[PSUBS-MAILIST] port grease

[Sean T. Stevenson via Personal_Submersibles]

In general, you want a 64 microinch finish on all window seat bearing surfaces, except for if you are using bearing gaskets in which case 125 microinch surface finish is acceptable where the gaskets are. High vacuum grease, if used, is not intended as a fluid thrust bearing (it is not completely encapsulated by seals), nor as a seal in of itself, but rather to provide thin-film lubrication to enable the acrylic surface to slide against the seat as it deflects under pressure, to prevent localized creep or fretting at the interface surface. As with all thin films, you need add only enough grease to make the surfaces shiny - excess grease will be extruded anyway. Air bubbles are really only a cosmetic problem, provided the encapsulating surfaces surrounding the bubble remain lubricated .  The seal is generally outboard, so it is just the thrust load squeezing the bubbles, and they will either compress and return, or simply be extruded out. You can try assembling the window into the
seat under vacuum to minimize them (no air, no air bubbles), in conjunction with minimizing the amount of grease.

If you have scratches on the bearing surface, the grease only helps insofar as it allows the window surface to slide over the scratch as it moves. A deep / wide scratch may allow the acrylic to creep and locally deform at that location, allowing it to get hung up as the rest of the window moves.  This will probably manifest itself as a particularly creaky window.

Use Dow-Corning High Vacuum Grease, applied under clean conditions to cleaned surfaces using a gloved hand or lint-free applicator, and wipe away excess grease before mating the window.

Sean


On July 20, 2016 10:27:53 AM MDT, hank pronk via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Hi Sean,I have a couple questions.  Can I assume that grease trapped in
>super small scratches in the port seat will carry the load of the
>acrylic.  These are swirls left after sanding and polishing the seat.
> How much grease do I actually put on the seat?  When I grease the seat
>with a very thin layer, air gets trapped in small bubbles even when I
>rotate the port.  I have seen these marks in the grease in pictures of
>other subs with conical ports.  Are those air bubbles a problem or will
>they squeeze out under pressure?Thanks' SeanHank
>
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