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Bafang / 8fun nylon planet gear replacement

November 2012 (with some updates)

I recently had my first breakdown of a hub motor. After about 3 years and 7000km of mountain riding, the nylon gears failed in the first ebike motor I ever bought. My brother in law pulled over to the side while climbing a steep hill to let a car pass, and when he tried to start again the wheel just gave a nasty buzzing sound and a bit of shaking when throttled.

The motor is a Bafang SWXB geared front hub model, which are very easy to open – only 6 philips-head screws holding on the side plate.



Inside we found that the 3 nylon planetary gears (which take drive from the little sun gear on the motor out to the ring gear on the hub shell) had their teeth mushed off.



After some correspondence with sales at Greenbikekit.com, I was able to order some replacement gears. There were some challenges working out what part went in what type of motor, but eventually I took delivery of a small pile of spare nylon gears for both the SWXB, and the other model motors I have in stock and have sold.

The SWXB – an older model of Bafang – takes 28 tooth gears, 38mm outside diameter. Some more recent SWXK and SWXH take 36 tooth gears. All gears came with sealed ball bearings inside them and replacement circlips. I paid US$5 each plus shipping, which came to about $25 a set.

I have no resentment about the failure of these gears after this much work by the motor. I’m sure they would have lasted much longer riding in normal terrain – up here my motors spend a lot of time hauling up hills at around 10km/h, resulting in high strains and high temperatures in the motor. Replacing the nylon gears every few years is a minor maintenance task, biggest problem being having to get home up some big hills without assistance when they fail. I’m not tempted to install steel gears to get a longer life. I expect this would result in much faster wear of the steel sun and ring gears and thus would shorten the repairable life of the motor.


Replacing the gears on the SWXB was quick and easy – only a few minutes work once the wheel was off the bike. For people doing a lot of hill climbing on e-bikes, I think it would be worth keeping spare nylon gears in stock – minimal cost and great saving of time and trouble once they fail and leave your bike broken down.

Since ordering the gears from Greenbikekit.com, I see that they now stock them on their website - previously I couldn't find them listed anywhere.

April 2013 update


Since repairing my old SWXB I've opened a few other Bafang motors. SWXH - the Bafang/8fun 250w rear motor - comes in 2 different models, as well as different speeds for different diameter wheels.

SWXH old-style (threaded side plate)

The older type of SWXH opens by turning off the threaded side plate which has the threaded freewheel boss on it. Here it is:

Opening this requires a special tool which keys into the 3 holes around the freewheel boss and a lot of force. Like a freewheel, it has a normal RH thread which tightens as you pedal, so it comes off anticlockwise.
I made a tool for this job, fabricated from mild steel:


This was held tight in place using the wheel nut and a big square washer:

It was so tight that I put the tool in the vice and turned the wheel (like removing a freewheel/spin-on-cluster):

 Tools can be bought from Greenbikekit.com. 

This SWXH had 28t nylon gears. Another which looked the same externally had 36t nylon gears.

I have had trouble with one SWXH when I needed to replace the spin-on freewheel, but the whole side plate spun off instead. Removing the freewheel required me to remove the side plate, and use the side plate tool on the inside of the side plate with a bolt holding all together. This sort of trouble makes the bolt-on side plates look good. 

SWXH new style: side plate with philips-head screws

Newer SWXH motors I've bought are much easier to open. 6 philips head screws hold on the side plate with the freewheel boss:
This seems like a big improvement, making servicing much easier. We have broken some of these bolts however, on Jasper's bike which is worked very hard on long, very steep climbs which he likes to do off the seat. It's worth checking them for tightness occasionally - no fun easy-outing the broken bolts from the motor body.

These new SWXH have 36t nylon gears (as far as I've seen so far). 

The other side of both SWXH motors look the same:


34 comments:

  1. I'm about to buy spare gears just like you, mine are 36 tooth thou.

    The thing is that I don't know the model of the Bafang motor.
    Are you positive that the spare wheels are the same (36 tooth) for SWXK & SWXH? What about the BPM model?

