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Vixen SXD tuning

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#1 inZet

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Posted 29 July 2014 - 03:21 PM

I would like to share my experience tuning the Vixen Sphinx Deluxe mount. I had problems balancing the mount because of what I felt extreme tightness of the axes, but also an irregular axes rotation. It was also very strange to me that after one hour of work the axes where smooth in their movements.

Posted Image

The first problem was the grease on the worm gear: two worms, two kind of grease? The first one was gluey; the second like moisture: both to be replaced, removed using gasoline, polished, and used litium-based grease.
When the worm gears where polished, the second problem arised: the bearings where not working properly. I don't know if they where broken or dirty: I've replaced them (6 euros both). It was like having a stepper motor instead of a smooth rotation: now they are ok.

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If you dismount the system, remember to remove the screw that blocks the axis only one side: the other side is blocked with a green glue, DON'T remove it, don't force it, it's the position regulator of the worm gear.

When you'll dismount the RA and DEC axes, you'll find a bearing ring like this.

Posted Image

These are not bearings but "buffer rings" made of cylinders. There is one ring at both sides of the axis. The sequence is: stainless steel ring, buffer ring, stainless steel ring. Grease between the components. If one states the SXD has bearings on the axes, THIS IS NOT TRUE.
To dismount an axis, you have to unscrew a blocking ring first. Pay attention to one side: a tiny screw (a grain) prevents the ring to be removed. Unscrew the grain first using an allen tool. I had to cut a piece of allen because was too long to be inserted in place.

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Remove the nut to dismount the RA worm gear. It is mandatory to dismount the motor, too.

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The motor should be remained dismounted to fine tune the distance of the worm gear from the wheel. Use an allen to tighten the two side screws and loosen the center screw. If the distance is too close the axes are hard to move. This is not bad in general, but unluckly Vixen used brass for both cogwheel and worm gear. If the pieces are too close, the risk is to seize or consume the gear. So the process is try and error.

When you'll mount all again, the axis should move like butter. Tighten the axes. This is good. If you can't move the axes, the motor will do. Their force is high compared to their size.
Once the SXD is fully mounted, power it on and look at the gears near the motors. The RA should move slowly, smoothly, without jerking.

That's it. It's not difficult, you can do it yourself, just take your time.

Gianluca
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#2 orlyandico

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Posted 29 July 2014 - 09:10 PM

1) if the worm felt like a "stepper" i.e. detents, it means the worm bearings were overly preloaded and have worn dents into the bearing race. The detents are the balls rolling in and out of these dents. Replacement (and not too high a level of preload) is correct.

2) those "buffer rings" are actually thrust bearings. They are used to provide support/low friction parallel to the shaft.
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#3 astrobug

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Posted 12 September 2016 - 01:38 PM

Gianluca-

 

Thanks so much for posting your Vixen SXD tuneup photos. I got one used (with the NexSXD upgrade) a few years ago. It has worked great for two seasons. But this summer, jerky motion in RA has caused problems for imaging. I am getting lots of streaked stars, even with PHD autoguiding. PHD is constantly sending guide commands, but there is still a lot of residual PE (max sometimes as high as 20", RMS is usually at least 3-5" after a few minutes of guiding).

 

My previous mount was an LXD-75, so I immediately suspected mechanical issues. Luckily I found your old post. I opened it up, and removed the spur gears from the motor shafts to check the smoothness of the worm rotation. The RA and DEC both have a too-tight, grumbly/clicking action as I spin them by hand. This appears to be the exact same problem you had (stepper motor feel instead of smooth rotation, probably caused by overloading these bearings).

 

To fix it, I will be replacing all 4 bearings in the worm blocks. I will also do a complete tear-down and tuning like you did: de-grease, re-grease, worm adjustment, etc.

 

 

A few questions before I start this project:

 

What size of bearings needed to mount the worms? 

 

What size allen tool is needed for the set screw ('grain') that prevents accidental loosening of the axis locking rings? 

 

grazie mille!

-Brett

 

 

 

 


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#4 inZet

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Posted 29 September 2016 - 01:55 AM

I'm sorry but I can't find the broken bearing, they're lost.

They where metric for sure, I had no difficulty to buy them in Italy.

 

The allen to turn the grain has a real measured diameter of 1.8mm but the box prints 1.5mm, and esagonal shape. The short side of the L should be 9mm long.

 

Ciao!

Gianluca


Edited by inZet, 29 September 2016 - 01:58 AM.

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#5 astrobug

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Posted 29 September 2016 - 03:44 PM

Gianluca-

 

Thanks for looking and the info on the allen key. I'll dismantle the mount and post back here with the bearing measurements for the benefit of any other SXD owners/tinkers. But that probably won't happen for a few weeks. (Need to finish a few repair projects on the house first.)

