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Is this a silly idea?

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#1 Space99

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Posted 24 February 2009 - 03:16 AM

Way is that all cheap eq mount using "barrel-type"
worm gear?
Is it cheaper to make?
Just look at the Losmandy G8/G11. Pressure plate, worm wheel, clutch. The worm has a full surface to contact the plate controlling the rotational torque of the scope.

The barrel type has a clamp, it has one contact point and transfer the rotation from that. Slightly pressing and offsetting the worm barrel.

Way not introduce more set screw at 120deg. around the worm
adjust it to the right amount for using it as a slip clutch?
It may improve the EQ5 clones...

Is this a sily idea? :tonofbricks:

Mick.

#2 Space99

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Posted 25 February 2009 - 02:54 AM

OK, let me rephrase the question.
What is the merit for both design:

barrel type worm gear clamp or
disk and pressure plate clutch.

Mick.

#3 brianb11213

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Posted 25 February 2009 - 12:01 PM

barrel type worm gear clamp or
disk and pressure plate clutch.

If you adjust the barrel type clamp to change the friction and the bore of the worm gear is not very, very accurately matched to the shaft diameter, you're going to change the periodic error profile. (Edit: and maybe add some backlash!)

Over 30 years ago I had a scope with a barrel type clamp - there was a grub screw through the worm gear which contacted the shaft through a nylon pad - it worked effectively enough but the drive was not exactly "precision" by the standards of the best modern mounts.

#4 Jared

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Posted 25 February 2009 - 12:54 PM

OK, let me rephrase the question.
What is the merit for both design:

barrel type worm gear clamp or
disk and pressure plate clutch.

Mick.


In my experience, either design can be done well or poorly. The advantage to the pressure plate clutch is that it allows you to set the amount of slip, so if you aren't using go-to or if you have optical encoders you can easily move the scope around without having to loosen the clutches. It doesn't matter so much if you are using go-to exclusively.

With the barrel type clamps, the performance seems to be best if multiple clamps are used--set at 90 or 120 degrees. Less expensive mounts usually just use a single clamp, and I suspect that you run the risk of shifting alignment slightly and affecting periodic error if you don't always clamp it with the same pressure. However, there is no way to isolate that issue from other quality issues that may affect periodic error consistency, so I wouldn't want to jump to conclusions.

My recommendation--I wouldn't worry too much about which design is used. Instead, I would look at the overall quality of the mount and assume that the design is appropriate for the intended use. If you are using go-to, the functional differences between the two designs are pretty minor.

#5 Skywatchr

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Posted 25 February 2009 - 02:42 PM

Personally, I would like to see a band type of clamp instead of the screw squashing the barrel onto an egg shape. I used to rebuild automatic transmissions and the spring steel bands with a friction material hold very well without a lot of pressure, and keep things round. That way the backlash and clearances can be set permanently on the worm without the need to be constantly tweaking for the PE.

Jeff

#6 Space99

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Posted 25 February 2009 - 03:57 PM

AP eq mounts using setscrews at 90deg. Anyone has a
"x-ray" drawing of the principle of that?
It would be educational to see how the pro doing it...

Mick.

#7 Jared

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Posted 25 February 2009 - 10:39 PM

AP eq mounts using setscrews at 90deg. Anyone has a
"x-ray" drawing of the principle of that?
It would be educational to see how the pro doing it...

Mick.


Actually, the AP clutch is extremely simple. The 3-4 set screws (depending upon the model of the mount) simply drive a rubber plug against the gear hub. There is a drawing available on the A-P website at this location.

#8 Skywatchr

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Posted 25 February 2009 - 11:02 PM

The rubber must be acting as the "friction material" so you don't need a lot of pressure for it to hold. That sounds a lot better than the single squashing screw lock. Oh by the way, your link just loops back to this forum..

Jeff

#9 Space99

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Posted 26 February 2009 - 01:01 AM

I wonder if it improve the barrel type worm arrangements,
by taping the case at 90 deg, and place a setscrew around
the bell. That would distribute the torque around better.
I still like the Losmandy style do.
Mick.

#10 Jared

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Posted 26 February 2009 - 12:13 PM

The rubber must be acting as the "friction material" so you don't need a lot of pressure for it to hold. That sounds a lot better than the single squashing screw lock. Oh by the way, your link just loops back to this forum..

Jeff


Funny that the link doesn't work... Perhaps because it's a PDF? Anyway, here is the full URL. The drawing is in the PDF...

www.astro-physics.com/tech_support/mounts/clutchpluginst.pdf

It's the instructions for replacing the clutch plug on an Astro-Physics 900, and it has a simple drawing of how the clutch plugs work.

#11 Qkslvr

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Posted 26 February 2009 - 01:25 PM

Have you measured your PE yet? Mine (CG-5) has a PE period of 10 minutes, one turn of the worm gear. Now it's one set screw might cause it's more PE error, but it'd have a period of many hours, and probably doesn't matter.

