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How do I go about correcting periodic error in my Celestron-Byers Worm Gear Drive?

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#1 Borodog

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Posted 26 June 2021 - 01:50 PM

Good afternoon. I have a 1985 C-B WGD C8. When I originally got it last year it tracked essentially perfectly with no detectable periodic error. Over the winter the original motor failed, though, and I had it replaced by a forum member, who did a fantastic job. However, I only shipped him the base with the drive in it, not the whole telescope. When I re-assembled the unit I seem to have done a poor job of it, as it now has periodic error. I know there are a couple of adjustment screws on the bottom of the unit, as well as 3 nylon "bolts" on the interior that I believe the 359 tooth gear may ride on, but I'm not sure. Does anyone know how to diagnose and adjust the unit to eliminate the periodic error? Or can someone point me to a reference?

 

Thanks very much for your time and any help. Both are greatly appreciated.



#2 tim53

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Posted 26 June 2021 - 02:35 PM

Are you sure you're talking about periodic error and not backlash?  Periodic error is inherent in all telescope drives, though Byers drives were better than most.  Backlash will show as a lag time when you move to an object and it drifts across the field before the backlash is taken up and the drive starts moving the scope again.  That can be adjusted on most mounts.


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#3 Borodog

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Posted 26 June 2021 - 08:31 PM

No, I mean the image slowly wandering back and forth during tracking (not the continuous drift of polar misalignment, either).


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#4 Borodog

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Posted 27 June 2021 - 09:31 AM

Mods, perhaps move this to Cats & Casses. I thought Classic Telescopes would be the best fit, but perhaps not.


Edited by Borodog, 27 June 2021 - 12:54 PM.


#5 tim53

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Posted 27 June 2021 - 09:53 AM

Sorry I haven't responded sooner.  Busy weekend for me.  I guess i'm still a little baffled as to how the PE would change at all when reasssembling the drive.  I would think that most or maybe all of the PE would be with the worm, so is it possible it can be put on the shaft in more than one orientation?  Or, if this drive has a spur gear between the motor and the worm shaft, maybe there?

 

Sorry I don't have more ideas, but I've never had one with the byers drive.

 

-Tim.


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#6 Borodog

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Posted 27 June 2021 - 12:59 PM

I think there are two possible sources. One, there are 3 nylon bolts in the base that I believe the 359 tooth gear rides upon. So (I believe) if you change their relative heights, you change the plane the gear is riding in, and hence the axis. I think. Two, there is an adjustment screw on the bottom of the base and changes the height of the worm gear relative to the 359 tooth gear.

 

To be honest, now that I describe it, I'm not certain how either of those could result in the drifting around I've been seeing since I reassembled it. confused1.gif



#7 Borodog

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Posted 28 June 2021 - 10:20 PM

Tonight I shot this video of the problem. I only roughly polar aligned, so that is the source of the background drift and is not the issue. I think the issue is pretty obvious, the question is, how do I correct it? In the image, RA is left-right, dec is up-down. Each frame is a 4 second exposure, or 96 seconds of real time for each second of video here.

 

 

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#8 GreyDay

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Posted 29 June 2021 - 09:10 AM

Look through the thread linked below, maybe something's amiss from the rebuild.

 

https://www.cloudyni...c8-disassembly/



#9 Borodog

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Posted 29 June 2021 - 09:50 AM

Perhaps some progress. From the original manual:

 

 

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#10 Borodog

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Posted 29 June 2021 - 09:52 AM

Ok, so the first 3 images are of my unit, the 2nd 2 showing the bottom of the Byers drive base unit and a close-up of the thrust adjustments referred to in the manual. The last image I borrowed from a tear-down album posted by CN user Psyches of his unit, which I referred to during my original tear-down to diagnose the motor failure. It shows the 3 nylon thrust bolts. I believe these are the most likely source of the problem. The question is, how to I go about adjusting them to correct it?



#11 GreyDay

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Posted 29 June 2021 - 10:11 AM

The question is, how to I go about adjusting them to correct it?

start with a circular plate that rests on the nylon bolts, place a circular bubble level in the centre. adjust accordingly to get a level starting point.

 

once level and rebuilt you can fine adjust from the outside of the base. like collimating the scope you'll only need fine adjustments to level the drive.


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#12 Borodog

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Posted 29 June 2021 - 11:06 AM

Thank you. Should those nylon contact points be greased?



#13 rmollise

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Posted 29 June 2021 - 03:11 PM

It had periodic error before you fixed it. 30" MINIMUM was normal for these. Assuming you reassembled everything correctly, examine the worm carefully for dried lubricant, dirt, etc. 


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#14 Borodog

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Posted 29 June 2021 - 04:30 PM

I promise you any periodic error was less than 10" before the drive failed. It's currently around 200" according to that video.