 

Ciao!

-Brett


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#6 Betelgeuze

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Posted 03 October 2016 - 07:01 AM

Hello,

I am reading this topic with great interest..
I have a Vixen Sphinx SWX, for almost 9 years and I really love it.
In the beginning I had to loosen the sticky RA axis, but that was all to be done.

 

But for 6 months ago it was starting to give strange RA jumps.
Before that I was easy able to do unguided subs for 5 minutes at 600mm.
I never did a re-grease in al lthose years...

 

So finally I gues that I have to take it apart, so this is the right topic on the right time.

 

My question is, I saw there is no grease at all on the brass motor cogs.
I read years ago on the Sphinx yahoo group, that this is’nt necessary.

What is the opinion from you guys?

 

If people have more photographs f and tipsrom the stripping proces of the mount,
that would  be verry welcome !

 

Thank you in advance!



#7 astrobug

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Posted 04 October 2016 - 01:20 AM

Yes, it sounds like we're having the same problem at the same time. I never tried unguided 5 minute subs at anywhere near 600mm. Maybe after the bearing replacement and tune-up. But 4 minutes at 135mm was no problem at all. And it guided beautifully while imaging at 900mm. But now I'm getting oval/streaky stars even while guiding. Feeling the RA worm motion by hand, I'm not surprised by this 'jumpy' motion in use. I am pretty sure that new bearings for the worms will solve the problem.

 

To respond to your question: I plan to grease to the spur gears ('cogs') between motor and worm. All the tuning instructions for the inferior equatorial mounts (CG-5, LXD-75, etc.) say to grease here. I don't think it would hurt anything to do this. Not greasing could cause excess noise, friction, wear, & (eventually) backlash. The quality of machining on the Vixen is high, so I will probably use a lighter, Teflon-based grease (SuperLube) here and between worm & wheel. Others here have suggested that the stickier 'glue grease' is often used on inexpensive mounts to take up the slop between gears.

 

I'll take pictures and post results here once I get around to this project.



#8 Betelgeuze

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Posted 04 October 2016 - 06:51 AM

When I turn the cog by hand, it feels uneven sometimes, like almost missing a tooth of the worm/gear.
And maybe it has something to do with te fact, I did 'nt used the mount from 2009 to 2015,
because I was fed up and frustrated with the light pollution and bad sky quality in my own country.
(this was afther I was on vacation in South France, star heaven..)

 

But last year I started to use the Sphinx again, because a friend could stop talking about Comet Lovejoy.. and so I got my interest back...

I had no problems in the beginning the first months, and later I rotate the RA axis all the way with the Starbook,
thinking it may be good idea to settle the grease again.. maybe this was not a good idea after all.
I had also succes before guiding my C8 prime focus on 2032mm focal lenght, so I really like the mount, and it never let me down.

But now I'm a be a bit scared it will never be as good as it was, if I take it apart.. but we will see.

 

About lubing, I read about super lube, but saw also messages that it dry out soon or fal apart...
The lube I was planning to use is Lithium based EP2 grease, it's good at different temperatures,
and I used it in the past to grease te baffle of my C8, and it's still feeling good after 8 years.

 

Another "problem" I'm aware of, is the construction of the worm wheel from my Sphinx SXW.
It's different from TS InZet's one.. I

The mine is the same as the one left on the picture.
I hope that I can take it apart safely, by loosening the big shiny bolt..

 

On the picture you see the difference.. Vixen-SXD-SX-worm-gear.jpg



#9 astrobug

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Posted 04 October 2016 - 02:57 PM

The differences in worm retention methods between SX and SXD(2) shouldn't matter too much. Mine is the older version SXD (like inZet's), and looks like the photo on the right. Looks like mine will require a special spanner-type tool. :(  The shiny bolt (or nut) you've got will only need a crescent wrench.  :)

 

Just make sure you don't bend the worm when you remove it. Also, as orlyandico mentioned above, don't overtighten the worm bearings when reinstalling with the new bearings.

 

Thinking about the bearings a bit more, it seems like the RA worm could experience very high axial loads. The counterweight shaft provides a very long lever arm. So an unbalanced scope, locked axis clutch during travel, or a child trying to hang off the telescope could have the same effect on the worm bearings as overtightening. If I can find some that will fit, I may end up installing angular contact bearings. Properly tightened, they should eliminate axial play, while enduring high axial loads better. I'll also think twice before I fully tighten the RA clutch (i.e. only while imaging).


