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How do dual speed focusers work?

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#1 anrran

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Posted 30 January 2013 - 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

#2 Sean Cunneen

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Posted 30 January 2013 - 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.

#3 Roy McCoy

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Posted 30 January 2013 - 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

#4 ccs_hello

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Posted 31 January 2013 - 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

#5 anrran

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Posted 31 January 2013 - 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...-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

#6 Jerry-rigged

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Posted 31 January 2013 - 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.

#7 don clement

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Posted 31 January 2013 - 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

#8 TVG

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Posted 31 January 2013 - 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

#9 anrran

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Posted 31 January 2013 - 11:39 PM

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.fallbrook...inci-technology

maybe it will help the visual learners out there like myself

Aaron

#10 Roy McCoy

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Posted 01 February 2013 - 12:52 AM

Really cool video Aaron.

#11 don clement

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Posted 01 February 2013 - 01:28 AM

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

#12 Pinbout

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Posted 01 February 2013 - 01:32 AM

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. :grin:

#13 don clement

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Posted 01 February 2013 - 01:46 AM

I think a hydraulic drive would be even better. :grin:


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!'

#14 Jon Isaacs

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Posted 01 February 2013 - 08:08 AM

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

#15 drprovi57

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Posted 01 February 2013 - 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.

#16 TVG

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Posted 01 February 2013 - 10:31 AM

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

Todd

#17 TVG

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Posted 01 February 2013 - 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

#18 Pinbout

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Posted 01 February 2013 - 10:44 AM

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. :lol:

#19 careysub

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Posted 01 February 2013 - 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.

#20 Z28500

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Posted 07 October 2013 - 10:36 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.cloudynig...ber=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.
:step:
Z

#21 don clement

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Posted 08 October 2013 - 01:00 AM

They are designed to be able to slip a little


I don't think slipping was by design.

Don

#22 polaraligned

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Posted 08 October 2013 - 06:49 AM

Do you or anybody else on this forum have a pic of a focuser with a lead screw design?


Try to google images of a Hedrick focuser. It is used on the Planewave telescopes and uses a leadscrew.

ON EDIT: here is a link;

http://planewave.com...cuser/#.UlPx...



#23 don clement

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Posted 08 October 2013 - 03:53 PM

Do you or anybody else on this forum have a pic of a focuser with a lead screw design?


Posted Image

#24 bremms

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Posted 08 October 2013 - 06:16 PM

I made a lead screw focuser years ago with the focus tube mounted to a dovetail block. It was butter smooth and rock solid. Made partly from a Gaertner scientific stage of some sort.
Don't care for Crayford focusers much. There is a reason I have two old AP units on my Refractors. I like Don's focuser too. Made a similar prototype in the early 80s but had no machining ability at the time.






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