Piston Ballast Systems

Hi

The valve is a doddle. Have a look at this..

https://youtu.be/2YsIfgtrXhM

Best John

That is very ingenious, John!

So the ball bearing is functioning as a non return valve and a (low) pressure relief valve? By low pressure I mean a few inches of water. It makes pump selection a doddle.

I wonder if a single one could also be used as a vent (to prevent over pressure) on an air filled bag system?
I confess I have never used a low pressure water pumped system or an airbag (I've only used pressure tank water systems and piston tanks.)but they are often talked about as solutions. But the necessary detail is often glossed over.

Low pressure water tanks + smaller piston tanks do offer benefits (as discussed earlier.)


David

PS I see that Arduinos can be programmed in C. I will give an update on my learning C shortly!

Yes to all of that.. keep in tough with your programming. We should exchange scripts as we go. I am not that good at it but it works so far..

Just an update on the piston tank saga. These are home made ones I did over the course of 2015. The top one is the one used for my Holland and the lower one is used in Resurgam and the Nordenfelts. Both are now pretty much standard designs using the proportional controller but I adjust the dimensions to suit the model.

They use a standard (Nitrile) O ring. The fit is all important and does involve a bit of trial and error. Spin the piston and take small amounts out of the groove with a needle file. My rule of thumb is that you have to be able to move the piston up and down the cylinder BY HAND. (If you can't it is too tight and could stall the motor.)

The basic construction is as made by German modellers. The two photos below show the idea (This tank was originally for Resurgam but was too undersized - err on the large size! This tank uses a silicone O ring - a bit softer and with less friction. Friction seems to be a problem with smaller diameter tanks, for some reason.)

An added thought. Very intersting about orings and pistons. I am in the process of building another one right now. I have found that if you cut the groove too deeply and there is not enough pressure, self sealing butyl tape can be used in the base of the groove under the o ring and keeps the seal nicely.

To get the best level of sealing with the level of friction appropriate for the surface finish of the cylinder, it's quite important to make sure that you cut the groove to the right dimensions as per a proper o-ring chart for dynamic conditions and not static fitment which has a higher % of ''squeeze'' and hence friction. A few minutes google-time will yield plenty of charts to get the right fit.


Trevor.

You are right Trevor. The surprising thing is how wide the groove must be to allow the O ring to "roll."

With my limited machining skills I have great difficulty in turning grooves to accurate dimensions. Any suggestions on how this is best achieved? Can you perhaps make a special tool to do it in one? (I use a narrow parting off tool - but it is not the best.)

David

a quick google search found this....


http://www.applerubber.com/oring-gland-calculator/

Hi

I am doing it now and putting together a video on it.  I think the best way to go is to start by machining a close sliding fit with the cylinder with practically no play.  This takes time and small cuts but is definitely worth it.   Then cut the O ring groove depth using the cross slide to 85% the width of the o ring, and the groove width to 120% width.  Then take a nice cut off the whole diameter to get a looser fit for the end cap..  maybe 5- 10 thousands of an inch.  Then youv'e got it.  The hardest thing to do using this method is to turn the initial sliding fit to work from.  The hardest thing to do is to measure the internal bore accurately as there are 'variables,' and this elimiates this problem.

Cheers

Hi John and a Happy New Year!

Very interesting what you say about starting out with the cylinder a good sliding fit.

I have perhaps been less than scrupulous about this in the past therefore with some rather "random" O ring fits.

The good sliding fit is the starting point for the other dimensions.

David

Here is a start. It worked pretty well

Just showing my growing collection of DIY piston tanks. The latest one is for the Pioneer. Several have been in use over a number of years. DIY does mean that they can be tailored to fit different hulls. Diameter is the key variable but length is also often a problem.





Sorry about the well-painted bins and the tie-wraps but they work!

David

david f wrote:Just showing my growing collection of DIY piston tanks. The latest one is for the Pioneer. Several have been in use over a number of years. DIY does mean that they can be tailored to fit different hulls. Diameter is the key variable but length is also often a problem.





Sorry about the bins!

David

Most instructive, sir.

David

In answer to John Wrennall's questions on Facebook:

SUMMARY OF MY PISTON TANK DESIGNS:
1.5 inch Outside Diameter tank (Pioneer model) uses MFA Como RE-280 motor with 100:1 ratio
2 inch OD tank (Resurgam and Holland models) uses MFA Como RE-280 motor with 30:1 ratio
3 inch OD tank (Charlie, R Class, Type XXI and U Class models) uses MFA Como RE-385 motor with 50:1 ratio.

All use M5 stainless threaded shaft.

Required motor torque depends a lot on the O ring fit. If you can't just push the piston in and out by hand you need to loosen it up.

Motors and gearboxes from MFA Como Drills.(UK supplier but also exporter.)

Gears and ball races from Technobots (UK supplier)