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Re: SCT OTA lubrication

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  • grouptele
    Bob and other fellow members, Thanks for the helpful feedback. First-hand recommendation for the lubricant is indeed welcomed. For now, the re-distribution of
    Message 1 of 20 , Aug 3 3:20 PM
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      Bob and other fellow members,

      Thanks for the helpful feedback. First-hand recommendation for the
      lubricant is indeed welcomed.

      For now, the re-distribution of the oil film seems to solved the
      issues; thus, I will await to see if it a long term solution.

      In the mean time, I would like to pose two (2) more questions:

      > When I was moving the mirror back/forth, it felt like the mirror
      guide tube was riding an a finely threaded surface. This effect is
      only noticeable when moving the mirror by hand at moderate speed, as
      the movement available with the focusing knob [thread pitch] is
      naturally much slower. Is such surface intentionally threaded or
      machine with a lead?

      > If/when I decide re-grease the tube on my own, how is the mirror and
      its mounting tube retrieved? When I was moving the mirror between its
      stops (I had removed the focuser bearing locking plate), the assembly
      would not dismount from the guide tube. Is there a retainer that
      prevents mirror overtravel?

      Victor.



      --- In sct-user@yahoogroups.com, Robert Piekiel <piekielrl@...> wrote:
      >
      > Hi Victor, I've never experienced "lube migration" either, as you
      call it, and it's surprising that you have it on a 2-month-old scope. I
      have 40-year-old Celestrons that are still operating fine, but when
      I've taken them apart for "maintenance" I ended up CLEANING the sliding
      mirror tube. If the fit is precise, it should work without any lube at
      all. If the fit is a bit looser, you'll need some lithium grease or
      something that doesn't creep. Grease will dry up over a period of years
      in extreme temps, so if you resort to lubing it, you may need to repeat
      it every few years. Good luck,
      > Bob Piekiel
      > author of "Celestron The Early Years."
    • Robert Piekiel
      Hi Victor, as to the threads on the baffle tube, they are just part of the fine-machining process that is done on the lathe when it is cutting in the
      Message 2 of 20 , Aug 3 3:29 PM
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        Hi Victor, as to the "threads" on the baffle tube, they are just part of the fine-machining process that is done on the lathe when it is cutting in the auto-feed mode. The tool puts many fine spiral grooves in the metal so it can feel like it has many fine threads if you push it a certain speed.

        As to removing the sliding tube from the baffle tube, if you've completely disconnected the focus spindle knob (bearings, nut(s) and everything else from outside the back of the tube), there may be a snap ring near the front edge of the baffle tube that keeps the mirror from sliding off. I know they did this with the earlier models, but haven't taken one apart like your newer C8. If you spot or feel this snap ring, just pry/pull it off and the mirror tube should slide right off the baffle tube.
        Take care, Bob


        --- On Sun, 8/3/08, grouptele <grouptele@...> wrote:

        > From: grouptele <grouptele@...>
        > Subject: [sct-user] Re: SCT OTA lubrication
        > To: sct-user@yahoogroups.com
        > Date: Sunday, August 3, 2008, 6:20 PM
        > Bob and other fellow members,
        >
        > Thanks for the helpful feedback. First-hand recommendation
        > for the
        > lubricant is indeed welcomed.
        >
        > For now, the re-distribution of the oil film seems to
        > solved the
        > issues; thus, I will await to see if it a long term
        > solution.
        >
        > In the mean time, I would like to pose two (2) more
        > questions:
        >
        > > When I was moving the mirror back/forth, it felt like
        > the mirror
        > guide tube was riding an a finely threaded surface. This
        > effect is
        > only noticeable when moving the mirror by hand at moderate
        > speed, as
        > the movement available with the focusing knob [thread
        > pitch] is
        > naturally much slower. Is such surface intentionally
        > threaded or
        > machine with a lead?
        >
        > > If/when I decide re-grease the tube on my own, how is
        > the mirror and
        > its mounting tube retrieved? When I was moving the mirror
        > between its
        > stops (I had removed the focuser bearing locking plate),
        > the assembly
        > would not dismount from the guide tube. Is there a
        > retainer that
        > prevents mirror overtravel?
        >
        > Victor.
        >
        >
        >
        > --- In sct-user@yahoogroups.com, Robert Piekiel
        > <piekielrl@...> wrote:
        > >
        > > Hi Victor, I've never experienced "lube
        > migration" either, as you
        > call it, and it's surprising that you have it on a
        > 2-month-old scope. I
        > have 40-year-old Celestrons that are still operating fine,
        > but when
        > I've taken them apart for "maintenance" I
        > ended up CLEANING the sliding
        > mirror tube. If the fit is precise, it should work without
        > any lube at
        > all. If the fit is a bit looser, you'll need some
        > lithium grease or
        > something that doesn't creep. Grease will dry up over a
        > period of years
        > in extreme temps, so if you resort to lubing it, you may
        > need to repeat
        > it every few years. Good luck,
        > > Bob Piekiel
        > > author of "Celestron The Early Years."
        >
        >
        >
        > ------------------------------------
        >
        > Visit the sct-user home page at:
        >
        >
        >
        > http://members.aol.com/RMOLLISE/index4.htmlYahoo! Groups
        > Links
        >
        >
        >
      • rbethman
        Victor, Don t even give a thought to the VERY shallow grooves in the baffle tube. That is a result of the manufacturing method. Now - as to mirror removal: 1)
        Message 3 of 20 , Aug 3 3:55 PM
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          Victor,

