Topics

New Build - Harmonic Drive --> 200:1 (main reduction) and 4:1 (HTD3M pulley off of NEMA 23 stepper)

bryan_ramsey_pterodyne
 

Hello all, Im just starting to get some momentum on my new build for Onstep. 

The basics for both axis are:

  1. 200:1 harmonic drive reducers (Harmonic drive.net thinks these are 50 to 60 years old!!) sourced on ebay.
    1. 22.5mm output shaft 14mm input. These are beasts and very smooth.  Oil bath type, which HD.net thinks they should run well at pretty wide temperatures.
  2. 4:1 - 72:18 HDT3M 15mm wide pulleys and belts.
  3. Shaft collars and mount parts to be machined by eMachineshop.com
    1. 70mm needle roller bearings to support weight and avoid flex on output shaft. Sandwiched between the output and the shaft collar.
  4. For now Im going use a wedge meant for a Celestron CPC 11 (It's a mitty wedge in the sketchup file for refernce) to simplify the machining. Depending on how solid that works I'll consider making my own equatorial wedge later
  5. Possible later addition of CW shaft, going to see how it goes without.  Just another piece sandwiched between the RA shaft collar and the HD unit, tapped for Astrophysics CW shaft.
  6. The NEMA 23 motors are either going to be .38A .9Nm 400 step @ 12V or 1.5A 1.16Nm 200 step @ 5.4V which I have already. 

     

I'm in Baja California Sur, Mexico at about 22 degrees north so wedges are limited to just the Celestron HD one.  Using a pre-made wedge will save me a lot of time and engineering for now.

design pics attached. The green parts are the HD reducers, I didn't model the drive mounts, just the foot that is why it looks like they are floating.
If anyone has any thoughts about the design elements, Id love to hear them before I start ordering parts at emachine shop. The biggest unknown for me is the axial load (right term?) on the output shaft.  It would be a complex amount (expensive) of machining to fit a tapered roller bearing as it would have to fit around the 70mm output cap.  The HD units spin very smooth and have no backlash or slop whatsoever.




bryan_ramsey_pterodyne
 

Parts ordered and some updated pictures

bryan_ramsey_pterodyne
 

Heading to Texas to pick up a 350MM RCT !! then on to Colorado to install that and Onstep in the observatory on my Titan mount.  THEN, returning back to South Baja with all the above parts to build this mount.  More pics to follow.. :) 

bryan_ramsey_pterodyne
 

Sadly the 350RC didn't fit in my observatory.  Pier too tall.  Hard to plan for that 2500 miles away. Anyhow, back from the US with my parts for harmonic build.

See attached..

bryan_ramsey_pterodyne
 

180mm Maksutov in tow..

azcharlie@...
 

That is beautiful!

Rick
Tucson

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bryan_ramsey_pterodyne
 

Thanks!

Im driving from Baja to Colorado next week so I hope to have some high resolution guiding tests, and PEMPRO done once I get there with all the gear. It will be interesting to see how well the gears do.

bryan_ramsey_pterodyne
 
Edited

Ive increased the voltage to 16 and now I can slew just fine with over 18Kg @ 2.2DPS no counterweight.

Pretty happy with this, although Ill probably try 24V.

Here is a video at 2.2DPS
https://youtu.be/QIpxVwxpnlw

JoAnn
 

very nice. Curious, why no counterweights, other than mechanical challenge? Can't wait to see how tracking and guiding goes. 

Howard Dutton
 

In theory this thing has basically zero backlash so it should guide like a dream.

Matthewabey@...
 

That is cool and a ton of power. Harmonic drives always have interested me, ive seen a few people use them but never any guiding results im curious to see how they work. 

Any way awesome setup.

bryan_ramsey_pterodyne
 
Edited

>>very nice. Curious, why no counterweights, other than mechanical challenge? Can't wait to see how tracking and guiding goes.

Most harmonic drive mounts I've seen don't have them, so I didn't design them in.  So far they don't seen necessary.  I'm sure at a certain payload the motors will stall and it would become a problem.  More I had hoped it wouldn't need them for simplicity.  Maybe someone with greater physics knolwedge could explain why.  Maybe it's just that so many more teeth are engaged throughout the rotation when compared to a worm, more torque is extracted

Howard Dutton
 
Edited

Harmonic drives are >80% efficient.  Timing belt reduction about 95%.   A worm-wheel is 20% if you're lucky.

On the other hand it seems to me that the asymmetrical forces need to be dealt with all the way to the ground or as it tracks (CG moves) and bends this way and that you'll have problems.

George Cushing
 

Very nice design. Great for an amateur build as none the machining is particularly difficult or expensive out sourced. 

Ant No
 

Can harmonic drives be printed?

Anthony

On 28 Aug 2018 13:38, "George Cushing" <pinyachta@...> wrote:
Very nice design. Great for an amateur build as none the machining is particularly difficult or expensive out sourced. 

George Cushing
 

Well if GE can print turbo jet vanes, I guess it's possible. https://en.wikipedia.org/wiki/Harmonic_drive

Ant No
 

Lol, I'm guessing they have all the latest upgrades on their diy 3d printer kit ;-)

On 29 Aug 2018 14:13, "George Cushing" <pinyachta@...> wrote:
Well if GE can print turbo jet vanes, I guess it's possible. https://en.wikipedia.org/wiki/Harmonic_drive

bryan_ramsey_pterodyne
 

This project has been on hold while I sorted out my main observatory. I never got time to test the harmonic mount under the stars in Colorado as I tore out the concrete pier and put in the larger telescope.  Now dealing with problems of collimation and focusing (which is really hard to do from 2500 miles away!)

Anyhow, Im back in Baja, and hoping to test this mount out soon weather permitting. Just posting here so people know it's in progress...

Ant No
 

Good to know Bryan. I've quite taken to the concept of harmonic drives.

I've found a few attempts at printing them. Two or three have some promise. One is quite a novel variation I'm still trying to understand properly, but looks interesting. They're all on thingverse.

Need to learn more about the different printing materials. Clearly, the ever popular, PLA is not the material of choice here.

Anthony

On 18 Sep 2018 23:27, "bryan_ramsey_pterodyne" <bryan@...> wrote:
This project has been on hold while I sorted out my main observatory. I never got time to test the harmonic mount under the stars in Colorado as I tore out the concrete pier and put in the larger telescope.  Now dealing with problems of collimation and focusing (which is really hard to do from 2500 miles away!)

Anyhow, Im back in Baja, and hoping to test this mount out soon weather permitting. Just posting here so people know it's in progress...