CG5 mount, clutches seized

[SteveBz]

Hi Guys,

I was lucky enough to buy a Celestron c8n with a CG5 mount on ebay for about £100.  The OTA was boxed and in nice condition.  Needs a bit a bit of collimation and for some strange reason has two dovetails, piggy-backed (a short one and a long one).

However, the CG5 was not boxed and had been batting about the sellers house for some time.  It was quite chipped with pain splatters and missing a few bits, like the polar alignment scope and bottom protective cap.  Not really the end of the world.

However, both the clutches are to one degree or another, frozen.  The dec clutch allows the dec axis to roll, but the polar scope "hole" (under the polar scope top cover), rotates with the OTA.  As it is apparently frozen at 90 degrees to the dovetail slot, it means no polar alignment using the scope, because when the main scope is pointing North, the hole polar scope is blocked by the "ring" inside the mount.  Sorry, I'm probably not using the right words.

The RA clutch is worse.  The OTA cannot be balanced with the weight bar because it does not freely swing.  If you use the RA slow motion control, you can rotate the OTA in the RA axis, but not freely.

It feels like I should strip it down and rebuild, but I'm feeling a bit nervous about what I might encounter that would prevent me putting back together again.  I should probably at least clean up the grease and re-grease. 

So is there anything I should look out for and any other maintenance I should perform when I get it apart?

What do you think?

Thanks

Steve.

[DRT]

Steve,

Here is a website that gives a detailed description of how to strip down and re-build a CG5: http://www.astronomyboy.com/cg5/

Good luck!

Derek

[RobertI]

I have disassembled my CG5 by following those instructions and it was very straightforward. You will probably find the offending problem(s) as you go through the disassembly process. If it's so serious that you cannot get it back together again, then you can look to source replacement parts or put a wanted ad for a CG5 for breaking. You won't be any worse off in my opinion as it doesn't sound usable in its current state anyway?

[spaceboy]

I suspect it is just a case of someone being a little over zealous when tightening the clutches on a cold night and now the warm weather is here everything has expanded a little.

So if all else fails put it in the fridge or freezer for a bit to loosen everything off then give the clutches a go. Personally I would remove the clutch levers themselves and get a spanner directly on the bolt to try loosening them as the cold can make some materials brittle.

HTH

[spaceboy]

Sorry for my confusion. Do the clutches un do? Or are the levers stuck solid so you cannot release the clutches??

[SteveBz]

Ok,  I should have been clearer.  The clutch releases but then the OTA doesn't rotate around the RA axis. For the dec, it does rotate, but the differential rotation of the polar opening fails.

I agree with RobertI, it's not usable except as decoration. I might as well use the astroboy instructions.  Do you think I can use any old silicone grease off eBay or amazon?

Thanks.

[spaceboy]

I think something has been over tightened at some point for the mount to not freely move with the clutches released. It will either be this ring nut  or this ring nut Note both are secured by 3 Allen set screws. Might be worth trying to loosen these off to see if it improves things before you go through the hassle of a complete strip.

 

Just remember if you do a complete strip to note the order the shims / spacers come off so they go back on the same way once re-assembled or the worm gears will not mesh perfectly and you will get increased play.

 

[DRT]

I bought one of these from Ebay for about £40 about a year ago and had an almost identical problem. When I started taking it apart it soon became obvious that everything had simply seized up due to thick, dirty grease that had solidified through lack of use. I got about 45 minutes into the strip-down and had managed to damage a couple of threads trying to prize things apart and then threw it in the wheely bin. Many parts of these mounts are made from low grade aluminium and they just can't take too much force before giving way.

If I were you I would look out for a nearly new EQ5 head

 

[SteveBz]

Oh wow DRT! I hope that's a one off. However I will say that some of Allen heads seam to have been tampered with.

[DRT]

14 minutes ago, SteveBz said:

Oh wow DRT! I hope that's a one off. However I will say that some of Allen heads seam to have been tampered with.

Unfortunately not. There are many reports of thread stripping and accidental damage to these mounts and it is very easy done. It is worth reading all the way through the instructions before you begin. Some of the parts require specialised tools so it is best that you ensure you have the correct tool set before starting. Improvising with what you have available leads to the problems I had

 

[SteveBz]

3 hours ago, spaceboy said:

I think something has been over tightened at some point for the mount to not freely move with the clutches released. It will either be this ring nut  or this ring nut Note both are secured by 3 Allen set screws. Might be worth trying to loosen these off to see if it improves things before you go through the hassle of a complete strip.

 

Just remember if you do a complete strip to note the order the shims / spacers come off so they go back on the same way once re-assembled or the worm gears will not mesh perfectly and you will get increased play.

 

Thanks for this SpaceBoy.  I'll do it. I'm travelling for a couple of days, so I'll try Thursday or Monday.

I've just been making a box to carry it in today.

