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AVX Question

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#1 DonH

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Posted 13 June 2015 - 10:20 AM

I just bought a new AVX and wanted to upgrade to a ADM dual saddle.
The DEC axis of the AVX was very tight making balancing a hit or miss proposition.
When I removed the Celestron saddle there was a thin nylon washer on the outer edge of
the Dec axis. Do any of you AVX experts know what this washer is for?
When I put the ADM dual saddle on and bolted it down the DEC axis was frozen in place,
it would not rotate. After much fiddling around and putting the Celestron saddle back on, I
had the same problem. I then measured the nylon washer and found it to be 0.027" in thickness.
I then measured the difference in higth of the Dec axis shaft and found it to be 0.024", thus when the two are mated the nylon washer is higher than the Dec axis shaft. I thought, wow, now I've got this nailed, just leave out the washer and all is right.
That lets me ask the question again, What is the purpose of the washer?
Attached are pics showing the problem.


Don Halter

Attached Thumbnails

  • AVX Problem172.JPG
  • AVX Problem171.JPG
  • AVX Problem173.JPG


#2 EFT

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Posted 14 June 2015 - 12:01 PM

This is the biggest problem area of the AVX mount.  That ring is bearing that is supposed to keep the ring gear from rubbing directly against the metal axis head/saddle.  Metal to metal contact is the least desirable contact in this situation (so you ask, "Why does the entire inner surface of the ring gear ride directly on the metal axis shaft?").  As you have discovered the thickness of this bearing (or the one on the opposite end of the ring gear) is problematic.  I prefer to avoid leaving nothing in there, but unless you have access to material of different thicknesses (in this case some 0.015" to .020" PTFE works well), there's not much you can do about that.  It's not the best situation, but you will likely be OK without the bearing in there, but keep it around just in case.  The other option is to raise the head a little to leave enough space for the bearing, but that is a challenge as well since you can only raise it a very small amount.



#3 DonH

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Posted 14 June 2015 - 01:37 PM

Ed,
Thanks for the info, much appreciated.
Question again,
Since I have 0.024" of clearance now between the axis head and the saddle, is there
that much vertical play that there is a chance of rubbing? If there is, the rubbing can only occur during balancing or when the clutch is released, but not while trackingWhen the clutch knob is tightened it locks the dec axis to the ring gear.
Am I correct in my thinking on this?

Since I started with a very tight Dec axis, almost like the mount was greased with
cosmoline instead of normal grease, this heavy stiction has not changed even when I
assemble without the nylon gasket. Without the saddle attached the Dec axis is
extremely tight. It feels like the Dec axis nut at the bottom of
the axis where the counter weight shaft screws in, is to tight, reminds me of my CGEM before I did the hypetune. In your opinion is it worth loosening this a bit or will I get to much vertical play?

The grease I see in the mount, giving Celestron the benefit of doubt is
and looks like and feels like Superlube. I guess the tolerance between the Dec axis
and the ring gear is so tight that even with the proper grease it will drag.

#4 EFT

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Posted 14 June 2015 - 06:25 PM

The problem is that, without the bearing in place, the entire shaft can move up and down since there is no longer anything constraining that movement.  Granted, it's not much, but it is there.

 

The axis is stiff because the entire inner surface of the DEC ring gear is in direct contact with the axis shaft.  The end(s) of the ring gear are also in direct contact with the plastic bearings.  Below the ring gear, the DEC axis (machined casting) is in direct contact with the axis housing and on the other end of the axis there are also a couple of substantial friction points as well.  There's also a lot of grease in there (whatever it happens to be) so the end result is very stiff movement.  There's a lot of friction there even though the surfaces are pretty smooth.  The CG-5 had the same issue but they actually increase the amount of surface to surface contact in the AVX.  Even if you remove the grease and replace it with less of something better, things will remain pretty tight.  It's a real struggle to get these things loosened up.



#5 DonH

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Posted 14 June 2015 - 11:00 PM

OK, Thanks Ed. I will leave it alone.

Don

#6 MCovington

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Posted 13 November 2017 - 10:34 PM

I had the same symptom, and the problem was at the other end of the dec axis.  The set screws on the retaining ring had come loose, and when I tightened the counterweight lock, I was also tightening the retaining ring and pulling the whole dec axis farther away from the saddle.  All was well until I replaced the saddle and took up all the slack at that end; then it froze.

The cure involved tightening the retaining ring only moderately tight, locking it in place with its set screws, and adding a washer so that the counterweight lock does not directly turn the retaining ring.

More here: http://www.covington...ex.html#x171113

171113-setscrew.jpg

 

171113-retaining.jpg

 

171113-washer.jpg


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#7 Whuppy

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Posted 14 November 2017 - 08:21 AM

I had the same exact problem with my AVX. Remove the plastic bearing. You'll find the dec axis will move much more freely. I realized that when I tightened the saddle it was sandwiching the bearing and causing it to literally lock up. As I loosened it, it got freer. When removed, there is no metal to metal contact. Keep in mind that you normally don't track in dec so wear is minimal if any at all. Slop is determined by the locking ring. It should only be tight enough to remove the end play. Mine has no slop, it's all tight, glides like butter and balances with ease. The difference is night and day. Also the 4 bolts holding the saddle on are supposed to be TIGHT. I realize some may not agree with this method, but it tremendously improved guiding in Dec for me...

#8 MCovington

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Posted 14 November 2017 - 10:34 AM

Remove the plastic ring under the saddle, you mean?  I've seen that suggested, and also replacing it with a much thinner one (so it is just a space filler, not load bearing).  (Earlier in this thread, in fact.)  Hmmm... I want to test it the way it is first.