#12 Space99

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Posted 27 February 2009 - 02:00 AM

Can you convert a barrel type( cg5, eq6, eq5...)
to a G8 style clutch system? Sure, you have to get rid of the polar-scope (no loss) and replace it with a knurled nut. The plastic shim should be cork or Teflon, and there it is.
Any mechanical person can chip in? Is this a bad idea?

Mick.

#13 Jared

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Posted 27 February 2009 - 09:59 AM

Have you measured your PE yet? Mine (CG-5) has a PE period of 10 minutes, one turn of the worm gear. Now it's one set screw might cause it's more PE error, but it'd have a period of many hours, and probably doesn't matter.


With PEC turned on it's +/- 2.5 arc seconds. With it turned off it's +/- 6 arc seconds. I don't think that much of the periodic error you are experiencing has to do with the design of the clutch system, though. There are much larger factors--such as how carefully the worm is cut, how carefully the drive wheel is cut (and how circular it is), precision of the gear box, quality of the bearings supporting the worm, how carefully the worm and gear are aligned, etc.

I think an awful lot of astrophotographers give too much emphasis to periodic error numbers. Assuming you are autoguiding, the total amount of periodic error is not in and of itself significant. Much more critical is the smoothness of the error--both periodic and random. If you have large fluctuations over minutes the autoguider can easily remove them. However, relatively small fluctuation of just a couple arc seconds can be a huge problem if they occur in just a second or two. Smoothness is much more important than absolute accuracy. The trick is that there is no single number measure of smoothness available, and even if there were would certainly vary with balance and weight, so people quote periodic error instead. In practice, there is actually a pretty good correlation between periodic error and smoothness of tracking. I just wouldn't spend a lot of time trying to minimize or improve the absolute PE of your mount. Instead, try to address the size and 'sharpness' of the tracking errors and you will get a good result regardless of the actual total amount of PE.

Again, all of the above assumes you are autoguiding. If you are not, then total PE becomes much more significant.

#14 Qkslvr

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Posted 27 February 2009 - 12:53 PM

Have you measured your PE yet? Mine (CG-5) has a PE period of 10 minutes, one turn of the worm gear. Now it's one set screw might cause it's more PE error, but it'd have a period of many hours, and probably doesn't matter.


With PEC turned on it's +/- 2.5 arc seconds. With it turned off it's +/- 6 arc seconds. I don't think that much of the periodic error you are experiencing has to do with the design of the clutch system, though.


My PE is ~ +/-10 arcsec over ~ a 10 minute period. I figured out that is you don't have the worm gear adjusted right, you get a big clitch at the end of every period. Just like if you unscrew a lid, when you get to the end if you keep going it drops back again.

With guiding on, my PE is about the same as yours with PEC +/- a couple arcsecs or so.

#15 Jared

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Posted 27 February 2009 - 01:43 PM

Have you measured your PE yet? Mine (CG-5) has a PE period of 10 minutes, one turn of the worm gear. Now it's one set screw might cause it's more PE error, but it'd have a period of many hours, and probably doesn't matter.


With PEC turned on it's +/- 2.5 arc seconds. With it turned off it's +/- 6 arc seconds. I don't think that much of the periodic error you are experiencing has to do with the design of the clutch system, though.


My PE is ~ +/-10 arcsec over ~ a 10 minute period. I figured out that is you don't have the worm gear adjusted right, you get a big clitch at the end of every period. Just like if you unscrew a lid, when you get to the end if you keep going it drops back again.

With guiding on, my PE is about the same as yours with PEC +/- a couple arcsecs or so.


+/- 10 arcseconds is adequate for almost any purpose. I wouldn't worry about trying to improve it. However, I'd be concerned that you aren't getting better results when guiding... You should be essentially seeing limited when using an autoguider (unless you are significantly undersampled or are shooting very wide fields of view).

#16 Skywatchr

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Posted 27 February 2009 - 08:31 PM

This link works fine. If you "mouse over" your previous link, look at the bottom of your browser (I use Firefox) and you'll see the link. It's about a mile long and starts with Cloudy Nights instead of astro-physics. :grin:

Jeff

Oh by the way, your link just loops back to this forum..

Jeff


Funny that the link doesn't work... Perhaps because it's a PDF? Anyway, here is the full URL. The drawing is in the PDF...

www.astro-physics.com/tech_support/mounts/clutchpluginst.pdf

It's the instructions for replacing the clutch plug on an Astro-Physics 900, and it has a simple drawing of how the clutch plugs work.



#17 Space99

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Posted 28 February 2009 - 01:56 AM

Folks, would it be a good idea to collect exploded diagrams
of eq mounts for reference? Anyone can chip in?

Mick.

#18 Space99

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Posted 01 March 2009 - 02:07 AM

How would a Losmandy G11 "pull-shaft" to worm compare
to a old-school Byers worm gear with spring-loaded set screws?

Mick.






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