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Dec Stall from Custom Park to CWD

 

Hi,
Last weekend got my G-11 Retro-kit installed but rain kept me from doing Cold Start. Did Quick Start got the Time/Location all set. Last night I was back out, rolled the roof and got ready for Cold Start. Set CWD using level, flipped on Gemini and my 1st chore was to set my Custom Home Park position. Slewed to my custom position, Pier Side W, Set Home Here. No problems so far. My next chore was to set my limits. My mount is in my custom home, so I press Park At CWD. The mount slews fine in RA all the way to CWD, but the Dec starts and stops, giving me a Dec Stall message. I tried manually pressing the Dec buttons, the Dec moves to the W, but when I press the East it beeps and says Dec Stall. I did not try and set the Custom Home to Pier Side E to see if I get the same behavior. One thing, while installing the Retro-fit kit I lost one of the grub/set screws for the Dec Worm Cover. Could this be the problem? Anyone know what size it is so I can get one from the hardware store?
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On Sat, Apr 12, 2025 at 08:14 AM, <fishonkevin@...> wrote:
Hi,
Last weekend got my G-11 Retro-kit installed but rain kept me from doing Cold Start. Did Quick Start got the Time/Location all set. Last night I was back out, rolled the roof and got ready for Cold Start. Set CWD using level, flipped on Gemini and my 1st chore was to set my Custom Home Park position. Slewed to my custom position, Pier Side W, Set Home Here. No problems so far. My next chore was to set my limits. My mount is in my custom home, so I press Park At CWD. The mount slews fine in RA all the way to CWD, but the Dec starts and stops, giving me a Dec Stall message. I tried manually pressing the Dec buttons, the Dec moves to the W, but when I press the East it beeps and says Dec Stall. I did not try and set the Custom Home to Pier Side E to see if I get the same behavior. One thing, while installing the Retro-fit kit I lost one of the grub/set screws for the Dec Worm Cover. Could this be the problem? Anyone know what size it is so I can get one from the hardware store?
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Hi,
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Stall messages are due to the motors being unable to overcome the mechanical resistance on that axis. The most common issues that cause that: unbalanced weight -- may need better balancing, or too tight a worm/gear mesh -- may need adjusting.
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On Sat, Apr 12, 2025 at 05:14 AM, <fishonkevin@...> wrote:
Hi,
Last weekend got my G-11 Retro-kit installed but rain kept me from doing Cold Start. Did Quick Start got the Time/Location all set. Last night I was back out, rolled the roof and got ready for Cold Start. Set CWD using level, flipped on Gemini and my 1st chore was to set my Custom Home Park position. Slewed to my custom position, Pier Side W, Set Home Here. No problems so far. My next chore was to set my limits. My mount is in my custom home, so I press Park At CWD. The mount slews fine in RA all the way to CWD, but the Dec starts and stops, giving me a Dec Stall message. I tried manually pressing the Dec buttons, the Dec moves to the W, but when I press the East it beeps and says Dec Stall. I did not try and set the Custom Home to Pier Side E to see if I get the same behavior. One thing, while installing the Retro-fit kit I lost one of the grub/set screws for the Dec Worm Cover. Could this be the problem? Anyone know what size it is so I can get one from the hardware store?
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What retrofit kit, the Gemini 2 controller kit or a full Gemini 2 upgrade kit?? ?
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--

Chip Louie Chief Daydreamer Imagination Hardware?

Astrospheric - South Pasadena, CA?
 

Paul, the mount was well balanced with the Losmandy DSC's before the upgrade. How would I judge if the worm/gear mesh needed adjusting.? I will check it tonight. Chip Louie, Full Gem 2 upgrade kit.?
 

Oh, you probably just need to loosen the outer worm block a smidge.? For original straight drive 2-piece worm blocks you need some backlash when warm.? It shrinks a lot when cold.??
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--

Chip Louie Chief Daydreamer Imagination Hardware?

Astrospheric - South Pasadena, CA?
 
