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Thread: Big Dave 3 Design & Build

  1. #1
    After several months of trawling forums, the RioBotz handbook and google, myself and two of my friends are ready to start building Big Dave 3, our first Heavyweight robot.

    Where we're at:
    We have the basics of a design in Autodesk Inventor. We're intending to get each of the plates (there are 8 main plates) cut out of 7075-0 aluminium. We have yet to decide on a material for the drum, which we're intending to get cut in sections and then bolt together to save cost & machine time.

    We've selected drive & weapon motors. Alien 8085's and Turnigy T203T's respectively.

    Drive: 5:1 via spur gears. Wheels are 11x7.10-5 GoKart tyres, driven with a keyway'd 25mm axle. We're hoping to get axles made up from a local lawnmower racer.
    Weapon: 6:1 via toothed pulley. 95% of full speed is just a hair under 11000RPM.
    Batteries: 8 x 8400MaH LiFePO4 batteries.

    Things we're slightly worried about:
    • Weight! Autodesk puts the framework (seen below) at 89Kg. Once we add in the bolts and electronics, I would image that would put us about 95Kg, without armour. The obvious solution to this is to mill pockets in the side and back plates, which could save us a good 10-15Kg. However, the machine time for that would be crazy expensive.
    • Weapon Drive - we're intending to use a toothed belt that's slightly overtensioned (.5mm), which might tear when the drum hits something.
    • Clearance, Clarence. There isn't that much room between the edges of the tyres and the side plates, nor is there much room between the drive motor and the gears. When running inverted, there's a 5mm ground clearance.
    • Bearings. None of us have the slightest clue how to design bearing housings in the side plates in a way that doesn't need machine time.




    Unfortunately, it's just a screen grab with none of the gearing shown, because trying to render something with that much detail screws up my laptop!

    Next steps:
    Buy everything once we're 100% happy with the design.
    Find a waterjet/laser cutting service, as well as a local machine shop/school/university that will allow us to cut pockets in our plates.
    Design & Print the battery housing.
    Start assembling!
    Drive Test & Tune.
    Weapon Test & Tune.
    Armour.

    Trivia: Back in 2004/5, we built/helped to build several autonomous robots for the "MicroMouse" competition: Lil' Dave, Dave, Big Dave & Big Dave 2. Thought we may as well continue the naming tradition.

  2. #2
    Great detail but brushless drive in heavyweight robots is still relatively un proven technology but best of luck.

  3. #3

  4. #4
    The drive has a WAY to low gearratio.
    Better using higher KV motors and a gearratio of 20-1 or so.

    Even Bosch 750's struggled with a 5-1 gearratio to 250mm wheels.

    And if you think speed is a problem.
    Bullfrog uses a 20-1 gearratio and 250mm Full foam wheels, with 170 KV 120mm diameter motors on 10S.

  5. #5
    Quote Originally Posted by maddox10 View Post
    The drive has a WAY to low gearratio.
    Better using higher KV motors and a gearratio of 20-1 or so.

    Even Bosch 750's struggled with a 5-1 gearratio to 250mm wheels.

    And if you think speed is a problem.
    Bullfrog uses a 20-1 gearratio and 250mm Full foam wheels, with 170 KV 120mm diameter motors on 10S.
    We can put a 15:1 ratio in without much bother, but even with higher KV motors (250, 80mm) I'd be worried about how slow we'd go - with luck, that might maybe touch 10mph. With using higher ratios, I'd also be a bit worried about trashing the gears at a higher ratio.

    What sorts of speeds should I be looking to realistically achieve?

  6. #6
    All three of my heavies have been geared from 12-18mph some go higher. Starting amps would be your issue if you gear for high speeds

  7. #7
    Gravity 3 was geared for 15km/h.
    Tough as Nails for a smidgeon faster, 18km/h
    Bullfrog can get up to 14km/h.


    Lets take a hypothetical setup.

    250KV, 37V 250mm wheels.

    With a 5-1 ratio you end up with ludricous speed.
    0.25m x pi x (280x37)x60/1000= 97km/h

    With 130KV motors, it's still more than 40km/h

    With 130KV motors and a 15-1 gearratio you're back in the normal range. Between 12 and 20km/h.

    Some machines are build to be faster. But getting the power to the floor is a problem.

  8. #8
    I cant see the pictures?

  9. #9
    Quote Originally Posted by maddox10 View Post
    With 130KV motors and a 15-1 gearratio you're back in the normal range. Between 12 and 20km/h.
    I get 11.2km/h with the 15:1 ratio with the 130KV motors, 16.4km/h with the 250Kv motors.

    Quote Originally Posted by adamclark View Post
    I cant see the pictures?
    Are you on a Mac? I've tried it on PC/Android/iOS and it displays, although it doesn't work on my work Mac.


    Has anyone ordered from Alien Power before? Almost everything shows as out of stock.

  10. #10
    Then the 250 KV motors would get my preference.

    I ordered in the past with Alien Power, and know of others.

    I ordered a lot more with Hobbyking.

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