74 replies to this topic

[AstroNikko]

I'm sharing my story here with the hope that if this happens to anyone else, they'll know what to do. Although, this should really apply to anyone with an AVX mount.

 

A little over 6 months ago, I upgraded the mount for my Celestron AstroMaster 130 from the CG-3 with RA motor drive to an Advanced VX mount. The Declination axis had always been a bit stiff for my liking, even after adjusting the tension on the axis. But until just a few days ago, it had never seized up on me. I did get a new scope for Christmas though, an Explore Scientific N208CF. I think the added weight of the scope plus the 2nd 11lb counterweight is what led to the issue manifesting itself so late. The AstroMaster 130 only weighed 12.2lbs, whereas the N208CF weighs 18.4lbs. Still well under the 35lb max payload capacity of the mount.

 

Luckily for me, the Dec axis seized up while I was balancing the scope, instead of putting that kind of load on the Dec motor. As it happened, I remembered back to a video I had seen earlier, even before I had upgraded to the AVX mount, where another AVX owner had this same issue with two different AVX mounts straight out of the box. 

 

The issue that causes the Dec axis of the Celestron Advanced VX mount to bind up and seize is due to the counterweight arm lock nut not having enough spacing between itself and the axis housing. The quick and easy solution is to simply invert the lock nut. The long-term solution is add a washer for spacing.

 

Knowing this, I was able to get my rig running smoothly again, and was still able to perform my imaging session that night. The long-term solution though, is to insert a washer into the recess of the lock nut to provide just enough spacing to prevent it from binding up and seizing again. A rubber garden hose washer will do the job perfectly. With the hose washer in place, the Dec axis glides super smooth now.

 

Please note, if the Dec axis for your mount is still stiff after adding the washer, the tension on the Dec axis may also need to be adjusted.

 

 

 

 

 

I hope this helps my fellow AVX owners here.

 

Clear skies,

 

Nikko

[AhBok]

You can also remove the PAS cover and just make sure the CW shaft is not screwed in far enough to interfere with the DEC shaft. Then screw the collar on tightly to make the connection of the CE shaft and mount rigid. This worked well for my former AVX.

[AstroNikko]

You can also remove the PAS cover and just make sure the CW shaft is not screwed in far enough to interfere with the DEC shaft. Then screw the collar on tightly to make the connection of the CE shaft and mount rigid. This worked well for my former AVX.

Please forgive me. I have to disagree with this suggestion to avoid confusion. This is something I've tried, as I've seen others do the same. Unfortunately, it doesn't work, and can actually exacerbate the problem, as I'll try to explain.

 

To be clear, the counterweight bar and Dec shaft should be in solid contact. Which is what the collar, or lock nut, is for. They should rotate smoothly around the Dec axis together as a single solid unit. By threading the counterweight bar into the Dec shaft as far as possible, it limits the amount of lateral movement in the counterweight bar due to any amount of play in the threading. The more lateral play the counterweight bar has, the more likely it is that the collar will make contact with mount body under the pull of the counterweights, causing it to rub and potentially seize up. As you can see from the image in my original post.

 

I believe this problem is due to a design flaw in the AVX mount. The Dec shaft should extend 1-2mm further beyond the lip of mount body to prevent it from making contact with the counterweight collar. By adding a washer in this case, enough spacing is added to prevent the collar from making contact with the mount body.

[WadeH237]

Good post.

 

The rule of thumb is that the large lock nut must not make contact with the mount housing.  If it does, as the dec axis turns, the lock nut will bind to the housing.  Since the dec axis is turning and the lock nut is not, it will either loosen or tighten the lock nut.  In the tightening case, it will tighten until the dec clutch slips and then it's bound.  It can bind so tightly that you cannot remove it by hand (hint: if this happens, loosen the dec clutch and turn the dec axis off the nut to release it).

 

I've seen this happen on one of my AVX mounts.  I've not had a chance yet to look closely at the other one, where I've not had this happen.  For the life of me, I cannot figure out why it doesn't happen on every AVX...

 

Anyway, I was planning on sizing a washer to fit around the larger dec shaft that the counterweight shaft plugs into.  But I just tried a hose washer, and it fits perfectly on the counterweight shaft and lock nut.  And it provides sufficient spacing to solve the problem completely.

