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dobsonian altitude bearings

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#1 jmillsbss

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Posted 07 July 2020 - 10:20 AM

I've been at this over 3 years now, and I still I have my first scope, the Orion XT10 Dobsonian. I've made some modifications to the azimuth bearing and got that pretty nice now.  The altitude needs some attention now.  I've noticed a common difference in the production scopes like mine and the top-tier scopes of the same size/focal length....the size of the altitude bearings.  Is that to have more tension or how does it hold a position without falling?  Mine has the springs but if you don't have either one of the springs hooked on the pin it falls right down.

 

My XT10 has 2 little 0.75" x 0.75" teflon (maybe?) buttons on the rocker "arms" (???) and an almost smooth veneer of some sort on the edge of a +/- 6" diameter (pressboard/particle board) "bearing".

 

Looking at a similar size high end scopes, like Teeter, Moonlite, Obsession, Starmaster, etc. etc., the bearings on those are quite a bit larger, half circle shape of perhaps 10" or more diameter.

 

What am I missing?  I'm wanting to lighten this thing up a bit and that would seem like a good time to do the same type modification on mine.  One other thing of note, this is the spring-tension dobs, not the one with knob tensioners.  I suppose I can, A, Get a longer similar tension spring or, B. Use a small piece of chain to use the factory spring and span across the new altitude bearing similar to current design?

 

Does it help smooth the altitude action or spread the weight better across the wider bearing?  There has to be an advantage.



#2 scottinash

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Posted 07 July 2020 - 10:35 AM

Probably someone will chime in here but you may find some very useful info on this archived thread:   https://www.cloudyni...e-dob-bearings/



#3 coopman

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Posted 07 July 2020 - 11:47 AM

Larger dia. alt. bearings make the scope better at staying where you point it and to resist moving due to imbalance issues from changing eyepieces.  I have an AD10 and made a tube box for it and put 13" dia. alt. bearings on it.  Now I have NO issues with unwanted movement or backlash.  The changes did make the tube assembly heavier, but I move it around with a hand truck anyways.  I had to rebuild the rocker box to accommodate the wider tube assembly. 

Have a look at what Bresser did with the alt. bearings on their Dobs.  That is pretty cool, IMO.   


Edited by coopman, 07 July 2020 - 12:05 PM.

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#4 Pinbout

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Posted 07 July 2020 - 11:54 AM

i liked how ES came out with larger alt bearings and tube rings so you can adj how the dob balances... Scott Roberts got 5 stars by me on the concept for inexpensive dob design...

 

https://www.youtube....h?v=wwPnHUEnRoo


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#5 jmillsbss

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Posted 07 July 2020 - 12:14 PM

i liked how ES came out with larger alt bearings and tube rings so you can adj how the dob balances... Scott Roberts got 5 stars by me on the concept for inexpensive dob design...

 

https://www.youtube....h?v=wwPnHUEnRoo

I saw that.  It's interesting that it has the capability to slide the tube forward on the Alt axis, but get it out there too far and it'll nose dive.  I agree, in it's current configuration, the base shoulda had taller "arms".  It would be a very flexible mount then, it would seem to me.   I have mine lifted up on a homemade base about 16".  I find I enjoy my Dob more if I'm standing.  The ES  alt bearings appear similar in size to their ultralight Dob lineup, maybe a bit smaller, but still looks like the bigger bearings are the way to go.  I'll have to build the whole base over anyway because the stock arms wouldn't be good for anything.  I'm thinking about it......



#6 stargazer193857

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Posted 07 July 2020 - 01:13 PM

The round things are called trunnions, the things on top.

Bearings are the bottom surface with Teflon.

Inexpensive dobs correctly have small trunnions because they are intended for Plossl eyepieces, and are balanced so there is less lever arm.


Big truss dibs have a higher lever ratio and heavier eyepieces, and so need bigger trunnions.

Explore Scientific solid tubes are marketed towards advanced users with Explore Scientific eyepieces, and so have big trunnions.

Orion dibs are marketed to entry level folk who likely will stick to Orion Expanse eyepieces.
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#7 SteveG

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Posted 07 July 2020 - 04:19 PM

Get some magnetic weights for your tube. It allows balancing changes, when going from lightweight to heavy eyepieces.



