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Massive lag errors in RA on my CG 4

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#1 Guest_**DONOTDELETE**_*

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Posted 12 August 2004 - 07:58 PM

Hey guys.

I bought a CG 4 about a year ago and it is a fantastic mount. Very solid. However I would like to use it for guided astrophotography which is doable with this class of mount, but mine seems to be a bit of a lemon. I realize that all mounts have some sort of lag, but should it be in the 5-10 second range? Not to mention the fact that there is a substantial backlash to it but only in RA. When I guide it in one direction, its very smooth, but when I go to reverse that direction it CONTINUES in the previous direction a good arc minute or two, hangs 5-10 seconds, then goes in the other direction.

I did some REAL careful observations of what seems to be causing the backlash problem (the lag is obviously due to the worm/gear mesh which can be minimized by tightening). The bearing that worm gear shaft turns in is held into place inside the worm gear housing via a rubber O-ring, probably to seal the grease in and to avoid the costs of machining a perfect hole for the bearing out of cast aluminum. But because it is merely a rubber O-ring, it compresses as the motor gear drives the worm gear shaft up or down. So what happens is the worm gear shaft actually rocks inside it's housing before turning. Its too hard to tell if this is the cause of that backlash though. But I know it also affects the lag quite a bit because time is wasted in the movement of the worm gear shaft.

I contacted Celestron about all this and they were sorta confused by it and just kept saying thats normal. The declination gear is what's normal exhibiting only a couple seconds of lag. The RA is just out of sight. Anyone else have similar problems and/or suggestions on how to fix this? I might try finding precision teflon washers to replace that O-ring to give the worm gear shaft better support. If any of this is confusing or needs more explanation, ill try and make an animated illustration of what's happening if I can.

#2 scope dog

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Posted 12 August 2004 - 09:45 PM

I have teflon o-rings for sale here. I know they fit the GP, CG type mounts that have the rubber o-rings and may fit yours. If you want I can send a set out to try. I had to have them made at a shop to that size. I tried everywhere but seems not to exist.



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#3 ccs_hello

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Posted 15 August 2004 - 11:20 PM

I've seen my Orion (EQ4/CG5) SVP mount's WORM shaft has the similar type of design/problem. David O. had suggested to change its rubber O-ring/brass bushing assembly to a bearing, which I did. In my SVP case, the bearings used are 6x11x4mm (or 6x10x2.5) bearings from RCBoyz. That fixed the WORM shaft wobbling. It's now play free, just have to be careful not to be too tight against the WORM to wear-out the WORM prematurely.

BTW, my LXD55 WORM shaft has no such problem. I recall there is no rubber O-ring inside during my hypertuning inspection.

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#4 ccs_hello

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Posted 20 August 2004 - 07:23 AM

I should've also mentioned that there is a well documented/known fix by replacing the rubber O-ring with a nylon washer. The washer is made by first obtaining a Nylon spacer from a large home remodeling supply (Crown Bolt Inc, 29/64" OD x 0.324" ID, UPC 030699872787). It's original length is 27/64" long, but will have to be trim down thinner significantly. The OD has to be reduced a bit to fit.

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#5 ccs_hello

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Posted 25 August 2004 - 07:43 AM

REF: Nylon Spacer fix:

overton.tamu.edu/rdb/CG5/wormgear_page2.htm

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#6 ccs_hello

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Posted 19 October 2004 - 11:31 PM

IMHO, the optimal mod is to use the Nylon washer on the non-screw opening side and ball bearing with aluminum (soda can) foil as shim on the opening side. Work best for both of my SVP and CG5 mounts.

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#7 scope dog

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Posted 23 October 2004 - 09:29 AM

That's good, The ones I'm selling were mesured by a CNC machine shop and produced by a o-ring/washer shop for an exact fit. If you want them I got them.
The older CG5 mount use metal, the GPDX use a teflon type. The small amount of flex acks like a anti-backlash device. However when overloaded turns into massive backlash. In fustration some tighten the worm shaft to where it became bent. If all play is removed damage may occur to the worm shaft. Because the worm gear is not perfectly round, when the clutch is engauge changes to the worm gear occur causing it to *bleep*, this is how the mount is design and thats when the none metalic o-rings come into play. If all devices are removed the shock and adjustment will depend on the flex of the worm-shaft, this is not desirable because of brass to steel contact may cause excessive wear or bend the shaft.
This tends to be the case when the mount is overloaded.

When adjustment is made and soft, or plastic type rings are use a close eye needs to trained on the gear side of the wormshaft. You will notice a gap. As backlash adjustment is made the gap will begin to close, stop. The other side needs to be with feel since no visual can be used. When making adjustment a gentel go between back and forth needs to occur, with a constant feel of tightness and play.

#8 ccs_hello

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Posted 23 October 2004 - 07:51 PM

Jim,

Thank you for the advice. This is just like one won't drive the fine tuned Lexus at 200 Mph on a bumpy road while towing a boat :lol:

I can understand the OTA load, mount mechanical precision, and other factors all play important roles on how much play/tightness a mount should be tuned/adjusted.

The other factors I learned:
- to reduce the jitter from the (mainly stepper) motor:
(more) play within the gear box, using one plastic gear, using rubber/nylon bushings (o-ring) in the worm shaft housing, using timing belt instead of using spur gear transmission all help a bit.
- sticky glue/grease

Of course, they are just my guesses. Black art and luck all involved.

I think they're all compromises. One cannot predict the actual conditions just like a car mfg. cannot dictate who should drive the car and the driving condition.

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