 

Nice work!

[AstroNikko]

Thank you! I’m glad this was helpful.

[AhBok]

Please forgive me. I have to disagree with this suggestion to avoid confusion. This is something I've tried, as I've seen others do the same. Unfortunately, it doesn't work, and can actually exacerbate the problem, as I'll try to explain.

 

To be clear, the counterweight bar and Dec shaft should be in solid contact. Which is what the collar, or lock nut, is for. They should rotate smoothly around the Dec axis together as a single solid unit. By threading the counterweight bar into the Dec shaft as far as possible, it limits the amount of lateral movement in the counterweight bar due to any amount of play in the threading. The more lateral play the counterweight bar has, the more likely it is that the collar will make contact with mount body under the pull of the counterweights, causing it to rub and potentially seize up. As you can see from the image in my original post.

 

I believe this problem is due to a design flaw in the AVX mount. The Dec shaft should extend 1-2mm further beyond the lip of mount body to prevent it from making contact with the counterweight collar. By adding a washer in this case, enough spacing is added to prevent the collar from making contact with the mount body.

I hear you and am glad that you found a solution that works for you. You should know though, that lots of guys do exactly what I suggest with their AVX and CG5 prior to that for years with no problem. Go to a big star party and see what other AVX users do. Perhaps there was a design change for yours. But again, if your mod works for you, that’s great.

[strelnikoff]

I haven't encountered this issue yet, but then again - my scope is not that heavy. Adding a washer is a simple solution.

 

But I'm trying to understand the problem - from what I understand here (from the post) is that counterweight shaft must be in contact with DEC shaft in order to rotate with it? I thought (didn't really looked into it) that counterweight bar is static and rotates around RA axis only - while DEC shaft/motor rotates scope around DEC axis? I have to look into my AVX... 

[WadeH237]

I'll see if I can describe it...

 

Forgetting the lock nut for a moment, the counterweight shaft itself screws into the declination assembly.  By this, I mean that it screws into an assembly that rotates with the declination axis.  For example, with the scope assembled, when the declination axis moves, the counterweight shaft, and the counterweights, moves with it.

 

So what about the lock nut?

 

The lock nut isn't really a nut.  It's a fairly large piece (at least as compared to just a nut) that screws onto the counterweight shaft before you screw the counterweight shaft onto the mount.  The lock nut is intended to brace the counterweight shaft against the declination assembly to stiffen the counterweight shaft.  When that happens, the lock nut, counterweight shaft, and declination axis all move together.  This is all as it should be.

 

The issue comes up when the lock nut makes contact with the mount housing (ie. the outside of the mount, which does not rotate with the declination axis) before it locks against the rotating part.  When that happens, the lock nut cannot move freely with the counterweight shaft and stays stationary with the housing.  When that happens, the declination axis and counterweight shaft will still turn together.  Since the stationary lock nut is threaded to the moving counterweight shaft, it will either get tighter or looser on the counterweight shaft, depending on the direction it's turning.

 

That's a bad state, because if the declination axis turns in the direction that tightens the lock nut, it will bind the whole declination axis.

 

You could avoid the problem by not fully tightening the lock nut.  This would leave the lock nut loose enough that the declination axis couldn't tighten it too much (unless if turns more than one revolution), but the lock nut would not serve any useful purpose in that case.

 

To fix the problem, we are inserting a washing inside the lock nut around the threads of the counterweight shaft.  That way, when you tighten the lock nut, the washer will make contact with the rotating declination assembly before the skirt of the lock nut can make contact with the housing.  That means that the declination axis, the lock nut and the counterweight shaft can move together without interference from the housing, and the lock nut can be fully tightened.

 

I hope that this helps to explain it.

[strelnikoff]

I'll see if I can describe it...

 

Forgetting the lock nut for a moment, the counterweight shaft itself screws into the declination assembly.  By this, I mean that it screws into an assembly that rotates with the declination axis.  For example, with the scope assembled, when the declination axis moves, the counterweight shaft, and the counterweights, moves with it.

 

So what about the lock nut?

