astromaster

 

I decided to write this page to about 10 years after I purchase my G11 Losmandy (a general description of mount is on the equipment page). In the 10 years the mount gave me one serious problem: bearings which hold the AR worm are perhaps undersized and tend to seize. On photography over 1600mm focel lenght the effect was shocking: sudden glitches and jumps so fast that no astro tracker could handle.

When I used the film the phenomenon made it impossible to achieve long exposures so just two years after purchase I was forced to exchange AR and DEC. The mount seemed again track as new, but after two years the problem arose again.

It is better to say that the G-11 gear wheels seems finished with a procedure that hardens the surface of the material and then anodised; the worm gears are rectified; both seems not lapped. In practice this means that you can exchange the worms: the only visible effect will be a different periodic errors. .

Therefore, after four years, I were with all bearings seized. Andrea and Maurizio came to my help and they replaced the four bearings bringing to a new life my G11.

In the next five years I used very little the mount and slowly I abandoned the chemical astro photography. Because digital exposure are shorter, made less dramatic the problem. Since 2006 I have taken up by photographing with continuity and I put in the head for a final solution to the problem.

I have sought on the Internet some ideas and solutions, but nothing. Indeed people still consider that the problem is mating between worm screw and gearwheel: perhaps they have never tried to feel bearings. Anyway up to now the only solution is to replace bearings.


As often happens, try one thing and I find another.

Many sites explain how to disassemble the G-11 for the purpose of healing and lubing it, but only some of them place the emphasis on how important this operation, and now I will explain why. The G-11 (when new) has this important feature: by moderately tightening RA and DEC frictions, motors remain always in grip and (fundamental!) the telescope can be moved smoothly and precisely with a hand . This feature essential for a mount without micrometric manual motions.

Without proper maintenance, this feature slowly disappears. The degradation is slow enough so you don't think that something has changed in the mount operation.

At the end  the possibility to move the telescope when frictions are tightened is lost completely, and to ensure a good tracking you must tighten the frictions to the limit of physical pain. It is for this reason that have been so successful the famous friction knob modifications.

Of many sites that speak of these arguments I selected those which I think are the best:

http://www.losmandy.co.uk/probs.htm

http://www.astro.uni-bonn.de/~mischa/mounts/index.html

http://astroshed.com/observatory/g11saga/g11saga.htm

http://www.tfh-berlin.de/~goerlich/cg11tune.html

http://overton2.tamu.edu/rdb/sbig/76-second.htm

 http://www.aozc64.dsl.pipex.com/tips/g11gemini.htm

After weeks studying, I decided that I was ready to make open-heart my dear G11.


I - what we need?

Never begin to disassemble something if you don't have a good idea of what you need!!!

Here is the list (I hope complete) of tools and materials that I have used:

1 - star screwdriver

2 - allen 5/64"

3 - allen 1/16"

4 - allen 5/32"

5 - allen 5/64"

6 - allen 3/16"

7 - allen 7/32"

8 - allen 3/8"

9 - allen 1/8"

10 - allen 3/32"

11 - allen 1/4"

12 - allen 1,5mm

13 - 2 paintbrushes

14 - hammer

15 - waterproof marking pen

16 - diesel fuel 200ml

17 - lithium grease 30g

18 - tatters

19 - old newspapers (a lot)

20 - blotting paper

21 - a big wooden pencil

22 - clamp

23 - 7/32" nut

24 - plumbier clamp

25 - dremel with a spherical mill

26 - various bowls

27 - a needlework spool string

28 - shaver oil

... and a lot of free time: my extimation is a total of 16 hours .

Let's start.


II - Initial operation

G-11 is made of 3 different blocks.

The polar block, in the bottom, contains the Azimut and Elevation regulations.

The AR Block is in central position.

The DEC block is on the top.

First of all we need to remove the motors. It is quite simple regardless of the type of motor mounted. These parties

may be put away because they don't need any maintenance.

 

The second operation is to remove the worms. The parties are to be treated carefully. Important: move the DEC with the heaviest downwards; in fact removing the worm screw the DEC axis tends to turn if not located in its lower position, leading to a serious friction between gearwheel and worm screw (and this would be bad!!!).

Loosen the two grains and remove the worm carter. Loosen and unscrew the two bolts that locks the cubes on which rotates the worm. Clean the surface of the worms and with a waterproof marking pen make a sign on it: this will allow you later to distinguish the two worms.

TO PULL OFF THE WORM BEARINGS I HAVE DONE A DEDICATED PAGE

Infact this is a complex operation which is necessary only if you want to replace the bearings.

The worm blocks are one of the few details of G11 that must be handled with extreme care.

If the curiosity is strong you can start to verify if the bearings are to throw: taking one cube between thumb and index, then slowly rotate the worm: if you are experiencing any stumbling the bearing have, quite for sure, to be replaced.

Anyway this test have to be repeated after a deep wash with fresh diesel.


