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Aug 21, 2018, 10:07 AM
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Brushless Motor for Heavy Platform


Hey guys,

I am trying to make a remote control platform capable of ~200 lbs at 5-7 mph. I know wheel chair motors are a common choice but I'm having a hard time finding them reasonably priced now. I'm also not sure how standard the interface is and I'm hoping to keep things simple (ie Rx directly driving ESCs).

I am also aware of the Sabretooth controller for brushed motors but I am leaning towards brushless motors....partly because of the increased efficiency and lack of maintenance but mostly just because I'm more comfortable with the RX to ESC interface.

I've found a fair amount of "lower" KV brushless motors but low KV for brushless is usually still pretty high.
This one for example:
https://www.banggood.com/270KV-Heavy...-p-1251447.htm

270KV with a 3s LiPo and 6" tires is ~50 MPH without gearing!

Clearly I need some sort of gearing and that is where I get a bit stuck. I have no idea what would be appropriate or even how to incorporate it. Chain, belt, or direct??? How do I connect it to the wheel and motor. The motor side makes sense....I assume just match shaft size to the gear I select but I don't know how to couple it to the wheel (I have not selected a wheel yet either). Are there common standards for these things? Does anyone have a good resource either for learning or getting parts? Ultimately I am trying to do this as CHEAP as possible so that constraint complicates things too.

tl;dr I need help with a robust drive train and I'm cheap.

Thanks for any help!

Chad
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Aug 23, 2018, 12:48 PM
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You need geared brushless motor (ordinary gearbox with brushless motor) or "wheel motor" (big low KV brushless motor integrated into wheel). Last one are used on electric bikes and scooters. Although, last ones aren't very cheap and usually use 36V and bigger voltage batteries.
Oct 21, 2018, 11:43 PM
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chickenLittle's Avatar
If your main consideration is "cheap" then I would suggest you look for surplus
cordless drill motors. The 1/2" size. At least 18 volts. You'd need at least two and
maybe four. The nice thing about these is that they usually have integrated
planetary gears that put out about the speed you need to run a vehicle.

Use small sprockets and chain to drive the wheels.

These motors likely cost you at least $50 each.

And you need high current motor controllers. Fifty, sixty amps at least.
Depends on your setup.

Another say $200 here.

So you'll still be lucky to get away with less than $500 total!

For a bot this size I don't think "cheap" is in the cards. Try relatively
inexpensive. :-)
Dec 31, 2018, 08:24 AM
Ken's CAD Models
dz1sfb's Avatar
Gear or belt reduction is the key to turning watts into torque. Here are a few vital factors to know to work out the bits.

1. Anticipated Battery Voltage - 3s = 12.6 v fullly charged

2. Desired Speed - 7 mph

3. Motor Selection

4. Wheel Diameter - 6"

From these four factors you can derive the information you are looking for.

Also know the 746 watts = 1 hp.

I have been playing with brushless conversion of Razor E300 scooters, and started with a Turnigy G46 670 Kv brushless motor. I was running on 24v and ultimately had to gear down from the motor 3:1 to get the torque up. Then I changed the final drive ratio from the gearbox to the rear wheel to 7:1. This yielded moving my 195 lb butt plus the 46 lb scooter to a spirited 20 mph. However the toothed belt in the primary drive exploded from the torque. That is a 740 watt motor.

I would say that you could do with a much smaller motor. After all both motors on banggood listed at 270 Kv are in the neighborhood of 2+ hp x 2 = ~5 hp to the wheels.

Let's crunch some numbers to see what kind of gear reduction you would need.

6" dia X 3.1415926 = 18.8495556" circumference. I'll round down to 18.85"

270 Kv X 12.6v = 3402 rpm

18,85" circ. X 3402 rpm = 64127.7 inches per minute with direct drive.

7 mph x 5280 feet per mile = 36960 feet per hour X 60 = 2217600 = feet per minute X 12 = 26611200 inches per minute / 64127.7 inches per minute = 414.972:1 drive ratio.
Wow that is quite a reduction, but the torque would be phenomenal.

This web site has calculators for gear ratios, torque output, and pulley spacing. http://www.sdp-si.com/
They sell components as well.

Ken
Dec 31, 2018, 08:47 AM
Ken's CAD Models
dz1sfb's Avatar
Here's a bit lower cost motor with higher Kv.
https://www.banggood.com/N5065-320KV...r_warehouse=CN

Still your going to need escs and here is an adequate low cost one https://www.banggood.com/Favourite-F...r_warehouse=CN . However, it only controls in one direction.

I have used Botbitz brushless escs that are flashed for robot use, giving reversible motor control. They are $85 each https://botbitz.com.au/collections/e...eed-controller

So, no matter how you slice it motors and escs are looking to be $96-$226 or more. Add on top of that gear box components and you could go up another $400.

Here's where your scrounging and fabrication skills com into play. Going with smaller wheels can reduce how much gearbox you need.

Ken
Dec 31, 2018, 10:59 AM
Ken's CAD Models
dz1sfb's Avatar
Found chain and sprocket calculator here http://www.botlanta.org/converters/d.../sprocket.html

Ken
Sep 16, 2019, 03:15 PM
Registered User
mike_o's Avatar
Ken, that is some solid advice, you're dishing out. I do hope the OP is listening.

Just to support your idea of a smaller motor, let me chime in: I use a $40 HobbyKing NTM 4258mm to power my electric assist bicycle. On a flat surface it pushes me and the bike (at 240lbs) to 17-18mph. I use a 10mm wide 3mm pitch timing belt and a large 3D printed toothless rim-mounted "sprocket". The belt doesn't last forever, but there is no slippage until it's worn to laces.

Use a decent VESC-type speed controller like the SK8, though.

Some notes on my project here: https://endless-sphere.com/forums/vi...58727#p1458727
Sep 18, 2019, 02:40 PM
Ken's CAD Models
dz1sfb's Avatar
Quote:
Originally Posted by mike_o
Ken, that is some solid advice, you're dishing out. I do hope the OP is listening.

Just to support your idea of a smaller motor, let me chime in: I use a $40 HobbyKing NTM 4258mm to power my electric assist bicycle. On a flat surface it pushes me and the bike (at 240lbs) to 17-18mph. I use a 10mm wide 3mm pitch timing belt and a large 3D printed toothless rim-mounted "sprocket". The belt doesn't last forever, but there is no slippage until it's worn to laces.

Use a decent VESC-type speed controller like the SK8, though.

Some notes on my project here: https://endless-sphere.com/forums/vi...58727#p1458727
Thanks for the advice. I still have my Razor E300S not fully functional. I like the idea of that programmable VESC though. In the process of assembling a 6S6P 2170 Li-ion battery pack for power. Need a better setup for the throttle with a sensorless motor though.

Ken


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