[PSUBS-MAILIST] gasket test results

[Sean T. Stevenson via Personal_Submersibles]

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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