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Problem with mount

 

Hi,

I'm having trouble with my G-11 mount and I want to get some feedback on what I plan to do to fix it.


My Losmandy G-11/Gemini-1 mount is 12 years old; it's mostly in original condition except that I've replaced both worms with Ovision worms.? It's in a rolloff roof observatory and I use it every clear night for variable star studies, using some software I've written to automate imaging (so I'm usually not in the observatory when it is running).


A couple of weeks ago, the Oldham coupler on the RA worm disconnected, apparently slipped on the worm shaft.? I re-positioned it and it seemed to work fine. The next day the scope wasn't moving in RA again, so I checked and saw brass shavings near the Oldham coupler.? Apparently I hadn't tightened it enough and it was sliding on the worm shaft, so I made the set screws as tight as possible and that seemed to fix it.


Less than a week later, the scope stopped moving in RA again. This time I found that the gearbox on the RA motor had broken. The RA motor worked fine, but the shaft on the gearbox spun freely.


After a moment of panic when the Losmandy web site said the gearbox replacement was out of stock, I found that Anacortes stocked replacement parts for Losmandy, so I ordered it with 2 day delivery.? It arrived yesterday and I installed it, and of course tested it out.


Gemini is now reporting "RA motor lags", and when the mount slews in RA my SCT seems to visibly wobble like something is impeding smooth motion.? I realize now that since I usually operate my scope remotely, I may have been getting the motor lags error for some time without realizing it, and that this might explain my problems with the Oldham coupler and the failure of the gearbox.? It also occurs to me that I had to replace the RA motor about 6 years ago, so maybe I've had a problem with my RA mechanics for some time.


I'm not much of a hardware person, but I think I need to take the mount apart and clean off any 12-year-old gunk that must be causing a load on the drive.? In hindsight, the lack of maintenance on my mount would explain why I'm having hardware failures.


Any words of advice before I get started?


Thanks,

-Joe
 

Joe,

12 years of service is pretty good.

But I would remove the motors and the covers on the worms. Then, with the scope package off the mount and the mount balanced, manually turn the worms - they should turn pretty freely with just your fingers. I sounds to me like something is binding in the gear train, probably at the worm to worm wheel interface but it could be one of the bearings in the Ovision bearing block. I am not familiar with the Ovsion design but you should check to make sure the lash (spacing) between the worm and worm wheel haven't shifted for some reason.

Of course you should also verify that the main bearings haven't seized. Do that by releasing the clutches and seeing how easily the RA and DEC axes turn.

Once you have confirmed that everything moves pretty freely over a wide (like 360 degree) range of axis rotation then install the new gearboxes. Based on your description it sounds as though something has seized up for some reason.

Good luck.

Mark C.
PS: If you had recently adjusted the worms in an effort to reduce backlash (which some carry to the point of a fetish) then I'd shine the Flashlight of Darkness on that - it is common for people to try to reduce the backlash without realizing that unless they do so at the point of closest mesh on the worm they will end up with binding when the worm rotates to that point hours later. I recently adjusted a (new from the factory but not Losmandy) mount where the worm axial loading was so tight you could feel the worm axis bearings rumble as you turned the worm shaft. And then when I rotated it through many revolutions so the worm wheel (on the axis)? rotated through 180 or more degrees I found a place where it bound. Obviously the assembler at the factory needed some training.
 

Hi Joe

Maybe can turn the worm by hand (with maybe an adapter) to see if the worm has a tight spot?

You can remove the Ra/DEC motors from the mount and run them connected to the Gemini ....just like if they were connected. and test the motor system.? If you still get lags then this needs to be investigated? further.

But since you mentioned issues with the couplings, worm and gearbox, there is probably no issue with the servo motor system.? I'm not a great fan of the oldham coupling but I think it helps smooth out shaft alignment errors, which cause the worm to have various PE errors.

Anyway...FWIW

Brendan
 

From your description, I'm pretty sure you have not got your worm axis lined up with your gearbox output shaft axis.??

So when the gearbox shaft turns, it is moving the Oldham pieces, and the gearbox mounting (plastic?) is getting moved back and forth.

So double check the alignment through the Oldham coupler.? Ideally you want that to only rotate, and all in one straight line, not shift the rod parts along the plastic center piece.??

As Brendan says, extra movement of that coupler, particularly at an angle, will put in PE frequencies.? Movement of the rods along the plastic might put in random jumps... all bad for imaging work.?

As I understand it (not having one!), the Ovision system should have a spring washer (Belleville washer) under the far worm end cap. That washer needs to be compressed...somewhat.? See if that is adjusted properly.

Hope you solve this. ..then tell us your story!

Michael?




On Dec 20, 2017 12:55 PM, "brenatlilydale@... [Losmandy_users]" <Losmandy_users@...> wrote:
?

Hi Joe

Maybe can turn the worm by hand (with maybe an adapter) to see if the worm has a tight spot?

You can remove the Ra/DEC motors from the mount and run them connected to the Gemini ....just like if they were connected. and test the motor system.? If you still get lags then this needs to be investigated? further.

But since you mentioned issues with the couplings, worm and gearbox, there is probably no issue with the servo motor system.? I'm not a great fan of the oldham coupling but I think it helps smooth out shaft alignment errors, which cause the worm to have various PE errors.

Anyway...FWIW

Brendan
 

Thanks everyone for your input.

I have my mount apart inside and I've found a few things. I probably did have the gearbox mounted off-axis when I replaced it the other day.? Inside, comparing the new gearbox with the old one, I can see that the mounting holes on the gearbox, where it goes over the threaded rods coming out of the mount, the holes on the new gearbox are smaller, which explains why I had so much trouble getting it on.? The old gearbox came with the holes slightly enlarged!

I'm enlarged the holes on my new gearbox now, and once I finish the cleaning and regreasing, I'll see how it works once reassembled.

I know about the adjustment on the Ovision worms and I will check that more, but I'm slightly concerned with how rough the RA worm feels now that I have it out, compared to the Dec worm. I'll check whether that is a lubrication issue or a tension issue with the adjustment screw.

Thanks again for all the advice.
-Joe
 

I recently found that with my G11 if the RA worm is too loose or has too much
backlash that I get motor errors lag ....with new Gemini II conroller...So it needs
goldy locks adjustment not too loose not too tight

Also Ovision recommends 1 to 2mm of end play with the allen adjustment



Greg
 

Greg...excellent site indeed!? You're a champ.

Brendan

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