Edited

Paul, and Chip Louie, I went ot hardware bought a variety of size grub screws to fit the Worm Gear Cover. Turns out it is size 6-32 3/16". I now have extras. But that is not the funny thing. I found what was causing my Dec stall. It was not balance, or worm/gear mesh. It seems my lost grub screw was inside the cover causing problems with the coupler. with that removed everything worked. I do need to redo my PA though. I did have a little slippage in the RA, I had to really tighten the coupler. The coupler is weird, it is not the Motor coupler on the Losmandy website.? It attaches more like a clamp with 2 screws, instead of 2 grub screw/@90* like the one on the website. See pic. Have you seen these couplers before? I may replace them. Lesson learned though, Don't play with screws in the dark.
Thanks for your help,
Kevin
 

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FWIW that¡¯s a far superior type of coupler than the standard Oldham coupler.

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From: [email protected] <[email protected]> On Behalf Of fishonkevin via groups.io
Sent: 13 April 2025 05:18
To: [email protected]
Subject: Re: [Gemini-II_io] Dec Stall from Custom Park to CWD

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[Edited Message Follows]

Paul, and Chip Louie, I went ot hardware bought a variety of size grub screws to fit the Worm Gear Cover. Turns out it is size 6-32 3/16". I now have extras. But that is not the funny thing. I found what was causing my Dec stall. It was not balance, or worm/gear mesh. It seems my lost grub screw was inside the cover causing problems with the coupler. with that removed everything worked. I do need to redo my PA though. I did have a little slippage in the RA, I had to really tighten the coupler. The coupler is weird, it is not the Motor coupler on the Losmandy website.? It attaches more like a clamp with 2 screws, instead of 2 grub screw/@90* like the one on the website. See pic. Have you seen these couplers before? I may replace them. Lesson learned though, Don't play with screws in the dark.

Thanks for your help,

Kevin
 

That's a disc coupler from Ruland.? I have one on my own mount on the Dec drive axis.? I've been trying to find a replacement for my own coupler, which has loosened up a lot (to the point of introducing a ton of backlash) and even after tightening the tiny, tiny disc screws may not remain tight.? I can find an almost-exact replacement on Ruland's website, but it doesn't have set screws but is rather a collar tightening model.? And it's priced at USD $108 for one.? I suspect that Losmandy may have had a custom-made model, or else the set screw version is no longer being made by Ruland.? ?I'm waiting (after more than 4 days) for Losmandy to return my call / voicemail to see what they can offer me.? The $50 Oldham coupler on their website does not appear to be long enough.? The Ruland disc coupler is about 24mm long and I have not found an Oldham coupler anywhere that is long enough to fit both the worm axis and the gearbox axis, and even the one on Losmandy's store website looks too short.? There are a couple of options I've identified but I still want to hear back from Losmandy in case they can offer the original Ruland coupler or at least guidance on the best replacement.
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-- Patrick Freeman
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¿ªÔÆÌåÓý

Patrick,

Tanya emailed that they were on a show in NY and will be back Monday.

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Am 13.04.25 um 13:37 schrieb freeman.patrick via groups.io:

That's a disc coupler from Ruland.? I have one on my own mount on the Dec drive axis.? I've been trying to find a replacement for my own coupler, which has loosened up a lot (to the point of introducing a ton of backlash) and even after tightening the tiny, tiny disc screws may not remain tight.? I can find an almost-exact replacement on Ruland's website, but it doesn't have set screws but is rather a collar tightening model.? And it's priced at USD $108 for one.? I suspect that Losmandy may have had a custom-made model, or else the set screw version is no longer being made by Ruland.? ?I'm waiting (after more than 4 days) for Losmandy to return my call / voicemail to see what they can offer me.? The $50 Oldham coupler on their website does not appear to be long enough.? The Ruland disc coupler is about 24mm long and I have not found an Oldham coupler anywhere that is long enough to fit both the worm axis and the gearbox axis, and even the one on Losmandy's store website looks too short.? There are a couple of options I've identified but I still want to hear back from Losmandy in case they can offer the original Ruland coupler or at least guidance on the best replacement.
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-- Patrick Freeman
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Thanks for the info, Rene!!!
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-- Patrick Freeman
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Edited