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    1. Dear Niklas,
      I'm not positive about the spare nylon gears - I've not actually changed them in a SWXK or SWXH, only the SWXB. However, I ordered spares for several different models of 8fun (there are different versions of both SWXH and SWXK), and only the 2 types of nylon gears were supplied. So I think H and K both take the same 36 tooth nylon gears.
      I have no advice to offer on BPM, as I have never used one. My expectation is that they would use a different planet gear again, as the hubs are bigger, and more powerful, so I would expect them to use bigger and stronger gears. There is a youtube video of someone opening a BPM, and the gears in that look big.
      Re identifying your motor, can you post some pictures? Have you opened the motor? The pictures above show SWXB, SWXK is shown on another of my pages. SWXH is a 250w rear motor with disc brake mount.
      Bruce.

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    2. Ok, now I'm getting really confused, you think that H & K take 36t because you ordered spares for them as well and got 36t, did I get you right?

      I've emailed Greenbikekit, and they don't really know what they are selling, because they usually just specify the model number when they order them from Bafang.

      So they forwarded my question to Bafang, and received this answer after three weeks:
      The latest news from bafang about the gears: the 28teeth gears are for all small 8fun motors(180W~250W), such as SWXH, SWXK, SWXK5, SWXU etc; the 36 teeth gears are for BPM, BPM2 and CST motors.

      I have a 250W, 36V, rear motor, without disc brake mounts. See pics: http://imgur.com/a/O2Hj1/

      The response from Bafang points out that I have a smaller motor to judge from the power specifications, but states that these use 28t which I obviously does not have :S

      I also asked Greenbikekit if the gears come with circlips, and they don't even know what those are.

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  2. Hi Niklas,

    It is difficult to get info from the Chinese suppliers. I often make purchases to find out the facts I'd prefer to get in writing....

    I've opened SWXK and SWXH and the nylon gears are 36t, OD 38mm.

    The motor pictures from your link look like it may be SWXB, as it looks very much like my SWXB front motor except it's rear. SWXH and K have disk brake holes. However with 36t nylon gears, perhaps it's some sort of intermediate model.

    I was sent new circlips with the nylon gear spares.

    So if your nylon gears look like they were 38mm OD, then the 36t gears I was sent for SWXK would likely fit.

    Bruce.

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  3. Update and correction to my previous post:
    Yesterday I opened an older SWXH with the threaded side plate (see update to page above). The nylon gears were 28t!

    So, I've found SWXH motors with both 28t and 36t, but so far the old style have 28t and the new style have 36t.

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  4. And now I've found an old style SWXH with 36t nylon gears. So it seems anything goes, but newer motors are more likely to be 36t.

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  5. Like your article! My issue is sourcing parts in America is like finding hens teeth. I'm looking for 33 tooth gears for a non-Bafang motor. GBK has these gears but trying to contact them through there online web site is problematic at best(no phone number). It's not like these gears are specially built as these are interchangeable through different manufactures. Come on these are hard working parts that need to have a source. Why not make them out of Delrin. At any rate there are millions of these bikes on the road WTF!

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  6. Pretty good post! What a wonderful idea! I have also seen the wheels at the store and they are quite pricey. Love your blog and all the ideas you share.


    Heavy Duty Casters With Brakes & Heavy Duty Casters Low Profile

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  7. Hello Bruce,

    Thank you very much for, amongst other interesting stuff, sharing your SWXB overhauling with us; great pictures too!
    But I have to disagree on one point with you: the motor I have, is a Bafang SWXB as well, probably slightly newer than yours (your s/n is BF807D11412, mine is BF1003D12464), but absolutely not "very easy to open". In fact, I cannot get it done. Yes, there are only 6 philips-head screws holding on the side plate, and having taken them out, I can now easily rotate the side plate, but there's no way I can lift it, open the motor, and replace the broken nylon planetary gears (as, judging by sounds and behaviour, I'm quite sure this is what's wrong with it).
    So rather than critisism of your work, this is a plea for help, as you know first hand what's inside the box. Could you think of anything preventing the side plate to be lifted, but still let it rotate? I have been looking for all sorts of clamps, rings and the like, but nothing...
    I hope very much you can get me on the track again. My only alternative would be buying a new motor, but at the moment the only thing harder to extract from under its cover, is money from my wallet...
    Thanks very much for any input you can provide!

