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628 Drive Problems

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#1 Atl

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Posted 03 December 2012 - 01:26 PM

I have a Meade 628 and I have been having an issue with it. During cooler weather the clutch slips. The RA knob is turning from the motor, but I have to tighten it down very tight to make it track from all positions. This makes it hard to adjust the telescope's position. I took the drive apart and cleaned the surfaces with degreaser. It was pretty clean to start, but I did it. It is still slipping. I have tried playing with the backlash screw and adjusting the angle of the worm gear but to no avail. Back during warm weather it was tracking great, but when it dips below 50 the clutch severely slips. I don't see how it could be broken...everything seems solid. I am thinking that I am missing something? It's got me pretty irritated I must admit. I have had thoughts of selling it to get it out of my sight, but it would be nice if I could fix it.

#2 rdandrea

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Posted 03 December 2012 - 03:39 PM

The RA knob is turning, but is the worm gear? I don't remember how the worm gear attaches to the shaft, but if there's a pin it might have sheared. Take off the drive cover and see if the worm itself is going around.

If it's turning, try torquing the three clutch pressure screws down a little tighter. Just be careful not to strip them.

#3 Atl

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Posted 04 December 2012 - 09:44 AM

The worm gear does turn when the RA knob turns. I verified that definitively. The worm gear also moves the large cog that it meshes with, but the teflon washers seem to require a lot of friction to engage the drive. If I torque the screws down tight enough to get the drive to turn it then becomes hard to move the scope by hand.

#4 Edward E

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Posted 04 December 2012 - 10:07 AM

Is the scope properly balanced in RA, Dec & tube? 80% of the time if a scope does not track it's a balance issue. Next thought would be that you need to tighten the three screws on the worm gear; over time the compression washers wear and you have to tighten the screws more to achieve a non-slip gear. Check to see that the set screws that connect the worm gear assemble to the RA shaft are tight.

#5 Atl

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Posted 04 December 2012 - 01:57 PM

My balance is good. I have been pretty picky about following the instructions in the manual. How tight should those screws actually be. I can engage the drive, but at that point the screws are very tight. Doesn't that put stress on the motor?

#6 rdandrea

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Posted 04 December 2012 - 04:41 PM

Doesn't that put stress on the motor?


Not if the scope moves. And not if the clutch disengages when you move the scope manually. You can put a lot of torque into a pinion gear from a worm, but because of the mechanical advantage of the worm, not so much the other way around.

You should be able to find a happy medium with the screws. It might change with the temperature (mine does a bit).

#7 Atl

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Posted 04 December 2012 - 05:04 PM

I guess that is the answer. I just need to tighten it down until it squeaks I guess...lol. I was just worried I would burn up the motor. Back in the summer it didn't require much tightening at all, but fall came along and killed that. It does disengage when the scope is moved.

#8 Edward E

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Posted 06 December 2012 - 12:08 PM

If your compression washers have been nearly flattened with time and use then it would take a lot more tightening of the screws to achieve the same compression on the worm gear assembly than would be required with new, "springy" washers and temp changes would have less of an effect as well. This is what I observed with my 826 mount from the time it was brand new to +10 years. Have a look at the old washers and compare them to a new compression washer of the same size so see the old ones need changing.

#9 planet earth

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Posted 06 December 2012 - 12:35 PM

On my 826 mount I need to tighten the 3 screws fully (not super tight) and then I back off the screws about 1/4 turn.
Make sure your polar shaft moves freely with the 3 clutch screws loosened off, say a full turn or so.
Did you test the scope balance, with the polar shaft horizontal and dec shaft at 45 degrees.If it moves it's not balanced. You'll notice many Cave's and other scopes have a extra weight system on the tube to balance the tube radially to compensate for the weight of focusar eyepiece and finder.
Check to make sure the worm gear turns freely and leave a touch of backlash. Make sure the worm and worm wheel turn freely when the clutch is loosened.

Sam

#10 planet earth

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Posted 06 December 2012 - 12:48 PM

Good tip on the spring washers Edward, I think I'll do mine now.
Sam

#11 Atl

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Posted 06 December 2012 - 06:45 PM

The big washers are teflon I believe. Where can you buy them?

#12 Edward E

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Posted 07 December 2012 - 05:40 PM

If we are talking of split (spring) washers, you should be able to get these in a well stocked hardware store such as "ACE" or "LOWES" or a good mom/pop hardware shop.

It's up to you but I would replace the plastic split washers with metal ones and keep the old plastic one in storage to refer to when needed.

#13 rdandrea

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Posted 08 December 2012 - 11:11 AM

I think he's talking about the big teflon washers that are sandwiched around the drive gear in the clutch assembly. Good luck finding those. There are lots of sources on the internet, but you have to buy large quantities.

#14 Edward E

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Posted 23 December 2012 - 12:01 PM

Any improvement or a fix on your slipping clutch?

#15 Atl

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Posted 23 December 2012 - 12:48 PM

None. The clutch either has to be so tight it won't move or so loose it slips. I am at a loss. I have been thinking of selling it since I bought a 12.5" dob...lol.

