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Equipment Discussions >> Mounts

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TopherTheME
sage
*****

Reged: 02/11/11

Loc: Rochester, MI
CG-5gt RA Rear Bearing Mod new
      #5395081 - 08/30/12 03:08 PM

After getting a lot of random error and what I like to call "star squiggles", I decided it was time to hypertune my CG-5. Upon disassembly of the mount I discovered its terrible rear thrust bearing design. The thrust bearing provides zero radial support on the back end of the RA shaft. Near the thrust bearing I found uneven wear tracks on the RA shaft where it was making metal on metal contact with the housing. I'm 99% sure this was the cause of my random error and squiggles. Furthermore, the CG-5 thrust bearing is really cheaply made and its very easy to see it has a short life (although mine is still relatively new). It seems the engineers at Celestron decided to put more money into the electronics and less into the mechanics.

After looking at this I decided the best course of action would be to nix the rear thrust bearing and replace it with a more stout radial bearing. Easier said than done as the pocket for the thrust bearing is a strange dimension and doesn't leave many options for replacing the bearing without removing any material. I actually couldn't find a radial or any other type of bearing that would fit in the stock thrust bearings location so I ordered a new 35x55x10mm bearing and decdided to machine a new pocket for it, Part# 5972K294 from McMaster-Carr.

Photo of stock CG-5 block and new bearing


Machining the new pocket is pretty simple. I just cut away a 2.160" diameter x 0.384" deep pocket into the back side of the casing.


The diameter of the new bearing is about 2.163" and I wanted it to be press fit like the stock front radial bearing. I did a slow finishing cut to a diameter of 2.162" to make the diameter one thou under sized and the contact surface smooth.


Here it looks like this turned out rather nicely. Turns out it really didn't. The machine I used has a little bit of backlash in the x-axis that I wasn't aware of so the pocket isn't a circle, its an oval.


After a little bit of sanding, I put the bearings in a cold chamber at -20*C and the housing in an oven at 80*C for about 20 minutes. After quickly pulling them out, both bearings dropped right in.


Here's the back end of the mount put together. One thing you can't notice in the picture is that theres a single layer of teflon tape going around the part of the RA shaft that makes contact with the rear bearing. This is because the back end of the RA shaft is slightly undersized in order to get the main RA bearing on when the mount is manufactured. The total gap is about 5 thou (radius) so the teflon tape takes up the gap nicely. I'll most likely replace it with some shim stock down the road.


I've got the rest of the mount almost finished up with new teflon washers all around, and all metal on metal surfaces have been sanded and polished and convered in a thin layer of SuperLube (although this stuff seems too thick to me though). Hopefully I'll have some time tonight or tomorrow to put it all back together and test it out sometime this weekend if the skies stay clear.

Thats all for now.

Edited by TopherTheME (08/30/12 03:09 PM)


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RTLR 12
Post Laureate
*****

Reged: 12/04/08

Loc: The Great Pacific NorthWest
Re: CG-5gt RA Rear Bearing Mod new [Re: TopherTheME]
      #5395135 - 08/30/12 03:39 PM

Very nicely done. It's too bad you could find a tapered roller for the application, but I think you have a winner! Let us know how it works out. I'll get one of my CG-5s ready for shipping to you for the mod in the mean time.

Stan


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EFT
Vendor - Deep Space Products
*****

Reged: 05/07/07

Loc: Phoenix, AZ
Re: CG-5gt RA Rear Bearing Mod new [Re: TopherTheME]
      #5396550 - 08/31/12 11:50 AM

Nice looking job. Getting the housing centered and held in place had to be a bit of a chore. You obviously have a relatively large mill to do this. I will be interested to see how it works out. My only concern would be if you did not get the front and rear bearing well aligned to each other, what affect that might have. Overall, it will have to be an improvement to the cheap thust bearing.

While the SuperLube seems thick, it works very well if used very sparingly and it stays in place well. I generally put things together (like the ring gear in the housing) and then pull them apart and wipe the gear off. What remains in the housing is all you need.


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TopherTheME
sage
*****

Reged: 02/11/11

Loc: Rochester, MI
Re: CG-5gt RA Rear Bearing Mod new [Re: EFT]
      #5396681 - 08/31/12 12:55 PM

Quote:

Nice looking job. Getting the housing centered and held in place had to be a bit of a chore. You obviously have a relatively large mill to do this. I will be interested to see how it works out. My only concern would be if you did not get the front and rear bearing well aligned to each other, what affect that might have. Overall, it will have to be an improvement to the cheap thust bearing.