          Don't even give a thought to the VERY shallow grooves in the baffle
          tube. That is a result of the manufacturing method.

          Now - as to mirror removal:

          1) It is NOT for the fumble fingered and/or faint of heart.

          2) The corrector MUST be removed from the front and placed in a very
          safe place.

          3) With the screws around the focuser knob REMOVED, press this forward
          INTO the OTA housing.

          4) The mirror can be CAREFULLY grasped at the outer edge and slid
          forward until it comes off the baffle - AFTER the focus arm is released
          from the focusing assembly.

          It REALLY sounds worse than what it really is. The corrector has a mark
          along its edge to put it back in position in relation to the primary.

          Ask Thad!

          Bob - N0DGN

          grouptele wrote:
          >
          > Bob and other fellow members,
          >
          > Thanks for the helpful feedback. First-hand recommendation for the
          > lubricant is indeed welcomed.
          >
          > For now, the re-distribution of the oil film seems to solved the
          > issues; thus, I will await to see if it a long term solution.
          >
          > In the mean time, I would like to pose two (2) more questions:
          >
          > > When I was moving the mirror back/forth, it felt like the mirror
          > guide tube was riding an a finely threaded surface. This effect is
          > only noticeable when moving the mirror by hand at moderate speed, as
          > the movement available with the focusing knob [thread pitch] is
          > naturally much slower. Is such surface intentionally threaded or
          > machine with a lead?
          >
          > > If/when I decide re-grease the tube on my own, how is the mirror and
          > its mounting tube retrieved? When I was moving the mirror between its
          > stops (I had removed the focuser bearing locking plate), the assembly
          > would not dismount from the guide tube. Is there a retainer that
          > prevents mirror overtravel?
          >
          > Victor.
          >
          > --- In sct-user@yahoogroups.com <mailto:sct-user%40yahoogroups.com>,
          > Robert Piekiel <piekielrl@...> wrote:
          > >
          > > Hi Victor, I've never experienced "lube migration" either, as you
          > call it, and it's surprising that you have it on a 2-month-old scope. I
          > have 40-year-old Celestrons that are still operating fine, but when
          > I've taken them apart for "maintenance" I ended up CLEANING the sliding
          > mirror tube. If the fit is precise, it should work without any lube at
          > all. If the fit is a bit looser, you'll need some lithium grease or
          > something that doesn't creep. Grease will dry up over a period of years
          > in extreme temps, so if you resort to lubing it, you may need to repeat
          > it every few years. Good luck,
          > > Bob Piekiel
          > > author of "Celestron The Early Years."
          >
          >

          --
          Bob - NØDGN



          [Non-text portions of this message have been removed]
        • P T Chambers
          I agree with Thad. The DC Hi Vac is intended for sealing interfaces in a high vacuum situation where outgassing would compromise the vacuum/experiment. Itt
          Message 4 of 20 , Aug 3 4:07 PM
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            I agree with Thad.

            The DC Hi Vac is intended for sealing interfaces in a high vacuum
            situation where outgassing would compromise the vacuum/experiment. Itt
            just does not outgas.

