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How tight should a dobsonian azimuth bolt be?

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9 replies to this topic

#1 MawkHawk

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Posted 24 June 2019 - 06:17 AM

OK, sorry for this dumb question. I used to know this when I built my first dob back in the 90's, but how tight should the azimuth bolt be? Should it be very lose such that stiction it completely handled by the bearing material or a bit tighter?

I'm replacing the bearing material and PTFE on my new-to-me Dobsutff 16".

Thanks in advance!



#2 Cames

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Posted 24 June 2019 - 07:38 AM

I don't know about tightening the az bolt. I would recommend a Nylock nut that resists turning even though it is not tightly cinched down. A bushing around the central bolt is also a good idea to keep the rotation a little cleaner.

 

More recently, the practice of stacking 'milk-jug washers' around the az bolt in order to carry a certain percentage of the load above the ground board has been helpful in reducing stiction at the bearings.  The stack of milk-jug plastic in the center point seems to glide effortlessly. The number of layers used determines the residual load on the peripheral bearings. You will need to experiment with the number of layers.  Too many will render the mount somewhat tipsy.

 

I prefer to use the gallon jugs that drinking water comes in. Same plastic only easier to clean.

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C


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#3 CrazyPanda

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Posted 24 June 2019 - 07:58 AM

It should not be tightened down as it will just introduce unnecessary force and add more friction. It just needs to hold the ground board on so it doesn't fall of. What matters more is ensuring the lateral movement has no slop so that it turns rather than shifts.

 

I also strongly recommend a teflon bearing or stack of milkjugs in the center to reduce the load carried by the bearings on the periphery. Makes a *huge* difference in how smooth the azimuth motion is. Just have to find the right thickness so that the dob doesn't want wobble.


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#4 Asbytec

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Posted 24 June 2019 - 08:03 AM

I set mine just tight enough not to wobble but not tight enough to impair movement. I just kind of feel it to be right. Kind of like the clips at the edge of the mirror, just loose enough to work and not too tight to crimp our style. Mine has a bushing and a lock nut. Need to try the milk jug washer thing at center. Some old CDs.


Edited by Asbytec, 24 June 2019 - 08:05 AM.


#5 Starman47

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Posted 24 June 2019 - 08:03 AM

In my experience with two dobs, the azimuth stiction should match the altitude stiction. This means that I do not notice any difference in the feel as I move side to side or up and down, The feel should be buttery smooth, So I tighten the azimuth bolt till I get the correct feel. 

 

So, this means testing a few times till I get the right feel.

 

P.S. The recommendation in the post immediately above this one is another way to say what I said.


Edited by Starman47, 24 June 2019 - 08:05 AM.


#6 Jon Isaacs

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Posted 24 June 2019 - 08:33 AM

It should not be tightened down as it will just introduce unnecessary force and add more friction. It just needs to hold the ground board on so it doesn't fall of. What matters more is ensuring the lateral movement has no slop so that it turns rather than shifts.

 

I also strongly recommend a teflon bearing or stack of milkjugs in the center to reduce the load carried by the bearings on the periphery. Makes a *huge* difference in how smooth the azimuth motion is. Just have to find the right thickness so that the dob doesn't want wobble.

 

waytogo.gif

 

The center bolt should not exert a downward force.

 

The difficulty with shimming the center bearing is that Dobs are wood and nothing is perfectly flat. Avoiding the wobble at high mags and yet actually having the center bearing carry some load is not easily done.

 

I don't worry much about friction, I worry about stiction.  Stiction is what causes a jerky motion. I don't worry too much about balancing the altitude and azimuth force, the altitude force is more or less  constant but the length of the azimuth lever depends on the altitude, near the horizon, the lever is long, the force low, near the zenith, the lever is short, the force high. 

 

Since I began using soap to lube the bearings, stiction has not been a problem and friction has been reduced somewhat. 

 

For my 16 inch, I designed and built a spring loaded center bolt. The spring pushes up on the rocker box and provides a constant force even if the ground board and/or the rocker box are not perfectly flat. The force is adjustable and can be adjusted with the scope assembled. It works.

 

Dob Center Bearing Design CN.jpg
 
The center bolt is secured by nuts top and bottom to the ground board so it's rigidly attached. The actual bearing is in the top plate.
 
Jon

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#7 Starman47

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Posted 24 June 2019 - 09:56 AM

Jon, If at all possible, would you please show a picture or pictures?  Rather than just the diagram



#8 MawkHawk

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Posted 24 June 2019 - 10:35 AM

Thank you for all of the info, everyone. You all have been very helpful. I do have  some spare PTFE so I'll try adding some near the center.

Thanks again.



#9 Jon Isaacs

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Posted 24 June 2019 - 12:00 PM

Jon, If at all possible, would you please show a picture or pictures?  Rather than just the diagram

 

A photo is not really possible because it only shows one piece.  And besides, my engineering is better than my workmanship.. 

 

The way it works is this:  

 

The drawing is a side view.   I have added labels.

 

There's a stud that is fixed to the ground board, it's dark gray in the drawing. 

 

There is a hole though the rocker box with a plate on top that it bolted to the rocker box.  The bolts are red in the drawing and allow for adjustment.  

 

The spring is between the ground board and the top plate, this tries to lift the rocker box and lightens the load on the outer bearings.  The spring needs to be quite stiff.  

 

Dob center bearing design 2019.jpg

 
In it's basic, conceptual form, the spring could placed between the rocker box and the ground board, it would serve the same purpose.  But it wound not be adjustable from the top.  
 
I hope this helps.  
 
Jon

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#10 Kunama

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Posted 24 June 2019 - 05:39 PM

I used a teflon pad at the centre bolt with a shim made from the side of a Coke can on my Dob, this gave just the right feel without any rocking being detected.

My rocker box base is 6mm thick aluminium plate. The centre bolt does not put downward force at all, it is locked into place a tiny bit of clearance.




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