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Close up images of an Criterion RV-8 Mount

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#26 AaronHenderson

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Posted 16 April 2021 - 09:03 AM

Any opinions about clutch material? The plastic water ring I installed kept slipping and I had to really tighten the clutch spring screws to get the RA axis to track. Is the original cork a better choice? If you have any sources of thin cork I'd be wiling to try that. I'm thinking the self adhesive stuff is non-starter, as I do not want its glue in my clutch mechanism.  Aaron



#27 mpsteidle

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Posted 16 April 2021 - 09:55 AM

At least on my Cave, it's just aluminum to brass with nothing in between and it works swell.  Criterion clutches may be different though.


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#28 apfever

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Posted 16 April 2021 - 10:19 AM

apparently a milk jug washer works. I haven't tried one yet. Make sure the inside hole is not binding on the little hub lip at all, free floating.


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#29 DAVIDG

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Posted 17 April 2021 - 09:39 PM

 Metal against metal is a bad idea for a slip cutch design  First the metal surface would need to be  precisely   machined flat to a high  degree. If not you have random points of contact so you not using the whole surface. The other issues is that metal  as a high degree of friction and  that limits  range of were the mount is easy to move and will track and too tight and the scope is difficult to move. If you have tried to adjust a Cave clock drive, either the scope feels too loose and the clock drive slips or the scope is difficult to move because the clutch is too tight. On all the Caves I have restored I added a gasket material between the clutch surfaces and now the I can easily adjust the tension on the clutch so the scope has the "right" feel and  also tracks well.

   There is  a good reason while Criterion and many other clock drives of similar design use a gasket  type material  as the clutch surface. The softer surface is compliant to the metal surface so now the whole surface is being used. The gasket material has a lower  coeff of friction then metal.  Both of  these properties give a larger range of pressure that  can be applied so you can adjust the clutch  so the movement of the scope has the right  feel ( not to loose or too tight) and it starts tracking as soon has you let the scope go.

  Even very high end clock drive like the ones made by Byers use a Teflon gasket in their clutch assembly.

 

                    - Dave 


Edited by DAVIDG, 18 April 2021 - 09:36 AM.

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#30 AaronHenderson

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Posted 18 April 2021 - 06:44 AM

 Metal against metal is a bad idea for a slip cutch design  First the metal surface would need to be  precisely   machined flat to a high  degree. If not you have random points of contact so you not using the whole surface. The other issues is that metal  as high degree of friction and  that limits  range of were the mount is easy to move and will track and too tight and the scope is difficult to move. If you have tried to adjust a Cave clock drive, either the scope feels too loose and the clock drive slips or the scope is difficult to move because the clutch is too tight. On all the Caves I have restored I added a gasket material between the clutch surfaces and now the I can easily adjust the tension on the clutch so the scope has the "right" feel and  also tracks well.

   There is  a good reason while Criterion and many other clock drives of similar design use a gasket  type material  as the clutch surface. The softer surface is compliant to the metal surface so now the whole surface is being used. The gasket material has a lower  coeff of friction then metal.  Both of  these properties give a larger range of pressure that  can be applied so you can adjust the clutch  so the movement of the scope has the right  feel ( not to loose or too tight) and it starts tracking as soon has you let the scope go.

  Even very high end clock drive like the ones made by Byers use a Teflon gasket in their clutch assembly.

 

                    - Dave 

Thanks Dave! This, in general was what I was thinking too. I took apfever's advice and cut a new clutch ring that was loose around the clutch bearing plate.  

 

I'm still having issues with the DEC axis not turning under load. I have the clutch screws tightened to the max and still will not rotate the scope. I have the scope very well balanced but it just won't track. I have a suspicion of what is causing the problem. Someone in the past for whatever reason, decided it was a good idea to drill a hole in the DEC shaft (see images below). I personally do not think it was a good idea LOL.  The problem with this hole is, when you tighten the set screws, they seek this hole and rotate the upper clutch plate and tighten it a bit, not allowing the shaft to rotate freely. So I either need to fill this hole by welding (i'm not a welder), probably not a good idea, fill it with a brass or steel rod epoxied in, tap a screw in and file it down (after removing it temper) or maybe get a replacement DEC shaft. 

 

-Aaron

 

IMG_2238 small.jpeg

 

It gets worse! I checked the back side more closely and I had to remove some epoxy and this is what greeted me.

 

IMG_2240.jpg

 


Edited by AaronHenderson, 18 April 2021 - 07:22 AM.


#31 apfever

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Posted 18 April 2021 - 07:36 AM

It is the Right Ascension (RA) axis that you are having issues with, not the DEC. That hole shouldn't be a problem. You should be able to use the axis. Your clutch assembly is in question.

