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Used Dob and loosening Azimuth knob.

dob DIY
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#1 ralphieboy

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Posted 25 November 2020 - 10:06 PM

Z Knob.jpg Z Lazy Susan.jpg

So I recently bought a used 12 Zhumell Dob recently and have been having issues with the Azimuth knob loosening regardless on how much I tighten. I have read the mega thread and a few other threads where suggestions include, gluing down bumpy Formica, adding a central bearing and new bolt with finer threads, and adding Teflon, among others but I’m trying to figure out if what I have is usable.

 

When I removed the AZ knob I noticed some odd items. The knob itself was filled with glue; presumably to keep the bolt attached to the knob. There was a split lock washer with the split glued / epoxied smooth; this looked to be done intentionally but would seem to defeat its purpose if it was to provide some spring resistance. There were also 2 smooth stainless square sheets on each side of large round bearing; I’m not sure if a lazy Susan requires all three parts but I have not seen any similar designs to mine on the web.

 

See attached: the spring washer was glued smooth, unless it leaked through when the knob was glued to the bolt.  The large wheel appeared to roll well, nothing broken, and you can see the ware marks in the center and under the large wheel.

 

So what are the steel plates for and is there any advantage to leaving them as is? If I leave them what else would be beneficial to getting the right movement? Would replacing those knob washers stop the loosening AZ knob?         

 

 



#2 TOMDEY

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Posted 25 November 2020 - 11:14 PM

The thing is ill-conceived. It's loosening up because there is nothing constraining it from doing so. The pin has to be locked to the ground board on the ground board side. There are plenty of ways to achieve that. One (of many decent approaches) is to rebuild so that the bolt "bottoms out" at just the right depth... still leaving freedom for the rocker box to swivel unimpeded. And that kind of washer is exactly the wrong thing... it is causing the bolt to grab the rocker box --- exactly what you don't want. Whoever came up with that doesn't understand fasteners. If that is stock Zhumell as shown (not modified by a previous owner) --- the designer sure ain't no mechanical engineer.   Tom



#3 Couder

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Posted 25 November 2020 - 11:18 PM

First: the lock washer should not be used as a brake. Second: the hole in the flat washer is too big for the bolt. If more surface area is needed, a fender washer should be used. Third: apparently the bolt pulled loose from the plastic and was glued in. 

Not sure what the plates were for - maybe to provide more surface area to use as a brake. Teflon, in my opinion, does not work as a brake, it is more like a bearing. Looks like you may have to rebuild part of your scope. If balanced properly, minimal braking should be required.



#4 Old Rookie

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Posted 25 November 2020 - 11:40 PM

Those stainless steel square sheets make a sandwich out of the lazy susan bearing.  Yours didn't come from GSO that way.  The GSO bearing has round sheets but no matter.  The lazy susan bearing really needs them for a bearing surface.  Without them, the bearing just rides on the formica of the ground board.  When the center bolt is tensioned properly, you shouldn't have any wobble between the ground board and rocker.  And the azimuth motion will be just right.  

 

That bolt you have screws into the ground board.  If you turn over the ground board, you may find that the threads in the nut imbedded in the ground board are stripped.  Those threads are like a capture and not supposed to loosen up once tightened.  I don't know how hard it will be to find and replace.  Or, you could gently knock it out, put a sleeve in there, and use a 3/8" bolt through the rocker and ground board with a friction nut on the bottom.  Parts courtesy of Ace Hardware.


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#5 junomike

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Posted 26 November 2020 - 08:08 AM

I like the added flat steel plates as they should keep things rolling smoothly during slews.

Easiest fix for the knob would be to add a second split-washer directly on top of the first split washer and then add a teflon washer between the flat washer and Base.



#6 Jon Isaacs

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Posted 26 November 2020 - 08:29 AM

So what are the steel plates for and is there any advantage to leaving them as is? If I leave them what else would be beneficial to getting the right movement? Would replacing those knob washers stop the loosening AZ knob?

 

 

My thinking:

 

The steel plates replace the stock steel plates.  These are stainless and look thicker which is a good thing.  

 

Your problem is that the azimuth adjustment is coming loose.  Here is a way to fix it.  

 

-  Replace the bolt with a bolt that threads through from the bottom up. This is basically a stud but it's locked in place. You could use a stud though you would probably have to make it.  

 

- On top use appropriate washers and either a lock nut or double nuts.  

 

This is similar to this adjustable preload bearing I made for my 16 inch.