    BTW. Surfing the net for clues I found this German supplier for Bafang gears -28, 36 and 42 teeth, in nylon or steel! To be found at: < http://www.ebike-solutions.de/de/Shop/Motoren/Motorersatzteile/Ersatzzahnraeder-fuer-Bafang-250W-Motoren >

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  8. Hi Rob,
    It sounds like you are very close to solving this problem.
    Firstly, check that you have removed all nuts and washers from the axle (sounds silly, but I've neglected this myself...).
    Then I suggest a gentle whack with a wooden mallet, or a piece of wood, on the end of the axle on the opposite side of the hub from the side plate, while holding the wheel. Screw the wheel nut onto the end of the axle first so you hit the nut instead of the axle itself, and remove the nut after the plate comes loose. Hold onto the axle on the side plate side with your non-mallet hand (sounds like you need 4 hands here), and it should come out into your hand with the hub contents - a little heavy.
    The whole guts of the hub, including axle, should come out into your hand with the side plate, leaving an empty hub shell.
    I recommend using the original nylon gears from GBK, as these are cheap, run quietly and won't wear the soft steel gears quickly. Make sure you count the teeth on the nylon gears before ordering. You can buy a whole freewheel including nylon gears, if you want to avoid wrestling with the circlips, at little additional cost.
    I hope this sorts things out easily. Let me know how you go.
    B.

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    1. Hi Bruce,
      Thanks very much for your prompt response, it's great that you are willing to spent time on other people's (silly) problems.
      In the end I got the motor open, pretty much the way you suggest: -whack it! (In the end I thought I had little to lose, so I applied the one thing that will always solve every problem: brute force ;-) ) It turned out both cover and axle where just stuck very, very tight on the bearings on both sides.
      Now that my motor is open, I regret not taking any pictures straightaway, as that would have shown the motors' inside as the most dirty heap of rust, squalorish grinded (ground?) bearingballs and indefinite particles anywhere to be found on the internet. It is a total miracle the bike has been running at all with this mess in its motor, and really, your motors' guts 'n' gears (as seen on the 3rd & 4th picture in your post) are very, very clean by comparison. Then again, your nylon wheels look just like mine: one broken, another with no metal ballbearing left at all, and all of them missing some teeth. I've spent a day removing the gears from the freewheel and meticulously cleaning all parts, and now the next obstacle hast presented itself: so far not even brute force has been able to remove the two remaining most inner parts of the nylon wheels' bearings that are stuck (with rust?) on the 8mm pins on the freewheel. I have a few days to figure out how to get rid of them before the new wheels I ordered will arrive, and tips are welcome!
      I ordered the wheels in Germany; the seller claims they are original too, and though the ones from GBK you advised are a bit cheaper, they are also one continent further away, what makes for longer waiting and much more postage. For now I go with three new nylon ones, no experimental tuning with metal gears yet, no filling the hub with oil; when I have an engine to spare, I'd like to try stuff like that, but for now I'll go no further than a drop of acidless lubricant here and there...
      Bruce, thanks again for your inspirational advice; I'll keep you posted on both progress and despair, as they may surface...

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  9. Great intel Bruce! Very many thanks for this, after 4 months and 1500 miles my 250W 8Fun rear hub suddenly lost a load of power and started making the nasty buzzing noise you described - I'm in the UK and not sure which size gear to buy for my model, so I got one of each from GBK, fingers crossed I'll be back to normal soon :)

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  10. Hello James, I hope the gears fix your problem. I'm a bit suspicious you may have an electric problem instead - if the motor is still running but has reduced power and is noisy. I recommend opening up the motor to see if it's crunched gears or blackened windings. I've had one motor that burnt when relatively new (like yours), apparently due to a manufacturing flaw, but mostly motors burn due to long slow climbs causing overheating (see my other posts).

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  11. Hi, first of all thank you very much for sharing these experiences and hints! This is exactly what makes the internet useful and valuable!

