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Old 04-01-2013, 12:33 PM
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pmrid (Peter)
Ageing badly.

pmrid is offline
Join Date: May 2008
Location: Cloudy, light-polluted Bribie Is.
Posts: 3,511
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 . 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 ( 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
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.

Last edited by pmrid; 04-01-2013 at 01:27 PM.
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