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Thread: Attaching Teflon pad for Dobsonian bearing

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    Default Attaching Teflon pad for Dobsonian bearing



    Hi All,
    I am replacing the teflon pads on my old 10" Orion Deep Space Explorer because they have been worn down to the point that the staples used to attach them are grinding up the nylon bearings (which no longer spin). This is an older model with sonotube OTA.

    I got the teflon from Scopestuff.com. They recommend using countersunk screws which I can easily do. However, I think this is a bad idea due to the weight of this scope. I think I'll be right back where I am in no time. I'm pretty sure it has extra weight under the mirror due to the really long tube necessitated by the slow focal ratio. Scopestuff also suggests using contact cement or some 3M doubles sided tape. I'd like to use contact cement since it is available locally.

    What do you folks think?

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    Default Re: Attaching Teflon pad for Dobsonian bearing

    Contact cement might be OK, although I always like to have a mechanical fix for teflon bearings: because teflon is designed to make sure nothing sticks to it! I have tried double-sided tape, and that seems OK, but I made sure I dug a hole for the teflon to sit in as well, so it can't fall off easily. If you use double sided tape, make sure you don't get the sort that has a sponge rubber layer between the sticky bits! I have also tried Araldite, but that was a waste of time.

    I think you will find that teflon is longer wearing than your original bearings if they were made of nylon, so if it is thick enough to countersink a screw a mm or more below the contact surface I doubt that you will have any problems. It would take a very long time to wear through a mm of teflon on a dob bearing... Maybe if you do a few Messier marathons every night for a few years...

    How old is the scope? the ones I can see on-line are from around the late '90's, so your original bearings may have been going for 20 years anyway!

    All the best,

    Dean
    Last edited by DeanD; 01-08-2018 at 11:25 AM.
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    Telescopes: 12" home-made dob, Celestron 150 f5, Tak TSA102, TV 76, Celestron 800CPC, ETX 125
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    Default Re: Attaching Teflon pad for Dobsonian bearing

    The standard recommendation that I've seen is to use Acid-etched teflon (PTFE). The acid-etched side is brown, and then you use contact cement on the brown side and the mounting surface. It's commonly available at ATM suppliers and even online metal/material shops.

    Then there are no screws/staples/metal to scratch your bearings.
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    Don
    -------------------
    Scopes: Criterion RV-6; 8" f/8 homebuilt; 10" Cave OTA; 12.5" Dobsonian
    Eyepieces: ES 82° 24, 18, 14, 11, 8.8, 6.7, 4.7; Orion Q70 32; Meade RG: Erfle 20; Ortho 16.8, 10.5, 7
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    Default Re: Attaching Teflon pad for Dobsonian bearing

    Don & Dean,
    Thanks for the very helpful replies! I did worry a bit about the contact cement sticking to teflon, but I was going to just rough it up with some coarse sandpaper.

    Dean,
    It is around 20 years old. Maybe a bit older. I've been told it was made by Discovery here in the USA for Orion. It is outrageously heavy because the long focal length required them to add a massive plate of lead under the mirror to get the sonotube to balance.

    The pads are teflon, while the bearings are nylon. Upon further examination, I see that they did not counter-sink the brads hardly at all. I grabbed the finest nail set I have and countersunk them. It is fine for the minute, so I plan to save the teflon I just bought and rebuild the mirror cell and build a new base.

    The mirror cell is crude and not elegant, but it works. The mirror is literally duct taped to the cell. It has particle board with the collimation bolt heads under the backside of the mirror. It has plastic spacers and dabs of silicone holding the mirror up off the bolt heads, then duct tape all the way around it! Don't laugh, it has lasted for 20+ years and the duct tape is still holding solid.
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    Default Re: Attaching Teflon pad for Dobsonian bearing

    Most of the pads I have seen were held in place with a countersunk screw.
    Scopes: Orion 130mm APO Triplet, Orion 8" f4 astrograph, 10" RC Mount: Celestron CGX-L Guiding: Orion 60mm f4 scope, Starlight Loadstar, PHD2 software - Cams: Planetary: ASI120mc, DSO:Atik 383L+, SGPsoftware Processing : Regisax, Deep Sky Stacker, Star Tools, Pixinsight Assistant: One very large cat
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    Default Re: Attaching Teflon pad for Dobsonian bearing

    Sounds like a bit of fun doing it up!

    Personally I don't have a problem with using silicon under a mirror to provide a bearing surface; especially a full-thickness mirror like yours appears to be. I don't like using particle board though! The duct tape no doubt kept moisture out of the particle board, as well as stopping the mirror moving around- and anyway, who's to argue if it has worked for 20 years!

    Have you thought about putting a strip of "ebony star" formica around the bearings to give smoother movement? What did they do for the ground-board bearings? Again, some "ebony star" and teflon bearings works wonders.

