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Sticky Dobsonian Mount


sophiecentaur

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I am not sure that my Skywatcher 200p mount is a free as it could be. It is wonderfully 'steady' with its wide, low base and the Ikea-style assembly is inspired.The trunnion type elevation is fairly well behaved when I get the tightness on the two side handles right. But the azimuth is not convincing and sticks at awkward times - particularly when looking high up. I removed the top from the base and there is a pair of large diameter circular (PTFE?) tracks that constitute the plane bearing surfaces. I have considered greasing the bearing but I am hesitating, pending any comments on the forum. The last thing I want to do is to soften / dissolve the bearing faces or get them clogged up with dusty gunge. 

I can't imagine my problem is not common. What do people do to improve the smoothness of this crude but elegant arrangement? 

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I have read that a small (as in tiny) fingertip dab of bike lubricant ( I hesitate to call this stuff  'grease') can help in making surfaces slippier.  Had I a functioning brain, I'd remember the names of the lubricants...but some more clearer-minded member will be along shortly, I'm sure, and can elucidate :)

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Dobson's Hole, Lol.......... learn something new every day here! and why does Vaseline spring to mind :lipsrsealed: , I suppose its the least harmful grease i know.

......again, I don't find any difficulty when viewing straight up, as some of you may have already read, my limited sky view,  from the garden,  is almost straight up ( and better for it, less atmospheric disturbance, shortest path and less light pollution ) and from my seated position, quite comfortable too at Zenith,  though you may need the neck of a certain  miss Regan MacNeil to align with the  straight through finder scope!

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I guess you'd have seen it but it's worth checking any metal work (e.g. staples) used to attach the PFTE are not sticking up above the level of the PFTE.  However if it's rotation when the scopes pointing high up it could just be the fact that you're applying a lot less torque so it's harder to overcome stiction.  One mod that people do is replacing the bearings completely with a lazy susan bearing there's some example threads on this forum, e.g.

 

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One problem with SW dobs is the centre bolt in the rocker box is not fixed solid so there is some lateral play. There is a mod to fix this but to do it yourself requires diy skills and tools to match. The second improvement is to add formica to the upper bearing surface for the Teflon pads to run on. this from memory can be purchased and all you have to do is stick it on. I can't remember where you can buy it but it isn't cheap. I will look through my old threads to see if I can find all the info needed and post later.

Didn't take long

No more off the shelf cut to size formica bearings http://suntactics.com/EbonyStar/ so you'd have to do more diy buying a sheet and cutting it to size etc.

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If you find Ebony Star hard to obtain - this is virtually the same and I use it on my Lightbridge - works a treat:

http://www.idsurfaces.co.uk/c-3379-international-decorative-surfaces-f1782-stardust.aspx

Here's where you can get the PTFE: http://www.awrtech.co.uk/miscprod.htm#PTFE

The Stardust only comes in large sheets and is quite expensive - so if anyone wants to team up and go halves or thirds on a sheet let me know cos I will be needing more in the not too distant future. Hth :)

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I started out with a Skyliner 150p and had a bit of tightness too. This simple mod solved it for me. Might be worth giving it a try.

The mod also suited my level of DIY skill, which is almost non-existent :icon_biggrin:

Andy.

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Fit a lazy suzan bearing. I got one from Axminster tools for about £15 if I remember correctly. At that price it's good for about 200 kg so it doesn't even feel the weight of the scope. Easy to fit and it will never stick again. An annual greasing and it will give you a life time of silky smooth azimuth movement.

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I added washers cut from old plastic milk cartons to the azimuth axis bolt of my 8" and 10" Skywatcher dobs, supplementing the teflon disk that is fitted. I experimented a bit to find how many were needed (I think 2-3 did the trick) and they seemed to make the motion much smoother and less jerky. I think they work by just taking a little of the weight off the 3 pads

I've owned a couple of dobs (8" and 12") which used the lazy susan type bearings but I found those too free moving if anything, especially if the scope is not entirely level or when a brieeze springs up. Also, after a while some of the lazy susan type bearings seem to develop a roughness and "rumble" bit in their motion, which I didn't like much.

Maybe mine were not good quality lazy susan bearings ? - the scopes were Revelation and a Meade dobs.

 

 

 

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could you possibly upload pics as I am struggling to imagine what you are describing. The bearings for most commercial dobs are three pads of white ptfe on the ground board which bear on the rocker box (i.e. the main unit of the base on which the OTA sits).

 

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