 

First, here is a picture of a clutch in good condition. The center bore of the clutch is threaded so there is no need of a nut. You mentioned a nut being used along with other holes being stripped, so I'm thinking your clutch is stripped with no threads in the center bore. I think your clutch can be made to work even if the threads are stripped but it will take some finesse. 

 

The thicker clutch plate is threaded, the thinner plate is a smooth slip fit on the RA threads. The threaded thick plate goes on first with the three set screws towards the mount. This does not have to go tight to the RA shoulder. The plate can be screwed to any position on the threads to help align it with the worm height from the backing plate. You can also shim the worm block on the back plate to raise the whole system.  

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#32 apfever

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Posted 18 April 2021 - 07:46 AM

In your case, you need to keep track of where that hole in your RA shaft is. Draw a line on the end of the shaft that represents the hole. You can position the clutch so that one set screw goes on the thin side of the hole and two set screws get centered on the thicker side of the hole. This would take a very small turn adjustment that shouldn't affect alignment with the worm, and don't forget you can shim the worm block assembly. You can also add washers between the plate and shaft shoulder that will let you thread the plate on to a snug fit where you want it.  

 

IF you have no threads in that thick clutch plate......

For starters, the bore needs to be a close fit to the shaft threads so the clutch is well centered on the shaft. Too much side to side slop can make the worm and gear go from being too loose to binding (run out).  You can wrap aluminum foil around the shaft threads to center up the clutch. The foil can then be cut with a razor and peeled off the end of the plate leaving a custom centered clutch. You have to do a series of dry assemblies with EVERYTHING including the worm block and backing plate, to figure out what height to place the clutch on the shaft. Favor moving towards the end of the shaft and you can shim the worm block for final fit. This only sounds arduous in words. It won't be much more work than a normal assembly. 

 

You are after:

1. Positioning the clutch at the right height on the shaft.

2. Getting the clutch centered on the shaft.

3.  Keeping the three set screws centered Away from that hole in the shaft.  

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Edited by apfever, 18 April 2021 - 08:13 AM.

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#33 apfever

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Posted 18 April 2021 - 08:36 AM

That thick plate (that is supposed to be threaded) needs to be TIGHT to the shaft. That is why it has three set screws. I wouldn't be concerned about running the set screws through foil if you have to shim the clutch plate. Everything else becomes normal once you get that bigger plate anchored to the shaft.   Ramble thoughts....make sure you have all the washers or other fittings you want between the RA and DEC housings. Any changes between the DEC and RA housings will affect the position of the RA shaft. Don't forget to have everything lubed up with grease where you want it - Before you do a final set screw tightening of the clutch into position. I do 'dry' assemblies with a drop or two of light machine oil until I'm ready for Final assembly. This just makes everything a lot neater since the shaft usually ends up going in and out a few times. It's easy to forget final lube. 

 

You stated you have room for a nut on that shaft. A nut won't work for anything. The only thing a nut could push on is the thinner outside clutch plate. BOGUS! That thinner plate needs to remain a slip fit on the shaft so clutch tension can be adjusted with the three small spring loaded screws.   


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#34 DAVIDG

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Posted 18 April 2021 - 09:33 AM

You need to start at the motor and work forward. Many times it is the motor that is the problem. It rotates at 1/10 rpm ie 1 rev in 10 minutes. These motors have a built in safety feature in that it use a Bakelite gear  that is design to break if the motor is stalled. So either you can get no rotation of the shaft, the shaft will rotate for few minutes and stop or it will rotate with no load but not when loaded. You should not be able to rotate the shaft of the motor by hand when it is removed from the clock drive. If so it is damaged. 

   The shaft of the motor has an adapter that is press fit on the end. It has "blade: sticking out that fits in a slot on the worm gear. You'll need to remove it and reuse it if you replace the motor. 

   To test the rest of the clock drive assembly, with the motor removed you can insert a small flat blade screw drive were the motor  go and into the slot on the worm. Now you can turn the worm by hand and see if anything is slipping or in there is any backlash or play.

 

               - Dave 


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#35 AaronHenderson

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Posted 18 April 2021 - 10:37 AM

Thanks guys! These are excellent! I’m a bit under the weather right now, but I’ll get to these specifically this afternoon if I’m feeling better, CN rocks!

#36 AaronHenderson

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Posted 19 April 2021 - 02:06 PM

Okay, feeling better today. 

 

I put the clock drive back together using the information provided above. Thank goodness the motor is functional and rotating correctly. There was a washer between the upper threaded clutch plate and the drive RA shaft that I believe was being pinched and not properly aligned causing my issue. I reassembled the lubricated (I used white lithium grease) and did a test non-loaded clock drive test.

 

I was able to line up the three set screws to avoid the hole in the shaft and they tightened nicely. Fortunately it was exactly at the point where the threaded clutch plate was tight on the shaft so no additional washer s or shims needed. 

