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Barn door tracker

  1. Jan 12, 2015 #1
    Has anyone here built one before. Ive got a nearly completed one but Im afraid its not stable enough to keep on track.
  2. jcsd
  3. Jan 12, 2015 #2


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    Doesn't it seem to you that it would be more helpful then for you to post pics and a description of yours and ask specific questions about it. I mean, what good is it going to do you to know that someone else has built one?
  4. Jan 13, 2015 #3


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    As phinds said, show us pictures of your construction and its possible people can then offer suggestions on improving it :)
    give details on its size, materials used, motor etc

  5. Jan 13, 2015 #4
    Because... I need to know if there is anyone in this section who can answer this question. No point in giving a bunch of details if the isn't anyone who can answer questions.

    Primarily I have a 3.5 rpm motor stepped down to1 rpm with connecting gears and a 1000 ohm potentiometer. The 1 rpm gear has a threaded rod running through the center that is bent to a radius of curvature of 7.15". I determined this by creating a chart comparing the change in length of the third side of the triangle created by the opening of the barn door to the arc length of a perfect circle.
    The problems Im noticing is that the tip running from the motor into the first gear is slipping and skipping. And some spots on the rod get stuck in the gear and the speed changes. Ill get some pics up soon. Im just looking for some general tips for ensuring proper guidance.
  6. Jan 13, 2015 #5
    I've never built a barn door tracker. I have done a fair amount of machining and other fabrication.

    Bending the threaded rod effectively changes the thread pitch. The threads on the outside of the curve spread out and the inside threads move closer together. From your description, I suspect your problem is there.

    Try using a straight or straighter rod. It could be straight as long as the ends could pivot a little -right? How much angular travel do you need?

    Or you could (triangle) file the threads around and around smoothing, rounding and reducing their size. That might reduce the interference enough. You will have more backlash but as long as the load stays in the same direction it won't matter.

    Other than that, pics or even a video would help us help you.
  7. Jan 13, 2015 #6
  8. Jan 13, 2015 #7
    The curve is necessary you need to have a change of length within about a thousandth of an inch of the arc length to get exposures over 30 sec. Without dramatic star trailing.

    Also with a straight rod the end connecting to the top peice has to be able to move freely horizontally. Which continuously changes the hypotenuse of the triangle.
    The earth rotates approx 1 degree every 3.997 min.
  9. Jan 13, 2015 #8
    I graphed the difference between the length of a straight rod and the arc length, the error exponentially grows as a function of time. There are other models that make up for the error by placing a second arm resting on top of the moving arm. This dramatically decreases error but requires some extra formulating I just haven't done yet.

    It was funny I did all this calculating using my classroom physics software and quite a bit of thinking and determined that the most efficient radius of curvature was 7.15 inches with the 32 thread per inch rod being threaded at 1rpm and calculating the difference in error to find the exponential rate of growth just to find all of this exact information in an old issue of sky and telescope from the 80s. :).

    Also I ordered some specific gears just to find that they were terribly flimsy and would have needed work to be threaded. So I went into my childrens large stockpile of kinex and found the exact sizes I was looking for, and the worm screw I put into the gear fit very snugly.
  10. Jan 13, 2015 #9
    Good pics and cool project! And I understand your dilema. I have questions.

    First. does it drive up opening the angle or down?

    How is the gear attached to the motor shaft? It appears to be a decent (1/4" ?) sized shaft. Does it have a flat you could use a set screw on? Is it splined?
    Worst case you could drill and pin it. That would solve the slipping and skipping but I still think the majority of your issues are coming from the curve.

    With the rod and red gear alone, the gear should spin smoothly along the rod. Any dragging or hanging will only be worse when loaded. A snug fit is working against you. Does the red gear have a threaded metal insert or is it all plastic?

    Would it be possible to scale up the radius and run the motor faster? That would give a gentler curve and reduce the error in thread pitch.

    In the pic the rod doesn't appear to be a smooth continuous curve. Or is that just an illusion? Where the windowsill is behind the rod it appears to be a shaper bend.

    Does the mesh of the gears give you any trouble? From the pics they look ok. But they aren't bevel gears are they?

    Under the red drive gear for the rod did you use a bearing surface? (UHMW or similar) That might help as well.
  11. Jan 13, 2015 #10
    It is 1/4 and has a flat I tried to rig it with a peice of plastic in the hole, it looks tight but whenever it has even a little resistance it starts slipping. Im not super familiar with rigging gears I was also having trouble coming up with a way to keep the red gear stable (it does start to tilt up occasionally) how would i go about finding a screw to hold the gear to the motor better and also hold down the red gear, i think that's where im having issues.

