I am considering designing an equatorial mount for astrophotography. I have been researching on the best way to do that and I am thinking on using a stepper motor with a gearbox. The thing is that it needs to turn at a really low speed, 1 rev/day. If I pick any Nema X motor I can get it to step 200/400 steps without a gearbox. The most reductions I have found are 1:100 (this is an example: http://www.ebay.com/itm/Nema17-23-34-Planetary-Gear-Ratio-5-1-10-1-15-1-20-1-30-1-40-1-50-1-100-1-Reduce-/201844830182?var=&hash=item2efee3afe6:m:mPuljSiQP2wMMZ69Cmf4BYA)

That means that I can get 20.000 steps/rev (400 step/rev motor) which would mean running the motor at 20.000/60*24 = 14 steps/min --> 4.34 sec/ step. I don't think the operation is smooth enough so it will might show in the pictures.

I have been researching on other similar projects and I have seen that they picked 1rpm as acceptable for that smoothness (0.15sec/ step for a 400 step motor). That means that I need a gearbox capable of around 1500:1. My questions is as follows: - Does it make sense to add 2-3 gearboxes in series to get that value? Like those ones and adaptors for the output shafts since they are different in size to the input shafts: http://www.ebay.com/itm/NEMA34-NEMA23-Stepper-Motor-Planetary-Gearbox-36-1-30-1-24-1-16-1-10-1-6-1-4-1/122395466919?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D44757%26meid%3D57a6af176f9b48e0b9f2042efc480bc1%26pid%3D100005%26rk%3D3%26rkt%3D6%26sd%3D122395466919&var=422871734736. An example could be adding first 30:1, second 30:1, 2:1 (1800:1 all together).

  • Would it be better to design a planetary/spur gearbox to satisfy those requirements?

Thank you. I hope this is the right forum. I haven't find one suitable for mechanics

  • $\begingroup$ Welcome to Robotics borias23, but I'm afraid that Unbounded Design Questions are off-topic because there are many ways to solve any given design problem. We prefer practical, answerable questions based on actual problems that you face, so questions which ask for a list of approaches or a subjective recommendation on a method (for how to build something, how to accomplish something, what something is capable of, etc.) are off-topic. Please take a look at How to Ask & tour for more information on how stack exchange works. $\endgroup$
    – Chuck
    Jul 13 '17 at 13:22
  • $\begingroup$ You're more than welcome to join us in Robotics Chat if you would like to discuss your design problem. A couple points to get you started thinking - (1) The problem seems to me to be more of an issue with the angular resolution than speed. How often are you taking pictures? How far does the earth rotate in the time between pictures? That would be where I would start, and I'd probably try to find an angular resolution 10x or better at least. (2) Regarding your speeds, I don't know how you're arriving at any of your numbers. You can step a stepper motor; it can turn at any arbitrarily low speed. $\endgroup$
    – Chuck
    Jul 13 '17 at 13:26
  • $\begingroup$ You could, for example, only step it one time per day, let alone one revolution per day. But this goes back to point (1) - what is the angular resolution you need? You need to find a conversion between your desired angular resolution and the angular resolution of the stepper motor. (3) - Backlash is likely to be your limiting factor here. Wind, floor vibrations, etc. will all occur, and if there is a high backlash in the gear train then the camera is free to move around within that backlash band. Again though, all points to consider - join us in Robotics Chat to talk about the project more! $\endgroup$
    – Chuck
    Jul 13 '17 at 13:29
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    $\begingroup$ Please don't link to ebay items. The listings will disappear in a few months, and then your question will lose important context. In future questions, if you need to link to a product, please find a non auction supplier, that way if the link dies, we might be able to re-link to the archive.org version of their page. $\endgroup$
    – Mark Booth
    Jul 13 '17 at 15:13
  • $\begingroup$ @Chuck Thank you for your fast and good answers. I havent used the forum that much. I will follow your advice and open a discussion on the Robotics Chat to follow with the conversation. $\endgroup$
    – borias23
    Jul 15 '17 at 9:20