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Hi Kevin,
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The original Oldham type coupler is a perfect choice for this application where the misalignment is only axial and is easily reduced.? The other coupler types also may allow for axial misalignment as well as angular misalignment. For the Losmandy mounts we only have to worry about axial misalignment.? So pick the type of coupler that have no or low changes in speed with axial misalignment and that's the best one to use.??
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If you do your research you will find that like the Oldham type each other type of couplers have advantages and disadvantages.? In testing done by many different people none of the alternatives, rigid, various disc types, helical types and bellows types are significantly better performing and often hurt tracking performance.? The reason may be that the Oldham is the best fit for axial misalignment while maintaining low difference in velocity changes due to axial misalignment.?
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So I think that while the Oldham tends to loosen over time, (this is often the problem found that fixes previously good tracking mounts), this is an issue with all of these miniature couplers.? The Oldham type coupler has minimal/no velocity shift and is easily axially aligned in place which helps reduce the already minimal changes in speed.? You can also order the perfect fit from Losmandy.? No need to machine or try to fit couplers with mismatched bores or lengths.??
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My tips for the Oldham couplers are to use a toothpick dab, as little as possible of SuperLube on the plastic coupler parts. Use removable thread adhesive on the grub screws, adjust the gearbox shaft with the Oldham already tight on the worm shaft and hold / float the gearbox side to align the plastic coupler parts with the metal parts of the coupler while snugging down the gearbox fasteners for best shaft alignment.? I also suggest keeping one complete spare Oldham and gearbox in your kit to keep the mount running if you don't want to be waiting for replacement drivetrain parts which sometimes have a long lead time from suppliers.? I'm just saying, you can control your own destiny in this small way.??
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--

Chip Louie Chief Daydreamer Imagination Hardware?

Astrospheric - South Pasadena, CA?
 

I stand corrected.? That's not the same Ruland disc coupler that is on my Dec axis, it's a spiral-cut beam coupler.? Mine is definitely a disc coupler, see attached photo.? Yours looks to still be from Ruland, however.? They do offer an aluminum beam coupler for about USD $50 that looks like it'll work, but I want to talk to Losmandy before I do anything else on mine.??

20250408_103403.jpg

 
Edited

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In thinking about the coupler type options out there I remembered a nice chart from Ruland that explains the different types of couplers they offer.? There are other brands of couplers but they all fall into one of the categories on the chart.? I have added a link to the chart below so you can understand the differences and why the folks who tested other coupler types experienced what they did.
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IME the requirements for the drive coupler based on the mechanical design of the drive system are as follows.
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Due to the drivetrain design the coupler must be able to accommodate some small amount of axial misalignment while maintaining constant velocity.? Additionally with this drivetrain layout there is no need for the coupler to accommodate any angular misalignment.??
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The coupler must have zero backlash.
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The coupler must be very stiff torsionally.?
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The coupler needs only be capable of moderate torque load.?
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The coupler doesn't experience significant bearing load so this is not an important factor.?
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For maximum drivetrain responsivness to changes in speed and direction for example during guiding, the amount of damping and inertia in the coupler should be as low as possible.?
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Ideally the coupler should not allow axial play in operation or changes in speed or direction. This is important for drivetrain responsivness as when guiding and platesolving centering.?
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IMO Scott selected the right type for the job based on the most desirable characteristics for the coupler types available.? Check the chart to see how your favorite coupler type stack up.??
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--

Chip Louie Chief Daydreamer Imagination Hardware?

Astrospheric - South Pasadena, CA?
 

On Sun, Apr 13, 2025 at 07:57 PM, Chip Louie wrote:
The coupler needs only be capable of moderate torque load.?
Tell that to the helical coupler I destroyed in an attempt to replace the Oldham coupler years ago. This happened because I had a slight binding in the worm/gear mesh that caused Gemini to increase PWM to near 100% at the start of a slew. That twisted the coupler with enough torque to snap it in half! I decided to keep the Oldham coupler after that :)
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