 

The lock nut isn't really a nut.  It's a fairly large piece (at least as compared to just a nut) that screws onto the counterweight shaft before you screw the counterweight shaft onto the mount.  The lock nut is intended to brace the counterweight shaft against the declination assembly to stiffen the counterweight shaft.  When that happens, the lock nut, counterweight shaft, and declination axis all move together.  This is all as it should be.

 

The issue comes up when the lock nut makes contact with the mount housing (ie. the outside of the mount, which does not rotate with the declination axis) before it locks against the rotating part.  When that happens, the lock nut cannot move freely with the counterweight shaft and stays stationary with the housing.  When that happens, the declination axis and counterweight shaft will still turn together.  Since the stationary lock nut is threaded to the moving counterweight shaft, it will either get tighter or looser on the counterweight shaft, depending on the direction it's turning.

 

That's a bad state, because if the declination axis turns in the direction that tightens the lock nut, it will bind the whole declination axis.

 

You could avoid the problem by not fully tightening the lock nut.  This would leave the lock nut loose enough that the declination axis couldn't tighten it too much (unless if turns more than one revolution), but the lock nut would not serve any useful purpose in that case.

 

To fix the problem, we are inserting a washing inside the lock nut around the threads of the counterweight shaft.  That way, when you tighten the lock nut, the washer will make contact with the rotating declination assembly before the skirt of the lock nut can make contact with the housing.  That means that the declination axis, the lock nut and the counterweight shaft can move together without interference from the housing, and the lock nut can be fully tightened.

 

I hope that this helps to explain it.

 

It does - and it's exactly what I thought it was. So far - I may have been lucky, I had no issues like this. However, I will... I WILL (promise to myself, out loud and public) get bigger scope and autoguider and all kinds of gizmo's - so this may become an issue. Great to know how to address this. 

 

In fact, it was kind of obvious from Nikkolai's photo - but I didn't know lock nut is not supposed to touch the mount body (and didn't cared honestly). Great stuff!

 

I actually really happy I'm here on this forum - in 2 days I sorted out few basic problems and learned a lot. 

 

 

Btw - I see you have an Astro-Physics mount - and AVX. Considering that Astro-Physics is ... well, kind of a Rolls Royce in mounts world (is it? so I've heard) - why do you still keep AVX? 

[WadeH237]

Btw - I see you have an Astro-Physics mount - and AVX. Considering that Astro-Physics is ... well, kind of a Rolls Royce in mounts world (is it? so I've heard) - why do you still keep AVX? 

Different mounts for different purposes.

 

I use the Astro-Physics mounts for imaging can carrying large scopes like my C14.  I use the AVX mounts for visual use with smaller telescopes.  They are much easier to carry around for casual use than the Astro-Physics mounts.

 

I have multiple mounts for visual because I often bring family and friends out to star parties and like to share views.  I bought the AVX mounts in 2017 for the solar eclipse when I brought 23 people to OSP with me.  During the partial phases of the eclipse, I had a PST on one of them, and a Herschel wedge with my 4" refractor with the other.

 

I keep one of the AVX mounts set up at home with a scope on it for casual use.  It is excellent for the purpose.

[Michael Covington]

See this, with pictures:   http://www.covington...ex.html#x171113

[AstroNikko]

I’m going to try this.

I haven’t been entirely happy with my current rubber washer solution. It compresses, and as a result I havn’t been able to keep the lock nut tight. Really do need a metal washer in this case.

Thanks for sharing!

[Michael Covington]

Be sure to pay attention to what is going on at the top end of the dec axis (under the dovetail shoe).  That is the key to it.

[AstroNikko]

Thanks for the heads up. I've had the dovetail shoe off once before in order to open up the Dec motor housing for tuning, but I don't remember seeing the plastic bushing you mentioned in your post. I'll definitely take another look, and pay closer attention this time. Unfortunately, it doesn't look like the Home Depot in my area carries the steel spacer bushing. I'm going to go check out the fasteners isle again to see if there's anything else there that might fit.

 

Thanks for posting and sharing that guide!

[AstroNikko]

Could also be called a machine bushing, but still not turning up any at my local Home Depot. Will need to go look around.

[AstroNikko]

See this, with pictures:   http://www.covington...ex.html#x171113

After reading through your post more thoroughly, it seems one would want to keep any sort of tension off the retaining ring. So why not use a smaller steel washer inserted in the recess of the locking nut, similar to the rubber hose washer I suggested in the original post here?

