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Calculate motor/gear ratio on single axis

  1. May 5, 2016 #1
    Firstly, let me explain that I am mostly inept at mathematics. Piecing together the required information is becoming a real challenge because of that.

    I am motorizing a dobsonian telescope. I have read many accounts of success and the steps involved. The places where detail is commonly left out are: defining worm gear ratio/size and stepper motor oz*in.

    I understand the basic problem here, I need to find out force required to rotate the base of the telescope at different distances from it's axis. With the current amount of friction(which will change after a small redesign), I have measured that it requires approximately 2 lbs 15 ounces of force to rotate the base 9 inches from it's axis.

    What I really need help with initially is how to extrapolate that data when moving closer to the axis. After that I can begin to calculate gear ratio, stepper motor strength and build my circuits. Some equation where I can input force required and distance from axis.

    Any help would be GREATLY appreciated
  2. jcsd
  3. May 6, 2016 #2


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    You have a force of 3 lb acting at a radius of 9 inches . This means you have a torque of 3 lb x 9 inches or 27 lb - inches .

    3 lb at 9 inches = 9 lb at 3 inches = 27 lb at 1 inch etc . The product of force and radius is always 27 .

    Generally though for most gear and motor calculations only the actual torque is needed .

    Post a picture or drawing of your telescope mount if you can .
    Last edited: May 6, 2016
  4. May 6, 2016 #3
    Thank you very much Nidum!
    Here is the link to a page with a 360 degree view:
    And a link to a page that has images and instructions to the same exact thing I'm attemtping:
    tbehind.png tleft.png
  5. May 6, 2016 #4


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    Thanks for that .

    Looks like an interesting project .

    Any particular design problems you want to discuss ?
    Last edited: May 6, 2016
  6. May 7, 2016 #5
    It's an exciting one, that's for sure. I bought a 3D printer and plan on printing my short-term/prototype gears out of nylon. I plan on using Onstep for the actual tracking and GoTo part of the programming. http://goo.gl/eDk5K4

    Most of the hard parts are completed already (meaning the software and testing by others). I've fitted the bottom axis with a lazy susan bearing like in the second link I posted. That brought friction down to 5 ounces instead of 3 lbs! Those friction pads work! So if I understand you correctly, that should equal 4.5 lb -inches? Or 72 oz*in?
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