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NexStar 8SE azimuth broken

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#1 Doc Willie

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Posted 21 July 2013 - 09:45 AM

My NexStar 8SE was blown over in a wind gust.

The azimuth now rotates freely by hand. When activated by the hand controller, the motor can be heard running, but the mount does not rotate.

Am I likely to be able to fix this myself. Where are parts available if necessary? Or if necessary where can it be shipped for repair?

#2 tim57064

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Posted 21 July 2013 - 10:45 AM

I think I would have been more worried about the scope than the mount. How did the scope fair? I think you may end up purchasing a used replacement mount,as it may be cheaper in the long run than shipping it back for repair.
If you are so inclined to repair yourself,disassemble it taking images of each procedure so as to remember where everything goes and see if you can find the broken part. Contact Celestron to see if you can get the part and if so, good,if not, you should be able to locate a used mount on here or on Astro Mart. Sorry to hear about your misfortune.

#3 Midnight Dan

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Posted 21 July 2013 - 11:50 AM

Hi Willie:

Hard to tell what might be wrong without opening it up. It sounds likely that a gear tooth was broken or something similar, but it could also be that a drive gear was moved on its shaft and can simply be repositioned and the set screw retightened. If you're handy with tools, I'd suggest opening it up.

If anything is broken, you won't find too many parts available for replacement. Like Tim says, you're probably better off getting a used one. Then you can keep your old one for spare parts for the next time the scope gets blown over. :p

-Dan

#4 Tel

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Posted 21 July 2013 - 01:57 PM

Hi Willie,

So sorry to hear of your misfortune.

As others have already stated, it's impossible to say at this stage what may or may not be damaged.

However, you do say that the azimuth axis now rotates freely by hand, which, clearly before this misfortune, it was not designed to do.

This therefore might indicate that the motor (drive pinion) has become dislodged, (or disengaged) from the main, large, static (cog) wheel: hence the motor turns, as evidenced by the sound you hear when the HC is operated, but, without the engagement of the two sets of intermeshing teeth, does not rotate the azimuth axis.

I'm afraid you will have no alternative but to open up the azimuth axis drive if you wish to investigate this (any) possibility yourself, but perhaps, the attached archived text might help. (?)

http://www.cloudynig...2309497/page...

Just a thought.

Best regards,
Tel

#5 Doc Willie

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Posted 21 July 2013 - 02:42 PM

Thanks for the recommendations. I'll open it up.

As far as the OTA, it seems to have survived quite well. No visible damage, and an out of focus star was quite even and circular. Looked at the (filtered) sun today with it, and it performed quite well.

#6 Doc Willie

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Posted 21 July 2013 - 07:37 PM

OK. Got it open. What I see is a small brass gear that is supposed to mesh with the large gear in the base of the mount. The small gear is rotating freely, and moving up an down easily on its shaft, with no visible means of securing it to the shaft. What next?

#7 Doc Willie

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Posted 21 July 2013 - 08:37 PM

On closer exam, I see the set screw, hidden by grease. The gear itself is slightly messed up, the teeth on the narrow side of the set screw opening are bent.

Looks like I need a new gear, unless I can put it back in and the damaged tooth is out of the way.

#8 Tel

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Posted 22 July 2013 - 01:41 AM

Hi Willie,

There are, (according to the above link), two set screws which hold the gear (pinion) to the motor shaft.

It might be possible to reshape that bent tooth and retighten depending on your overall assessment of the damage, otherwise it does sounds like a new gear is required.

Is though the motor shaft and gear still in line with the toothed disc, (i.e. the az. motor/shaft/gear have not been bent or otherwise damaged as a result of the impact) ?

Best regards,
Tel

#9 Arctic_Eddie

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Posted 22 July 2013 - 07:19 AM

It's not likely that you can return the small gear to the original shape. As that gear meshes with the large gear in the repaired area you will see a tracking error. It could lead or lag depending on how you bent the teeth back into position. Damage to the large gear will show a problem only in one small region of azimuth. This you can probably ignore. There are many sources of small metric gears. It's likely that you can find a replacement. Try this source.

https://sdp-si.com/w...ml/products.htm

#10 Arctic_Eddie

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Posted 22 July 2013 - 08:26 AM

I have a spare AL motor/gearbox assembly which may use the same final gear. These are the specs for that hubless gear.

