or gray color scheme. Looked around a bit but could not find the exact color description.
Anyone has a tip? Thanks

[Guest]

Well it's not a scale boat, so paint it any scheme you like most. One thing worth noting is that grey can really disappear well in a swimming pool.

Car gears will be good, they'll probabyl be moulded from POM or Nylon, both which will stand up well to the rigours of use. Wouldn't worry too much about noise, once in the water, it will damp down a lot- you'll only upset the fish!

The quietest piston tanks I ever saw (heard?) were ones made by Ron Perrott. They used two stages of toothed belt drive instead of gears. However they were tricky to set-up, and Ron no longer uses belts on his tanks preferring helical gears.

[warpatroller]

That is one thing I have noticed over the years about RC subs, they are much NOISIER for their size, than the full size subs they are modeled after. These models are not what I would call stealthy.. That said, will my models be stealthy and quiet underwater, probably not.. If my father were still around, to help me, they just might, as he was a mechanical engineer, Noise and Vibration expert, for the US Navy..