#8 jmillsbss

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Posted 07 July 2020 - 08:07 PM

The round things are called trunnions, the things on top.

Bearings are the bottom surface with Teflon.

Inexpensive dobs correctly have small trunnions because they are intended for Plossl eyepieces, and are balanced so there is less lever arm.


Big truss dibs have a higher lever ratio and heavier eyepieces, and so need bigger trunnions.

Explore Scientific solid tubes are marketed towards advanced users with Explore Scientific eyepieces, and so have big trunnions.

Orion dibs are marketed to entry level folk who likely will stick to Orion Expanse eyepieces.

Well the plossls lost property rights as soon as I figured out what the difference was between 50 degrees and 82 degrees!


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#9 jmillsbss

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Posted 07 July 2020 - 08:11 PM

Get some magnetic weights for your tube. It allows balancing changes, when going from lightweight to heavy eyepieces.

Right. I have 2 pound and 1 pound and can hang more off the back for low altitude targets.  And I can slide them up or down the tube. The nose is heavy with an HRCC and a 22 Nagler.  I can balance fine, I'm just curious if larger trunnions are any different. Sounds like it would be better.


Edited by jmillsbss, 07 July 2020 - 08:12 PM.

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#10 Starman1

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Posted 10 July 2020 - 06:55 PM

.  I find I enjoy my Dob more if I'm standing. 

Disadvantages to standing:

  • you get tired faster so can't last as many hours.  I've noticed "standers" usually close up shop really early compared to seated observers.
  • you are less stable standing so your head moves back and forth and in and out relative to the eyepiece a lot more.  Most eyepieces are more of a problem to use.
  • the eyepiece will often be at positions where you have to bend over, hurting the back.
  • you won't spend as much time on each object, so you'll see less.

Now, if a ladder is used on a big scope, you can hook an arm over the ladder, or lean against it, so it's not the same issues as just standing.

But standing?  Just doesn't make sense compared to a chair.


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#11 Starman1

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Posted 10 July 2020 - 06:59 PM

The round things are called trunnions, the things on top.

Bearings are the bottom surface with Teflon.

Inexpensive dobs correctly have small trunnions because they are intended for Plossl eyepieces, and are balanced so there is less lever arm.


Big truss dibs have a higher lever ratio and heavier eyepieces, and so need bigger trunnions.

Explore Scientific solid tubes are marketed towards advanced users with Explore Scientific eyepieces, and so have big trunnions.

Orion dobs are marketed to entry level folk who likely will stick to Orion Expanse eyepieces.

The very low prices of the Explore Scientific First Light 8" and 10" dobs say that is wrong--they are aimed at exactly the same customers as Apertura, Orion, GSO, SkyWatcher and others.

It's just that the people at ES understand the advantage of a larger altitude trunnion.  Instead of increasing the friction with a spring, they use a larger trunnion.


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#12 n2dpsky

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Posted 10 July 2020 - 08:24 PM

I've been at this over 3 years now, and I still I have my first scope, the Orion XT10 Dobsonian. I've made some modifications to the azimuth bearing and got that pretty nice now.  The altitude needs some attention now.  I've noticed a common difference in the production scopes like mine and the top-tier scopes of the same size/focal length....the size of the altitude bearings.  Is that to have more tension or how does it hold a position without falling?  Mine has the springs but if you don't have either one of the springs hooked on the pin it falls right down.

 

My XT10 has 2 little 0.75" x 0.75" teflon (maybe?) buttons on the rocker "arms" (???) and an almost smooth veneer of some sort on the edge of a +/- 6" diameter (pressboard/particle board) "bearing".

 

Looking at a similar size high end scopes, like Teeter, Moonlite, Obsession, Starmaster, etc. etc., the bearings on those are quite a bit larger, half circle shape of perhaps 10" or more diameter.

 

What am I missing?  I'm wanting to lighten this thing up a bit and that would seem like a good time to do the same type modification on mine.  One other thing of note, this is the spring-tension dobs, not the one with knob tensioners.  I suppose I can, A, Get a longer similar tension spring or, B. Use a small piece of chain to use the factory spring and span across the new altitude bearing similar to current design?

