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Gruber
HobbyMasters
M.L. Toys
KidsWheels
HobbyMasters M.L. Toys
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.
User avatar
By Hammer-fm
#147061
Got some video of this rig running on 36V. It's temporary -- the wife thinks it's too fast and I agree. I did not get any of the really good video -- drifting sideways at 10-11mph. Too dangerous in a truck like this, but a fun experiment.

F150 has hybrid plastic/rubber tires (will post in the Wheels section about this soon)

User avatar
By Sagacious715
#147313
Since you've got me interested in this now... :) How much of a pain was it to drill all the holes in the right places and line everything up correctly?

If I were crazy and pretended I had enough time I'd probably do this mod with two VEX 775Pro's /w 15T pinions, a hefty 36V ESC and build myself a 10S 18650 Li-Ion battery pack to squeeze in a few extra volts @ 42V. :)
User avatar
By Hammer-fm
#147316
It wasn't too bad, but there are definitely a few more steps involved than the usual motor swap :D. I've started in on a second one using 16T gears, in a 19T small-axle gearbox for the Mustang, and I'm taking more pictures of the process. The first one was a good learning experience -- this iteration is to try to make it more streamlined.

The nice thing about using an aluminum plate is that you can oversize the holes for the motor shaft and the motor screws by a small amount and then move the plate around until you get the spacing you want for the motor pinion vs. the plastic gear. The motor's snout is inserted into an exact fit hole in the alumnium, so it moves with the plate. Once you're happy with the alignment you tighten down the motor retaining screws (with washers under them to spread the load out). I then drilled additional holes through the gearbox and aluminum further away from the motors, and installed machine screws. This ensures the plate can't move with respect to the gearbox case, and it also stiffens up the gearbox case itself. You can see these in the picture that shows the back of the motors/gearbox -- they're the three nuts tightened up against the aluminum in a triangle pattern (one is also visible in the front-side view as the flat-head machine screw; the other two are underneath the gear). My main concern was ensuring the plate was fixed in place relative to that 2nd-gear shaft.

Later if you decide you want a different gear ratio, you can -- within limits -- just loosen up the screws and move the plate around, then retighten in the new position. The motor doesn't really create that much torque, so if you have a screw that is 1-2" from the origin of the torque (eg. center of motor), it doesn't need to exert a lot of force to prevent things from moving.

I'm hoping to get more done this weekend and should be able to post more of a step-by-step guide.

I'm really expecting the limit will be the interface between the shaft and the hub for one of these gears -- they're going to be spinning 3-4-5x the stock speeds and that should put some serious heat into them. So far I haven't had any indications of problems but I am still only maxing out at 12-ish mph on a 13.5" tire -- with the Mustang that gear's rotation speed will be 45% higher. Since your 2nd gear apparently lasted quite awhile running similar RPMs, maybe this isn't really a limiter. We'll see :).
User avatar
By Sagacious715
#147319
Awesome, a step by step guide would be great...would definitely lessen the unintended screws ups and trying to do it for the first time.
As for my 2nd gear(s), we haven't had a single problem with one yet.
Also.. I have to ask two questions. would you mind sharing your magical engineering spreadsheet?? :) and do you have a write up or anything on this homebrew motor controller of yours?
User avatar
By wired
#147320
Every once in a while somebody goes that new direction and takes things to a totally different level. I am happy to see another one of those moments here and now. Big thumbs up on several levels with this one. Thanks for sharing. I'm sure you have inspired others with this pioneering modification. Great...just great.
User avatar
By chart2006
#147323
Impressive! I'm using a 24V 1000W ESC with 2x 775s (16T pinion w/ steel 1st gear) in my son's F150 Raptor and so far not overly impressed with it for off-roading. I may have to try your "quad" 775 route if further modifications don't provide the torque I'm looking for.

As a side note something you may consider doing is what I did to the wheels - see the photos below. I'd just do the rear tires as I'm running into rubbing issues with the front. The tire rubs the body (not even the fender well) to the point where it completely stops the truck. This only happens though when turning on a hill. Unless you have a mod in mind to prevent it that's why I say just do the rear for now. I'm probably going to either widen the wheel base or raise the front an inch or two. The tire size is 16x8.00-7 and here's a link to the tires I bought https://www.amazon.com/gp/product/B06X9Y2YL3/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1. Sorry Amazon Cloud's Photo service doesn't allow me to embed photos so I can only share them using a link.

https://www.amazon.com/photos/share/lpHsfbLQYY3ayQ4pGY1mb3JKgChDm6FltpwsTufdROa

https://www.amazon.com/photos/share/mrQMw5WhO5RKIoYqcApAM3zslGmmX13nonaHJBoqj56

https://www.amazon.com/photos/share/FY14hgamGyt7KzRFRvUkjIfdEcewI9VCoHTIdXoT7Tj
User avatar
By Hammer-fm
#147333
I don't have anything written up on the homebrew controller yet. It's a modification of the "DIY ESC with no features" that was posted on the board a few years ago -- higher frequency (3khz), overkill MOSFETs (4xIRFB4110), simpler overcurrent protection. I do want to get it posted sometime soon, when I have more time for writeup :).

As for the calculator: The old version was very crude and not really external-friendly. I spent a few hours last night and this morning creating a better one. This also builds estimates of how much heat is generated in the motor when running, estimates peak power output, and attempts to model things like wiring resistance. It's not perfect but should provide some useful guidance.

I've posted it here:
https://drive.google.com/file/d/0B7sByZqUm8XxYTl2WHBKQUU5elE/view?usp=sharing

I recommend actually using Excel, though Google Sheets seems to handle most of it ok (charts are a bit messed up).

Hopefully a few people can try it out and provide feedback (especially on usability/directions; I'm an engineer and not a customer-facing one at that :lol: ).
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