III - How to remove DEC block

Locate the four bolts that join the DEC block to the AR block.

Remember that without worms the axes are free to rotate. and that this operation must be made after removing the worm, otherwise you can ruin worms and the wheel. Loose all bolts and then remove the lower two . The lower part of the DEC plate is completely flat, so removing the other two bolts the block will slip: be careful, it is a 7 Kg piece!!!.

We have DEC blok free.

 


IV - How to remove the AR block

This is what remains of our G11.

Find and unscrew the big bolts that joint the AR block to the polar block; don't be afraid if anything moves: the two blocks are held together by a brass pin that must be off using a hammer and a wood or plastic tool. Hummer gently until the pin begin to come out the other side. When the pin is out for more than half of its length you should be able to take it away with one hand; with the other hand hold the AR block. AR that becomes free.

Now you have finished the first part of the work separating the G11 in its three main blocks.

Now we can disassemble the three blocks individually.


V - Polar block

Take upside down the polar block and unscrew the big bolt in the centre.

Now the metal disk with impressed the mount serial number is free. Take the block side up, and unscrew the two black butterfly bolts; now the base of the polar block is free and you can see 4 more bolts.

Unscrew these bolts: the azimuth regulation plate is free; now you can disassemble the elevation mechanism.

After the bearings here lies the second big problem with this mount: this piece is hurt.

The elevation is adjusted using the knob on the right which moves back and forth the brass pallet that you see in the middle.

Point 1: the pallet is quite shorter the distance between the two side walls; this becomes a play that can get a degree and a half in elevation adjustment. This gives no real problems, either in photography since the mount is 'hanging' by gravity, but a solution is rough up to the rest of the mount.

Point 2: The big black knob is screwed to the worm and is set with a grain pressed directly on the thread, so that once the grain lifted, the knob does not unscrew!

Point 3: the race-end is represented by black cylinder (left of the brass washer) and it is also fixed with a grain directly on the thread. This makes particularly difficult to disassemble the whole because, while the two bolts at the left end are easily accessible, the four bolts on the right are covered by the big black knob.

And here you have to be patient and lucky: replace the brass pallett with a 7/32" nut, screw it completely so the big black knob is blocked; put a few drops of oil in the grain hole and try to force the knob clockwise and counterclockwise. The thread of the knob will ruin a little but this is not a not a big problem because I will create a grain home on the worm ( as it should have Mr. Losmandy).

Coming off the knob, there is a surprise: in its base there is a hidden thrust bearing. With such a mechanism it is completely unusefull!

Now you can remove the four bolts. Using a plubier clamp and the 7/32" nut you can pull off the end-race. Also in this case we have to make a grain place to facilitare reassembly.

That's the result.

Difficulties encountered during polar block disassembly are the only ones throughout all the process.

Now stop and arrange the pieces in a logical sense. It will be useful to take some photos or mark some parts: at the end we will have a fifty pieces (bolts & nuts excluded) and it will not be an easy task to remember your mind the right assembly sequence.


VI - AR block disassembly

It's a very simple task. Completely unscrew the knob which blocks the axis: five parts now are free (remember the order).

At this point you can pull the axis which remains linked to the round base. On this base you can see a white nylon plate: pull out it with great care being careful not to bend or deform. It should be left to soak in water and soap because it has time to fully defatted.

Meanwhile we can inspect the surface of the axis; the brown tracks are nothing but solid grease, are sticky to the touch.

By placing a finger in the holes where the axis moves one can assess the needle bearings condition. In my case they were completely blocked by old grease and no needle was free to rotate. The hour circle comes away with a little effort: it is also pasted with old grase and are unusable since long time.

Someone has unlocked the circles putting a few drops of oil between the rim and the housing protects the gear, thus making the circle is actually unlocks, but almost certainly the oil, too fluid, ends up in the particular area where the nylon plate is placed, totally defeating its  functionality.

Identify three allen bolts fixing the gear housing; loose them a little and then turn the block so that the gear is pointing up, remove bolts and the housing. At this point you can pull the gear (it should be protected from any accidental bumps so I have wrapped in the rags).

Under the gear there are four washers and a thrust bearing. The last washer, the thicker hides three other allen bolts; remove them and you can remove the plate that supports the motor and the worm.

The block containing the two needle bearings is composed of at least eight other pieces, but these are difficult to separate.

For the purposes of cleaning and greasing it is better leave the block as it is.


VII - DEC block disassembly

DEC block disassembly process is quite the same as AR block.

 

And with this we have finished disassembling the G11.

Time required: about three and a half hours. Move on to the next stage of cleaning.


VIII - Dgreasing, cleaning and drying

I divided the pieces into four groups:

A - worm assemblies

B - Bearings and parts containing bearings

C - nylon parts

D - all the rest (bolts & nuts included)

Nylon washers are left to soak in a bowl with water and mild soap. After a while wash it by hand and dry with a clean cloth.