 

Then I made sure the shaft was moving freely before I re-attached the motor assembly and it was free and moving properly. I added a counterbalance to offset the the DEC shaft weight turned it on. It tracked perfectly. Next, I will add a 6" Newtonian scope to see if it handles the load of a telescope. This smaller scope should be no problem for this mount. I'll let you all know how it comes out.

 

-Aaron



#37 YourNotSirius

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Posted 01 May 2021 - 10:02 AM

Okay, feeling better today. 

 

I put the clock drive back together using the information provided above. Thank goodness the motor is functional and rotating correctly. There was a washer between the upper threaded clutch plate and the drive RA shaft that I believe was being pinched and not properly aligned causing my issue. I reassembled the lubricated (I used white lithium grease) and did a test non-loaded clock drive test.

 

I was able to line up the three set screws to avoid the hole in the shaft and they tightened nicely. Fortunately it was exactly at the point where the threaded clutch plate was tight on the shaft so no additional washer s or shims needed. 

 

Then I made sure the shaft was moving freely before I re-attached the motor assembly and it was free and moving properly. I added a counterbalance to offset the the DEC shaft weight turned it on. It tracked perfectly. Next, I will add a 6" Newtonian scope to see if it handles the load of a telescope. This smaller scope should be no problem for this mount. I'll let you all know how it comes out.

 

-AaronI

We never EVER use white lithium grease! If I remember correctly it contains Lithium Hydroxide (LiOH) which is commonly obtained by the reaction of lithium carbonate with lime. This becomes a lithium salt, or soap of stearic and other fatty acids. This soap is used as thicken agent in lubricating greases. Over time, the actual lubricant/carrier dries out and leaves behind a white cake of soap. The lubricity is actually quite poor when compared to synthetics. Also, the temperature range is very narrow. It dries quickly in hot weather and becomes very hard in cold weather. It's use as a lubricant for moving parts has long been superseded. It was fine for a model T in the 1920s but, really not very good for telescopes or other equipment in the modern day.

 

We have several contemporary choices which we use on everything astronomical with far better results than can be achieved with white lithium grease. We use ZEP-2000 on most roller bearings. In fact, on just about every bearing. It contains PTFE, has a very high rate of adhesion, can handle extreme pressures, has a temperature range of -106F to +600F and is not prone to water washout. Our telescopes still turn freely at -25F in the dead of winter when using this product.

 

For gear trains we use good old Super-Lube Grease. It is much like Zep-2000 except it is a much thicker product. It is a synthetic NLGI grade 2 heavy duty lubricant with PTFE. The carrier base fluids are combined with PTFE micro powders to form a high quality lubricant. This gives a superior protection against friction, wear, rust and corrosion. One of the nice things about it is that it does not run, drip, evaporate, form gum like deposits, melt, or separate as do lithium compounds.

 

For breaking adhesion on corroded components, we use Corrossion-X. This stuff is amazing! It is heavily used in the aviation and marine industries. If you have something that is corroded and nothing else works, this stuff is the bee's knees! We've used it on items that spent years by salt water and after applying it and letting it sit overnight, everything came loose. At first, the old man though the bolts had snapped. He was thrilled that after breaking the initial bond they were finger loose! He's used it exclusively ever since. It will seek into corroded components using a chemical change that promotes capillary action which draws the solvent into corroded threads, shafts, and so on.

 

I guess I kind of turned this into a cross between a rant and a chemistry lecture. Sorry about that! It's just that we have learned to loathe white lithium so much that I felt it was my duty to help inform everyone about how got away from it and why. Those products that we use have made a big difference in how our telescopes and just about everything else we own perform so much better. That includes our cars, trucks, tractors, mowers, machinery and even our fishing tackle!

 

If anyone finds this to be of any value then my input was worth it. I hope that it does so.

 

Clear skies,

 

Q


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#38 apfever

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Posted 01 May 2021 - 01:22 PM

I like good ol' lithium grease, some of those old tubes on the garage shelves. Stuff.

Super-Lube is good for some things. I've found it separates in the tube with a clear kind of fluid. It also doesn't have a lot of what I'd call 'heft'. Some things like focusers are still scratchy with it, where a drag grease would be better. 

 

All things considered on Aaron's scope, I wouldn't be taking anything apart to replace the grease. It sounds like a nice working job, good job Aaron. You can experiment with grease next time, but..I like good ol' lithium grease, some of those old tubes........


Edited by apfever, 01 May 2021 - 07:37 PM.

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#39 AaronHenderson

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Posted 03 May 2021 - 12:06 PM

I appreciate the WLG advice. I think I'm going to keep it in place for now and If I have any issues with lubrication or sticking going forward, i'll definitely clean it up and use something better. I've been crazy busy lately and not had a chance to work on the scope. I should have some time this weekend and might just get a chance to work on it. I'lll keep you all posted. 

 

-Aaron




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