Converting a hubmotor to a middrivemotor

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Converting a hubmotor to a middrivemotor

Postby crossbreak » Sun Nov 04, 2012 7:28 pm

Hi folks,

The idea is to use the axle of a geared hubmotor as output shaft. No one ever did post anything about this in the past ( I searched a lot), that's why I wanna show how it might work. I use a 24V 250W front driving MBG Goldenmotor for this mod (reviewed the 36V rear version here http://endless-sphere.com/forums/viewtopic.php?f=3&t=4892&p=654910#p652141 . The mod is ONLY possible with outrunner geared hubmotors.

This mod has some advantages over mounting a sprocket to the hub (rather than the axle) for using it as a middrive:

-the gear ratio of the hub gear is increased by one (original: 4:1, modded: 5:1, that's an increase in torque of 25%)
-the hub can be used to mount the motor unit to the frame (to get a light, rigid and NOT ugly looking mount)
-the heat produced by the stator can be conducted to the hub to increase cooling and so the continuous power (by a so called "thermal bridge" / heatbridge)
-possibility of using sprockets with less than 18 teeth (to speed up the motor and thus raise power and efficiency)

The aim is to achieve the double continuous output power of a regular mid-mounted geared hub motor (edit: I know now it's about triple without additional heatsink).

Warning1! The peak output power is NOT influenced by this conversion. If you could drive your Hub@1kW for a minute, that does not mean you can drive it with 3kW after conversion! But you can drive it with 1kW continuously without overheating it.

Warning2! You might think now that you could still double your output power by changing the plastic gears for metal ones, this is true, but high torque leads to low efficiency due saturation of the magnetic path in the motor! This is why I do not recommend using metal gears. I simply replace my plastic gears once in a year, they are cheap and run much more quiet.

Warning3! Up-Speeding your motor does raise output power, but it has limits. As a rule of thumb, 250Hz field-change-frequency should not be exceeded for motors with 0.5mm laminations, since eddy current loss raises a lot at higher rpm. This means you should not run your rotor faster than:
*1000rpm for the older MAC, 2000rpm for newer 0.33mm lamination motors, as well as the BMC hubmotor which are identically constructed
*2000rpm for the Bafang BPM
*1750rpm for the Bafang SWXB and SWXH
[/color]

EDIT: the conversion described below was abandoned, since I developed a much better one (read second post) this text below just shows how this idea was born. The first succesfull conversion like that was done on a used and very cheap Bafang SWXB front hub motor, but can be done with many others as well or even better

To achieve this,
1.the phase wires are cut and pulled out of the axle
2.the axle is cut through behind the planetary gear
3.the "new" output shaft is centered by a brass guide pin and a brass washer
4.the left side axle is shortened
5.the stator is screwed to the hub (by a so called "thermal bridge" / heatbridge)
6.the "new" output shaft gets another bearing on the output side of the hub (this is necessarily to not over stress the guide pin)

Today I worked 3 hours and got point one, two, three and four done. The remaining points will follow next sunday. I needed no special tools for the mod, except for a brass cartridge and a 15mm inner dia pipe from the hardware store. I used my lathe to get a better result for the output shaft. It could possibly also be done with an ordinary hand drill, a file and sand paper. Enough talk, here are some pics (graphic found on pg 22):
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Last edited by spinningmagnets on Mon Sep 08, 2014 9:39 am, edited 42 times in total. View post history.
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Re: Converting a hubmotor to a middrivemotor

Postby crossbreak » Sun Nov 04, 2012 7:28 pm

This Post will be updated frequently to summarize progress


This is the official conversion:


If one removes the key that fixes the stator on the axle, the axle can rotate and there is no need for any additional bearings. After you removed the key and screwed the stator to the hub-housing with an alloy-piece, the stator is fixed on the hub-housing, while the axle is free to rotate. The axle will turn in reverse direction after conversion.

remove_key.jpg
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Pic that shows the turning directions from page 14
Image