Bore diameter = 5mm

Number of teeth = 18

Body length = 11mm

Body material = brass

Body diameter = 10.45mm

Pitch diameter = ??? but slightly less and likely 9.5mm ( 10.45 * 0.91 )

Pitch = 48 ( Number of teeth divided by pitch diameter in inches )


I did not find the right gear at the above link but there are literally hundreds on that site just in brass.

#11 Arctic_Eddie

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Posted 22 July 2013 - 09:27 AM

If your gear is long like mine then you can slide it on the shaft, or reverse it, and use an undamaged area. The shaft has only one flat so one of the setscrew holes will be empty.

#12 Doc Willie

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Posted 22 July 2013 - 10:42 AM

If your gear is long like mine then you can slide it on the shaft, or reverse it, and use an undamaged area. The shaft has only one flat so one of the setscrew holes will be empty.


I was thinking of this. Does the big azimuth gear mesh at the bottom or top of the brass gear?

#13 Arctic_Eddie

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Posted 22 July 2013 - 11:50 AM

Not sure about the AZ gear as mine is a replacement AL unit. However, the gear meshes at the opposite end as the setscrew. As long as the large gear doesn't hit the screw, it can mesh anyplace along it's length. The small gear can also be reversed if that will help.

You should also note the AZ position of the damaged portion of the large gear and avoid using it in that region. Even though the small gear has been replaced, there will still be a tracking error in that region as the bent teeth will cause a lead or lag when engaged. It will be a small angle, a few degrees.

#14 Doc Willie

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Posted 22 July 2013 - 08:41 PM

Fixed it!

I think. I put the gear back on, with the set screw on the other side, where the hole was undamaged. Put the gear as far on the end of the shaft as I could, so the damaged tooth would be out of the way. It seems to be working, at least mechanically. Will have to wait until the next clear night to field test it.

Thanks for all your help.

#15 Midnight Dan

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Posted 22 July 2013 - 08:43 PM

Excellent! We'll keep our fingers crossed!

-Dan

#16 tim57064

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Posted 23 July 2013 - 01:45 PM

Glad to hear it! Now let us know when you test the tracking. :waytogo:

#17 Peter9

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Posted 23 July 2013 - 03:20 PM

Hope your mount stands up to the test and it is truley fixed.

Regards. Peter.

#18 Doc Willie

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Posted 24 July 2013 - 04:23 PM

Just took it out, and tested it with the 60mm solar scope. Seems to be tracking fine. Hope it holds up over time.

Just in case, I ordered a spare drive gear.

Cloudy Nights folk are the best!

#19 Arctic_Eddie

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Posted 24 July 2013 - 05:01 PM

Where did you find a drive gear, Celestron or elsewhere? My searches never found an alternate supplier.

#20 Doc Willie

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Posted 24 July 2013 - 06:37 PM

One of the contributors of the Yahoo NexStar group had a couple.

#21 Arctic_Eddie

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Posted 24 July 2013 - 07:00 PM

Good deal.

#22 Geo.

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Posted 26 July 2013 - 09:09 AM

Hey Doc, you should have the gear by now. I should tell you that Celestron uses some form of adhesive in addition to the set screw to retain it. I had to let the drive (out of the mount!) cook a few minutes on the stovetop* before I could remove it. Not sure I'd super glue it on though, think I'd rather have it slip than take a quater Moon out of the driven gear.

*With the wife out of the house....

#23 Arctic_Eddie

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Posted 26 July 2013 - 09:14 AM

Loctite has a breakable glue just for that kind of application.

http://www.homedepot...2-209728/100...

#24 Doc Willie

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Posted 30 July 2013 - 11:11 AM

Had the scope out last night, and the mount performed normally. Conditions were not great, but the stars did not look right - could not get a pinpoint image on bright stars. Maybe the collimation is off. Will try a formal star test soon.






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