 

In the case of your suggested fix, the washer is still in contact with and applying force on the tension ring. The solution I suggest here in the original post provides enough spacing to prevent the counterweight lock nut from making contact and binding with the retaining ring, or the outer casting.

 

Your guide does address the issue of the dovetail shoe binding with the outer casting. This is something I still need to inspect on my mount.

[strelnikoff]

I'm patiently waiting to see how this will be resolved before I implement solution to my AVX

[Michael Covington]

I think yours is indeed the better solution -- nothing extraneous contacting the retaining ring at all.  Empty space is better than a well-lubricated washer! 

 

I will take another look at all of this when I can.  Thanks for posting.

Edited by Michael Covington, 09 January 2019 - 07:25 PM.

[AstroNikko]

Haven't had much luck with hardware stores, so broadening my searching to auto parts as well. Currently looking for an M16 aluminum drain plug gasket. Looks like NAPA has one in stock that might do the trick. Going to pick one up today to try out.

 

For future reference, the counterweight lock nut has an M16 thread, while the inner recess of the lock nut has a diameter of 26 mm.

 

The rubber hose rings work well to provide the spacing needed, but they get mashed up pretty good under the tension of the lock nut.

 

 

 

New washer (left) included for comparison.

[fgraus]

You could get some 3/4" ID PVC pipe and cut off what ever thickness washer you need.

[AstroNikko]

It looks like the M16 aluminum drain plug gasket is the way to go. It has an outer diameter of 23 mm, and a thickness of 2 mm. Which provides plenty of spacing to prevent the lock nut from binding with the retaining ring and outer casting. Being made of aluminum, it also doesn't crush the way the rubber washer did.

 

 

 

Works like a charm, and only cost $0.85, including tax.

[strelnikoff]

It looks like the M16 aluminum drain plug gasket is the way to go. It has an outer diameter of 23 mm, and a thickness of 2 mm. Which provides plenty of spacing to prevent the lock nut from binding with the retaining ring and outer casting. Being made of aluminum, it also doesn't crush the way the rubber washer did.

 

 

 

 

 

Works like a charm, and only cost $0.85, including tax.

Also - being made of aluminum/aluminium - you may want to check it later on. The material is notorious for surface abrasion and contact wear under stress/dynamic load. You will be probably fine for a while. 

It would be awesome if there were teflon rings to use - but those too will wear eventually.

[AstroNikko]

Thanks. I’ll keep an eye on it. I know these crush under a decent amount of tension, but wasn’t sure that would be the case here with hand tightening.

I did see nylon and synthetic fiber gaskets as well, but I haven’t given any of those a try yet. Figured they may not hold up as well as aluminum. The copper ones were all too thin to provide adequate spacing.

[Michael Covington]

Also - being made of aluminum/aluminium - you may want to check it later on. The material is notorious for surface abrasion and contact wear under stress/dynamic load. You will be probably fine for a while. 

It would be awesome if there were teflon rings to use - but those too will wear eventually.

As I understand it, with his washer choice, as opposed to mine, the parts on either side of the washer are not moving relative to each other.  The washer is just taking up space to make sure the outer part (hand-tightened lock knob) sits far enough from something it might otherwise contact.

[AstroNikko]

As I understand it, with his washer choice, as opposed to mine, the parts on either side of the washer are not moving relative to each other.  The washer is just taking up space to make sure the outer part (hand-tightened lock knob) sits far enough from something it might otherwise contact.

Yep, that's pretty much it. Once the lock nut is tightened down, there's no further movement.

 

I suspect any wear that might happen to the washer would be due to repeated loosening and tightening of the lock nut. Personally, I don't normally have to move the mount all that far, though. Just from the house to the backyard, and back again. I don't usually bother removing the counterweight bar when moving the mount head. I might remove it when traveling any real distance, though. Such as to a dark sky site. I still can't imagine the washer wearing out, even with regular loosening and tightening of the lock nut. Also not sure if you can manually apply enough torque to the lock nut to be able to crush the washer.

 

I'll keep an eye on it though, and report back here if I notice any wear at all to the washer itself, or the surfaces it comes in contact with.