This alloy-piece is called heat-bridge or thermal bridge since it conducts the waste-heat from the stator to the hub, which leads to much increased cooling of the stator. This piece fixes the stator in the hub, prevents it from turning. The screw holes must be machined very concentric, so the stator can not offset from the center. There must be a center alignment, we use the screws for that. Since we ca not use a The pic below shows the stator of a bafang SWXB, which was relatively simple to convert, due it's simple design. This really is a very powerful motor, sadly it's one of the loadest hubmotors on the market, due it's oversized and durable gears. Still I can recommend it for people who want only 1kW. It's still more quiet than the RC middrives I have seen.

my first Heatbridge /Thermal Bridge:
Image
Image

The axle still slips on the stator bore, so there shall be some space between these two components. We can make this space by by polishing/filing-down the surface of the shaft, till the axle can rotate without friction. You can also drill the stator bore to a greater diameter, but make sure your heatbridge centers the stator perfectly before you do this.

After a first test assembly to see if it all moves right, the axle has to shimmed right to axially align both rotor and planet carrier. There can be friction and noise which are induced by bad alignment. If you do this conversion, you should buy some shims (12 and 15mm inn dia) for alignment. Sry I've got no pics of the shims.

You may now add an additional Heatsink:
This pic from page 10 show how a heatsink for the stator could look like. thx bob for the pic. there are more if you look at page 10. This way the available continuous output power must be more than 4 of times the original (ie the MAC can output about 1800Watts, a small 250W bafang can do almost 1000W continuously):

Image

This conversion also enables the option to use both ends of the shaft. This way it can be used as a Jackshaft without using a freewheel crank[/color] Explanation what this does can be found in the wiki: http://endless-sphere.com/w/index.php/E ... figuration .

spinningmagnets writes a step-by-step how-to on page 19: viewtopic.php?f=28&t=45245&start=450#p719603 (work in progress, almost complete)

Drive system with dual-freewheel Jackshaft -> much better drive-ability and no more need for crappy freewheel cranks
Wiki article: http://endless-sphere.com/w/index.php/E ... eel_Config:
Image
Image

Speeding up the Motor to get more continuous power

The less magnets a motor has and the thinner the laminations are, the faster we can spin it to gain a good efficiency an thus continous output power. power loss/ heat /inefficiency (=all the same) limits output power in the first place. On the other side, more poles means more torque for a motor of the same size.

Dues these facts, we can compare middrive motors (there is a much better and detailed list by miles i can't find ATM):
The Bafang BPM has less magnet poles (16) and can be speeded up further than the MAC (32poles, it has equal 0.5mm laminations, the BMC and upgrade MAC have 0.3mm). The BMC/upgrade MAC are a good choise for upspeeding, but these are more expensive. The Bafang BPM has a prospective advantage in Power/Weight ratio over the old MAC due this (edit: in fact they are equal), the BPM and the BMC should be about equal (edit: The upgrade MAC/BMC are supperior). Compared to RC motors like the CA120-70 and other low induction motors gain a even higher power/weight ratio - since they are available with 0.2mm thin laminations, just for comparison. BUT low induction motors have special requirements and often fry controllers, if not treated right (which is really hard as I had to experience). Still RC motors can be a good choice for a middrive if driven fast, which means a high reduction ratio.

Gearing a middrive: http://endless-sphere.com/w/index.php/G ... _mid_motor

List of people here who try this conversion:

in order of appearance
bob
with pics of his mounted heatsink here: viewtopic.php?f=28&t=45245&start=135#p672235

mr.electric
pics starting here: viewtopic.php?f=28&t=45245&start=135#p671774

studebiker
with some issues and useful explanation about disassembly here viewtopic.php?f=28&t=45245&start=210#p678511

whiplash
his new build thread: viewtopic.php?f=28&t=46568&p=681062#p680899

spinningmagnets
who shows us how it works using a MAC motor viewtopic.php?f=28&t=45245&start=450#p719603 (work in progress) and viewtopic.php?f=28&t=51603

waynebergman
http://www.endless-sphere.com/forums/vi ... 67#p730659


Sprocket Adapters
I used 18T 9-speed motor sprocket from an old cassette, it just got worn out very quick. IGH sprockets and chainwheels are much more durable and fit the purpose. Next step is an all left hand drive that uses custom sprocket adapter to make common freewheels and sprockets fit the flatened double-D 12mm shaft of the humbotor. I'll start to sell these at cost soon... should look like this in the end:
Image
Image
Image