            It does have a silicon content and the worries about silicon are real.
            However, it exhibits no "creep" like some silicon products so it is safe
            with the notable exception that if you get it on your hands and touch the
            mirror/glass all bets are off. Best to use cotton/thin rubber gloves
            that you can toss out afterwards.

            It also has a huge temperature range without much of a change in viscosity
            and will stay where you put it.

            I am sure there are other alternatives but most of the greases I use on
            other stuff I can smell so I dont trust them as much. By the way, the
            DC Hi Vac is not my choice of grease for other areas of the scope. Wrong
            stuff. I use white lithium for most other applications.

            Not a "cult" exactly as there are real reasons for using the stuff. That
            said, it is hard to get the "right" amount on the primary baffle. Too
            much and your primary "oozes" into position which is not good. Too
            little and it doesnt do much.

            ---------
            Phil Chambers [ptchamb-at-svpal.org] (S.F. Bay Area - Calif. USA)

            On Sun, 3 Aug 2008, Thad Floryan wrote:

            > --- In sct-user@yahoogroups.com, "Dennis Persyk" <dpersyk@...> wrote:
            >>
            >> I agree with Greg.
            >>
            >> The cult following of DC High Vacuum grease stems from concerns that
            >> ordinary hydrocarbon greases would "outgas", and thus coat the inner
            >> optics with a thin grease layer. There are no reported cases of
            >> this ever occurring.
            >
            > If the grease has an odor, it's outgasing (as do most of the so-called
            > recommended greases); simple to test -- just smell it. :-)
            >
            >
            >> DC High Vacuum grease is an expensive alternative.
            >> It does have the requisite high viscosity, high adherence, and
            >> exhibits little creep. So do more common bearing greases.
            >
            > Which outgas similar to how such stuff outgases in cars and forms
            > a coating on the inside surface of the windshield and windows.
            >
            > I have two tubes of the Dow Corning High Vacuum Grease and there is
            > absolutely NO perceptible odor -- it doesn't outgas.
            >
            >
            >> Roland is a pretty knowledgeable glass pusher, so I tend to take his
            >> warnings seriously. Silicone fluid has a very high affinity for the
            >> silica in glass, and I believe that his concern is that silicone
            >> fluid could permeate the AR coating and undermine it. Again, no
            >> reported cases.
            >
            > And thus no need for concern unless one applies it (grease) sloppily
            > and dribbles it onto the primary and/or corrector.
            >
            >
            >
            > ------------------------------------
            >
            > Visit the sct-user home page at:
            >
            >
            >
            > http://members.aol.com/RMOLLISE/index4.htmlYahoo! Groups Links
            >
            >
            >
            >
          • gnowellsct
            Victor: This photo-essay should answer most of your questions about how it all works. regards Greg N
            Message 5 of 20 , Aug 3 6:04 PM
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              Victor:

              This photo-essay should answer most of your questions about how it all
              works.

              regards
              Greg N


              --- In sct-user@yahoogroups.com, "grouptele" <grouptele@...> wrote:
              >
              > Bob and other fellow members,
              >
              > Thanks for the helpful feedback. First-hand recommendation for the
              > lubricant is indeed welcomed.
              >
              > For now, the re-distribution of the oil film seems to solved the
              > issues; thus, I will await to see if it a long term solution.
              >
              > In the mean time, I would like to pose two (2) more questions:
              >
              > > When I was moving the mirror back/forth, it felt like the mirror
              > guide tube was riding an a finely threaded surface. This effect is
              > only noticeable when moving the mirror by hand at moderate speed, as
              > the movement available with the focusing knob [thread pitch] is
              > naturally much slower. Is such surface intentionally threaded or
              > machine with a lead?
              >
              > > If/when I decide re-grease the tube on my own, how is the mirror and
              > its mounting tube retrieved? When I was moving the mirror between its
              > stops (I had removed the focuser bearing locking plate), the assembly
              > would not dismount from the guide tube. Is there a retainer that
              > prevents mirror overtravel?
              >
              > Victor.
              >
            • gnowellsct
              http://www.astromart.com/articles/article.asp?article_id=594 whoops forgot the URL ... and ... its
              Message 6 of 20 , Aug 3 6:05 PM
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                http://www.astromart.com/articles/article.asp?article_id=594

