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Lazy susan for an altitude bearing?

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#1 gaidash

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Posted 10 May 2013 - 03:38 PM

Does anybody have experience using lazy susan in a vertical application? I am thinking of using one of these

http://image.made-in...m-Lazy-Susan...

for altitude bearings, but not sure how well they function vertically...

#2 Pat at home

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Posted 10 May 2013 - 03:43 PM

There might not be enough rolling resistance to be really useful. Dob altitude and azimuth bearings need a bit of rolling resistance to hold the scope in position, but not so much as to make it difficult to move.

#3 gaidash

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Posted 10 May 2013 - 03:46 PM

Tensioners can do the job, breaks, etc. I am more worried that this is not being designed for a vertical load... The issue of stiffness is not important either: there are 6 plus holes on each ring and if bolted to stiff walls this will be sufficiently stiff.

It's how the balls roll inside when this is placed vertically, and how that changes friction, that worries me.

#4 Mirzam

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Posted 10 May 2013 - 04:17 PM

That's a pretty nice looking bearing. Where did you get it?

JimC

#5 gaidash

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Posted 10 May 2013 - 04:24 PM

The E-site and the A-site are full of them....

#6 Mirzam

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Posted 10 May 2013 - 04:36 PM

I know how to find things on the internet. Sometimes the quality leaves a lot to be desired, which was why I was asking about your specific source.

JimC

#7 gaidash

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Posted 10 May 2013 - 04:44 PM

Ah, sorry, no, no, I have not bought them yet. I am rebuilding my dob into double truss design, and will definitely buy one for az. bearing, but am also considering two for altitude bearing: the geometry of the scope works out nicely with them on a double truss.

From what I understand absolutely all of them come from China, so the quality will be about the same. I read lots of reviews about them, most say they work nicely, and tightness can be improved with thick grease.

#8 Dick Jacobson

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Posted 10 May 2013 - 06:58 PM

They function very poorly in a vertical position. The problem is that all the balls jam together when the bearing is tilted. This is not a true "ball bearing" where the balls are held in separated positions in a race. Instead, it's just a bunch of balls running in a groove. The friction is very low (about 1%) when horizontal, but it tends to increase abruptly by a factor of about 5 when the bearing is tilted. I tried to use one as a polar axis (tilted 45 degrees) but the irregular friction made it unacceptable.

They work great as an azimuth bearing although you will probably need to provide additional friction. They are also great for a rotating tube on an equatorial, where the uneven friction isn't a big problem.

#9 Pinbout

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Posted 10 May 2013 - 07:01 PM

what you'd really be doing is using a thrust bearing as a roller bearing which is for a different vector and will fail sooner than what its designed.

#10 tim53

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Posted 10 May 2013 - 08:37 PM

They function very poorly in a vertical position. The problem is that all the balls jam together when the bearing is tilted. This is not a true "ball bearing" where the balls are held in separated positions in a race. Instead, it's just a bunch of balls running in a groove. The friction is very low (about 1%) when horizontal, but it tends to increase abruptly by a factor of about 5 when the bearing is tilted. I tried to use one as a polar axis (tilted 45 degrees) but the irregular friction made it unacceptable.

They work great as an azimuth bearing although you will probably need to provide additional friction. They are also great for a rotating tube on an equatorial, where the uneven friction isn't a big problem.


Yes to all the above. They do make good rotating rings, but the friction varies by the orientation and "bunching" of the balls.

Posted Image

The supports shown are a bit light, so I plan to make new ones. These are the 17" bearings. They make all kinds of different sizes, including some really small and inexpensive ones. I bought several from VXB for experiments and other scope projects: VXB lazy susan bearings

-Tim.

#11 gaidash

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Posted 11 May 2013 - 02:12 AM

OK, thank you guys for the input, that settles it for me...

#12 KevinS

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Posted 11 May 2013 - 08:20 AM

I experimented quite a bit trying to make slewing rings out of these bearings.
I could never get enough even friction to make it acceptable.
I ended up with scuffed plastic 6 inch Alt bearings riding two teflon pads and a formica Az plate on a central bearing and edges riding three teflon pads. If this sounds a lot like the traditional Dob design, it is. It works amazingly well.

#13 ausastronomer

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Posted 11 May 2013 - 09:15 AM

Just FYI it may be worth noting that the only telescope manufacturers who use these lazy susan bearings (as bearings) on any dobsonian telescopes are the Asian mass producers.

I cannot think of one custom / professional truss dobsonian telescope maker anywhere in the world who utilises these bearings. There is probably a very good reason for this.

Cheers,

#14 gaidash

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Posted 11 May 2013 - 10:14 AM

I don't think any makers use them at all. GSO and the like come with a different one (which probably also costs way less). But you can google up a lot of homemade dobs with them in the base with satisfactory results.

#15 KevinS

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Posted 13 May 2013 - 03:19 PM

gaidash,

I owe you some pics but couldn't send them in a PM.
Here is the first try at lazy susan slewing rings.
Inverting the belts changed the pitch gap and reduced the contact area so they jumped almost every motor gear turn.

Attached Files



#16 KevinS

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Posted 13 May 2013 - 03:23 PM

I gave up on the bearings but reused the belts, pulleys and motors for a belt drive system. Here's the AZ drive before assembly (upside down view)

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#17 KevinS

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Posted 13 May 2013 - 03:25 PM

The AZ plate in the previous picture gets placed on top of this ground board.

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#18 KevinS

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Posted 13 May 2013 - 03:28 PM

The ALT drive was even simpler. I must have been tired of trying to be fancy. Just hung the motor and drive pulley out on a bracket with some tensioners.

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#19 KevinS

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Posted 13 May 2013 - 03:29 PM

Finally, I ended up with a basement floor of parts...

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#20 KevinS

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Posted 13 May 2013 - 03:35 PM

Ended up looking something like this.
It actually works very well except I JB welded and bolted the ALT pulley to the stock Orion ALT bearing which is very solid but ungainly looking.
I much prefer the Dob design for holding long OTAs. With a focus motor there's no noticeable vibration in the EP. Even bumping the OTA or wrestling a camera into the focuser is no problem with the scope encoders as SiTech will recover the correct position.

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