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anrran
member


Reged: 04/03/11

How do dual speed focusers work?
      #5653223 - 01/30/13 09:14 PM

Hello

I did a Google search on this but it mainly came up with sites just trying to sell focusers. I am just curious about how they work. I assume they may use some sort of planetary gear design, but I saw on the MoonLite focuser website that they use a "ball bearing planetary reduction design". I couldn't really find any information on that either. Does anybody have any information on these, or could maybe point me in the right direction? I currently do not own one so I can't take it apart to see how it works. If you have pictures of the inside of one, that would be nice too.

Thanks,

Aaron


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Sean Cunneen
Let Me Think
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Reged: 08/01/07

Loc: Blue Island Illinois
Re: How do dual speed focusers work? new [Re: anrran]
      #5653384 - 01/30/13 10:52 PM

I don't have pictures, but I can give you an idea of the principle. Basically instead of gears, you use ball bearings to redduce the speed(usually 10:1). You have a plate that contacts a ring of bearings (or sometimes just one) around it's equator while the bearing contacts a ring about a minor axis of the sphere. That way 8 or ten turns on the minor axis of the sphere results in one rotation of the focus shaft. Same principle as a car's CVT transmission.

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Roy McCoy
Pooh-Bah
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Reged: 10/13/05

Loc: Glendale, AZ
Re: How do dual speed focusers work? new [Re: anrran]
      #5653414 - 01/30/13 11:24 PM

Hi Aaron,

They are a planetary gear that use balls instead of spurs.

The shaft is the sun and the balls are the planets.

See step 7 or 8 of Pollux's web page.

Best Regards,

Roy


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ccs_hello
Postmaster
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Reged: 07/03/04

Re: How do dual speed focusers work? new [Re: Roy McCoy]
      #5653773 - 01/31/13 08:10 AM

The design is carried over from an old era:
radio or measurement instrument fine tune dial.

I'll post a picture later on.

Clear Skies!

ccs_hello


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anrran
member


Reged: 04/03/11

Re: How do dual speed focusers work? new [Re: ccs_hello]
      #5654004 - 01/31/13 10:18 AM

Thanks every one for their replies. Wikipedia has a link to a Chinese website that I believe is the same focuser that's on the Pollux site. It shows CAD drawings of the unit. You have to translate the page to English.

http://bbs.astron.ac.cn/thread-54988-1-3.html

I guess one thing that I don't understand is how the bearings have enough friction against the shafts to transfer the rotation from one shaft to the other. I know that gears would use teeth to do this. I thought ball bearings were used to minimize friction. Would the bearings slip every once in a while, causing the ratio to change? Or are they pretty good at keeping the ratio constant? Does it have to do with how tight that nut is? Also, ccs_hello, a picture would be greatly appreciated.

Thanks
Aaron


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Jerry-rigged
sage


Reged: 01/30/12

Loc: Coastal Texas.
Re: How do dual speed focusers work? new [Re: anrran]
      #5654046 - 01/31/13 10:39 AM

I have not messed with a duel speed focuser, but in a previous hobby-life, I built and rebuilt ball-bearing differentials for R/C race cars. Also, as I understand it, Crayford focusers also kind of use the same principal. If there is pressure between both surfaces and the ball, it will act like a gear. Apply too much torque, and yes, the balls will slide. If there is too much slippage, you will need to increase the pressure between the plates/balls.

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don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: How do dual speed focusers work? new [Re: Jerry-rigged]
      #5654280 - 01/31/13 12:57 PM

Ball bearings are much easier to fabricate than precise quantized involute surfaces of gears. However the additional step of adding secondary planetary reduction wouldn’t even be necessary if a smooth easily made lead screw had been used in the first place.

Don Clement


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TVG
member


Reged: 05/03/12

Re: How do dual speed focusers work? new [Re: don clement]
      #5654684 - 01/31/13 04:56 PM

What do you mean a smooth easily made lead screw? Please explain further or even post a pic/diagram if able.

Thanks,
Todd


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anrran
member


Reged: 04/03/11

Re: How do dual speed focusers work? new [Re: Jerry-rigged]
      #5655328 - 01/31/13 11:39 PM

Quote:

If there is pressure between both surfaces and the ball, it will act like a gear. Apply too much torque, and yes, the balls will slide. If there is too much slippage, you will need to increase the pressure between the plates/balls.




Your explanation clears it up for me, thanks.

Also, I found this cool video of a ball bearing cvt

http://www.fallbrooktech.com/nuvinci-technology

maybe it will help the visual learners out there like myself

Aaron


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Roy McCoy
Pooh-Bah
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Reged: 10/13/05

Loc: Glendale, AZ
Re: How do dual speed focusers work? new [Re: anrran]
      #5655402 - 02/01/13 12:52 AM

Really cool video Aaron.

