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Old 18-12-2010, 03:59 PM
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Timing belt modification for EQ-6 mount -another one

Finally I found some free time to finish something I started almost a year ago, but never had enough time and motivation to finish: the belt modification of my EQ-6 mount.

I bought the mount 2 years ago, and I was quite happy with it, especially after I did so-called Darren’s modification (http://projects.gbdt.com.au/eq6-1/) – which gave me Go-To ability (my EQ-6 was standard mount).
However, one of the problems that bothered me a lot was error introduced by transfer gears (47 teeth), resulting in sudden 2-3 arcsec jumps, very visible on photos taken with anything longer than 500mm.
The nature of those jumps are a potential problem for autoguider as well, so I decided to replace the transfer gears with timing belts and pulleys, as soon as possible.

My current design uses reduction from motor shaft by 20/80 teeth gears (I might omit this stage in the near future if it proves un-necessary or bad idea), and 12/60 timing pulley pair (2mm pitch). All shafts (including stepper motors) have ball bearings.

The pulleys were machined to 8mm thickness, to fit into worm housing (the diameter of 60-teeth 2mm-pitch pulley plus belt is exactly the same as 47-teeth transfer gear.
However, because the pulley is wider than gear and the slot on the motor housing of the mount is mis-aligned (still OK for gears), I used the file to widen it a bit – 20 minutes hand-job.

The whole thing turned out pretty nice, apart from miscalculation for belt length – so I couldn’t fit both screws for AR motor because the slot for screw on the motor flange is not long enough.
I will replace the belt with a shorter one when it arrives (sometimes in January).

And, now I need some time (and clear skies) to test the whole thing.
I also plan to lap the worm gear pair with diamond paste if PE turns out too large.
Attached Thumbnails
Click for full-size image (Timing_belt_coupling.jpg)
60.6 KB246 views
Click for full-size image (DEC_belt.jpg)
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Click for full-size image (DEC_belt_slot_inside.jpg)
48.3 KB196 views
Click for full-size image (Slot_filed.jpg)
101.7 KB181 views
Click for full-size image (Dec_motor_in.jpg)
64.6 KB205 views
Click for full-size image (AR_motor_in.jpg)
57.9 KB199 views

Last edited by bojan; 24-12-2010 at 06:34 PM.
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Old 24-12-2010, 09:51 AM
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PE analysis (based on 10 minutes measurement session) is attached.

The whole thing turned out not too bad, considering that the alignment was pretty much "ad oculos" (visible trend in PE).
Also, later I realised the worm gear meshing was not done properly (too much backlash).
Lapping may help to drastically reduce the PE even further, as shown on the graph done by Filip Lolic - he started from similar PE as mine but ended up with 1/4 of the value, after treatment with the diamond paste (0.5um, http://www.zvjezdarnica.com/forum/in...ach=7183;image. The details of his work can be found on EQMOD Yahoo group).
Also, I still have original bearings - it is not clear the new bearings will help, according to articles posted on EQMOD group (http://f1.grp.yahoofs.com/v1/kPATTXv...eplacement.pdf)
Attached Thumbnails
Click for full-size image (PE_60_12.jpg)
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Click for full-size image (F_60_12.jpg)
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Last edited by bojan; 24-12-2010 at 11:53 AM.
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Old 07-03-2011, 08:55 PM
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Finished (for now)

The final result of the belt modification is here.
Thanks to Bert (Avandonk) who kindly offered me his spare hand paddle (because the processor in mine stopped working, together with the processor on the main board...The main one was easy (Element14, or formerly Farnell helped) but the one in hand paddle would have been more difficult problem to solve without Bert's help).

Interestingly, there is a low frequency signal present (green line) , it must be the flaw associated with worm gear (I can't imagine it is due to fluctuation of crystal frequency).
But, overall the PE curve is smoother than before and definitely correctable by autoguider.
PE amplitude is around 9arcsec (5.6 RMS)- all original bearings and no additional grease.
Now, I only need time to start using it... well, retirement is hopefully getting closer
Attached Thumbnails
Click for full-size image (belt_60_12.jpg)
130.1 KB101 views
Click for full-size image (Low_freq.jpg)
118.8 KB72 views
Click for full-size image (spectrum.jpg)
112.1 KB56 views

Last edited by bojan; 08-03-2011 at 08:28 AM.
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Old 08-03-2011, 07:29 AM
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Steve,

The fist pic shows PE with high pass filter applied. The green line is filtered out low frequency part (probably worm gear problem, since the dec drift (not shown on graphs) was linear).

The second pic presents raw data, and the third is frequency spectrum (the peak in the middle is due to worm(PE), the right-most one is that low freq component)

Last edited by bojan; 08-03-2011 at 07:47 AM.
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Old 29-10-2011, 11:56 AM
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I guess I know what I'm doing during bad weather now.
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Old 29-10-2011, 02:34 PM
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Can the same job be done to an HEQ5 in a similar way?
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Old 29-10-2011, 03:56 PM
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Quote:
Originally Posted by mswhin63 View Post
Can the same job be done to an HEQ5 in a similar way?
Yes, it can be done. on the attached image you can see details of one way to do it.. it's relatively simple job.
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Last edited by bojan; 29-10-2011 at 04:32 PM.
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Old 29-10-2011, 06:19 PM
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That's a really practical mod - impressive - and seems to be working well. Maybe you've seen this. Some great ideas and info. Nice to get rid of troublesome gearing altogether.
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Old 29-10-2011, 07:48 PM
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Quote:
Originally Posted by rcheshire View Post
That's a really practical mod - impressive - and seems to be working well. Maybe you've seen this. Some great ideas and info. Nice to get rid of troublesome gearing altogether.
Thank you for this link.. no I haven't seen this page yet, very interesting it is..
However I am using friction drive on my bartelised dobson for quite some time (it was partly inspired by Mel's web pages, but some details and solutions are not found elsewhere, not that I am aware of similar), see here:
http://www.iceinspace.com.au/forum/s...friction+drive
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Old 29-10-2011, 09:11 PM
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Thanks for the information, that link will be great for a future product (building a large EQ mount).
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Old 30-10-2011, 12:41 PM
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The friction drive is really interesting and unique solution. No slippage. The big issue with metal to metal friction drives is the pressure applied and risk of failure of the small spindle. It would be a lot of fun building a super accurate mount for AP using these ideas.
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Old 30-10-2011, 01:53 PM
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Quote:
Originally Posted by rcheshire View Post
The friction drive is really interesting and unique solution. No slippage. The big issue with metal to metal friction drives is the pressure applied and risk of failure of the small spindle. It would be a lot of fun building a super accurate mount for AP using these ideas.
Long time ago I have read a book about big telescope mechanical design.. Almost all of them use friction reduction (200" Mt Palomar is one of them if I am not mistaken)
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