Here you can get such an adapter: viewtopic.php?f=31&t=52572


Link to the geared Hubmotors wiki page: http://endless-sphere.com/w/index.php/E ... _Hubmotors


Steps you need for converting the SWXB, SWXH, SWXU and Goldenmotor MBG

1. glue the stator to the side cover with epoxy putty, spray a bit of WD40 on the stator carrier, so that the putty does not glue to it, but takes a perfect copy of the shape.
Image

2. dirll holes for 6 holes with a 3.2mm drill. Then M4 cut threads
Image
Image

ONLY IF everything fits fine, proceed:
3. Remove the key, polish/enlarge the stator holder center hole using a file
download/file.php?id=101495

4. Drill a hole for the cables, pull them through the hole
Image

5. screw the stator holder to the side cover, put in the shaft and rotor, connect a controller+battery+throttle, let it spin....check if anything rubs
[sorry no pic]


6. assemble the hole unit, test again if if anything rubs...let it run for a few minutes and check nothing gets hot due rubbing.

7. Make a mount for your bike, drill holes into the hub housing to fasten it on your mount.
Last edited by crossbreak on Tue Jan 27, 2015 7:58 am, edited 71 times in total. View post history.
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Re: Converting a hubmotor to a middrivemotor

Postby spinningmagnets » Sun Nov 04, 2012 9:11 pm

Brilliant! I've thought in the past (based on other builders inputs) that the outrunner inside the geared hubs was a great size for BB-drive. I haven't had the time or money to play with one, and based on the pics you've posted, you are just the builder for the job!

One of the reasons this is a great idea is that the cell_man MAC geared hubs can be had in 5 different Kv's.

edit: heres the ES list of geared hubs viewtopic.php?f=3&t=4892

MAC
8FUN Bafang
BMC
Fusin
Tongxin
Ananda/Cute Q128
MXUS
eZee
Golden Motor MBG36
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Re: Converting a hubmotor to a middrivemotor

Postby crossbreak » Sun Nov 04, 2012 9:25 pm

spinningmagnets wrote:I haven't had the time or money to play with one, and based on the pics you've posted, you are just the builder for the job!

One of the reasons this is a great idea is that the cell_man MAC geared hubs can be had in 5 different kV's.


Thx for your support. This mod is somehow self suggesting if one got a hubmotor in his hands. It's just the how which is hard to think of. I got this motor for a hundred bucks shipped and just saw that I could have gotten a used bafang 250W motor for half of that :? so yes the fear of wasting these hundred is still in the back of my mind, so I wrote myself a howto to prevent errors ;)
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Re: Converting a hubmotor to a middrivemotor

Postby crossbreak » Sun Nov 04, 2012 10:05 pm

the brass guide pin is 3mm in dia ATM. I know that fiction losses can be significant, so i kept is small. I wanna figure out if this guide pin is a weak link. I still have a hubmotor on my bike, so if this pin breaks i dont have to pedal all the way home :mrgreen:
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Re: Converting a hubmotor to a middrivemotor

Postby Cyclebutt » Thu Nov 08, 2012 10:48 am

Interesting, will follow for sure.
Though the river tells no lies, many do not trust it.
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Re: Converting a hubmotor to a middrivemotor

Postby John in CR » Thu Nov 08, 2012 6:29 pm

Here's a Fusin gear hub I did similar, except I retained the fixed axle for easy swingarm mounting. I used a piece of pipe as my output shaft that fits to the sun gear. It's that long so it extends out of the housing which is now stationary. Output shaft is supported by bearing on the end. The spinning rotor which I affixed some blades, which turns the unit into a centrifugal fan with input at the wire side straight at the exposed stator and windings.

Yes, 2x rated power is a good estimate if you get fresh air in. I elected to dump the planetaries in favor of a #25 chain. It works fine, but project set aside for high power interests. Geared hubbies still have the problematic issue of thick and/or cheap steel laminations due to the pursuit of cheapest solution instead of a better motor. I'm working with a factory to end that though with a proper geared hubbie leaving mid-drives the solution only if you're going airborne or require multiple speeds. A high efficiency motor in a geared hubbie with proper heat dissipation is long overdue, and I'm sure the motorcycle companies will come out with a high powered version (maybe even with 2 speeds) in the not too distant future....Come on batteries, it's time to deliver what was promised in '08, '09, and '10.