#16 dgreyson

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Posted 24 December 2012 - 12:38 AM

Are you sure the clutch is actually slipping? Maybe the clutch assembly it'self is rotating on the shaft, or the equatorial head is slipping on the other end of the shaft.

It sounds like something is contracting when it gets cold enough and is slipping loose then, just my W.A.G.

#17 Edward E

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Posted 26 December 2012 - 09:54 AM

Not good. It is staring to sound like the aluminum ring gear had worn into the plastic grove to the point that it no longer fits snuggly. If so, it's either replace the plastic or find a new ring gear assembly. Check with Don Rothman at Astro Parts Outlet he has a lot parts for these older mounts or you could build a new worm gear/clutch assembly. Edmonds Sci "All About Telescopes" has a good design to use.

#18 tim53

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Posted 26 December 2012 - 10:43 AM

I haven't worked on one of these in a very long time, but used to build them at Meade 30 years ago.

Pics would help a lot for seeing what you are referring to. There were steps we would take while assembling these to make sure everything was working before be buttoned them up and shipped them out. I might be able to remember the steps if I could see a pic, especially where you think the problem is.

-Tim.

#19 planet earth

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Posted 26 December 2012 - 12:20 PM

Here a quick drawing:
Basically C and F are tightened with the 3 screws, with the worm wheel E: in the middle, and D: 2 thin teflon disks on each side. H is a teflon ring that goes into the wormwheel bore.
B and C are setscrewed to A the polar shaft.
Sam

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#20 dgreyson

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Posted 26 December 2012 - 12:55 PM

My drive dosent have "D" but does have "H" between the innder diameter of the RA gear "E" and a concentric step on "C" inside the diamater of "E" which is it's bearing "race"

#21 Geo.

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Posted 26 December 2012 - 04:34 PM

The problem I've seen is that the set screws (3 as I recall) that lock B onto A will slip out of the land cut in A and move up A binding against the axle housing. But I remember that clutch pressure was applied by a large handwheel at the end of A.

#22 tim53

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Posted 27 December 2012 - 01:28 PM

From ancient memory, I recall that the worm wheel is a ring that rides in a step in c and f. The two Teflon disks D are rings that also ride in the step as clutch surfaces between the hubs and the worm wheel. Adjustment should be VERY sensitive, such that minor fractions of a turn of the clutch screws can make a big difference in the amount of friction applied. The lock washers were used to prevent the screws from loose ing on their own. They don't act as springs

When we put the mounts together, we tested them as we assembled them on a bench. First, we made sure the motor was turning by putting a tape "pointer" on the shaft and watching it turn. Then we put the motor on and marked a tooth of the ring gear to see if it moved while we did other things. At this point we might have left the clutch loose, I can't remember. But you could do that test with the whole scope assembled or without the ota or counterweight.

Remember, the worm wheel only turns on e in a day, so you need to wait a while to see the motion. Alternatively, you could set up on the sky and see if its tracking pretty quickly. There will likely be some backlash at first, but so long as the worm and worm wheel are making contact and the motor is running, you should see the scope start tracking your object in under 30 seconds or so (a lot of backlash!)

Tim

#23 apfever

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Posted 27 December 2012 - 02:21 PM

Atl,

I'm a sad case for documenting my work, and getting it all posted here where it counts. I've completely done this drive and whole mount from inside out. The string is here:

http://www.cloudynig...4966686/page...

There are pictures and process on the drive that I've never posted. Everything was redone and aligned with calipers and micrometers. This is a busy week and I have to jump on an Orion XT8i today. I'll get a few pictures now. I'll get you through this thing dot and tittle. Mine is complete, correct, in good shape, and works excellently. Originally this had an 8" on it. It now goes with a 10" that was offered on this mount from the factory. here's a picture of the basic clutch. The lock ring (far right) is first on the RA shaft and sets the end play in the mount housing.

The clutch starts with the large hub (center). This gets a teflon washer, ring gear, another teflon washer, and cover (upper left). All this is CLEAN, no lube, certainly no grease! I do use a special dry Teflon lube on some of these, but don't remember if I used it on this one. I could use some lube on my brain. Note that gear ring center!!! it has a small teflon strip around the inside. This goes between the gear fing and the hub. It makes the ring tight on the hub. The worm gear ring is thus isolated from everything with a washer top and bottom and a band around the center.

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#24 apfever

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Posted 27 December 2012 - 02:29 PM

Random picture:

The worm had more endplay than it should. It should be right at zero without binding. I put a socket over the end and used my metal vice to squeeze the brass (porous bronze?) bushing in till the worm rotated freely with just a hint of resistance at the tightest. The loosest position was probably in the tenths for clearance (.0001" = a tenth).

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#25 apfever

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Posted 27 December 2012 - 02:47 PM

After positioning the bushing for end play, I installed the retaining bolt with just a touch of Red Locktite on the threads, and a micro hair amount at the contact point between the bolt head and bushing.

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