                whoops forgot the URL

                --- In sct-user@yahoogroups.com, "gnowellsct" <tim71pos@...> wrote:
                >
                > Victor:
                >
                > This photo-essay should answer most of your questions about how it all
                > works.
                >
                > regards
                > Greg N
                >
                >
                > --- In sct-user@yahoogroups.com, "grouptele" <grouptele@> wrote:
                > >
                > > Bob and other fellow members,
                > >
                > > Thanks for the helpful feedback. First-hand recommendation for the
                > > lubricant is indeed welcomed.
                > >
                > > For now, the re-distribution of the oil film seems to solved the
                > > issues; thus, I will await to see if it a long term solution.
                > >
                > > In the mean time, I would like to pose two (2) more questions:
                > >
                > > > When I was moving the mirror back/forth, it felt like the mirror
                > > guide tube was riding an a finely threaded surface. This effect is
                > > only noticeable when moving the mirror by hand at moderate speed, as
                > > the movement available with the focusing knob [thread pitch] is
                > > naturally much slower. Is such surface intentionally threaded or
                > > machine with a lead?
                > >
                > > > If/when I decide re-grease the tube on my own, how is the mirror
                and
                > > its mounting tube retrieved? When I was moving the mirror between
                its
                > > stops (I had removed the focuser bearing locking plate), the assembly
                > > would not dismount from the guide tube. Is there a retainer that
                > > prevents mirror overtravel?
                > >
                > > Victor.
                > >
                >
              • Thad Floryan
                ... Heh, oozes is a very apropos description. :-) I first did my 8 LX200 and had too much on the baffle tube. Took the primary sled a l-o-n-g time to
                Message 7 of 20 , Aug 3 6:08 PM
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                  --- In sct-user@yahoogroups.com, P T Chambers <ptchamb@...> wrote:
                  > [...]
                  > Not a "cult" exactly as there are real reasons for using the stuff.
                  > That said, it is hard to get the "right" amount on the primary
                  > baffle. Too much and your primary "oozes" into position which is
                  > not good. Too little and it doesnt do much.

                  Heh, "oozes" is a very apropos description. :-)

                  I first did my 8" LX200 and had too much on the baffle tube. Took
                  the primary sled a l-o-n-g time to settle downwards by itself. Well,
                  that's how we learn.

                  :-)
                • grouptele
                  Greg N and others, Excellent feedback. All clear now. Good hunting, Victor. ... it all ... the ... the ... mirror ... effect is ... speed, as ... or ...
                  Message 8 of 20 , Aug 3 6:10 PM
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                    Greg N and others,

                    Excellent feedback. All clear now.

                    Good hunting,
                    Victor.


                    --- In sct-user@yahoogroups.com, "gnowellsct" <tim71pos@...> wrote:
                    >
                    > http://www.astromart.com/articles/article.asp?article_id=594
                    >
                    > whoops forgot the URL
                    >
                    > --- In sct-user@yahoogroups.com, "gnowellsct" <tim71pos@> wrote:
                    > >
                    > > Victor:
                    > >
                    > > This photo-essay should answer most of your questions about how
                    it all
                    > > works.
                    > >
                    > > regards
                    > > Greg N
                    > >
                    > >
                    > > --- In sct-user@yahoogroups.com, "grouptele" <grouptele@> wrote:
                    > > >
                    > > > Bob and other fellow members,
                    > > >
                    > > > Thanks for the helpful feedback. First-hand recommendation for
                    the
                    > > > lubricant is indeed welcomed.
                    > > >
                    > > > For now, the re-distribution of the oil film seems to solved
                    the
                    > > > issues; thus, I will await to see if it a long term solution.
                    > > >
                    > > > In the mean time, I would like to pose two (2) more questions:
                    > > >
                    > > > > When I was moving the mirror back/forth, it felt like the
                    mirror
                    > > > guide tube was riding an a finely threaded surface. This
                    effect is
                    > > > only noticeable when moving the mirror by hand at moderate
                    speed, as
                    > > > the movement available with the focusing knob [thread pitch] is
                    > > > naturally much slower. Is such surface intentionally threaded
                    or
                    > > > machine with a lead?
                    > > >
                    > > > > If/when I decide re-grease the tube on my own, how is the
                    mirror
                    > and
                    > > > its mounting tube retrieved? When I was moving the mirror
                    between
                    > its
                    > > > stops (I had removed the focuser bearing locking plate), the
                    assembly
                    > > > would not dismount from the guide tube. Is there a retainer
                    that
                    > > > prevents mirror overtravel?
                    > > >
                    > > > Victor.
                    > > >
                    > >
                    >
                  • gnowellsct
                    Well it s certainly got a large number of adherents on this group and that is why I forayed into Dow Corning. Precisely because of the wise folks here who
                    Message 9 of 20 , Aug 3 6:12 PM
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                      Well it's certainly got a large number of adherents on this group and
                      that is why I forayed into Dow Corning. Precisely because of the wise
                      folks here who advocated it. The number of enthusiasts for DC High
                      Vac over on C14 seems to be lower. Since many greases work, the only
                      question is which works best; we know that Celestron does *not* use DC
                      High Vac, and we know that outgassing is not an issue for its optics
                      in tubes that are thirty and forty years old.

