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Old 04-01-2013, 11:33 AM
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pmrid (Peter)
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Pimping my G11 - some notes for others

As a summer project I decided to do a full strip and refurbishment on my G11. I had come to suspect one or both needle bearings in the RA axis were either gludged up or worn. So replacing them was a first step.

I had also been unhappy with a degree of instability in the motor/gearbox assemblies. With light finger pressure, I found that there was a degree of movement that had to have an impact on the worm drive. When I looked closely at the gearboxes, I found that these were Losmandy originals. They are a plastic-bodied gearbox and I found that the two halves of the plastic case were very poorly joined - probably the clips holding them together. So I decided that gearbox replacements was a second step.

The choice of gearboxes was limited to replacments with new Losmandy or with McLennans. I chose the latter and was able to buy them locally.

These gearboxes are not a drop-in replacements though. They are metal and solid making them a great choice. But the drive shaft is smaller that the Losmandy and the direction of rotation is opposite. So fitting them was a fiddly job but not beyond my basic skills. You need to do both gearboxes I think so that when you're finished, you can tell your Gemini that the mount is now a MI250. That takes care of the direction of rotation.

While I had the thing down to this level, I decided to replace the clutches, worm bearings and Oldham clutches as well - it seemed silly to get this far and not do them.

There are plenty of resources tellng you how to disassemble a G11. SO I won't repeat that here.

The replacement of the gearboxes proved to be the most fiddly. It required getting hold of some 3/16" x 0.014" brass tube and some M2.5x16 cup-head screws. That was easily done though Small Parts in Brisbane. For the gearboxes, see RS components at http://australia.rs-online.com/web/g...=tc&id=aboutRS . They are Item 336-444 "Gearbox,spur,ovoid,synchronous ac motor,10rpm" at A$48.50 each.)

Also needed was a small pipe cutter (borrowed one from Wally at Burrum Heads - thanks again Wally) and was given a pinion-puller to do the job. (once again, a generous gesture from Wally). The trick with the pinion puller is to be sure to get one that has a variety of shafts that will be able to press down on the drive shaft of the motor which are 2mm. So you should be careful if you want to get one on eBay.

The brass pipe had to be cut into lenghts of 8mm and a 3mm hole drilled in one end 2.5mm from the end. That was fiddly.

Then, the short length of tube has to be fitted over the top of the drive shaft from the gearbox. The process of cutting them with the pipe cutter tends to squeeze the ends in a bit so I had to nurse a drill bit to open them up a little to make the fit possible. Not a biggie. Make sure the 3mm hole in the end is big enough to allow the grub screw (one of them anyway) on the Oldham couple fit through it because it has to go through the tube and lock onto the flat endge of the gearbox shaft. I actually went to 3.5mm with my holes.

You'll also need some LockTite or similar to fasten the supplied pinions that come with the gearboxes onto the motor shaft. There are heaps of good locking compounds out there and it only takes a drop.

Putting it together is straight forward.

The needle bearings for the RA and Dec axes were a very precise fit. I bought mine from McMaster Carr in the states (http://www.mcmaster.com/?orderview=new#) but since they don't ship internationally, I also had to use a VIAddress US address to have them reshipped. That was an added cost. If I knew more about bearings I am sure I could have found equivalents in OZ. The same is true with the worm bearings which also came from McMaster Carr. The bearings required are:

1 Item ID 57155K377 Mini High-Precision Stainless Steel Ball Bearing - ABEC-5, Double Shielded, 1/4" ID, 5/8" OD, .196" Width. 4 required if you want to do both worms) US$ 4.63
each
2 Item ID 5905K29 Steel Needle-Roller Bearing, Open for 1-1/4" Shaft Diameter, 1-1/2" OD, 3/4" Width (you can either add bearings or replace them. I decided to add one per axis so I bought 2) US$7.78 each.

These bearings are supplied dry so they will need some grease. I used a Lithium-based grease from SuperCheap Auto. Purists would no douby frown at this recklessness. While I had the mount stripped down to this level, I also soaked the old bearings in some diesel and made sure they were clean of old gludge and moved well before they were dried and regreased.

Fitting the bearings was not hard. Some people replace or add an extra bearing at the bottom end of the race and others at the top. I chose to add one at the top although there was enough room to have done one at the bottom as well. They needed some help from a wooden mallet to carefully position and them work into the bearing shaft. SLow and careful and it is fine.

Reassembly was not an issue and what remained to be done them was the worm adjustments as usual. I found the axes were both a lot smoother and the backlash in the worms much better controlled.

Of course the weather continues to be awful so I haven't been able to give it a live test under stars yet but it moves well in the dry runs so far.

Hope this is of some help or interest to anyone looking to do the same thing.
Peter.

Last edited by pmrid; 04-01-2013 at 12:27 PM.
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Old 04-01-2013, 01:51 PM
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RickS (Rick)
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Glad to hear the transplant was successful, Peter
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Old 04-01-2013, 01:56 PM
icytailmark (Mark)
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how often should you regrease a G11?
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Old 04-01-2013, 02:09 PM
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pmrid (Peter)
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Quote:
Originally Posted by icytailmark View Post
how often should you regrease a G11?
Depends on:
1. whether you've done it since new. If the mount is new and has never been cleaned and regreased, I would do it now. If it has already been cleaned and regreased at least once, see 2 below;

2. what grease you have used last time. All greases are not equal. If you used a good quality grease and the mount hasn't been wet or exposed to really duty environments, it should be fine for a couple of years. I'm sure there are mounts out there that have never been opened up and perform flawlessly.

3. Rely on your fingertips to tell you if a regrease is needed. The axes should spin by light finger pressure without binding or stuttering. If the axis feels like there is a section where it is tighter than the rest, the time has come to do a quick strip, clean and regrease.

With the G11, it is so beautifully modular that this process takes no time at all. The hardest part is redoing the worm gears every time.

Peter
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Old 04-01-2013, 09:38 PM
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RobF (Rob)
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Sounds like you're keeping busy Peter, even if there have been clouds about
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Old 05-01-2013, 08:10 PM
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midnight (Darrin)
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Thanks Peter for a valuable resource.

I am looking at doing some work over the next few weeks including regreasing and upgrading my FLT110 focuser. The DDG focuser on my FLT is a joke imo when the DSLR goes on.

Cheers,
Darrin...
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Old 05-01-2013, 09:32 PM
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pmrid (Peter)
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Quote:
Originally Posted by midnight View Post
Thanks Peter for a valuable resource.

I am looking at doing some work over the next few weeks including regreasing and upgrading my FLT110 focuser. The DDG focuser on my FLT is a joke imo when the DSLR goes on.

Cheers,
Darrin...
G'day Darrin. I put a large format Moonlite on my 110. With their steppers and controller, it works a treat. Not cheap but worth it.
Peter
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Old 06-01-2013, 01:43 AM
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midnight (Darrin)
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Quote:
Originally Posted by pmrid View Post
G'day Darrin. I put a large format Moonlite on my 110. With their steppers and controller, it works a treat. Not cheap but worth it.
Peter
Interesting! I have upgraded my LXD55 8"SN and little 80mm Meade to Moonlite and love it. I will definitely investigate the Moonlite option and what Ron can do. I'm trying to use the FLT as my principle imaging scope for 2013. Just that DDG focuser is really letting it down at the moment.

Many thanks Peter!

Cheers,
Darrin...
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