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Thread: Spur Gears; Brushless Motors

  1. #1
    Adobe Evangelist emisnug's Avatar
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    Spur Gears; Brushless Motors

    I realised that it's been a while since I actually posted on here! A little stumbling block I've run into today - I'm currently designing a combat robot (because Robot Wars is back!), and realised that I don't actually have a clue how brushless motors work etc. I've done some reading up and am now less naive about them, but can't seem to figure out a way of estimating torque. We're gearing our motors way down (5:1, output shaft speed around 1750RPMish) but I have no realistic idea if the motor can provide enough oopmh/will actually accelerate the bot in a reasonable fashion.

    Anyone got any recommendations on how to work it out?

    The motors that we're hoping to use for drive are 6KW/170Kv models, (Link here), and we're using some 11'' Go Kart tyres because they were free and we're poor.


    Afterthought: Spur Gears - for proper tooth fit, it's (PCD + PCD)/2, right?
    No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.

  2. #2
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    You want enough linear force to break traction at stall, in a pushing match. Traction on the floor is pretty terrible historically though. Soft foam filled tires are your best bet. Relying on pneumatic is a dubious proposition, of course.

    Bldc motors function like pmdc motors with respect to torque. You should be able to calculate stall torque from the estimate of max powe at 50 percent maximum speed.

    http://www.linmotors.com/_images/BL16B17-08_NEW_HR.jpg

    Check out the riobots manual if you haven't already. Really good resource for how to think about a winning design.

    I really liked the new battlebots series, but that's an "only" hobby. Brushless tech has come SO far since my initial interest in combat robotics, where big pmdc motors like magmotor and eteks dominated. The power to weight and efficiency on the motor you linked is excellent and 2 of them would motivate a 100kg bot very well.

  3. #3
    Adobe Evangelist emisnug's Avatar
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    One of the ideas behind using GoKart wheels (apart from easy availability of parts) is that our traction area in relation to the size of the bot is relatively large. What we're hoping is that the slip on the floor will act almost like a clutch, and with luck we won't just sit there and do a burnout.

    Thanks for the info regarding stall torque - looks like we should be able to operate the motors with an OK degree of safety.

    The Riobotz manual has been a hell of a good resource - I even bought the book off of Amazon! The 30-30-25-15 rule that's talked about a few times has been thought about with ours, although not very well! We're currently:

    15.5Kg - Drive System
    28Kg - Weapon
    44.5Kg -Armour
    10Kg - Electronics

    Although that it all CAD driven weights, we're thinking that it needs to go on a diet before we send the parts off to get cut. The idea way would be, of course, to mill pockets in the side plates, which could save us an easy 10Kg, but we're also trying to design it with as little milling as possible. Currently, we have 10 channels to be milled, and that's looking like a few hours of machine time that we'll have to pay for, which drives the cost up by a huge amount.

    I think that the Etek motors still hold the domination crown! Although I think that Tombstone/Last Rites/Whatever he's calling it now goes through 4/5 of them a year.

    Here's a picture of where we stand at the moment. Just a rough thought, but close enough for illustration purposes. What's not shown are the batteries or gears, because I can't render them without the laptop throwing a hissy fit. 10/50t on the drive motors, 12/72t on the weapon motors.



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    Last edited by emisnug; 09-25-2016 at 07:35 AM. Reason: Pics didn't work
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