 

Does it help smooth the altitude action or spread the weight better across the wider bearing?  There has to be an advantage.

I'm pretty sure your Orion was made by Synta, in which case, this may help.  I did this mod a few years ago on my SkyWatcher FlexTube 10'.   It was cheap and made a nice difference.  It's not high-end by any stretch but cheap and can be done in an hour.

 

https://www.cloudyni...e-dob-bearings/


Edited by n2dpsky, 10 July 2020 - 08:24 PM.

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#13 CrazyPanda

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Posted 11 July 2020 - 08:52 PM



I've been at this over 3 years now, and I still I have my first scope, the Orion XT10 Dobsonian. I've made some modifications to the azimuth bearing and got that pretty nice now.  The altitude needs some attention now.  I've noticed a common difference in the production scopes like mine and the top-tier scopes of the same size/focal length....the size of the altitude bearings.  Is that to have more tension or how does it hold a position without falling?  Mine has the springs but if you don't have either one of the springs hooked on the pin it falls right down.

 

My XT10 has 2 little 0.75" x 0.75" teflon (maybe?) buttons on the rocker "arms" (???) and an almost smooth veneer of some sort on the edge of a +/- 6" diameter (pressboard/particle board) "bearing".

 

Looking at a similar size high end scopes, like Teeter, Moonlite, Obsession, Starmaster, etc. etc., the bearings on those are quite a bit larger, half circle shape of perhaps 10" or more diameter.

 

What am I missing?  I'm wanting to lighten this thing up a bit and that would seem like a good time to do the same type modification on mine.  One other thing of note, this is the spring-tension dobs, not the one with knob tensioners.  I suppose I can, A, Get a longer similar tension spring or, B. Use a small piece of chain to use the factory spring and span across the new altitude bearing similar to current design?

 

Does it help smooth the altitude action or spread the weight better across the wider bearing?  There has to be an advantage.

I would say the primary purpose of big bearings is to keep the rocker box as short as possible. A short rocker means less weight and less flex.

 

Consider this typical Teeter classic design

 

The center of that bearing is basically at the top of the rocker box, which is roughly where the balance point of the scope is.

 

So now imagine that bearing only being 6" in diameter. What would have to happen?

The sides of the rocker box would have to be taller in order to "reach" the bearing. Well, stiffness of a board decreases by the cube as its length increases.

 

So a sideboard on a rocker that's 2x longer without being made any thicker, would flex 8x more. At high magnification when you're trying to track an object, that kind of flex will be noticeable. You would go to make a small movement, and the rocker might flex before the stiction in the azimuth bearing gives, and when you let go, the scope would just spring back to where it was. Or it will flex a tiny bit proportionally to the dynamic friction of the bearings, and then spring back a tiny bit when you let go.

 

Alternatively, you could keep a short rocker and just move the 6" bearing down further in the mirror box, but in order to compensate for this, you would need add MASSIVE amounts of weight to the backside of the dob, increasing total weight considerably more.

 

The secondary effect of course is that big bearings increase force of friction. This has an advantage of allowing a variety of light weight and heavy equipment to work without the use of a counterweight system. You don't want too much additional force of friction, but enough to make it so that you can take a 1.5 pound eyepiece out to put another eyepiece in without worrying about the scope swinging upward when no eyepiece is in the focuser, is a huge quality of life improvement.


Edited by CrazyPanda, 11 July 2020 - 08:57 PM.


#14 Jon Isaacs

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Posted 11 July 2020 - 09:34 PM

The purpose of the large diameter bearings is to increase the force required to move the scope in the altitude axis. It not only makes balancing easier but a certain amount of resistance is necessary for optimal tracking.

 

Analytically, the force required involves the diameter of the bearing, the weight of the scope, the distance from the center of the bearing to the where you're applying the force, the coefficient of friction of the bearing as well as tangent of the angle of the pad's contact.  There are a lot of factors...

 

But large diameter bearings/trunnions inherently increase the tracking force..