    The gears are working together perfectly together they are plastic. The rod is a near perfect curve for about the middle six inches, the rest is a little off. But because it takes a min for a full rotation it takes 32 min for an inch. Any exposures over 12 minutes suffer from sensor heating problems, light pollution ect. To take many photos i won't ever need more that 2 to four inches of uninterrupted regular threading. (Im thinking of attaching a polarity switching switch and rotating the mount.

    Oh and I have it closing to help keep the red gear in place but because the axis has to be oriented to true north i can reverse polarity and rotate the mount 180 degrees.

    The red gear isnt threaded so i screwed a double threaded screw (inside and out) tightly into it and it spins smoothly (most the time on the threaded rod)
  12. Jan 13, 2015 #11
    Thanks for your detailed assistance this far as a physics teacher and astrophysics minor some engineering problems can slip me up lol. I had to build the foucault testers while making a telescope mirror.
  13. Jan 13, 2015 #12


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    Hi Scott

    I can see a number of mechanical problems that will cause instability .....

    1) your hinge area is too small for the width of the section you are hinging ... you need a wider hinge to make the platform more stable
    2) Motor rubber banded in position ... seriously ... that should be an obvious stability problem ;)
    3) plastic gearing you could almost get away with, but when combined with really coarse teeth is a recipe for problems
    you will find there is too much slack / play in the meshing of the teeth
    finer toothed metal gears are a much better solution :)
    4) and on top of all this .... how much camera and lens weight are you expecting this construction to be able to handle ?

  14. Jan 14, 2015 #13
    It has held up to 9 lbs without any noticable effect on the system and the motor has a mount but im waiting to perma fasten it till I solve the gear slippage problem.
    The primary problems are slipage of the gear to motor connection and the other gear rising up off of the platform. montoyas7940 mentioned a fix for both of these problems but Im little confused as to the parts needed and where to find them. ☺
  15. Jan 14, 2015 #14
    Solving the slipping black gear is easy since you have a flat on the shaft.

    Drill a hole between the teeth of the small unused gear portion of the black gear and secure it to the flat of the shaft with a set screw in the hole. Set screw diameter doesn't matter much since you aren't using that portion of the gear, 3/16" or 4mm should be about right. Be careful not to drill the hole too large as it is an easy mistake to make. The hole should be about equal to the minor diameter of the set screw. Google drill and tap size for more detail. Get one with a coarse pitch since it has to grip plastic. You might have to actually tap threads into the set screw hole but you could possibly get away with brute force. Filing (triangle file) a notch along the set screw length will make it somewhat self tapping as it creates a cutting edge and a way for the plastic chips to move out of the way. Or you could just buy a cheap tap (I probably would). Your profile doesn't tell where you are. Do you have a Lowes or Home Depot or really any decent hardware store nearby? You could order everything online but I think you will get a better feel for the sizes you need if you carry the gear with you to the store.

    Just pausing to say I really hope I am not insulting your intelligence with this. I don't know how much you know and if I have, I apologize.

    Anyhow, on to the red gear. Two possible solutions to the riding up problem depending on the cause. One is rotating the motor and black gear so they are inline with the red gear instead of perpendicular. (Motor off the side maybe) The other is to sandwich the red gear between bearing surfaces. Something simple such as delrin sheets top and bottom separated by spacers (washers, short pieces of tubing or whatever) or delrin sheet on bottom and a couple of square rods across the top (also spaced so the gear can rotate). Even if you rotate the motor and black gear I think a bearing surface under the red gear would help.

    I hope this is helpful and coherent. I've been up 26 hours, going to bed now.
    Good luck!
  16. Jan 14, 2015 #15
    Perfect! Exactly what I needed to know. Thanks!
  17. Jan 14, 2015 #16
    Get some sleep lol
  18. Feb 24, 2015 #17
    Finished build three of the barn door tracker. Ive been able to track up to 10 min so far with absolutely no trails ☺ 20150216_234253.jpg
  19. Feb 24, 2015 #18
  20. Feb 24, 2015 #19


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    great to see the final product :smile:
    Gosh, Haven't been through Kennewick, WA since 2006
    well away from the major cities, you should be able to find some dark skies :smile:

    Last edited: Feb 24, 2015
  21. Feb 24, 2015 #20
    Wow! I am always amazed at the success of some amateur astronomers. The fact that you built it from scratch makes the end result even more impressive.
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