A nice mechanical drive

edited January 2008 in General
If, like me, you have a liking for clever mechanical approaches to KAP you might like the Geneva drive, developed for early cinema projectors, allowing the film to be held briefly in the gate before the next frame is moved on. A splendid way to create a geared intermittent rotary drive from a slow rotating continuous source like a dumb servo.

I can certainly see a KAP use for such an elegant old mechanical design. I know current autoKAPs do this with electronics but this is just so lovely.

And thanks to Wikipedia for such a well prepared animation.

Geneva Drive

PS I'd love someone to fabricate a few of these for AutoKAP. (I'd pay for a couple myself.)


  • edited January 2008
    I'd be happy to do the fabrication.

    Sorry, I hadn't read your post thoroughly, and assumed you were planning on using some sort of spring drive. After a few minutes of head-banging trying to figure out how to pack enough winds on a spring to make it worthwhile, I finally re-read what you'd written and saw you mentioned a dumb servo. This is entirely doable, and should give hours of run-time. It would be neat to either use the geneva cam itself, or to put a second cam on the servo shaft to actuate the shutter so it's all driven from a single motor.

    An alternative to the dumb servo would be to use a low-speed gear motor like any of the Solarbotics GM series motors. The smaller ones have a shaft that's easy enough to adapt for this purpose, and they're pretty forgiving on voltage range.

    In any case I think this would be a fun project, and I'd be more than happy to do the design and fabrication for it.


  • The dumb servo (servo converted to simple low speed geared motor) would be an ideal drive for such a device, otherwise I may use my simple AutoKAP drive with elastic band and silly putty friction bearing set for a reasonable revolution rate to match the drive.
  • The lost art of pure mechanics. Killed by the microprocessor. I mourn it's passing. I found that one in a 1904 book titled "Mechanical Movements and Mechanical Appliances" that my grandfather left me. Amazing what you can do without silicon. :-)
  • Very inspiring.
    Here's a link to a PDF with a drawing on the second page that will help to construct this.
  • Aaaaah, not entirely lost. For what it's worth, the first time I ever worked on a geneva mechanism was on an infrared camera mounted on the back-end of an adaptive optics system. The thing never really did work out, unfortunately, because it's actually quite hard to get a zero-backlash lock on a geneva, and that's what was called for. For the replacement I voted for a solenoid driving a ratchet wheel with a sprung ball engaging a V-shaped detent to lock it in each position. The arrangement kinda looked like a steam engine. Unfortunately interest in the instrument waned before I got to build the steam engine wheel design.

    For KAP, I think the geneva would be a lot better suited than a racheting solenoid because of the ramp-up and ramp-down on the rotation rate. It should make for very little residual rotation in the rig at the end of a move, and might not even need additional damping.

    Seriously, I'm in. I've got some 25mm diameter Delrin rounds lying around looking for something to do. If I can make everything work out with that, it'd be easy enough to make up several sets with the material I've got on-hand.

    I'll design around a Futaba spline, but I may also try an under-sized 3mm hole to use on a gear motor.


    P.S. If anyone's interested in seeing that pure mechanics are still alive and well, take a look at "Home Shop Machinist" and "Machinist's Workshop", two publications from Village Press. A third publication of theirs focuses on CNC machining, so it's not PURE pure mechanics, but a lot of the stuff that gets produced doesn't have a smidgen of silicon in it. Neat stuff. But I'm getting way off-topic.
  • edited January 2008
    Count me in for a small "6 station" version (ie as per the wikipedia animation and illustration below), rather than the 4 station type if you can do it. Happy to have in Futaba splined and another shaft adapted for 3mm. I love the creative enthusiasm and depth of knowledge and skills available around here.

    6 station Geneva mechanism
  • This could be great for a motor free concepts.

    I'm not sure about the positives and negatives of the different designs. The dwell time, angular velocity of the driven wheel, and contact surface during dwell will be different depending on the size and number of stations. The driven wheel for the four station model looks very fast to machine. You could gear the output however you wanted. I think you would have to make a simple jig to complete the slots in a 6 or 8 station model. Not difficult. Especially if you are making a few.
  • To some extent the dwell time and angular velocity can be adjusted by changing the speed of the drive motor. But yeah, it'll depend on the geometry of the geneva.

    I have to admit, I was going to cheat somewhat on making the thing. Rather than using a jig or rotary table, I was going to CNC the geneva wheel and the drive cam. Making duplicate sets shouldn't be a problem.