 

The GSO correct tension springs first seen on the XT-6 and XT-8 twenty years ago, use an alternative scheme to achieve this same effect. Rather than increasing the diameter of the trunnions, they used springs to more double the load on the bearing which effectively increases the tracking force in proportion.

 

I measured each spring at working length as providing 25 lbs of added force. For a scope weighing 35 pounds, this increased the effective weight from 35 lbs to 85 lbs so for the 5 inch diameter trunnion bearings, this provides the same tracking forces as a 12 inch diameter trunnion.

 

I've had my 10 inch GSO for 17 years. I track at 800x on close doubles without too much trouble.. 

 

Jon


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#15 stargazer193857

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Posted 11 July 2020 - 09:55 PM

I never liked the look of altitude clutches. I always thought they were a poor design and that springs are better. One of the reasons I held off buying one of these new dobs.

#16 Jon Isaacs

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Posted 11 July 2020 - 10:14 PM

So a sideboard on a rocker that's 2x longer without being made any thicker, would flex 8x more. At high magnification when you're trying to track an object, that kind of flex will be noticeable. You would go to make a small movement, and the rocker might flex before the stiction in the azimuth bearing gives, and when you let go, the scope would just spring back to where it was. Or it will flex a tiny bit proportionally to the dynamic friction of the bearings, and then spring back a tiny bit when you let go.

 

 

 

The difference in height is only 1/2 the increased bearing diameter. Going from a 5 inch bearing to a 10 inch bearing shortens the rocker box by 2.5 inches. This has a trivial effect on the stiffness but doubles the "frictional" forces. And the larger trunnion is also more flexible..

 

Jon



#17 CrazyPanda

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Posted 12 July 2020 - 01:09 PM

The difference in height is only 1/2 the increased bearing diameter. Going from a 5 inch bearing to a 10 inch bearing shortens the rocker box by 2.5 inches. This has a trivial effect on the stiffness but doubles the "frictional" forces. And the larger trunnion is also more flexible..

 

Jon

Note that you can double the frictional force of a small trunion's bearings by placing them farther apart. I can't be bothered to do the math right now, but increasing frictional force on small trunnions is usually not a challenge, so I can't imagine that being the major driver of going to bigger trunnions.

 

Larger trunnions are only going to be more flexible at the points where they are not laterally supported by the mirror box, which is almost always when the scope is aimed very, very low. A classic Teeter or Obsession's larger bearings are almost fully supported for most working altitudes, so their larger sizes introduce no additional flex.


Edited by CrazyPanda, 12 July 2020 - 01:11 PM.


#18 stargazer193857

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Posted 12 July 2020 - 02:15 PM

Note that you can double the frictional force of a small trunion's bearings by placing them farther apart. I can't be bothered to do the math right now, but increasing frictional force on small trunnions is usually not a challenge, so I can't imagine that being the major driver of going to bigger trunnions.

Larger trunnions are only going to be more flexible at the points where they are not laterally supported by the mirror box, which is almost always when the scope is aimed very, very low. A classic Teeter or Obsession's larger bearings are almost fully supported for most working altitudes, so their larger sizes introduce no additional flex.


When people cut lightening triangles out of trunnions, I wonder if that is a good place to do so, given how they carry the full weight of the OTA, especially when aimed low. Also, the wood left over is not in its strongest orientation.

#19 Starman1

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Posted 12 July 2020 - 05:03 PM

If the trunnions are large enough, like on scopes similar to the Waite Renegade, they can be connected at the front end (under the scope when pointed to low altitudes) and be correspondingly stiffened.


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#20 CrazyPanda

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Posted 12 July 2020 - 05:11 PM

When people cut lightening triangles out of trunnions, I wonder if that is a good place to do so, given how they carry the full weight of the OTA, especially when aimed low. Also, the wood left over is not in its strongest orientation.

Given most trunnions are made from baltic birch or similar ply, there's likely 13 or more layers of plywood glued at opposing grain orientations. And since the scope swings nearly 90 degrees, there's really no "best" orientation.

 

I would argue that most trunnions are probably a lot wider (vertically) than they need to be. As long as they are anchored to the mirror box such that there is only a relatively short span between the anchor points, you should be able to carry fairly heavy loads without issue.

