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Lazy Susan...no thanks.


elliot
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I've just received a 12" Lazy Susan bearing to see if I can ease the Azimuth movement on my 10" SkyWatcher Dob.  Before I drilled any holes I thought I'd just put it in between the baseboards to see how it moved with the weight of the scope. Sure, it moves very freely and of course I'd have needed some kind of clutch to stop it flying around in the wind, but when it gets down to really fine movement, the cheep pressed steel bearing really isn't up to the job. I couldn't get the super-fine movement I had with the Teflon pads, it was just a wee bit too jerky. The Teflan pads are not as easy flowing but you get a load of control with Teflon if you apply some carefully applied muscle. 

That's two dob mods that haven't worked for me so far, caster wheels and now the lazy susan bearing. Next upgrade will definitely be something simple like an eyepiece.

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I must admit I've never quite seen the practicality of caster wheels on a Dob mount unless they very positively lock into place with no flexion or wobble - I get that they make moving the larger Dobs into a basic viewing position easier, but surely such things would work against the movement of the scope?

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I've owned a couple of dobs with a "lazy susan" type bearing on the azimuth axis but I've always preferred correctly spaced teflon pads for smooth motion with the right level of sticktion.

The scopes with lazy susan type bearings seemed too loose in a breeze and a little rough in motion when I was using the scopes at high magnifications.

My current 12" dob uses well sized and positioned teflon pads and it's spot on :smiley:

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I'm testing a lazy susan bearing with furniture glides - the glides are a skoche taller than the lazy susan. The reason why I went with this design is that the GSO LS starts to stick after a few years. It will be interesting to see how I get on (If I ever manage to finish the dob build)

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13 minutes ago, Craney said:

" sticktion  "   .....  I'm liking that word, is it official Dob-speak ??

It's "Stiction" as in "Static Friction" not "Sticktion" which sounds like it has something to do with small tree branches :)  It basically refers to the fact that two surfaces in contact can require more force to overcome friction when they are not moving than is needed to keep them moving thereafter.  The main effect with a dob base is that you try to nudge the tube along and it won't move, so you push harder and then it suddenly moves and overshoots.

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46 minutes ago, IanL said:

It's "Stiction" as in "Static Friction" not "Sticktion" which sounds like it has something to do with small tree branches :)  It basically refers to the fact that two surfaces in contact can require more force to overcome friction when they are not moving than is needed to keep them moving thereafter.  The main effect with a dob base is that you try to nudge the tube along and it won't move, so you push harder and then it suddenly moves and overshoots.

I might have used the wrong term then. A good dob bearing will stay put when you want it too but move smoothly without jerking when needed. What ever that property is called, Moonshane / Shane put just the right amount of it into the dob mount that he built for me :icon_biggrin:

 

oo12dob01.JPG

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At Stoneleigh  this year I looked at all the Dobs big and small. The optics were never in question, but every one on display had some degree of stiction. The small ones also suffer with being too light weight so the whole base moves when you turn them. The bigger ones sit quite solidly at least. I find it so odd when products are displayed that clearly have faults.

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1 hour ago, TSRobot said:

At Stoneleigh  this year I looked at all the Dobs big and small. The optics were never in question, but every one on display had some degree of stiction. The small ones also suffer with being too light weight so the whole base moves when you turn them. The bigger ones sit quite solidly at least. I find it so odd when products are displayed that clearly have faults.

I know what you mean. When I was at Stoneleigh this year I had a good look at some large refractors. With one exception they were all undermounted and would have been hopeless to use at anything like high power. I realise that the mounts were being used to display the scopes but some folks I overheard clearly thought that they were demonstrations of what would be suitable for the scope :rolleyes2:

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I remember saying to one nice sales chap about a small 150 Bresser dob that maybe it just needed a little Mr Sheen to get it moving. He didn't seem to understand what the problem was. I was actually pondering buying it right then, but wanted to see how well it turned. Hey ho ?

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There was a nice article in "Telescope Making" magazine [years ago now] regarding the static friction versus the dynamic [moving] friction of Dobsonian bearings. [Stiction]

There is a fine line between the two forces best served by PTFE [US. Teflon] and Formica where the effort of the slightest push exactly matches the freedom to keep moving. 

There is a sense of light, buttery smoothness without loss of control, or sensitivity to wind, regardless of the telescope pointing angle or the direction of travel.

Only at the zenith does the friction seem to rise because the azimuth bearing cannot enjoy the usual "leverage" applied instinctively by the instrument user at the eyepiece.

This is true of both the Dobsonian mounted reflector and refractors mounted on Berry-style, counterbalanced, offset forks.

A suitable lever could replace shoving the instrument itself when viewing only at the zenith.

Perhaps a vertical pole extending upwards from a corner of the usual Dob supporting "fork" or side boards for maximum leverage.

This would provide a standard torque around the azimuth bearing regardless of the uprightness of the OTA itself.

The azimuth push pole would only be needed when viewing at the zenith. So could be simply slid into and out again of its location tube or rings.

Then discarded again when not in use. A refractor would need an inverted pole for zenith viewing and could even be cranked outwards for ease of use.

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According to simply bearings, the pressed steel bearings need a dose of lubricant to smooth out the balls motion and also add some damping, at which point they will glide. I suggest some dabs of normal lithium grease will provide the smoothness and sticktion you want. 

I don't see the point of using a bearing like this and then raising the base onto ptfe gliders such that the bearings aren't running in their races properly - there's no advantage of a bearing there at all other than to provide a centre of rotation.

I like the idea of transport bearings if you have a hard enough base surface. 

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I've always thought people wouldn't pay hundreds of pounds for a decent commercial mount if you could make something just as good with a bit of wood and some teflon pads at a fraction of the cost. I've had a home made 8.75" f7.2 Newtonian for thirty years which has been on several home made dobsonian mounts - all scrapped for the reasons mentioned on this post. Partly my own fault for not following proper designs or plans but I've wasted untold time on this and this post has reminded me not to waste any more time on dobsonian mounts. I've read some articles which indicate that you really need to know what you're doing to design and build a good dobsonian mount otherwise you can waste a lot of time and effort.

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11 hours ago, Davidv said:

I've always thought people wouldn't pay hundreds of pounds for a decent commercial mount if you could make something just as good with a bit of wood and some teflon pads at a fraction of the cost. I've had a home made 8.75" f7.2 Newtonian for thirty years which has been on several home made dobsonian mounts - all scrapped for the reasons mentioned on this post. Partly my own fault for not following proper designs or plans but I've wasted untold time on this and this post has reminded me not to waste any more time on dobsonian mounts. I've read some articles which indicate that you really need to know what you're doing to design and build a good dobsonian mount otherwise you can waste a lot of time and effort.

I'm not sure what you are doing wrong but none of my Dobsonian builds has offered anything but smooth, effortless movement.  :thumbsup:

Except one. Which was a plywood equatorial with large disk bearings. The cantilever forces required the bearings be bolted through and tightened.

This  increased the friction beyond the acceptable. It might have worked better on a stiffer pier than my 4" x 6' steel pipe.

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