                      But as I say, different strokes for different folks. My experiments
                      with the c8 established to my satisfaction that I wouldn't be putting
                      DC High Vac on teh c14 any time soon.

                      You know, it ain't like these tubes are airtight in any case. There
                      is some circulation in there....for better or for worse.

                      regards
                      Greg N


                      --- In sct-user@yahoogroups.com, rbethman <rbethman@...> wrote:
                      >
                      > Amen!
                      >
                      > I do NOT do cults! I DO use the DC grease. It's been in there for
                      over
                      > four years!
                      >
                      > No migration, no discernible mirror shift.
                      >
                      > Best thing made for SCTs!
                      >
                      > Bob - N0DGN
                      >
                      > Chris Peterson wrote:
                      > >
                      > > "Cult" is a bit strong! <g>
                      > >
                      > > What is special about the DC grease is that it is a true gel. I don't
                      > > know
                      > > of any other greases that have that property. It doesn't exhibit
                      "little"
                      > > creep, it exhibits _no_ creep. It also has near constant viscosity
                      over a
                      > > huge temperature range.
                      > >
                      > > The trick to using it to manage mirror shift is to apply a very thin
                      > > film-
                      > > just a few thousandths thick. Unless you do something very wrong, the
                      > > stuff
                      > > never comes anywhere near your optics, so I don't think that's much
                      > > concern.
                      > > FWIW, there are good solvents for silicone oils. Most are
                      chlorinated or
                      > > fluorinated hydrocarbons, and can only be obtained from industrial
                      > > chemical
                      > > suppliers. But they will completely rinse silicone oils from glass
                      and
                      > > most
                      > > plastics.
                      > >
                      > > Chris
                      > >
                      > > *****************************************
                      > > Chris L Peterson
                      > > Cloudbait Observatory
                      > > http://www.cloudbait.com <http://www.cloudbait.com>
                      > >
                      > > ----- Original Message -----
                      > > From: "Dennis Persyk" <dpersyk@... <mailto:dpersyk%40att.net>>
                      > > To: <sct-user@yahoogroups.com <mailto:sct-user%40yahoogroups.com>>
                      > > Sent: Sunday, August 03, 2008 2:45 PM
                      > > Subject: [sct-user] Re: SCT OTA lubrication
                      > >
                      > > >I agree with Greg.
                      > > >
                      > > > The cult following of DC High Vacuum grease stems from concerns that
                      > > > ordinary hydrocarbon greases would "outgas", and thus coat the inner
                      > > > optics with a thin grease layer. There are no reported cases of this
                      > > > ever occurring. DC High Vacuum grease is an expensive alternative.
                      > > > It does have the requisite high viscosity, high adherence, and
                      > > > exhibits little creep. So do more common bearing greases.
                      > > >
                      > > > Roland is a pretty knowledgeable glass pusher, so I tend to take his
                      > > > warnings seriously. Silicone fluid has a very high affinity for the
                      > > > silica in glass, and I believe that his concern is that silicone
                      > > > fluid could permeate the AR coating and undermine it. Again, no
                      > > > reported cases. Silicone fluid has no "solvent", and can only be
                      > > > partially removed from glass by repeated rinsings with very low (1-2
                      > > > cs) silicone fluid. Some fluid always remains in the glass matrix,
                      > > > along with a few monolayers on the surface.
                      > >
                      > >
                      >
                      > --
                      > Bob - NØDGN
                      >
                      >
                      >
                      > [Non-text portions of this message have been removed]
                      >
                    • Don D'Egidio
                      Greg, It depends on the amount of image shift when focusing. I have a 21 yr. old C8 that has a small amount of image shift, about 20% FOV TV 13mm Plossl (~ 4
                      Message 10 of 20 , Aug 3 7:48 PM
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                        Greg,

                        It depends on the amount of image shift when focusing. I have a 21 yr. old C8 that has a small
                        amount of image shift, about 20% FOV TV 13mm Plossl (~ 4 arc/min).