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#10 inZet

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Posted 05 October 2016 - 02:51 PM

Don't use grease on brass cogs, the ones you see when the carter is pulled off. The gear teeth are designed to push, and they can slip. Use it on buffer rings, worm gear, and large gears in RA and DEC; the teeth are helicoidal, designed to screw each other.

I use a litium-based grease used for... automobiles! 8 euros for 1 Kg (!). This grease can resist to high&low temperatures, extreme tortion, and has the right viscosity to turn on axes remaining attached. Given the use on our mounts, it will last *forever*.


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#11 astrobug

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Posted 07 October 2016 - 01:44 PM

Got it: no grease on the cogs (aka spur gears). I have reasonable quantities of Superlube and white lithium grease leftover from my LXD-75 mount tuning days  :p  There has been a lot of discussion and contradictory information about the best grease to use on telescope worm & worm wheel gears. I'm going to heed the advice of a professional mount tuner and use Superlube:

 

http://www.cloudynig...-2#entry7197503

 

Now, I'm not saying white lithium grease is a bad thing to use. It's way better than the 'glue grease' so common in import telescope mounts. And frankly, our uses (low speed, low temperature) aren't very demanding of the lubrication. As long as there's _something_ there to prevent metal-on-metal contact that hasn't solidified, it will probably work. Just be sure to check on a yearly basis and replace as necessary.

 

In other news, I dismantled my SXD mount far enough to remove the DEC worm bearings. Here are the worm bearing dimensions:

 

ID 6mm

OD 13mm

Width 5mm

 

This translates to a '686' bearing size. I have no idea if the SX uses the same size. I ordered 8 so I can do both the RA and DEC now, plus have a spare set in case the problem recurs. I went for ceramic balls & stainless steel races, ABEC 5, which cost a total of ~$30 US on eBay. This seemed like a good compromise between smoothness, corrosion resistance, & price. I'll try to remember to take (and post) photos of my tuneup. But will probably be another week or two before that happens.


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#12 Betelgeuze

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Posted 08 October 2016 - 12:29 PM

The first problem was the grease on the worm gear: two worms, two kind of grease? The first one was gluey; the second like moisture: both to be replaced, removed using gasoline, polished, and used litium-based grease.

 

 

I'm searching the internet for all kind of tips and re-grease story's, just to be shure I do the right thing to my mount.. :)

I bumped in to this discussion, explanes maybe why there are 2 different kind of grease used in the mount ?

 

I'm not sure if im allowed to post a link, ( otherwise the moderater can delete it of course)

 

https://stargazerslo...ng-mount-grease



#13 astrobug

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Posted 21 November 2016 - 05:46 PM

Well, I didn't get any time to really work on the mount until October 30th, and I'm just now getting around to the write-up. The good news is that we are into our usual rainy season, so I haven't lost any observing/imaging time by waiting this long.

 

Disassembly went smoothly, but took a bit of time to figure out how to proceed on some steps. In particular, the counterweight shaft retention nut was stuck on the DEC shaft, so I had to remove the whole thing together, then separate the nut from the DEC shaft. Also, the RA worm adjustments are very difficult to access while the motor is attached.

 

I used lemon-scented Simple Green (a biodegradable household cleaner which is sometimes used to degrease bicycles) and a toothbrush to remove the old grease. I then dried everything off thoroughly.

 

I replaced the bearings in the worm blocks, and snugged them up to "just right" (worm rotates freely, but without end play). Then reassembled everything with a stingier amount of lubrication (Superlube) between the moving surfaces. After reassembly, I kept the covers off so that I could watch for 'cogging' of the spur gears that connect motor & worm shafts while running the motors at all speeds. It seemed to work very well in this test.

 

Naturally, I didn't get a good clear night to try it out 'for real' until a few days ago. Everything seems much smoother and quieter. The guiding performance is much improved (max was<2", RMS was <0.6"). No elongated stars in 4 minute exposures. But there was a lot of backlash in RA; it took ~5 seconds of holding the button down go from tracking to eastward movement. Plus there was a noticeable 'wiggle' in RA when handling the scope or counterweight shaft. Darn, I thought I had everything adjusted "just right"! (At least the DEC axis was ~perfect, no backlash to speak of...but of course, it's the easier one to adjust :p).

 

So yesterday, I opened it back up, and checked the RA adjustments. Strangely enough, there was quite a bit of axial 'end play' between the RA worm & block. So I snugged up the worm bearing mountings again, and readjusted the mesh between worm & worm wheel. I will have to wait for another clear night to check the backlash, but it seemed to work well good indoors. I'm just hoping that the RA worm doesn't loosen up in its block again. There may have been some thread locker in there from the factory (that I didn't replace), and if the problem recurs, I will likely add some removable thread locker (blue Loctite) to keep the worm bearings in place.

