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Thread: Flux [featherweight]

  1. #1
    A while back at a friends house I came across one of these old fashioned tonka toys - a chunky steel thing which surely wouldn't be allowed anywhere near a child these days.



    Clearly I wasn't about to take one to a sandpit but it was the first time I'd seen rubber tracks at an almost-perfect scale for a featherweight. Here it is by my beetle for comparison.



    And so, after finding two of these monster-toys on eBay and spending silly amounts of money on various other components here's where I am:



    The observant among you will notice it's essentially a larger version of my beetleweight. I actually started planning and compiling parts for this one long before fractal but the beetle was much easier (and cheaper) to build. I was at one point hoping to finish this machine by September but instead have decided to take it slowly but surely, as is usually the route to more successful designs.



    Instead of the four powered axles on the beetle this will only have two, with two dead axles at the front. One of the most complex elements of this design is likely to be track tensioning, something that I really don't want to be rushed on - there will most probably be two additional suspension arms per side.



    Here's a shot to give you an idea of the arm/weapon actuator. It's a combination of an MFA como drill motor and homemade gearbox (although the standard motor will likely be swapped out for another 540 sized one). The overall reduction is 831:1 (the three stages being 11:1 > 40:1 > 9:17). Hopefully it will be pretty damn hard to back-drive given the worm stage.



    The drive gearboxes are the new-style 26:1 reduction banebots ones with 540 style motors fitted. The motors are 9.6v hobby-racing style motors to be run off a 14.8v battery. The wheel diameter over the track is approx 74mm (which compared to the body height of 38mm should give decent clearance). You'll notice the toothed drive-hub for transmitting power to the track, this took a lot of maths and patience to get right!



    Here's a size-comparison with the beetle. It's pretty compact for a featherweight, and I've still yet to build to the new 13.8kg rules, thus I'm going for a mostly steel and hardox welded construction (inc. all hardox scoop). Having got the hang of welding this should be fun - when I've got the time and money to finish it all!

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    The first stage is quite simply one of these http://www.technobotsonline.com/re-540- ... -11-1.html which is an easy way to get the motor speed down to a reasonable rate for the worm stage (which isn't well suited to the ~10,000rpms of the motor). With the como drills range the ratios ‰ 11:1 usually have larger MOD spur gears than 50:1 and above, hopefully better able to take the torque.
    Then the worm stage is comprised of one of each:
    http://uk.rs-online.com/web/search/sear ... &R=5216963
    http://uk.rs-online.com/web/search/sear ... &R=5216890
    for a reduction of 40:1. Worms allow a great reduction in a small space, at the price of lots of friction. You can see in the above picture roughly how I'll be housing this, of course with lots of bearings and additional shafts
    The final stage is simply a chain stage (9T sprocket to 17T sprocket) which will be rigged on the side of the housing, I may get some bigger sprockets though as I'm dubious that 6mm chain will stand up to shocks on the arm.

    This set-up, using the standard motor and ignoring inefficiencies would provide ~22rpm no load output at 14.8v with ~10kg/m stall torque. The motor I'll probably be using is rated at approx 40% greater rpm at ~3 times the stall torque (with an awful lot more current and heat!) http://www.robotmarketplace.com/product ... P5737.html

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  6. #6
    Looks awsome Ewan. Always wanted to make a tracked robot but never found any decent tracks the right size, and making metal ones is a lot of effort. Also when they get damaged its going to be expensive to keep replacing them.

    But fair play for doing it.

  7. #7
    Yeah it's looking great. Nice work with the wheels as well. Whenever I see a tracked robot it always makes me want to make one, although I probably couldn't do a decent attempt.

    But I'd love to stick some on my grabby-robot when I rebuild it, something meaty like on this Aussie bot:
    http://www.robowars.org/forum/album_pic.php?pic_id=1516

    Anyway, will love to see it in action when it's done

  8. #8
    Is this thing still being made?

  9. #9
    Over summer hopefully, university takes up all of my time at the moment.

  10. #10
    My Easter holidays have begun and so has the race to get this machine finished for the Yeovil Robochallenge event. Here's how it looks today (apologies for poor quality, I'll use my camera instead of my phone next time!):





    So the set-up will be two slightly modified 9.6V GR01's for drive (built into the chassis rails), a 14.8V 4000mah Li-po for power and a Scorpion XXL (in a heat-sunk & fan cooled enclosure) as the speed controller. The weapon/scoop is powered by a GR01 slaved to a worm reduction stage (hence at 90 degrees to the weapon shaft) and then slaved again by chain to the shaft, a total reduction of approx 1200:1 from the motor. I've had the worm-stage housing laser-cut from aluminium which isn't in this picture.

    The end result should look pretty similar to my beetleweight:



    Tomorrow I'll start cutting up the 4mm Hardox which will make up most of the exterior, then begin welding the scoop, front axles and side arms together. Lots to do!

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