Fusin motor rotor side and new shaft.JPG
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Re: Converting a hubmotor to a middrivemotor

Postby amberwolf » Thu Nov 08, 2012 7:35 pm

This is interesting....

Should it be even easier for the TongXin motors, which have bolt-on axles?
viewtopic.php?p=511519#p511519
where I began the pondering fo using it for a middrive, and you (Crossbreak) posted some info and advice and such there, too.

Another option I will have to ponder. :)
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Re: Converting a hubmotor to a middrivemotor

Postby crossbreak » Thu Nov 08, 2012 7:44 pm

Thanks John, this is something what I've searched for but didn't find. You did not use the planetary reduction anymore I assume. That's sad, since it's made so well, except for the weak plastic gears, but these can be replaced.

I'm not willing to charge the hub with air. This way dust could get in (which is what killed my tongxin drive) and it would still be poorly cooled since the stator lacks of cooling surface. What I want is to thermaly connect the hub to the stator by 3 alloy sheet pieces with thermal grease. I did not d any real calcs yet, but from estimation I can tell that heat dissipation should be increased by 3-4 by this.

The stator laminations don't look that thick. I assume 0.6-0.8mm. Could be well enough for 1800rpm i hope.

About batteries: We arrived at 270wh per kilograms this year. Compared to my cheap lipos, which get about 125wh/kg, this is more than double. There is an increase of almost 100% within the past 5 years. What should we bother about?

I'm waiting keen for 2-speed hubs. I guess it will take some years till they come to market.
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Re: Converting a hubmotor to a middrivemotor

Postby crossbreak » Thu Nov 08, 2012 7:48 pm

amberwolf wrote:This is interesting....

Should it be even easier for the TongXin motors, which have bolt-on axles?
http://www.endless-sphere.com/forums/vi ... 19#p511519
where I began the pondering fo using it for a middrive, and you (Crossbreak) posted some info and advice and such there, too.

Another option I will have to ponder. :)


I'm sorry but I abandoned my tongxin. Compared to the bafangs it just sucks IMO, both efficiency and power. Of course a tongxin can be converted much easier, I did that last year and you already posted into my old thread ;) (just look at my signature :) The bafangs can be almost silent, too - if they are driven by the right controller.
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Re: Converting a hubmotor to a middrivemotor

Postby amberwolf » Thu Nov 08, 2012 7:53 pm

crossbreak wrote: Compared to the bafangs it just sucks IMO, both efficiency and power. Of course a tongxin can be converted much easier, I did that last year and you already posted into my old thread ;)

You're right, and I am just braindead. I would use my recent lumpy forehead incident as an excuse, but really it's just a faulty brain. :oops:

What did you do with the TongXin?
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Re: Converting a hubmotor to a middrivemotor

Postby crossbreak » Thu Nov 08, 2012 7:58 pm

nothing, I repaired it once. It broke again within 100km, obviously I did not get all of the dust out. Now it just lays around. I'd give it away if this was not the first working drive I ever made ;)

It still works, it's just load as hell :( :(
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Re: Converting a hubmotor to a middrivemotor

Postby crossbreak » Thu Nov 08, 2012 8:21 pm

a teaser what it could look like in my test build, fits into the triangle just perfect:

Edit: This triangle mount is not very stiff. It is much stiffer to mount the motor in front of the BB
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Re: Converting a hubmotor to a middrivemotor

Postby Whiplash » Thu Nov 08, 2012 9:12 pm

Wow! Cool idea! I bet I could do the same with a MAC!! Double the power would be awesome! I could pull an easy 3K watts!!
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Re: Converting a hubmotor to a middrivemotor

Postby crossbreak » Thu Nov 08, 2012 10:34 pm

Whiplash, I already thought you'd never show up ;) I test this on a cheap motor, not going to ruin $300 before I tried it :mrgreen:

I bet I could do the same with a MAC!!


feel explicitly asked to try!
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Re: Converting a hubmotor to a middrivemotor

Postby Whiplash » Thu Nov 08, 2012 10:40 pm

Haha, I'll put it on the LIST.. ;-)
Power is a fascinating thing, the more you have, the more you want, but the real power is having the restraint not to use it all at once...............Um...Yeah..

