Re: [PSUBS-MAILIST] O rings and pressure

["Brian Cox" <ojaibees@ojai.net>]

  O rings:

     Looks like we may have a debate going here !   Ok, I can see not wanting to put a groove in the shaft and there by possibly disrupting the integrity of that shaft, and a larger shaft would result in more pressure. 

 

Pressure:

        Of  course you will need air supply for life support so you can't get around having those air lines to deal with inside the sub.

 

         I had planned on not having any actual pressure lines (for ballast) coming into the sub at all, that's what the thru hull shafts are for.  However I do need to look at a pressure gauge and unless that is located outside and visible from my viewport then I would have to bring a HP line into the sub to look at the gauge.  But for ballast air it seems like it would be unnecessary to regulate that air down to a lower pressure, the regulator would be just one more thing to go wrong. However, If you are blowing your hard ballast with 2000+ air you better make sure your water exit valve is open or you would rupture your ballast tank. Also the pressure in that ballast tank is going to climb very fast depending on the size of your water exit valve. so maybe lower pressure would be better.

 

 

Brian

----- Original Message -----

From: Dan H.

To: personal_submersibles@psubs.org

Sent: Thursday, November 20, 2003 5:29 AM

Subject: Re: [PSUBS-MAILIST] O rings and pressure

Brian,

 

I see you already have good recommendations for some O-ring information sites.  I would like to comment on some of the other recommendations you were given though. 

 

True, many times an O-ring is installed in the shaft rather then the housing.  As was mentioned, It reduces the effective diameter of the shaft though and caused critical stress points because of the nature of the O-ring groove itself.  Also, if you require a .500 inch diameter shaft for the torque your applying and groove the shaft, you have to use a shaft of maybe .625 or .750 inch to have .500 where the groove is.  The larger shaft size is like a piston in your hull.  With the water pressure on the outside and ambient pressure in the inside, the larger shaft diameter has greater longitudinal forces on it and will be harder to rotate at depth.

 

Machining internal O-ring grooves is more difficult then machining grooves on a shaft but polishing a bored cylinder is more difficult then simply buffing the shaft to a smooth finish.  

 

For most through hull applications put the grooves in the housing not the shaft.

 

I also think you should use a high pressure regulator on your air supply.  If find a time when you need 2000 pounds pressure to blow ballast, your in deep ( I wasn't thinking water here).   Regulate your air pressure down so you can blow ballast at depth with some extra pressure for a safety factor.  The HP air won't freeze since the majority of the moisture is removed if you have them filled at a dive shop.  But you just don't need to work with that high pressure in a P-sub. 

 

Dan H.

 

----- Original Message -----

From: Brian Cox

To: Personal_Submersibles@psubs.org

Sent: Wednesday, November 19, 2003 10:07 PM

Subject: [PSUBS-MAILIST] O rings and pressure

Hi,   I have a dumb question,  I just machined my first sub part, sort of a test part, a thru hull piece that will have a shaft going thru it to turn a valve from the inside of the sub to the out side.  I machined an inside groove for an "o" ring,  now are there  recomended specs for this "o" ring ?  Like so many thousandths thickness etc..   and should the shaft be notched as well? 

 

Second dumb Question:  Is all of your air pressure comming off the high pressure cylinders (2000 psi) always dropped down to a lower pressure (200 psi)  with a regulator before it is used to blow out a ballast tank?  Is that high pressure too high to use for that (possiblly freezing up lines?)

 

Brian Cox