[PSUBS-MAILIST] port grease

Thu Jul 21 14:11:56 EDT 2016

The window is not really supported by the grease. As the applied force increases, the pressure within the lubricant film increases, and as this exceeds the viscous friction within the fluid, it will just extrude out the open end until the window is sufficiently supported by the metal of the window seat, or the reduced clearance increases the viscous friction in the fluid to match the applied forces. When you have bubbles, your fluid is a heterogeneous mixture of air and grease, but it is all at the same pressure, so it offers the same support to the window (if the bubbles are not open to atmosphere at the edges). The acrylic doesn't care whether air or grease is providing the support. The window isn't really floating on the fluids though, except possibly at low external pressures where the viscous friction in the grease is sufficient to hold it in place against extrusion. Under higher pressures, the thin film lubrication layer is only nanometres thick where the window is actually
making contact with the peaks of the metal bearing surface, and that contact area increases with increased pressure. Having the grease film just allows the acrylic to deflect radially in response to load, instead of building larger internal stresses near the mating surfaces. You can use such windows without grease at all, but they will have higher friction at the interface, and thus creak more as the higher stresses are built up and relieved when radial forces exceed the coefficient of friction.  With gaskets, there is typically no relative movement at the mating surfaces, and it is the gasket that deforms to accommodate the relative movement.

Sean

On July 21, 2016 9:51:58 AM MDT, hank pronk via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Sean,Thank you, you are the man!  I probably had to much grease, but
>that aside, if there are air bubbles, would that not impact the load
>bearing.  The point location of the bubble will not support the port.
> It is with that logic that I assumed a super fine scratch would not
>impact the load bearing capacity.  Yes I was hoping the grease gods
>would off set my fine scratches.   Hank 
>
>On Thursday, July 21, 2016 9:39 AM, Sean T. Stevenson via
>Personal_Submersibles <personal_submersibles at psubs.org> wrote:
> 
>
>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. Youcan
>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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