    I have a very simple and practical question: how do you take the screwed on freewheel off? I have a customer's bike in for service, with a Bafang rear hub and 8 speed threaded freewheel (not a cassette).
    With the electrical cable coming right out of the hub axle, and the multi-connector at the end of the cable, I can't figure out how to get my freewheel removal tool in there. Am I just plain stupid and lacking fantasy? Is there some special procedure? A special tool? (like a two part / split freewheel remover???)

    Thank you anyway, hoping for an answer.
    Pascal (Belgium, mechanics instructor in an educational second hand bike workshop)

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  12. Edit/update: I just saw a bike with apparently the same motor - but with a small diameter cilindrical connector on the cable that comes out of the axle. That one would be no problem changing the freewheel on... "mine" has got a fat (28x18mm) rectangular multiconnector.
    I thought I'd found the solution"... I was about to cut a slit in my freewheel remover to slide it over the cable and into the freewheel - one of my apprentices made me understand that the freewheel wouldn't fit over the connector!

    What is this b******t? Is this a first generation motor, where they launched the thing before thinking about future repairs? And then changed the connectors on the next batch, when they understood you can't throw away a wheel when only the cogs are worn?

    I guess I'm in for some cable cutting and soldering after the freewheel change :(
    Pascal (again)

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  13. Hello Pascal, thanks for sharing your challenge!
    Most of the Bafang rear motors I've bought have no connector in the power cable. This isn't great - it means disconnecting the cable from the controller when you need to change freewheels, change nuts or washers, or get the wheel right away from the bike for maintenance.
    I was very pleased when I bought some Xiongda motors this year that they included an in-line connector for rear hub motors, and would make the cables whatever length I chose. The in-line connectors are small - small enough to fit the axle nuts over them.
    Your situation with an oversize connector sounds difficult. It seems strange that the manufacturer would have installed the in-line connector after spinning on the freewheel, and after spinning on the axle nuts. Perhaps the plug can be dismantled?
    The other problem I've had removing freewheels is on motors with spin-on side plates. On these the circular side plate with the freewheel thread (like the first motor photos in my April 2013 update above) can unscrew off the hub instead of the freewheel unscrewing off. For this I've been able to use the 3-pronged tool (pictured above) on the inside of the side plate, with a large bolt to hold the tool in place. Sorry no photo.
    I wonder if this big plug of yours is a repair job - doesn't sound at all like something the Bafang factory would do.
    Good luck!

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  15. Hello Bruce. Thank you very much for sharing with us your experience and expertise on Bafang e-motors. I have just purchased a BMP 2 48 volts 500 MW motor mounted in a front wheel from BMS batteries to convert a home made LWB recumbent. Upon receiving the wheel I noticed that it is very hard to turn by hand (in the correct direction), thus it can hardly freewheel. BMS batteries told me this is how the motor is made. I am new to this but I know that that geared motors such as this one should freewheel. I suspect that somehow the axle is on too tight. From your experience dismantling the SWXH is there a way to adjust the axle tightness like on an ordinary hub ? Thanks in advance for your help.

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    1. Hello Romarin. You are right, these geared hub motors are made to freewheel forwards, including Bafang BPM. The most frequent problems, especially when new, are with the internal freewheel unit. Your problem sounds like the freewheel is jammed, which has happened to me several times. To get a better understanding of the freewheel, have a look at this page of mine:
      http://bruceteakle.blogspot.com.au/p/blog-page_16.html
      If the freewheel is jammed, you will feel and hear the gears whirring when you spin the motor forwards, the same as it is when you turn the motor backwards. Sometimes this is just a sticky freewheel roller and resolves itself in use, sometimes it is due to a broken roller which needs replacing (usually by replacing the whole freewheel unit).
      You could try giving the motor a few modest bumps while trying to roll it forwards, which might free a jammed roller. If this is unsuccessful, and you are unable to negotiate a resolution with your supplier, it would be easy and cheap to open the motor (the 6 screws on the RHS) and replace the freewheel unit. Then you have spare planet gears at least, and perhaps can get the freewheel going later.