Alignment was a bit of pain. Took about 45 minutes to get the housing flat and center and only about 20 minutes to cut. The bearings aren't perfectly aligned, the rear bearing is off by about a thou or so but with the undersized section of the shaft on the rear bearing it seems OK. The RA shaft spins very freely and is very smooth.

I'll try removing more of the superlube from the Dec axis and try and loosen it up more. I actually put a graphite based lock oil for the ring gear on the RA axis. Super slick and spins very freely.


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skywolf856
sage


Reged: 01/25/08

Loc: SE Michigan
Re: CG-5gt RA Rear Bearing Mod new [Re: TopherTheME]
      #5403696 - 09/04/12 07:30 PM

Toepher,
I probably drive right by you every day.
Let me know how this mod works out for you.


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TopherTheME
sage
*****

Reged: 02/11/11

Loc: Rochester, MI
Re: CG-5gt RA Rear Bearing Mod new [Re: skywolf856]
      #5404179 - 09/05/12 12:05 AM

Update: I got the mount all put together and have been playing around with it a bit indoors. The RA axis already shows major signs of improvement in terms of friction. A light push of my finger sends the RA axis spinning. I now have to adjust the counter weight to within about 1/16" in order to get the mount to balance. The Dec axis is noticeably better too. Much smoother and easier to spin although super stiff compared to the RA axis.

I was hoping I could test it this week but the forcast calls for clouds and rain the rest of the week. I should have tested the mount tonight but the clear skies caught me off guard as the weather report said it was suppose to be cloudy.


Quote:

Toepher,
I probably drive right by you every day.
Let me know how this mod works out for you.




Will do. You might drive by me every day, depends on where you're driving.


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Falcon-
Post Laureate
*****

Reged: 09/11/09

Loc: Gambier Island, BC, Canada
Re: CG-5gt RA Rear Bearing Mod new [Re: TopherTheME]
      #5404212 - 09/05/12 12:49 AM

Very cool mod - this has me looking at my own CG-5 with speculation.... alas that I do not myself have direct access to the tools you do!

Now the question is of course, what will you do about the Dec housing? <evil grin>


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skywolf856
sage


Reged: 01/25/08

Loc: SE Michigan
Re: CG-5gt RA Rear Bearing Mod new [Re: Falcon-]
      #5404782 - 09/05/12 12:18 PM

Toepher,
I work in Auburn Hills and live in New Baltimore.
That puts you in between.


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TopherTheME
sage
*****

Reged: 02/11/11

Loc: Rochester, MI
Re: CG-5gt RA Rear Bearing Mod [Re: skywolf856]
      #5415442 - 09/11/12 02:47 PM

Well I finally had some time to test out the mount. Overall, I'd say there's a pretty drastic increase in performance. Unfortunately I couldn't get the mount properly balanced and the polar scope was slightly off axis so my polar alignment was a little off.

GoTos: I first tested the GoTo function visually. Before all objects would be somewhere in the field of view at about 30x but never in the center. Now everything is dead center in the eyepiece, even at 100x. I think this has more to do with re-greasing and finishing the Dec axis and not so much with the modification to the RA axis but its a welcomed improvement.

Tracking: At first, I didn't really see much of an improvement in tracking. I was using my AT72ED and as other CG-5 users with the scope probably know, you can't balance it with a standard 11lb. counter weight. I also found that my polar alignment was off due to the position of my polar alignment scope changing after rebuilding the mount. Once I did a second polar alignment and I moved up the weight as far as I could until it was slightly rubbing on the casing things got better.

I took several exposures up to 5 minutes to see how the mount performed. I noticed that I was getting bad frames, but they were consistently bad. That is, the star trails were due to a poor polar alignment, not the error in the mount. After a lot of shots I've found that I get no more star "squiggles" and the mount is much more consistent. I still get some frames with random jerking or movement every once in a while but I'm not sure if thats because of an unbalanced mount, strong winds, or the mount itself. Further testing is needed. One thing is for sure though, my keep rate just went up significantly. When shooting 90sec exposures (which is what I normally shoot due to light pollution) my keep rate from last night is about 95% compared to the usual 60-70%. I think with a decent polar alignment and balanced mount I could push this thing up to 120-180ish seconds.

Here's a single shot of M33 uncropped, 90sec ISO 1600


And here's that shot cropped at 100%


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