    Hey, quick question: What should the exit shaft look like? Or should that be left as an open-ended question for now? I'm tempted to make it something that'll play nice with Brooks's picavet cross since that's what I'm using on my rig. But I can leave it pretty generic.

  • For me, the exit shaft to picavet or other suspension would be fine as a 3mm hole which if necessary could be drilled out to a little larger then I'd probably go for threaded shaft and locknuts . So Generic rather than specific for me....
  • Tom, please put me on the list for one too. Thanks.
  • For the moment, I'm figuring out a system for taking motorised ground shot panorama's. This system could be very interesting, because it has 6 positions, for which I need to take 6 separate shots.
    If you have more info on your project, Tom, please let me know.

  • 3mm through hole it is, and I'll stick it in a 5mm or wider shaft so it can be re-drilled or tapped as necessary.

    Yvan, I'm guessing for ground shots you'd need something beefier than I'm designing, but I'll happily post all the drawings and details once the project is done. Glad you mentioned this, because I hadn't even thought about extending this to a ground-based panorama setup! It'd be perfect, especially if the rotating mount is tripping the shutter.

    Peter, no problem. If anyone else wants one, let me know.

    I haven't had a chance to start work on the design yet, but I'll post some prototype drawings as soon as I've got them done.

  • Computer animations, CAD drawings, CNC machining? Seems an odd way to eliminate a microprocessor on a KAP rig. I'm of a mind to design an electronic controller with a small LCD that can display that wikipedia animation while it's controlling everything electronically. ;-)
  • Hahahahaha! Touche, David. And when I've finally built that Holtzappfel Rose Engine, I'll be in the market for an LCD animation of that, too.


    P.S. No, I'm not planning to build a Rose Engine. But cripes what a gorgeous machine.
  • Electronics and mechanics; two completely different languages. Combining them is one aspect that makes KAP so interesting.
  • edited January 2008
    Alrighty, here's the first pass:


    5mm diameter output shaft with a 3mm hole, and I did my best to put the output shaft at the symmetric point on the servo so you can just rotate the servo 180 degrees, bolt the Geneva on top, and still be in balance.

    The top and bottom plate will be 1/16" 6061 aluminum, and the Geneva wheel and cam will be Delrin. The pin will be a 1/16" dowel pin. I've still got some work to do, like beefing up the lower plate a little around the cam wheel, and making the broach to cut the Futaba splines in the cam wheel. But otherwise it should work as-drawn. Also, I doubt it'll need all four bolts through the plate, so it should be possible to just use two diagonally opposing bolts and not interfere with the bolts and standoffs that hold the pan servo onto a BBKK rig.

    One bit that's not drawn in is any way for the cam pin to hit a switch to trigger the shutter. A lever switch would probably be the best bet, set on the opposite side of the cam away from the Geneva wheel. But I'll try to leave enough meat on the plate for that to be added later.

  • benedict:

    If you're taking orders, now or in the future, I'd love to get on the list.

  • David just to answer your point, "Seems an odd way to eliminate a microprocessor on a KAP rig."

    For me KAP is an adventure in ideas, it is a ever-fascinating field of play in which to try out ideas, creativity, expansion into other fields. it is an area in which thankfully, there are few rules and the people who practice it are utterly commited to sharing their concepts and ideas, building on them, and congratulating each other on the results of their efforts.

    A fact that I endlessly delight in is that there are no right and wrong ways to do KAP, and whilst microprocessors and complex electronics will do it and may be the ideal approach to consistency and accuracy, other approaches using wooden cameras, or mobile phones from stunt kites, or elastic bands and silly putty all have validity. Strangely enough too, each slightly different approach ends up being a little niche within the whole, and that collection of niches and approaches creates that fabulous diversity of KAP we see today.

    I realise your comment was not really an attack on these fundamentals of KAP, more a sideways look at the computer machining technology and bringing it to bear on such mechanics.

    I was thinking anyway to write something like this from for some time to remind all KAPpers that there are many ways to KAP heaven, and each time the latest KAP electronic invention comes out, it also has the potential to stifle the inherent creativity in KAPpers. If all the thinking is done for you by an electronic box, it is possible to miss the chance to do it a simple way to similar end results. And likewise, whilst I delight in the hundreds of KAPpers brought to KAP by Brooks' kits, I also sometimes feel a little pang that some of those folks have missed one of the greatest pleasures of KAP (for me) - learning workshop tools and techniques and building rigs from scratch.