                        My 12 yr. old C11 had much more. It was almost half the FOV of a TV 35mm Panoptic (~ 25 arc/min). I
                        took it apart and used the HiVac grease and brought the image shift down to slightly less than half
                        the FOV of a TV 10.5mm Plossl (~ 5 arc/min).

                        Don

                        ----- Original Message -----
                        From: "gnowellsct" <tim71pos@...>
                        To: <sct-user@yahoogroups.com>
                        Sent: Sunday, August 03, 2008 9:12 PM
                        Subject: [sct-user] Re: SCT OTA lubrication


                        Well it's certainly got a large number of adherents on this group and
                        that is why I forayed into Dow Corning. Precisely because of the wise
                        folks here who advocated it. The number of enthusiasts for DC High
                        Vac over on C14 seems to be lower. Since many greases work, the only
                        question is which works best; we know that Celestron does *not* use DC
                        High Vac, and we know that outgassing is not an issue for its optics
                        in tubes that are thirty and forty years old.

                        But as I say, different strokes for different folks. My experiments
                        with the c8 established to my satisfaction that I wouldn't be putting
                        DC High Vac on teh c14 any time soon.

                        You know, it ain't like these tubes are airtight in any case. There
                        is some circulation in there....for better or for worse.

                        regards
                        Greg N
                      • eddwen2001
                        Well I use the exotic DuPont 240AC for the same reasons Chris recommends Dow Corning High Vacuum grease plus it has the benefit of also being a lubricant.
                        Message 11 of 20 , Aug 4 7:13 AM
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                          Well I use the 'exotic' DuPont 240AC for the same reasons Chris
                          recommends Dow Corning High Vacuum grease plus it has the benefit of
                          also being a lubricant. Therefore, I can use a bit on all aluminum
                          threads, etc. Expensive, but my tube is about 30 years old.

                          By the way, I use it because, as a manufacturer, I had my company use
                          it during assembly of critical components.

                          Clear skies,

                          Edd Weninger
                          Overgaard AZ

                          --- In sct-user@yahoogroups.com, "gnowellsct" <tim71pos@...> wrote:
                          >
                          In addition to these main three lubricants, there is a sub-culture of
                          more exotic lubricants that defies categorization. The thought here
                          is that we users can afford to splurge on high end lubes that cost $30
                          or $40 or $60 for small quantities whereas manufacturers will shun
                          them as a regular production item.
                        • Thad Floryan
                          ... DuPont Krytox (NOT for the baffle tube) is even more expensive, and for a good reason: it s arguably the best. For an eye-opening lube test, check this
                          Message 12 of 20 , Aug 4 7:58 AM
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                            --- In sct-user@yahoogroups.com, "eddwen2001" <Eddwen@...> wrote:
                            >
                            > Well I use the 'exotic' DuPont 240AC for the same reasons Chris
                            > recommends Dow Corning High Vacuum grease plus it has the benefit of
                            > also being a lubricant. Therefore, I can use a bit on all aluminum
                            > threads, etc. Expensive, but my tube is about 30 years old.
                            >
                            > By the way, I use it because, as a manufacturer, I had my company
                            > use it during assembly of critical components.

                            DuPont Krytox (NOT for the baffle tube) is even more expensive, and
                            for a good reason: it's arguably the best.

                            For an eye-opening lube test, check this out:

                            <http://www.sandsmachine.com/grease_t.htm>

                            The variant of Krytox that matches the DTBG cited at that web site is
                            GPL-227 which has a sodium nitrite additive for corrosion resistance
                            (re: moisture and oxygen). A two ounce tube is US$35.41 and an eight
                            ounce tube is US$111.48 [at Mcmaster-Carr]. Krytox is great for a
                            worm:worm-wheel and critical bearings.
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