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don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: How do dual speed focusers work? new [Re: TVG]
      #5655448 - 02/01/13 01:28 AM

Quote:

What do you mean a smooth easily made lead screw? Please explain further or even post a pic/diagram if able.

Thanks,
Todd




The friction roller used on most Crayford type focusers has a maximum reduction of perhaps 15:1. If one were to replace the friction roller positioner with a leadscrew and nut(s) for focus position, a much greater reduction ratio could easily be had and thus negating the need for a secondary planetary reducer. Leadscrews are easily made and are inherently smooth. The nut(s) average many threads unlike the essentially single point or limited area contact of the friction roller or even rack and pinion. I use a leadscrew with backlash compensation with my focusers and have no need for secondary planetary reducer. Also I use a self-lubricating material for the nut(s) with a SS leadscrew so no grease or oil is needed.

Don Clement


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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: How do dual speed focusers work? new [Re: don clement]
      #5655454 - 02/01/13 01:32 AM

Quote:

I use a leadscrew with backlash compensation with my focusers and have no need for secondary planetary reducer. Also I use a self-lubricating material for the nut(s) with a SS leadscrew so no grease or oil is needed




I think a hydraulic drive would be even better.


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don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: How do dual speed focusers work? new [Re: Pinbout]
      #5655468 - 02/01/13 01:46 AM

Quote:


I think a hydraulic drive would be even better.




There is a big difference between what one imagines would work better and that of a design that has been built and proven to work.

Don Clement

Speaking of grins: `Well! I've often seen a cat without a grin,' thought Alice; `but a grin without a cat! It's the most curious thing I ever saw in my life!'


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: How do dual speed focusers work? new [Re: anrran]
      #5655729 - 02/01/13 08:08 AM

Quote:


I guess one thing that I don't understand is how the bearings have enough friction against the shafts to transfer the rotation from one shaft to the other.




In a ball bearing, the friction is low because because the ball bearings turn, there is no slippage. In the micro-focusers that use ball bearings as planetary gears, the bearings are loaded and provides the friction between the steel bearings and the steel outer race and inner shaft. Adjustment is critical, if there is not enough load, the micro-focuser may slip, if there is too much, it may be rough or notchy. They all seem to use heavy knobs for the micro-focuser, it helps smooth out any roughness.

The advantage of the two speed design over other techniques like a lead screw that also provide fine focusing is that one can have both course focusing when swapping eyepieces and yet have the precise fine focusing required to get the best possible views at high magnifications.

As an aside, I have an old Swift Binocular microsope with two speed focusing, it must be 40 years old but it appears to be the same design as Feathertouch and others use, it's totally awesome, super smooth, tight, precise. It's rack and pinion and could be adapted to use in a telescope but I would hate to sabotage such a nice piece of equipment.

Jon

Edited by Jon Isaacs (02/01/13 08:13 AM)


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drprovi57
super member


Reged: 06/13/06

Loc: Virgina USA
Re: How do dual speed focusers work? new [Re: don clement]
      #5655931 - 02/01/13 09:41 AM

Don,

I was curious about lead screw designs - I know of another focuser that use lead screw design but that they can have play backlash or slight play - also they can take forever to move a long distance making "v-curves" (e.g. FocusMax) take a long time to generate.


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TVG
member


Reged: 05/03/12

Re: How do dual speed focusers work? new [Re: anrran]
      #5656055 - 02/01/13 10:31 AM

Thanks for the cool video, that cleared my understanding up nicely.

Todd


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TVG
member


Reged: 05/03/12

Re: How do dual speed focusers work? new [Re: don clement]
      #5656073 - 02/01/13 10:37 AM

Don,
Thanks for the explanation. Do you or anybody else on this forum have a pic of a focuser with a lead screw design?

Thanks again,
Todd


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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: How do dual speed focusers work? new [Re: don clement]
      #5656091 - 02/01/13 10:44 AM

Quote:

There is a big difference between what one imagines would work better and that of a design that has been built and proven to work.






that was funnier than you alice in wonderland quote.


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careysub
Carpal Tunnel
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Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: How do dual speed focusers work? new [Re: don clement]
      #5656351 - 02/01/13 01:18 PM

Could you comment on the advisability of using a common V-thread screw instead of a true lead screw in a focuser?

Especially with a large knob, as many seem to like, the drag of the V-thread would seem to be a minor issue.


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Z28500
sage


Reged: 02/27/12

Loc: N. 30.1 W.-95.6 Texas
Re: How do dual speed focusers work? [Re: careysub]
      #6123838 - 10/07/13 11:36 PM

Several month old thread but here goes:
Here is a tread with a design I drew of a crayford dual speed focuser:
http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Number=6122834&...

Crayfords actually are relatively simple, they are designed to work with just the right amount of tension, on the shaft to the tube, and with dual speeds, correct pressure on the bearings. They are designed to be able to slip a little which eliminates breakage if turned too far.
As of this date I think the drawing is the last post so far. The rest of the thread is of an F/15 I built.

Z


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