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Re: [PSUBS-MAILIST] No Machining Hatch Sealing Concept



Thanks Chs Hugh
Glen SA
----- Original Message -----
Sent: Sunday, September 06, 2009 9:20 PM
Subject: RE: [PSUBS-MAILIST] No Machining Hatch Sealing Concept

Glen,  Tried and true.  Follow the plans.  Everything is there for a reason.  You will never wear it out from opening and closing the hatch and diving.  You may damage it from dirt or UV or just ageing.  The Captain put it in to retain the ring, so you stretch the o?ring slightly over the dovetail. A bit of grease also helps to retain O?rings, however it also attracts dirt.  I have studied his plans and his genius is in his simplicity. ( I always complicate things).  IT can be argued that you can use a slightly oversize ring with an external dovetail but if you get it wrong you squash it in here and it pops out somewhere else.  Phil Nuytten has a very nice little addition which is a plastic cap to cover the hatch face for when the hatch is open to stop damage from people climbing in or when it is being serviced.  Chs Hugh

From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of glen brown
Sent: 06 September 2009 23:51
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] No Machining Hatch Sealing Concept

 

Hi Hugh

Thanks for the info .Would you advise as per the K350 plans the hatch seal groove which is a half dovetail groove to have the dovetail side of the groove on the water  pressure side or as the plan stipulates on the low pressure side which could cause (I think)more ware on the O-Ring due to the sharper corner.

All the best

GlenSA

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

From: Hugh Fulton

Sent: Sunday, September 06, 2009 8:00 AM

Subject: RE: [PSUBS-MAILIST] No Machining Hatch Sealing Concept

 

Just a clarification with Seals and O?rings etc.   On a hatch don?t use anything that is new and unproven.   Sorry Brent.  Same goes for Domes. Sorry Alan.

When looking at any Seals book you see O?ring grooves with Corner radiuses and tapers.  A common fallacy is that the bottom of the groove needs to be radiused.  Not so.  These tolerances etc were done before tip tools were invented.  The angle which states zero to 5 degrees, or some such, is because inspection would reject an otherwise acceptable groove because of slight machining anomalies.  An industrial acceptable groove must have a tolerance on it.  This should allow for slight tool wear which will occur on corners of the older used high speed steel ( HSS ) and also for slightly inaccurate tool setting. ( How do you set a tool so it is guaranteed parallel to zero tolerance.?)  When a turner is grinding a tool for making an O?ring groove they will do so typically on a grindstone.  Not all machine shops have Tool grinders.  Now they are almost a thing of the past with tip tools and specialist Tool sharpening services available.  The Taper also does not need to be there.  It shows a maximum of 5 degrees and this is not a target.  It is a tolerance. You can?t get a negative angle in a lathe on a groove unless you have tool bend (which can happen) or a cross slide out of tolerance unless you deliberately want one and then of course it can be machined.  (Cross slide angle to bed has a tolerance as well and should have a concave tolerance but not convex tolerance) The angle has no effect.  There is an exception. You can have negative angles such as a dovetail groove for O?ring retention without any adverse effect.  E.g. Kittredge hatch.  Corner O?rings and machining are OK and that is a 45 degree angle.  The important things are clearance, squeeze and surface finish.  Metal to metal is the most desirable with an elastomer as the bubble tight backup.   IF there is a clearance such as a piston to cylinder type of seal with an O?ring then the corners should be ?broken? with a small radius so as not to cut the O?ring when it squeezes between the diameters.  When machining it is normal practice to ?break all corners/edges? . In relation to the bottom corners and radii, if a tool has sharp corners they won?t last as long as it cannot get rid of the heat so tips are made, or ground, with a built in radius.  Heat removal, longevity and aiding surface finish.  It is one thing to machine a groove, it is another to machine high production items with a groove to a tolerance and surface finish. Swarf can damage surface finish. Swarf control is addressed in the modern tip designs.  It is not necessary to have a radius in the bottom of a groove.  Just pay attention to breaking the edges without too much radius.  Radii in corners are a good thing though to eliminate stress multipliers.

 

Corner O?rings.  The books give dimensions which are OK if the fluid can energise them but in low pressure or pulsation applications the sizes should be amended.  Normal O?ring to cavity ratio is 80% but for a pulsing situation or low pressure situation then we find 90% is a better ratio.   O?ring hardness and surface finish equals friction.  Harder seals for greater pressure but for most the standard 70 durometer is more than adequate.

 

Therefore the hatch situation is a typical static seal application.  Tried and proven!   It will cost very little extra to put in an O?ring groove as opposed to machining it flat and then adding other ?inventions?. 

 

I have had supposedly experienced people try and understand o?ring grooves and the machining tolerances and wrongly interpret them.  I resigned off a standards committee because a bunch of people sat and listened to a ?would be engineer? who got up and lectured on O?ring grooves and drew a round bottomed groove!!, and the others all listened to him.  They even put it in the first issue of the standard!!  Chs, Hugh

 

From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of ShellyDalg@aol.com
Sent: 05 September 2009 03:17
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] No Machining Hatch Sealing Concept

 

Hi Brent. Nice drawings.

A couple of thoughts though.

This rendering shows basically a flat gasket trapped between two ridges. It won't operate like an "O" ring. It operates like a flat gasket. It would probably seal at low pressure but loses it's sealing capacity at greater pressures.

If a guy wanted to weld rings instead of cutting grooves ( for an "O" ring ) it seems it would be a lot easier to use square stock. The grooves specified by the Parker "O" ring book call for rounded corners at the bottom. A TIG weld would give you that radius. I considered that option when first building my hatch. I just didn't think it would be nearly as strong as a machined groove. For the $300 cost of the machining versus buying, welding, and other work associated with the welded rings approach, It's pretty cheap for a nice clean means of sealing the hatch.

I don't think the mating surfaces need to be perfect, as the "O" ring will seal up a fairly large crack, and as the pressure increases the crack gets smaller. Eventually the two mating surfaces are touching and the "O" ring is forced into an almost rectangle shape. It still seals.

I think it's pretty well established that an "O" ring is the only way to go, but it's good to keep looking at different ways to skin a cat.

Frank D.

 




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