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Gruber
KidsWheels
Need new motors? Grind a gearbox? Adding teeth to a pinion?
User avatar
By Hammer-fm
#146403
Hi all,

I'm new on the boards but have been messing with our two power wheels for the last 6 months and found the community posts very helpful. I thought I'd share a unique modification that people may find interesting -- using dual motors in each gearbox.

It spawned out of the failure of a 1-week-only 15T gearbox that I'd put in my son's Hurricane. The vehicle was using a 24V battery and 3 step-down (DC/DC) converters to provide variable speed (6-20V) and current limit (40A max). Following a fairly long session of continuous driving (me chasing the boys firing a nerf gun at them), we heard some unusual noises and parked the vehicle. We found that the motors were too hot to touch (but still functional) but we had spun the 1st gear pinion shaft in the gearbox case.

"Stupid 1st gear" was my first reaction. Part of the problem here is that once the motor heats up it transfers a lot of that heat into the Nylon gear and either strips the teeth or overheat the shaft:gear interface.

Instead of buying a steel 1st gear, I thought I'd do some calculations on whether it was practical to just remove it. The 2nd gear is same pitch but has a wider face (6mm vs. 4mm). The reduction provided by the 1st gear is about 2.88:1 (72 tooth input, 25 tooth output). What if I used a 11T pinion on a 7-series motor and drove the 2nd gear directly? Since I'm an engineer, I put together a spreadsheet to analyze a few possibilities :D

For the Hurricane in its current state this would have been a disaster as the 40A current limit would cut the available torque at the wheels by too much, so I decided I'd try it on the still-stock-wired F150 that we had a round (the Hurricane ended up getting a Mabuch 8514 w/16T pinion and has been happy since @ 22V). I found a 7-series motor that pulled about the same current as stock but provided 33% more torque. Then I thought: Could I fit two of those in each gearbox?

The answer is yes! I drew up a template in a CAD program and drilled out the appropriate holes in some 1/8" alumnium sheet:
CAD template:
Image
Backside view (motors wired in series):
Image
Front view showing both pinions:
Image


The Nichibo motor I'm using has a longer shaft than the Mabuchi motors and so the extra offset wasn't a problem. The motors are wired in series and so the vehicle runs on 24V but pulls about the same current as stock 12V. Running the motors in series means that each one sees the same current and should deliver the same torque to the gear. Since they're both driving the same gear there's no load inbalance issues. eg -- it's not like the classic wheel-spinning open-diff behavior you get from the two sides on a stock power wheels in 1st gear.


Also, since the power and torque is distributed between two motors, the driven gear (2nd gear) doesn't have a lot of tooth load -- it's actually not much harder on those teeth than the stock setup would be. The 2nd-to-3rd interface obviously has a lot more torque on it, and I expect the gears downstream to be the ones breaking.

The spreadsheet projection was that the end result at 24V would be about 2x the speed of the stock setup, with 50-60% more torque. The speed matches up pretty well -- the F150 has smaller tires and the spreadsheet suggests that it should be the same as the Hurricane (16T/Mabuchi 8514 @ 22V) and they're even in a straight line. I don't have a way to really measure the torque but there's certainly no lack of it. The tires have road-bike tire rubber on them and it will spin those at launch and leave 2-3' long black marks on the pavement. There's no comparing the holeshot on the F150 with this vs. the Hurricane with the current limiter.

I've been running this for a bit over a month (probably 10hrs of actual drive time, mostly on sloped pavement) and had one gear failure: A tooth lost on a final drive gear. The "interface" gear -- 2nd gear -- so far looks good on both sides. I told me son it was ok to start it in 2nd rather than babying it, so we haven't been taking it easy on it 8-)

I haven't tried running 36V into it, but the motors should hold up fine. I would be concerned about the 3rd/4th gears in the box breaking; the fact that it's already spinning the tires may mean that I'm already hitting maximum stress though. When I get another spare box I may try this :).

This could be done using two 5-series motors as well. Also, if I were doing it again I would have used a 14 or 15T pinion. The 11T is more reduction than is necessary given the torque of the motors. If you were using an ESC with limited current (say, 60A), you're already limiting the maximum torque and may want a smaller reduction.

BTW, if you look closely you can see that I'd previously tried a single-motor variant of this (there's a hole in the gearbox between the two motors). I had attempted to use a single Banebots 24V motor (Mabuchi 5033). That approach worked but didn't have any of the advantages of dual motors: higher torque, better thermals, etc. Alas, I didn't provide proper venting on the face of the motor and it overheated at 36V.

Comparison of stock motor @12V vs. 1st-gear delete scenarios of Banebots 24V (Mabuchi 5033) vs. Dual Nichibo 775-7013F
Image

[Edited 7/13 to clean up picture links]
Last edited by Hammer-fm on Thu Jul 13, 2017 11:50 pm, edited 1 time in total.

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