 

Anyhow, long story short, I think my mount is now fixed, and I'm anxious to start using it again. FYI, photos & some comments on the tear-down are available here:

https://goo.gl/photo...JChbp6oAFxmdXz9



#14 Betelgeuze

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Posted 11 February 2017 - 09:34 AM

So how it ended on my SXW....

 

First apologize, for entering a topic about the SXD, while my mount is a SXW.... (well it is a Vixen..) ;)

 

The difference between a SXW and SXD is also totally clear to me....

There are no bearings at ALL in the SXW, only in the Dec clamp-head.

 

I still wonder how its possible the mount works on just telflon bearings.

 

Disassembly went also verry smoothly, I was in no hurry, double checking every action,

except one part of a small nightmare, I mixed up writing down the number of teflon washers on te RA and Dec axis.  :foreheadslap: 

At reassembly this finaly become clear, but before that, it gives me some sleepless nights. 

I cleaned off all the old grease, which was'nt in a very bad shape after all, and put the small parts in a ultrasonic cleaner.

I also inspected en re-grease the motor gear head, first one motor to compare the movement and sound under full load, and re-grease the encoder axis

 

Reassembly was another story, this took a long time..

Most of the time I spend adjusting the axial free play of both the worms, and the gear mesh adjustment.

I was very precise on this, and after re-adjust everything for 5 times, before I was satisfied.

And I also spend a bit on the Ra and Dec axis free play adjustment.

 

The second test under the stars, was visually checking the sound, movements and goto's.

Everything operated perfectly, sound was smoother,

buttons on the Starbook reacting very direct, and the goto's were spot on. :waytogo:

 

A few weeks later I did a Pec measure with my ED80 and OAG/Lodestar under bad seeing.

Using the part of the guidelog from PHD2 using the guide assistant, so no guide commands are sending to the mount, and load the log into PECREP.

(shame on me I did'nt do a Pec measure before I took the mount apart, so I have nothing to compare)

PECREP showed the Ra  Pe was  Pe + 4.35" and - 4.22" en on the Dec + 2.44" and - 2.69". 

 

It was worth the time I spend, I gues.

 

Thank you all for the inspiration !  :flowerred:

 

 

 

 

 

 

 

 



#15 astrobug

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Posted 19 February 2017 - 11:34 PM

Betelgeuze-

 

I'm glad to hear that your mount is now tuned to near-perfection! I didn't take my motors/encoders apart to re-lube, but that would probably be a good idea. I should also test my unguided periodic error; +/- 4.5" would make me very happy indeed!

 

-Brett



#16 Betelgeuze

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Posted 20 February 2017 - 07:18 AM

Yes you should be very happy, when you indeed did the PE measurement 100% right.. :x

 

 

It seems to easy..

Just use a unguided log and load it into PECREP and fill in the "right" numbers...

 

But....

 

The out coming of my measurement seems not 100% reliable due a few facts..

 

It seems a sort of I sold the skin before I shot the bear.... (Dutch saying... ;) )

 

To make a long story short...

 

After the above measurement I wanted to do another measurement. 

I dived in deeper (but NOT deep enough ;) ) into PE measurement, and read the advise was to capture at least 5 worm periods, so an average measurement get better honest results.. so I did last week...

The PE after that was even lower than the above numbers, that seems to be good to be true to me,

so I started a new thread, if I could trust these numbers...

 

the long story : http://www.cloudynig...pe-measurement/

 

It came clear I made a mistake with filling in the wrong numbers at the declination correction, and when entering the right numbers, it seems also the mount was tracking at 2x sidereal speed (?)

So there was a significant drift in RA, and the guide camera angle needs to be adjusted.

Now the overall estimated PE is not to bad I believe, (beside a inexplicable 240s fundamental)

but difficult to estimate with these factors.

 

I couldn't find the setting in the Starbook to set it on 1x sidereal speed.

First I was thinking maybe it was the Ascom driver... it was indeed on 2 x..

But you can set it only from 2 to 792 guiding-rate x sidereal speed.. see below...

 

guiderate ascom.jpg

 

I also didn't tracked a guidestar near 0 declination, but 24 declination, thinking Pecrep was taking care of that with the Dec compensation function...

 

I'm gonna try to make a new measurement if I can find the solution of the 2x sidereal tracking, and take a guidestar near 0 declination...

 

In the meantime may I ask, did any of you guy's took PE measurements, and how they looked, and how did you overcome the 2x sidereal tracking behavior ?

 

Sorry for the inconvenience...

 

A long story after all...

 

 

 

 

 

 

 




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