 

Dob center bearing design 2019.jpg
 
Jon

 


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#7 ralphieboy

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Posted 27 November 2020 - 02:39 AM

Thank you all for this information. For now, it seems like the plates will stay. As suggested, I looked at the bottom nut and it was stripped and loose when inserting the bolt. So, I will be heading to the hardware store and rebuild bottom up; I will try to use the spring design diagram. Thanks again.


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#8 ralphieboy

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Posted 27 November 2020 - 06:12 PM

Jon, 

 

How well does your spring loaded central bearing design handel the weight? It seems that a larger more dispersed surface area, like the steel plates, would distribute / handle the weight better than a single central bolt / bearing. I'm not sure but really trying to figure out if the steel plates / lazy Susan stays in my Dob or not. Do you have other supporting material, Teflon, Velcro or felt placed away from the central bearing? 

 

What did you use for the top plate? It looks like the plate has drilled holes and maybe a couple of Allen adjustment screws. If so, do you find it easy to adjust these; I am wonder if I need to consider tools / knobs. Being new to Dobs I have not learned how often the AZ adjustment would be required and mine is stripped. 

 

From the drawing I see that you do not have a rocker box bushing around / supporting the central spring; is this correct? I don’t want to over engineer…ok maybe I do want to but if its not required I would not have to use a bushing.

 

Any additional comments would be greatly appreciated. 

Ralph



#9 Jon Isaacs

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Posted 27 November 2020 - 07:46 PM

Ralph:

 

There base uses Teflon against a laminate. The spring just takes some of the load to lighten the action. 

 

With your steel rollers,  you don't need that contraption. You just need the center bolt secured to the ground board, the bearing sleeve, a washer and a lock nut.

 

I just posted it to show the center bolt secured to ground board. It probably caused more confusion than clarity.

 

Jon



#10 ralphieboy

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Posted 27 November 2020 - 08:28 PM

Jon,

It’s all good; the diagram helped and gave me some ideas. I did see some threads where they threw out the old lazy Susan parts and went with a central bearing and Teflon slides. I will rebuild / replace the central bolt as suggested and keep the sheets for now. Thanks.
Ralph


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#11 Jon Isaacs

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Posted 28 November 2020 - 04:50 AM

Jon,

It’s all good; the diagram helped and gave me some ideas. I did see some threads where they threw out the old lazy Susan parts and went with a central bearing and Teflon slides. I will rebuild / replace the central bolt as suggested and keep the sheets for now. Thanks.
Ralph

 

:waytogo:

 

Ralph:

 

Sounds like a good plan.  If you find the lazy Susan design doesn't work for you, that's the time to think about Teflon-Laminate bearings.

 

Jon



#12 BradFran

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Posted 28 November 2020 - 05:39 PM

It was modified by the previous owner. Here is a link to the Zhumell instruction manual showing how the bearing was from the factory. The lock washer wasn't originally there, but was probably added to replace the missing thrust bearing, a washer with ball bearings in it, that sits between two other flat washers. Some people have reported more success with a needle bearing and recommend it as an upgrade. Here is an example of one for $3. Be sure to see if your bolt is 8mm or not, you need the right size hole for your bolt.

 

These top tensioning blots are not very good, but get the job done. The bearing Jon describes is superior, but might take some tweaking to work with the lazy Susan in your 12".

 

There is a good mega thread in this forum about modifying the Zhumell Dobs, lots of good info you might want to delve into.



#13 FlyingV74

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Posted 28 November 2020 - 08:40 PM

I’m a new Z8 owner, trying to figure out how to get consistent azimuth motion. I have learned the hard way that over-tightening the center bolt will cause the ball bearing to dig into the steel washers surrounding the bearing and create a groove. This is obviously a very bad thing. I’m of the opinion that a properly gauged/installed spring washer would prevent this from happening and would keep center bolt from coming loose. Of course from monkeying around with roller skates, I am of the opinion that a roller skate bearing would probably work better. Anyone who has monkeyed with tightening the nuts on roller skate wheels will know what I mean.

If you haven’t already, definitely take a look at the Z8 assembly instructions. Based off what I see in your photos, it looks like the center piece of tubing that centers the lazy Susan and upper azimuth plate is missing. Without this in place, you can greatly over-tighten the center bolt, which is what it looks like has happened in your case. That is why the center of the lazy Susan has circular scratch marks all over it. That should not be happening unless the center bolt is greatly overtightened.

Hope this helps.


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