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    2. Just to clarify: With a hub motor you can't adjust the axle tightness like an ordinary hub. The bearings are sealed. It is conceivable that you could have a problematic bearing, but this is very unlikely unless the motor is old and the main bearings have been damaged by weather and dirt.

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    3. Hello Bruce. Thank you very much for your prompt and detailed response. It is much appreciated. The wheel spins forward without engaging the gears, but is does not freewheel, it stops as soon as you remove force as if something was too tight. Bms battery, the supplier, is trying to convince me that the BPM motor are built like that. If the tightness cannot be adjusted I can only conclude there is a bad bearing somewhere, but it is brand new and for the life of me I cannot imagine how they could have built the motor into the wheel without realizing that there was a problem.

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    4. Hi Romarin, have you had any breakthru on this problem? There is a tradeoff between price and quality control on these motors! I encourage you to open the motor and see what you find. They are simple machines and have a limited number of places where the resistance could be created. I've never replaced a hub motor bearing yet. Perhaps the problem is as simple as a missing axle washer or a tight dust seal on a bearing.

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  17. How do you get the old nylon bearings off? We removed the clips and they won't come off the base. Is this supposed to happen?? Or is there a trick to it?

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  18. How do you get the old nylon bearings off? We removed the clips and they won't come off the base. Is this supposed to happen?? Or is there a trick to it?

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    1. I replaced the gears on my Bafang CST motor on Saturday, and I had the same problem. I removed the gear carrier from the hub (it just tapped out with a drift). I could then hold the gears in a vice, and tap the shaft out of the gear. They are a bit of an interference fit, and I could not get a gear puller into the confines off the hub. THe new gears just push on, a soft tap with a soft mallet overcoming any resistance.

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    2. Thanks Dave for your advice. I'm not actually sure how I pulled my nylon gears off, but I'm imaging I used something like a pair of chisels to lever under the gear (or gearless bearing) from each side. The freewheel plate is robust as are the gears, so this would be fine too.
      Dave: did your CST motor strip its gears? After how much use, and with what volts and amps? I'm curious about how long these big motors' gears will last. My BPM has done 7000km on its first set of gears, but is running on 36V and max 18A.

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  19. What type of grease should be used on my new nylon gear replacements from greenbikekit? I have a 250 watt, front mounted hub, if that matters. New geesrs are 28 teeth, a perfect match to the old ones. Appreciate your help in all this!!!

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  20. Also, I broke one of the circlips for the gears. Do you have a recommendation where I can get another one? I use this bike for my daily commute so I need it back to life ASAP.

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    1. Hello Naomi, congrats on getting so far in repairing your own hub! Re grease: I don't know what the best grease is. I've used grey molybdenum grease (I use for the car) because that's what the original grease looked like. I'm also unsure how much of a problem it would be if grease gets on the windings (I suspect no problem). I understand the nylon gears are to some extent their own lubricant, but the grease makes it even better.
      Re circlips: I expect the circlips are standard industrial parts, probably available from businesses that sell bearings. I suggest measuring the diameter of the groove bottom and phoning, or dropping in to a bearing supply shop with the freewheel unit. Also when I remove or install circlips I wrap a rag around the job in case the circlip slips and flies away (happens easily). Good luck!

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  21. Does anyone have a Bafang 750 watt/48 volt geared hub motor ? I was wondering how many teeth the 3 nylon gears have and what are their dimensions and if any company sells replacement nylon gears or metal gears .

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    1. Hello Dave, perhaps this is a Bafang BPM motor? See my page: https://bruceteakle.blogspot.com.au/p/blog-page_8.html
      Replacement nylon gears and freewheel + gears units are available from Greenbikekit.com, and presumably elsewhere. The later model Bafang hub gears such as the CST (with cassette freewheel) may have the same gears and freewheels but I'm not sure.

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  22. Hi
    does anyone know if the freewheel plates from GBK will be ok to fit 28t gears and be suitable for 2010 SWXB regards

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    1. I'm very sorry not to reply earlier, your question slipped past me. I can't promise that the freewheel plates from SWXB are the same as stocked at GBK, but my guess is that they are the same. I've sold my old SWXB motor so I can't check. Is your freewheel broken?

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