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Thread: Deathly Hallows - FW - Alligrater

  1. #1
    Deathly Hallows's Avatar
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    Next project in the pipeline a featherweight.

    Early stages of design - but rough concept as follows:

    MY1016 drive motors for plenty of grunt
    1/2" thick HDPE wedge body with perhaps an aluminium floor (for weight saving)

    I have been struggling with a weapon concept for a while, current plan is..
    Weapon a circular saw blade
    Weapon drive will be a brushless motor

    I have a few ideas to make the bot a bit more interesting.
    For the Alligrater concept the HDPE will be green.
    I intend to add serrated bars on the ramp too.

    The motors and HDPE are heavy so I am a bit worried about the weight. Calculations rough at the moment but should iron out the basic design in the next week.

  2. #2
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    HDPE isn't particularly heavy in of itself. The Honey Badger 2.2 uses 10mm HDPE which, all told, probably adds up to only 1kg, maybe less. If you want it green though, why not look at RG1000, which is a recycled variant of UHMWPE.

    Looking forwards to seeing this one
    Team Kaizen

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  3. #3
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    Quote Originally Posted by Deathly Hallows View Post
    MY1016 drive motors for plenty of grunt
    You know that those motors what about 2.5kg each right ? drill motors are the norm for drive motors or a brushless system. Not saying a scooter drive system is not impossible but that the only robot I know of using one is a RAM bot

  4. #4
    Deathly Hallows's Avatar
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    I know they're heavy I hope I can get it to work. Saw a pair forĀ£30 and couldn't resist!
    But I have some drill motors handy too if needed.

    Rough calculations:
    Drives x2 = 4kg (I think the ones I found weigh 1.9kg each)
    6S LiPo = 1kg (depending on size - I estimate 4000mAh should suffice but have budgeted for larger)
    Wheels = 1kg (these are heavy tough ones quite wide, could save weight here)
    Brushless motor = 0.31kg (Turnigy XK series inrunner)
    RX & ESC x2 = 0.2kg (Turnigy/FLYSKY RX, FeatherTwo, Hobbyking 80A)
    Armour/chassis = 6kg
    Bearings, weapon disc and misc. = 1kg

    I'd like a self right mechanism in there too but perhaps that would push it over the limit.
    It's a bit of an experiment, and may need adapted. If it's anything like Evil L'Weevil it'll be a under my estimates. EKW came in a full 100g less than my calculations.

    Once the motors arrive, I'll weight them, finalise the dimensions of the chassis and update my calculations. The wedge slope needs to be a low enough angle (35' would be good). I am concerned I have over cooked it, worst case I use this pair of motors for an axe weapon system on another concept I have.

  5. #5
    Maxamuslead's Avatar
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    ah so you got the 250w ones, have you designed a gear reduction for it ?

  6. #6
    Deathly Hallows's Avatar
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    Quote Originally Posted by Maxamuslead View Post
    have you designed a gear reduction for it ?
    I am currently thinking about a chain drive, if I can fit a larger sprocket on the motor's drive shaft, I can get a 4 or 5:1 ratio.

    http://www.gearsandsprockets.co.uk seem to have lots of options. Double grub screw fix to the wheels axle, counter sunk for extra bite.

    I'd fit some kind of tensioning arm/idler to hold it in place.

    The big question on the drive for me at the moment - is how to fix the drive shaft to the wheels (from Screwfix). If I want the axle supported at both sides I need a round shaft at both ends, but to fix in the wheel a hex would be best as I was thinking about the thermal insertion method. I don't have a lathe to mill down a hex bar at both ends. Do you know if anyone supplies something like this or have an alternative idea?

  7. #7
    Maxamuslead's Avatar
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    any reason why you can't use a dead shaft ? that way you can attach the sprockets directly to the wheels?

    Or if you want a live shaft why not a plate which is attached to the shaft and bolt the wheel to that?

  8. #8
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    Quote Originally Posted by Maxamuslead View Post
    any reason why you can't use a dead shaft ? that way you can attach the sprockets directly to the wheels?

    Or if you want a live shaft why not a plate which is attached to the shaft and bolt the wheel to that?
    I am learning stuff every day! I love this hobby.

    So a 'dead shaft' would mean the shaft is fixed (not rotating) the sprocket is fixed to the wheel so when the sprocket turns the wheel does too. At these low speeds for short duration then no bearings needed I would assume, just a dab of grease or PTFE. Though I could insert something to reduce the bore to allow a narrower gauge of rod/axle.

    Did I understand correctly? I don't have an engineering background so I could be totally off mark

    Thanks for your input - very much appreciated.

  9. #9
    Maxamuslead's Avatar
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    Bang on, I dont have a engineering background either but have always had a health interest.

    I would look into using some bronze bushings at the very lease as it would help reduce friction prevent as much wear on the wheel hub.

  10. #10
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    Since the last post I have been trying to source parts and get the layout sorted in my head. Unfortunately my son is revising for his exams and he is hogging the laptop, so not had the time I need to work in CAD.

    Oilite bushes purchased they will work fine.
    I bought a 25H chain seems a perfect fit for the existing MY1016 sprockets. But I am out of my depth when it comes to the names and codes for sprockets and chains. I was looking on Gears and Sprockets for a suitable one to mount on my wheels. I thought I may find a cheap one on eBay but the mini dirt bike and mini atv ones look too big. The outer diameter of the wheel is around 150mm and I would like something nearer 90mm. This may limit the ratio I can achieve without more complexity than I want, >5:1 would be nice. 2750 -> 550 would still be very fast. Would need to configure the controller for logarithmic to make it controllable at low speed.

    Is 04B compatible with 25H or do I need to look elsewhere? Suggestions appreciated
    I'll need an idler sprocket on a slide too, or at least i think i will so I can adjust the tension/allow easy removal replacement.

    Cheers
    Last edited by Deathly Hallows; 30th April 2018 at 13:59.

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