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Gear train for torque and holding power

  1. Jul 1, 2010 #1
    I have a project where I am using a hobby servo with 133 oz/in of torque which currently has a 32 tooth timing pulley on the output shaft ard driving a 102 tooth timing pulley connected to a camera platform that has a 950g camera sitting on it.
    The issue is that the camera weight needs to be all on one side of the pivot point.

    The servo can drive this ok in one direction but only just in the other, and there is also the problem of holding power when the servo is not moving. So I need a gear box or gear train to make the load easier on the servo and also hold position when the servo is not moving.

    Would someone be kind enough to tell me how I can work out how many and what size gears I would need, or maybe know of a small gearbox that will do the job.
    I want to keep this all as light and compact as possible.

    I did find these gearboxes but I need to able to use a hobby servo and not a DC motor.

    http://www.hobbyplace.com/robotics/gearbox.php" [Broken]

    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jul 1, 2010 #2
    One would need to know the lever arm of the camera mount as well as an estimate of the friction losses etal to calculate anything reasonable. However you have an _almost_ working system with about a 3::1 ratio... This makes for about 390 oz/in on the output drive shaft (ignoring losses) moving 35oz... giving me a ballparkish 5" arm with 50% loss... So... kicking up the pulley ratio to 4::1 might do your trick.

    Have you considered counter-weighting the camera platform so it is better balanced?

    The only reason I have all this off the top of my head is that I'm building (the second iteration) of a similar thing using a couple of heavy-duty geared stepper motors from All Electronics: http://www.allelectronics.com/make-a-store/item/SMT-94/STEPPER-MOTOR-WITH-GEARHEAD//1.html [Broken] -- A 133 oz/in for a Hobby servo sounds pretty heafty and expensive, maybe you should consider my motors instead?
    Last edited by a moderator: May 4, 2017
  4. Jul 1, 2010 #3
    Thanks schip666, I could try a 4:1 ratio but I'm thinking that that probably won't help the holding situation when there is no power., maybe a worm gear would do the trick?
    https://sdp-si.com/eStore/" [Broken]

    I have thought of counter balancing, but the counter weight would be in the picture.
    Last edited by a moderator: May 4, 2017
  5. Jul 1, 2010 #4
    The hobby-servos move to a position and hold it under power...IIRC...you will have
    heat issues though I think. There might be enough friction in the gear/belt-train to at least slow the un-powered drift to an acceptable amount. As to counter-weights, I'm imagining something that tilts your camera up-down, so there might be someplace to put a weight that is not exactly opposed to the weight and out of the line of sight.

    I don't know what you are going about here so this may be un-solicited advice (well actually...it is...)... The reason I'm building powered camera mounts is to get the center of rotation aligned with the optical center of the system -- which leads to little/no distortion when trying to stitch images together. This has the added advantage of more/less centering the load with a lens-heavy still camera. Thought food...
  6. Jul 1, 2010 #5
    Sounds like we are trying to do the same thing, or at least something very similar. I'm also building a camera mount that have the camera positioned so you get minimal distortion when stitching. With this in mind, most of the camera weight will be positioned behind the pivot point.
  7. Jul 2, 2010 #6
    How about a separate, mechanical brake or lock?
  8. Jul 2, 2010 #7
    Good idea on the brake... in fact the motors I mentioned have a brake installed which I am removing because I think the friction will be sufficient.

    However on the camera positioning issue -- One wants the optical center of the system to be at the rotational center. The optical center is usually about where the iris of the lens is, and most of the weight of a DSLR is in the lens itself. So there will be some off-balance weight towards the body of the camera, but not as bad as it could be. In my case I am doing two-D pan and tilt, so the lens is centered on both axes (axisisses?, whatever...). Using hobby-servos never occurred to me, but my current project needs to run as fast as possible so they probably wouldn't work anyway.

    For the OP's project: Are you doing one or two D motion? If it's just panning you shouldn't have much trouble as all the weight is carried by a good bearing...you do have a good bearing, right?... Inertia should be the only real problem. For the tilt we have the off balance issue where we need to brute-force the motor.

    And why aren't you using the Giga-Pan like everyone else?
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