Worm blocks will be the last to be assmbled and should be put in a bowl and brush occasionally with diesel, especially in the bearings area. Initially use only the first brush to clean the worm assembly; after use this brush for other parties only. Wash bearing blocks with a little diesel, and remove the dirt diesel (you can use it to clean the other parties). Use the second brush only for worm blocks fine cleaning.

After a couple of hours, you can wipe with a damp cotton (no debris, please!!!). Drying must be repeated several times to remove all the diesel excess in the bearing housings. The thread of the worm must be dried thoroughly using embroidery wire to remove the oil from the valleys. At the end of drying you must test rotation of blocks around the worm. Select the one that gives fewer signs of galling: It should be mounted to AR.

Other bearings and bodies with bearings should be carefully brush with diesel.

Bearings should be left half an hour immersed in diesel; the blocks with four needle bearings (two per axis) must be continuously brushed to remove any dirt inside the holes. In particular the four bearings must be sprayed with generous strokes of diesel because the dirt is left behind bearing parts. Once a bearing starts to rotate, alternate a stroke and a series of rotations of the bearing itself. The deal ends when no more dirt comes out. Then a final round of stroke with clean diesel and we have finished.

These parts can be placed on sheets of paper because it absorb most of the diesel in excess, then you may dry it with kitchen paper towel. The drying must be repeated several times because diesel tends to remain hidden and flows slowly

Axes are to be brushed firmly to remove grease deposits; after that, together with gears and the whole group D parts can be washed quickly with diesel, dried and washed in the dishwasher.

Do not worry; it is anodized aluminum, stainless steel and brass: they do not suffer at all a good wash. Final rry by hand is recommended.

We have finished this step that lasted about five and a half hours.

Now we must put all this stuff.

 


IX - Greasing and reasssembling

Reassembly is easier than disassembly: just carry out the disassembly in reverse order. I will focus only on parts to be greased and steps that require more attention.

Thrust bearings need grease on both sides. When you are sure grease is well inside the bearing remove the excess on the outer and inner border circle and place the bearing between two washers; then add the third. Keep your hands as clean as possible. if you have too grease like in this photo, repeat the operation.

The excess grease tends to invade the neighborhoods, and here we are very close to the area where the nylon washer works. Even small traces of grease in this area can affect the operation.

When you put in place the nylon washer take care not to bend it, and that the contact areas are perfectly clean.

Gear needs grease to penetrate between the teeth. When this is done all the excess grease must be removed carefully. Make the same thing for needle bearings.

Axes don't need grease; once put in place clean the part that comes out (which will inevitably be impregnated with grease). The hour circles shall be greased at the bottom side only.

There are no specific warnings to put RA and DEC blocks toghether.

 Put a bit of grease on the spring for the knob of the dovetail plate: this will make it sweeter and immediate movement of blocking and unblocking of the dovetail.

The polar block is reassembled without problems, once places for grains have been made.

 Here you can overdo the pallet and the worm with grease. The top of the pallet have to be grased: it runs  under AR block to reach the proper height. This makes sweeter movement driven by elevation knob.

The plate that contains the level bubbles (fixed with silicone) needs grease: this allows a fluid motion in azimuth regulation.

 Now again we have our three blocks: polar, AR and DEC. We can proceed with reassembling.

 Another bit of grease is usefull on the pin that links AR and DEC blocks. The inclusion of this particular is a bit cumbersome because we must maintain aligned all holes. There is no need to force it, just a little patience aligning the blocks. Also make sure, before you insert the pin, that the elevation pallet is conveniently positioned in the AR block throat. Repositioning DEC block is difficult just because it is heavy: better get help.

Well: apart from motors and worm blocks we reassembled all in about four and a half hours.


X - Reassembling motors and worm blocks

You will have the impression that something's wrong than before because the mount seem to rotate barely touched with a finger: worms are still missing!

Worms refitting is the most boring operation. They do not seem to ever go out: too loosen, too close, more or less off-axix, in short may be frustrating. Then be prepared to repeat such operation dozens of times before reaching a decent compromise. It's certainly the most difficult operation ever and its success can not be finally checked unless a visual test.

The only possible test now assess the play between worm and gear, rotating the worm by hand in both directions. A full gear rotation is necessary to evaluate possible stake.

Once you lock the worm you can grease and put a drop of oil for electric razors on each bearing. Verify the motor shaft couplings are well aligned; a little bit grease helps right coupling.

This last step takes more or less two hours.

  The final satisfaction, however, is large: mount locks with just a moderate close of the two knobs, and moves smoothly with one hand (of course load must be well balanced). A visual test should be done as soon as possible. I used a 2X Barlow on my 8" LX200R and a 9.7mm ocular: about 410 magnification. Once balanced load I have observed in daylight a distant object to evaluate possible jumps, backlash, and stiction. Some further refinement on gear- worm distance could be necessary after this test, but the final evaluation have to be carried out on stars.


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