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Re: Converting a hubmotor to a middrivemotor

Postby Whiplash » Thu Nov 08, 2012 10:47 pm

Couldn't you simply pull the wires from the axle out the side cover and trim off the axles? You could leave one longer and the back side cut flush but just let it spin NO?
Power is a fascinating thing, the more you have, the more you want, but the real power is having the restraint not to use it all at once...............Um...Yeah..

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Re: Converting a hubmotor to a middrivemotor

Postby crossbreak » Thu Nov 08, 2012 10:55 pm

No, since the stator is fastened on the axle by a key. An option would be to get the key out and do it just as you said. I could not get out the key without probable damage to the stator.

One would have to add a proper bearing between the axle and stator. Guess this is much more work than just cutting the axle and inserting the guide pin. The "outside bearing mod" which i'll do on sunday will be even stiffer and almost wont let the output shaft flex at all.

Of course the stator has to be fixed to the hub, otherwise it would turn. the pictures I will post on sunday will make this clearly visible.
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Re: Converting a hubmotor to a middrivemotor

Postby Whiplash » Thu Nov 08, 2012 10:59 pm

Oh I see, it would spin too and rip out the wires right? So how ARE you keeping the stator from spinning then? Is THAT the reason for the split shaft? I think I get it now. Just don't have it in hand right now to study it..
Power is a fascinating thing, the more you have, the more you want, but the real power is having the restraint not to use it all at once...............Um...Yeah..

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Re: Converting a hubmotor to a middrivemotor

Postby crossbreak » Thu Nov 08, 2012 11:02 pm

I will screw the stator t the hub. This way it wont spin and it can transfer its heat to it, which is most of the trick to push these motors to 2kw+
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Re: Converting a hubmotor to a middrivemotor

Postby amberwolf » Thu Nov 08, 2012 11:12 pm

Whiplash wrote:Oh I see, it would spin too and rip out the wires right? So how ARE you keeping the stator from spinning then? Is THAT the reason for the split shaft?


However it is done, you have to do something to mount the stator so it doesn't spin, if for no other reason than you have to have something to react against to get the torque from the motor into the output shaft. (as well as not twisting up the wires, though that could be done by using slip-rings).
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Re: Converting a hubmotor to a middrivemotor

Postby crossbreak » Thu Nov 08, 2012 11:46 pm

amberwolf wrote: However it is done, you have to do something to mount the stator so it doesn't spin, if for no other reason than you have to have something to react against to get the torque from the motor into the output shaft. (as well as not twisting up the wires, though that could be done by using slip-rings).


1/5th of the torque is taken by the added stator screws, the other 4/5th of the torque are taken by the ring gear. hope this helps.

BTW, spinningmagnets, thanks for this link: http://www.emissions-free.com/catalog/i1.html
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Re: Converting a hubmotor to a middrivemotor

Postby k-harvey » Fri Nov 09, 2012 2:46 am

Shikes youre a better man than me . I 'll have a go at most mechanical jobs but that would be too much . Amps etc can baffel me I make up batteries with BMS etc help others out who get caught out, but sure can get too much depth for this poltyfilla type , thats me----Great job you did ---shows that hub drive is flat towns bike.I had & sold a non geared hub drive for a loss but to a flat town man, always thought if I had it as a geared I might have hung onto it.
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Re: Converting a hubmotor to a middrivemotor

Postby crossbreak » Fri Nov 09, 2012 7:25 pm

I try to find out what gear ratio the BMC/MAC/PUMA motors have. I found out that the planet gears have 54teeth. Does someone have a pic of such a sungear? I'd say from the pic's I found that it should have about 30 or 32teeth, but I'm not sure. Can someone help?
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Re: Converting a hubmotor to a middrivemotor

Postby spinningmagnets » Fri Nov 09, 2012 8:08 pm

The emmissions-free website listed them as 5:1 but that may not be exact. Here are some internal pics of the MAC viewtopic.php?f=4&t=22103

Here is a video showing some of the internals, he mentions that the MAC and the BMC planets/clutch are interchangable, so the tooth-counts should be the same for both. viewtopic.php?f=2&t=38710&start=15
Last edited by spinningmagnets on Fri Nov 09, 2012 8:09 pm, edited 2 times in total. View post history.
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