    I'm delighted we have someone here that has the potential to machine such parts, because although I could surely do it, most of my fabrication on a geneva mechanism would be labour intensive and take me some hours to do.

    When I see something like the geneva mechanism, or a solar powered houseplant turner in a garden shop, my whole mind leaps ten feet to the left and screams, "you could use that for KAP !"

    I have KAP rigs "enough to cobble dogs with " (wonderful old saying from my mother) and in truth need no more, but as long as I have hand and mind, I'll go on thinking up new ways and approaches. There is little need - just the fun of doing it.

    I know from your sensational ring rigs that you have a similar inventive approach to KAP problems Dave, so I know your comment was not really pooh-pooing simple mechanics.

    I think the geneva mechanism is going to end up on two rigs one a ultra light elastic band powered, and the other with a dumb servo and some other ideas incorporated.

    Love the 3D Tom. Looking great.
  • edited January 2008
    Wow. I'm as blown away by the model as by the upcoming device.
    and of course, add me to the list too, esp after the shut switch jumps aboard.

    As to Simon's chagrin that lots of newbies are missing the creative part, I agree -- but selfishly, that gives me a lot of exploring and learning and tweaking to enjoy myself. If it weren't for that part of KAP, I wouldn't be here several times a day.
  • Simon, I think you recognized my comments as the good-natured jab they were intended to be. And lest there be any doubts, I do admire an elegant mechanism as this certainly is. Why else would I carry around your autokap thingy that I bought at auction at KAPiCA 2006?

    Tom, I'm impressed by how quickly you volunteered to do the fab and put together a cad drawing. Methinks I've made a serious error not posting a sketch of a next-generation tilt ring mechanism to this forum. Perhaps someone here would have built it and shipped it to me by now.
  • edited January 2008

    Could you post a 2D drawing of the wheels with measurements? I'd be interested in trying out a few different variations for a wind-powered rig I'm building around the gear box of a disco ball. I love the creativity here.
  • Tom,
    Please put me down for one. The compactness of your drawings of the device are great.
  • The Geneva design is brilliant, but how do you plan to synchronise firing the trigger with the still moment of the rotation?
    Tom, put me on the list as well as I really would like to test such a device. I have been using a small electric motor ( 1RPM) for a few month combined with a 5 seconds intervalometer on a RicoH GX 100 but I have to go through hundreds of shots for a decent panorama. This design may be the answer.
  • edited January 2008
    Great piece of work Tom. Put me down for one too!

    One thing slightly worries me though. The drive is going to have to turn the Geneva mechanism very slowly for this to be comparable to a TUCIT or Gentled360 (i.e. one step every 5-10 seconds say). Will this be possible without a lot of extra gears?
  • edited January 2008
    The dumb servo I have here does some 15 rpm on a 1,5 V battery. Or it takes some 4 seconds for 1 rotation. With a "6 station" Geneva that would end up to some 24 seconds for 360 degrees for the camera.
    At 3 V it does 32 rpm.
  • Just some random thoughts...

    Do we have any idea how long a 1.5 V battery would last running continuously?

    I wonder how the vibration and friction would be between the drive wheel, the driven wheel, and the servo.
  • I've run a small dumb servo on an AAA for over an hour. Certainly with rechargeables and a couple of spares in pocket, thats enough for me.

    I read somewhwere that high speed Geneva drives need good lubrication and I can imagine foreign bodies could interfere with them. I expect for the proposed usage this shouldn't be much of an issue. There is potential for a fair bit of friction between the flat surfaces.
  • Tom, please add me to your list; though I'm still figuring an application, I can not pass up such a thing of beauty.
    Thanks for your efforts.
  • edited January 2008
    One thing slightly worries me though. The drive is going to have to turn the Geneva mechanism very slowly for this to be comparable to a TUCIT or Gentled360 (i.e. one step every 5-10 seconds say). Will this be possible without a lot of extra gears?

    If the pin on the driving wheel (or another bit rotating in sinc) simply triggers an "on button" for the Gent360-LED at or just before the beginning of the dwell period it should work fine. A simple gentLED shutter or equivalent would be all you really need though. With the right camera and set up you could take exposure bracketed bursts during each dwell period.
  • I think one of the nice properties of Delrin is that it's self-lubricating. So hopefully friction won't be much of a problem in Tom's design.
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