 

Look at this aluminum bearings from Obsession: https://www.obsessio...ndreas-Hybl.jpg

 

Let's assume the OTA weighs 100 pounds. Each pad exerts 25 pounds of force on the trunnion. If those bearings are anchored every 8" or so, that 8" span (which is curved, mind you, making it stronger), only has to resist 25 pounds (and it's really only 4" on either side of the pressure point). That's hardly anything. A baltic birch trunnion wouldn't have to be *that* much wider to get the same stiffness out of it.

 

In that design, only the very ends are unsupported, so there would probably be some flex noticeable at high mag when that scope is aimed at a few degrees above the horizon, but I don't know many people doing high power observing of objects that close to the horizon, so to me it's a non-issue.

 

Now, if you had fewer anchor points, a truss-style trunnion like Teeter would probably be a safer bet, or you could just increase the width of the trunnion a little more.

 

Things change when you get into the "low profile mirror box" design like Waite Research and similar designs. Those trunnions are largely unsupported so they not only have to be thicker, they have to be wider. But that's a lot of layers of ply, so strand orientation is not an issue. Whether hollowing them reduces their strength enough that it's counter-productive, I can't say.


Edited by CrazyPanda, 12 July 2020 - 05:13 PM.

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#21 Jon Isaacs

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Posted 12 July 2020 - 06:03 PM

Note that you can double the frictional force of a small trunion's bearings by placing them farther apart. I can't be bothered to do the math right now, but increasing frictional force on small trunnions is usually not a challenge, so I can't imagine that being the major driver of going to bigger trunnions.

 

 

Have you tried it?  The evidence is that scopes with large bearings or spring loaded bearings track better. I can remove the springs and it tracks poorly and there's no room to move them further..

 

If the small bearings could provide the proper friction for tracking, there would Dobs with small bearings that were excellent trackers, I haven't seen that.

 

The stiffness is a non-issue.  There are plenty of ways to stiffen the base. My standard 10 inch Dob base is plenty stiff and it could be stiffened easily if needed.

 

I've seen scopes with tall boxes, thin, thick bearings, short boxes that that track nicely.. not seen one with small bearings unless there were springs to increase the frictional force.

 

Jon


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#22 stargazer193857

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Posted 12 July 2020 - 09:23 PM

If the trunnions are large enough, like on scopes similar to the Waite Renegade, they can be connected at the front end (under the scope when pointed to low altitudes) and be correspondingly stiffened.


I just now realized how that cross bar stiffens it so much.

#23 Oberon

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Posted 14 July 2020 - 04:52 AM

This is bouncy...

 

gallery_217007_5817_31131.jpg

 

This is not bouncy, this is stiff and smooth...

 

med_gallery_217007_5817_320420.jpg

 

Same trunnions in both photos, unfinished and finished.

Structure (such as braces) make all the difference.

 

gallery_217007_5817_87674.jpg


Edited by Oberon, 14 July 2020 - 04:53 AM.

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#24 Jon Isaacs

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Posted 14 July 2020 - 09:56 AM

Same trunnions in both photos, unfinished and finished.

 

It's amazing just how stiff that paint layer is.. :) 

 

Starsplitter altitude trunnions are carbon fiber over a foam core. 

 

6446676-Birthday Dob CN.jpg
 
Jonathan's scope is an example of a general engineering rule, light weight requires more careful design, more attention to detail. You look at a a Caterpillar tractor, lots of big plates of steel, they need the weight to get the traction. They'll add it if need be.  A D11 weighs 230,000 pounds.  A commercial airliner, every pound matters so lightweight materials and very careful design.
 
Jon

 


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#25 cuzimthedad

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Posted 14 July 2020 - 12:57 PM

This is bouncy...

 

gallery_217007_5817_31131.jpg

 

This is not bouncy, this is stiff and smooth...

 

med_gallery_217007_5817_320420.jpg

 

Same trunnions in both photos, unfinished and finished.

Structure (such as braces) make all the difference.

 

gallery_217007_5817_87674.jpg

What a beautiful telescope Jonathan


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