    If you are going the whole hog with the re-build, you might be able to reduce or remove the lead weights by increasing the height of the sides and moving the bearings up the tube. As another option, when I built my 12" truss-tube dob I added tension springs to act as "virtual weight"; and these work very well.

    All the best, and good luck!

    - Dean
    Last edited by DeanD; 01-08-2018 at 10:40 PM.
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    Telescopes: 12" home-made dob, Celestron 150 f5, Tak TSA102, TV 76, Celestron 800CPC, ETX 125
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    Default Re: Attaching Teflon pad for Dobsonian bearing

    Hi Dean!
    So I took it apart and cleaned the mirror last night. A lot better, but it's a real mess. Definitely needs to be recoated. Going to reassemble with silicone and duct tape for testing.

    Your suggestion is awesome and that's exactly what I'm hoping to do. Build a lightweight, open mirror cell with fan and move the bearings up and build a new base. I'm curious about your tension springs. How did you make them? Do you have any pictures?

    Thanks!

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    Default Re: Attaching Teflon pad for Dobsonian bearing

    Quote Originally Posted by lign View Post
    Hi Dean!
    So I took it apart and cleaned the mirror last night. A lot better, but it's a real mess. Definitely needs to be recoated. Going to reassemble with silicone and duct tape for testing.

    Your suggestion is awesome and that's exactly what I'm hoping to do. Build a lightweight, open mirror cell with fan and move the bearings up and build a new base. I'm curious about your tension springs. How did you make them? Do you have any pictures?

    Thanks!
    My approach with the springs is very "agricultural", but it works. It involves four short springs (about 20c each at the hardware store) joined together and attached to the scope by two hooks! I don't need them usually, but I connect them up if I am loading the focuser with my ES 20mm 100° hand grenade! I don't have any detailed piccies, but I will take some soon and let you know when I upload them to my album. (Remind me by pm if I haven't done it in the next few days!) There are a few pictures of my scope on my album page here (but you can't see the springs): http://www.astronomyforum.net/member...-s-12-dob.html - I just noticed that they are very low res, so let me know too if you want a closer look. The last one shows a 30cm carpenters' square sitting on top for scale. My secondary and focuser cage is very light-weight, and you will notice there are curved spider vanes: this eliminates diffraction spikes so I see round stars on a good night. I also made the whole thing so that there are no screws etc. that need to be added to put it all together (or lost in the dark!). Even the side bearings stay on when it is folded away, although they swing down for storage, and the truss pieces are permanently joined together, so they fold up as a group. The scope is a 12" f5 with a very nice GSO full thickness mirror, and I have a built in cooling fan.

    All the best with the re-build!

    - Dean
    - Dean
    Last edited by DeanD; 01-10-2018 at 04:42 AM.
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    Telescopes: 12" home-made dob, Celestron 150 f5, Tak TSA102, TV 76, Celestron 800CPC, ETX 125
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    Default Re: Attaching Teflon pad for Dobsonian bearing

    Hi Dean! Thanks for the info and the pics in your album. They are a bit small, but I can see you have an absolutely incredible telescope that you should be insanely proud of building! I'd love to see more photos. I don't hope to do anything that elaborate. I got a NEW TOY a couple days ago (Orion Skyview Pro 127mm Mak-Cass) to use while I'm rebuilding the 10" Dob. Someone on here commented that I might not have the rebuild done in time to use it with my daughter who graduates in a few months.

    Anyway, today I finally got around to gluing the cleaned mirror back on the cell with silicone. Of course, it's going to be 39 degrees here tonight (that's Arctic weather for here in Florida) so I plan to bring it inside from my shop so that it will cure properly to be able to use tomorrow.

    Wow, what a beautiful clear night it is tonight. Spent an hour or so out there but that's all we could take of the "cold." Lol.

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    Default Re: Attaching Teflon pad for Dobsonian bearing

    The Golden Rule for Teflon pads is 15lb per square inch.

    So, for the az pads, if the OTA + rocker box = 45lbs, then the area of each pad would be 1sq in. You also need to take into account the hollows of the countersunk holes, so the actual size of the pads will be a little larger. This Golden Rule will stop premature wear and provides the optimal weight to area ratio for the best quality of action.

    I have never needed to change the Teflon pads because of wear. Sounds like your original pads were not Teflon, as Dean mentioned. Particularly if they were correct size for the instrument. Hence the screws that hold the pads in place have never come close to damaging the laminate.

    Also, regular cleaning of the laminate with methylated spirits and an application of good quality car wax to the laminate will keep the action of the dob up to its best. The Teflon pads should only be wiped clean with methylated spirits - do not apply wax to the pads. The reason that Teflon is used as a bearing material comes from the very material itself. Apply any coating onto the pads, and you no longer have the Teflon doing its job.

    If you do go down the adhesion method, be aware that Teflon is not the easiest material to glue.

    Alex.
    Last edited by bladekeeper; 01-14-2018 at 05:21 AM. Reason: per request...
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