Edited by MCovington, 14 November 2017 - 10:37 AM.

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#9 MCovington

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Posted 14 November 2017 - 08:07 PM

Celestron tells me that loosening the 4 bolts under the saddle, and then using Loctite to secure them, is also an acceptable solution.  But I prefer mine.  I'd rather adjust the tension at the end where it's supposed to be adjusted.



#10 EFT

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Posted 14 November 2017 - 08:22 PM

Do you mean the four bolts that hold the saddle on?  They use way too much thread lock on way too many things in the first place.



#11 MCovington

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Posted 14 November 2017 - 09:36 PM

Yes; it doesn't make sense to me to threadlock those bolts.  I think it is much better to tighten them down and adjust from the other end as I did.



#12 EFT

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Posted 14 November 2017 - 11:29 PM

I certainly don't use thread lock on those bolts.  If someone wanted to use thread lock there, it should only be medium strength (blue) at the most.


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#13 Whuppy

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Posted 15 November 2017 - 01:06 PM

Remove the plastic ring under the saddle, you mean? I've seen that suggested, and also replacing it with a much thinner one (so it is just a space filler, not load bearing). (Earlier in this thread, in fact.) Hmmm... I want to test it the way it is first.


Yep, that's exactly what I mean. I didn't notice any thread locker but I do remember the bolts "popped/cracked" when I loosened them. I just didn't understand why when I put the saddle back on the dec axis got so tight. I did back them off a bit but could never get good free movement that didnt leave the saddle a bit loose. That's when I took the plastic bearing out and things got way better.

This is the way I see it...

If you notice, the 4 bolts screw into center part of the drum that sits slightly higher than the outer ring. The only time that bearing comes into play is when you release the clutch such as when balancing. When the clutch is tightened then both pieces are locked together and the bearing does nothing. If removing the bearing causes metal to metal contact, I'm sure my Dec axis wouldn't move as freely as it does and I only put a small coating of Super lube on the outer ring before putting the saddle on.

I know this goes against the "norm" and can seem a bit radical but it only takes a minute to try.

#14 Whuppy

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Posted 15 November 2017 - 01:10 PM

Yes; it doesn't make sense to me to threadlock those bolts. I think it is much better to tighten them down and adjust from the other end as I did.


That makes sense? When I put mine back together I started with the locking ring and then put the saddle on. Perhaps doing it your way allowed more clearance for the bearing under the saddle?

#15 EFT

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Posted 15 November 2017 - 01:20 PM

This is exactly why, when I HyperTune an AVX, I put it back together with a PTFE bearing that is half the thickness of the original plastic bearing.


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#16 SonnyE

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Posted 15 November 2017 - 01:31 PM

I certainly don't use thread lock on those bolts.  If someone wanted to use thread lock there, it should only be medium strength (blue) at the most.

I can see no reason to use thread locker of any strength on these.

Mine have never loosened from my fiddling with them. And I have never felt any need for Loctite blue there.

 

A tractor, or lawn mower, then sure. But not my AVX.... wink.gif



#17 EFT

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Posted 15 November 2017 - 02:08 PM

I agree.  I have never seen a loose saddle on an AVX (I have seen them numerous times on a CGEM/DX).


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#18 MCovington

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Posted 15 November 2017 - 02:23 PM

 

Yes; it doesn't make sense to me to threadlock those bolts. I think it is much better to tighten them down and adjust from the other end as I did.


That makes sense? When I put mine back together I started with the locking ring and then put the saddle on. Perhaps doing it your way allowed more clearance for the bearing under the saddle?

 

That's exactly it.  The rotating middle part was sitting too low in the non-rotating outer part until I adjusted it.  I decided to adjust something that was obviously designed to be adjusted, and whose set screws had loosened, and which had been thrown out of adjustment by something I understood (the counterweight bar lock), rather than trying to re-engineer the saddle mount.

The plastic ring bears very little weight or tension when the other end of the dec axis is adjusted the way I did.



#19 MCovington

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Posted 15 November 2017 - 02:24 PM

This is exactly why, when I HyperTune an AVX, I put it back together with a PTFE bearing that is half the thickness of the original plastic bearing.

Where do you get those?



#20 MCovington

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Posted 15 November 2017 - 02:25 PM

I agree.  I have never seen a loose saddle on an AVX (I have seen them numerous times on a CGEM/DX).

Hmmm, I also have a CGEM.  Should I check its saddle and maybe threadlock it?



#21 SonnyE

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Posted 15 November 2017 - 07:29 PM

 

I agree.  I have never seen a loose saddle on an AVX (I have seen them numerous times on a CGEM/DX).

Hmmm, I also have a CGEM.  Should I check its saddle and maybe threadlock it?

 

If I may interject here....

I would only go so far as to see if any have loosened.

And advise they be checked now and then before mounting a telescope.

If they loosened, or appear to be loosening, then maybe a tiny smear of Blue loctite to see if it helps. The thinking here is: one could try a larger amount on the second go.

However... Loctite training expresses that less is more.

So don't "paint" or "saturate" the threads.

Like Brylcreem, a little dab'll do ya. cool.gif

 

Please excuse my interruption...  wink.gif

 

Also, Dr. Michael, Thanks for the critical info on the adjustment ring. I was going to ask about the size for the Allen wrench, but see in the text it is 1.5 mm. waytogo.gif


Edited by SonnyE, 15 November 2017 - 07:35 PM.

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#22 SonnyE

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Posted 15 November 2017 - 07:43 PM

I agree.  I have never seen a loose saddle on an AVX (I have seen them numerous times on a CGEM/DX).

Thank You for sharing your knowledge on the AVX, Ed.

Your input is appreciated. wink.gif

I wanted you to know that.


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