[PSUBS-MAILIST] gasket test results

[hank pronk via Personal_Submersibles]

Sean,
I just did a test with a gasket 1/3 the thickness and it worked nicely.  I feel in my case it is important to use a gasket to help protect against point loading.  My feeling was, if the gasket is a bit thicker it would create better load uniformity.
It sounds like that may not be the case.
Hank --------------------------------------------
On Wed, 7/8/15, Sean T. Stevenson via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

 Subject: Re: [PSUBS-MAILIST] gasket test results
 To: "Personal Submersibles General Discussion" <personal_submersibles at psubs.org>
 Received: Wednesday, July 8, 2015, 12:45 PM
 
 This is one reason why the durometer / hardness of
 the gasket material is important - dimensional stability.
 Most elastomers recommended for gasket service exhibit 100%
 volume displacement, meaning whatever volume is lost to the
 compression is displaced / extruded out the gaps. This is in
 contrast to something like cork, which will actually just
 take a (slight) compression set without extruding.  Anyway,
 higher durometer elastomers will retain their shape and
 resist extrusion better, but the tradeoff is their ability
 to fill voids and scratches on the sealing surface. This is
 what lubrication is intended to improve, as by reducing the
 static coefficient of friction at the interface you allow
 the material to find its lowest stress state as it conforms
 to the microscopic geometry. This would be similar to the
 way we lubricate O-rings, I.e., just enough to make them
 shiny. You don't want so much lubricant film thickness
 on your gasket !
  that you
 create hydrodynamic thin-film lubrication of the viewport at
 large, allowing it to easily slide across the bearing
 surface.  Vance mentioned steel bearing surfaces, which
 works because 1) He was operating commercial vehicles with
 windows and window seats machined to exacting tolerances, 2)
 acrylic creeps under high pressures and will conform to
 surface, and 3) lubrication was used to facilitate the
 acrylic / steel interface finding it's low stress state
 at the microscopic level.
 Gasket thickness need only be large enough to
 accommodate the machining tolerance limits, scratches / pits
 on either surface, and differential contraction. They
 prevent local stress concentrations on the acrylic bearing
 surfaces where high spots exist. Gaskets will also provide
 some compliance which increases impact tolerance somewhat,
 but I don't think this is a common design consideration.
 In any case, as gasket thickness increases, so too must its
 hardness for the same dimensional stability. If your gasket
 is moving too much for comfort, make it a bit thinner, or
 the next available hardness.
 Sean
 
 
 On July 8, 2015 6:15:00 AM
 MDT, hank pronk via Personal_Submersibles
 <personal_submersibles at psubs.org> wrote:
 I did my testing this
 morning and found some scary results.  I bonded EPDM gasket
 material to my port test chamber and tested a port to 500
 psi.  The gasket extruded about 1/16 of an inch.  When I
 greased the surface of the gasket and pressured it to 500
 psi, the gasket extruded 1/2 inch.  When the pressure is
 released the gasket returns to its original position.  
 When the same test is done without bonding, I
 get the same result.  
 
 My
 next test will be with the bearing surface machined to
 create tiny ridges for traction.  
 Hank
 
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