 

When I had it disassembled I degreased and regreased it. The gear mesh is well greased.

 

I now doubt the thrust adjustment has anything to do with the problem, unless the 359 tooth gear is perhaps pressing unevenly into them and sticking and slipping. The issue here is that the scope is surging in RA over a short duration, not that the RA axis not aligned with the base normal.



#15 Borodog

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Posted 29 June 2021 - 10:32 PM

Actually, the scope is not surging ahead in RA, the target is, i.e. the scope is dragging in RA before catching up. This really makes me suspect either sticking and slipping, or some kind of backlash in the gears. The period of seems wrong for sticking and slipping, though. There is the tiniest fraction of a mm of play in RA relative to the worm gear, but I don't know how to adjust it out. There are 2 flat head screws in the bottom of the base; one fastens the worm gear holder and the other fastens the tension spring holder. They seem adjustable, but I have no idea how to set either of them. The one seems like it can adjust the height of the worm gear relative to the 359 tooth gear, but I don't know what height it should be at; it isn't obvious. I can't test until after dark, and by then it's too dark to see to make adjustments. Very frustrating, because this scope used to track perfectly. :O/



#16 Borodog

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Posted 29 June 2021 - 10:36 PM

One thing I noticed is that with the cover off of the base unit, the motor and worm gear wobble slowly. When the cover is on the motor is held fast and the worm gear appears to turn smoothly. Not sure what to make of that.



#17 GreyDay

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Posted 30 June 2021 - 02:22 AM

One thing I noticed is that with the cover off of the base unit, the motor and worm gear wobble slowly.

There shouldn't be any wobble, wobble suggests something is "out of round", which in anything geared means misalignment or bent/worn gear shafts. Check the motor/worm and worm/RA gear connection, it sounds like a load/no load situation where two parts are alternating between too tight and too loose.


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#18 Borodog

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Posted 30 June 2021 - 07:14 AM

Here’s a time lapse showing the wobble. I suppose I could loosen the set screw on the coupling and then retighten in an attempt to straighten the coupling. The problem with this is that the Allen head set screw seems to be a non-standard size; I don’t have anything that fits it.

I guess my next move is to try to adjust the worm gear higher so that it sets more firmly in the 359 tooth gear.

It also may be that the setting circle is sticking and slipping against the cover plate, which would point back to the nylon thrust bolts needing adjustment.

 

Edit to add the link (oops):

 

https://www.youtube....h?v=1fwwxjdjLAg


Edited by Borodog, 30 June 2021 - 11:10 AM.


#19 Borodog

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Posted 30 June 2021 - 12:53 PM

Narrowing in. Not the setting circle sticking and slipping on the cover plate; of course the setting circle doesn't move relative to the base during tracking (duh).

 

Not the nylon thrust bolts; they seem well adjusted.

 

Most likely source of the problem is simply backlash between the worm gear and 359 tooth gear. The worm gear was riding a bit high relative to the 359 tooth gear (I believe I indicated low before but that was backwards), so I centered it. I also tightened the tension spring nylon screw a quarter turn; just slightly tighter but not enough for the nylon to touch the worm gear support. Seated the worm gear nice and tight against the 359 before installing the cover plate. Used a small piece of gaffer's tap on the motor's gearbox housing just to give it a bit of traction against the inside of the cover plate. Testing seems to show that the very tiny bit of backlash I observed before between the gears seems to be gone. Won't know for sure until I can star test the tracking tonight.


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#20 rmollise

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Posted 30 June 2021 - 03:10 PM

I promise you any periodic error was less than 10" before the drive failed. It's currently around 200" according to that video.

 

When I had it disassembled I degreased and regreased it. The gear mesh is well greased.

 

I now doubt the thrust adjustment has anything to do with the problem, unless the 359 tooth gear is perhaps pressing unevenly into them and sticking and slipping. The issue here is that the scope is surging in RA over a short duration, not that the RA axis not aligned with the base normal.

I believe you. But that must be the best one Celestron ever made. 


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#21 Borodog

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Posted 30 June 2021 - 03:15 PM

I believe you. But that must be the best one Celestron ever made. 

It was FANTASTIC before the original motor failed. If I can get it back to the 30" you mentioned, that will be "good enough", but I would still sorely miss the dead still performance it used to have. :O/

 

Edit: I've also been in communication with another poster who used a guide scope and camera (also my method) to measure the periodic error in his Byers drive to 4".


Edited by Borodog, 30 June 2021 - 03:24 PM.


#22 Borodog

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Posted 01 July 2021 - 09:53 AM

Found this old thread with someone having the same problem. However they were talking about a Powerstar III and not a Super C8, and I can't make sense of what they were describing about the pivot in the worm gear support; I don't see anything like that in my Super C8.