[SteveBz]

21 hours ago, DRT said:

Steve,

Here is a website that gives a detailed description of how to strip down and re-build a CG5: http://www.astronomyboy.com/cg5/

Good luck!

Derek

Nice to see that you can download it as a pdf. I didn't see that the first time.

[Roy Challen]

Often, the ring nut threads are damaged by attempting to undo the ring nut with without undoing the 3 set screws. 

Take photos as you go- saves trying to remember which order everything goes back in.

[spaceboy]

Did you get the mount sorted in the end Steve ?

[triton1]

I also had a similar problem with mine with the Dec whatever you do don't force it,I ended up using my missus hairdryer and got it as hot as I could and it finally freed up it was the old grease that sized it.After a polish and regrease it has been mint ever since.

[SteveBz]

On 15/07/2016 at 05:44, spaceboy said:

Did you get the mount sorted in the end Steve ?

Hi Spaceboy,

Thanks for asking.  I have a friend who is a more experienced astronomer than I am and who has a mount cleaning kit.  He's been away all this week, but I hope we'll get together soon and go through it.  I do, actually, now have some silicone lube and the instructions for pulling apart and putting back together.  I'd be up to doing it on my own, but it would be more companionable to do it with a friend.

So I'll wait.  If I don't hear from him soon, then I'll do it on my own.

Thanks,

Steve.

[SteveBz]

Hi Guys,

OK, I just stripped down the dec.  Actually, after washing with a stiff artists brush and white spirit, it's all in quite good condition.  However, here's one of the problems: the worm gear is stuck on the dec shaft.  It will not properly engage with the collar, leaving the worm drive not quite pulling the worm gear, nor will it totally slide off the dec shaft so that I can clean the inside of it.

I suspect it has been repaired by a previous owner and a similar, but not identical, worm gear installed by force.

Photo attached, you can see the worm gear, then the large washer, then the collar, which is too tight to insert properly.  You can put it about half on, but then it is too tight to turn.  I think this explains my problem with the dec drive.

Thanks

Steve

[spaceboy]

What you need to do is get hold of a helper, mallet rubber, nylon or wood and a piece of scrap wood. First off my guess is the part is fine but who ever installed it didn't put it on nice and square in the first place and then as you say forced it the rest of the way. So try to figure out where it is slightly off. Maybe a ruler will give you an idea if you can't see it by eye. Boil a kettle and pour it over the gear then with the helper holding the DEC shaft you have a go at gently but firmly tapping the gear square then off. Keep the piece of scrap wood up against the gear and hit the other end with the mallet. This reduces the impact and risk of hitting anything you don't want to hit. Once you have it off just get some wet & dry from B&Q or the like and give both inside edge of the gear and outside of the shaft a once over. You don't have to go mad just make sure you sand everything evenly. Again I am pretty sure this has been installed slightly off square. It is easily done just as it is cross threading fine threads.

Once you have given everything a once over with varying grades of wet and dry just rub your fingers over the surfaces to feel for any high spots. If it is all smooth then you can attempt to put it all back together again just paying a great deal of attention to it going on square and if it becomes overly tight.  Another important thing is note what shims go where as these are of varying thicknesses and play a part in things lining up.

[spaceboy]

P.S just try not to damage the shim so do your best not to have it in the way when pouring boiling water or hitting the gear.

[SteveBz]

Spaceboy, what do you mean by shim? The plastic washer?

[spaceboy]

Yep although I think in fact it may be made of Teflon or something similar. Hence proceed with caution as it wouldn't be easy to replace.

[SteveBz]

I have continued in this thread here:

In spite of my skepticism the kettle trick worked like a dream. It looks like two eq5s from different scopes stuck together. The dec shaft seems to be the foreign one.

I worry that wet and dry will create unevenness. Should I try, or try to get someone with a lathe?

What are the pros and cons?

Steve.

 

[DannyLee]

I imagine the surface of the shaft will probably be hardened so a lathe tool may not touch it. in any case you are probably only trying to remove a few hundreths of a mm so I would go for emery cloth soaked in  oil  and used as a long thin strip wrapped around and run up and down the area concerned. Keep cleaning the shaft off and frequent trial fits til you get it just right. I have used this method many times in model engineering and if there is any unevenness created it is too small to be an issue. Little and frequent testing is the key and when you have it correct clean all the bits to death to get rid of any emery before greasing it up

Dan

[SteveBz]

Hi Dan,

I didn't even know you could still buy it. Halfords 2.99 a cording to Google. Great idea.

I'll try tomorrow. It'll certainly do 2 of the three edges. The third is a little sill only 2 or 3 mils wide. I think maybe the cloth would smooth it's edge. What do you think?

Tx

Steve.

 

[SteveBz]

Hi Dan,

I didn't even know you could still buy it. Halfords 2.99 a cording to Google. Great idea.

I'll try tomorrow. It'll certainly do 2 of the three edges. The third is a little sill only 2 or 3 mils wide. I think maybe the cloth would smooth it's edge. What do you think?

Tx

Steve.