 

https://www.cloudyni...ear-time-lapse/

 

Several posters do suggest it could be the coupling between the motor shaft and the worm gear. However, I don't think I can do anything about that, as the Allen head set screw in the coupler is just ever so slightly larger than my 1.6 mm bit; the 1.6 mm bit feels tight, but if I attempt to loosen the set screw it slips, and I don't want to round off the edges in the screw head. I'm trying to decide what to do about that. Threadlocker on the bit maybe?

 

In any event I did some experimentation last night. I had adjusted the two flat head screws to to center the worm gear on the 359 tooth gear better, but no luck. Periodic error still there.

 

I did confirm that the period of the error is exactly the worm gear's rotation period (4 minutes exactly), so that seems to rule out any sort of sticking and slipping.

 

However, I took the cover plate off to get a better look at the wobble while tracking Arcturus and a funny thing happened. With the cover plate off, the motor torques itself completely out of the housing until it is caught by the setting circle. In this configuration, the periodic error is almost completely eliminated! I think that this implies that it is the reaction force back on the motor, which creates a clockwise torque about the worm gear support, is what is eliminating the error. I tried to mimic this by tightening the tension spring nylon screw. I made it probably unreasonably tight; so tight that I don't want to leave it that way. It is certainly much tighter than when the unit ran perfectly prior to motor failure & replacement. However, with the cover plate on the periodic error was back, although perhaps (?) improved somewhat. So I'm at a loss for what to do with this knowledge.

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#23 Geo.

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Posted 08 July 2021 - 10:35 AM

I recall that these motors always have decomposed foam adhering to the bottom. I've never paid it much attention, but it may have been there originally to prevent the motor from "climbing." The PowerStar and Ultimas had a bracket that tied the motor to the worm block to stop such movement.


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#24 Borodog

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Posted 09 July 2021 - 07:36 PM

Stopping the climbing only helped in the peculiar case of allowing the motor to crank itself out of the base and rest against the setting circle. I used a piece of foam rubber above the motor with the cover plate on to prevent the rotation, and the periodic error was as bad as ever.

 

I've essentially given up on this problem for now. I've done everything I know to do that I can do for the moment. More important matters are currently pressing.



#25 jkmccarthy

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Posted 20 July 2021 - 03:15 PM

Here’s a time lapse showing the wobble. I suppose I could loosen the set screw on the coupling and then retighten in an attempt to straighten the coupling. The problem with this is that the Allen head set screw seems to be a non-standard size; I don’t have anything that fits it.

I guess my next move is to try to adjust the worm gear higher so that it sets more firmly in the 359 tooth gear.

It also may be that the setting circle is sticking and slipping against the cover plate, which would point back to the nylon thrust bolts needing adjustment.

 

Edit to add the link (oops):

 

https://www.youtube....h?v=1fwwxjdjLAg

Apologies for not discovering this thread earlier, but what I see in your timelapse video on YouTube above strongly suggests to me that either the replacement motor shaft is bent, or else maybe the motor shaft is slightly undersized compared to the bore-hole at the end of the worm into which it's inserted (and the motor shaft might therefore be setscrew-clamped in a cockeyed orientation such that the axis of the motor shaft is not centered-on and parallel-to the axis of the worm).  I cannot see any wobble of the worm as it rotates inside its bearings (which I take as a positive sign), but obviously the motor itself wobbles as the motor shaft spins the worm.

 

Have you shared this video with the CN'er who replaced the motor for you, and sought their opinion on which alternative (bent motor shaft, or non-axial alignment after motor-shaft-to-worm setscrew clamping) they regard as most likely?   If the latter, somewhere here over the past couple of months I read someone's description of a motor drive rebuild where thin-wall tubing was used around a motor shaft increase its diameter to achieve a more precise fit into the bore-hole of the part being driven ... possibly even because the replacement motor (that matched the original motor's rotation period, electrical inputs, and fit within the available volume) did not exactly match the original motor's shaft diameter, but was slightly undersized.   Obviously if the person who replaced the motor measured the shaft diameters of both the original and replacement motors with a micrometer and the shaft diameters were identical, then this isn't the cause.   Do they recall encountering any difficulty separating the original motor from the worm, such that in trying to pry (or tap the motor with a mallet) the two apart perhaps the interior surface of the bore-hole may have been marred, creating (or increasing) the possibility that the new motor's shaft isn't installed to be coaxial with the worm?  Last question that comes to mind concerns the provenance of the replacement motor --- was it new, NOS, or a used motor pulled from some other device, and how carefully (?) --- and what this might mean in terms of the possibility that the replacement motor's shaft could be bent.

 

Hope you can get things straightened out ... best of luck !

 

      -- Jim


Edited by jkmccarthy, 20 July 2021 - 03:16 PM.



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