Help finding a solution to Bafang gear stripping

[docnjoj]

Thanks for your efforts, JohninCR! I do appreciate the road testing, and at least we will see if the rest of the motor holds up!
otherDoc

 

[John in CR]

I just opened mine back up to double check the wiring before mounting it on the bike. I've reharnessed a hubbie before, and everything is so tight and compact in the Bafang that there's no way I want to rewire this one. I'll be adding some kind of extra protection where the wires exit the axle to help ensure everything inward from a few inches out can't get damaged.

John

 

[Knuckles]

I have bad news and good news.

Bad News: I pushed the Bafang (w/ metal gear NO BEARING) hard at 72V and 25A. Running great until motor seized. Upon inspection the metal gear (NO BEARING) had seized on the post.

Good News: No damage at all to any of the gears. It was just a lack of proper bearing in the metal gear. Still have the metal gear (pried it off) and the post is still in good shape.

But I will not use it again until I can get a bearing for the metal gear (expensive for me in NY I am sure).

I’ll put the third nylon gear back in and put this motor on another 26” build limited to 36V and maybe 20 amps. But no more than that for me for now until the Bafang can get a proper metal gear w/ bearing. Such a shame too. Bafang has nice performance at 72V.

But at least this test shows me how well a single metal gear “protects” against the destruction of the nylon gears.

btw ... KHK has a Hong Kong Factory ... http://www.yuenfatgears.com/index_e.htm

 

[Nogwin]

I now have a few hundred more miles on the Bafang since the last update. I think it's around 2200 km and spent this week running at 80V. I am very careful to pedal first and keep the watts under 1000W during a hard start (typical is around 600W), then settle in for around 300-450W continuous.
Managed 34mph yesterday on the flats WOT for 5 miles, and was a blast. I was pedaling pretty good at the time as well.

I'll settle for a more reasonable 66V for now, but 80V sure feels good. I think like all things, the gears will break at some point, but I will still continue to be light on the throttle.

A friend who works at our machine shop at work will press in bearings for me. He said to provide him with the bearing and 1.25 gear. He and I will do this one weekend in the near future. I'm very concerned about the gears and feel like I'm the last one out there with plastic gears. Now I'm pretty worried


Nog

 

[voicecoils]

most of us don't have metal gear(s) yet.
You're the lucky oddball not getting any gear stripping at high voltage. Your pedaling habbits are the main factor saving you IMO. Does your commute have any hills?

 

[fechter]

The motor current has more to do with it than voltage. Most controllers will give the motor more current with a higher battery voltage, so I guess there is a connection.

If you ran a 20 amp limit, I bet the gears would never strip.

 

[voicecoils]

The motor current has more to do with it than voltage. Most controllers will give the motor more current with a higher battery voltage, so I guess there is a connection.

If you ran a 20 amp limit, I bet the gears would never strip.

I stripped my second set of gears at 80v and under 20A, at least I was not seeing any high spikes on the CA from the 28A PIC ecrazyman controller. Maybe the sample rate wasn't fast enough for me to catch it.

I only road a few km's so far at 80v and 9-10A limited on the CA. Haven't stripped them that way but the motor/gears sound very unhappy on starts or hills. The motor sounds best when run out and stabilised at WOT, even with the motor spinning on it's own without the gears, case or wheel attached. When riding at 80v10A I found that I rarely had enough flat long sections of road to get the motor into the WOT sweet spot. The gear sounds on starts or hills did not inspire confidence at this voltage, but maybe I've just become paranoid.

May do more testing at that voltage once I've got a spot for my batteries in the main triangle. Otherwise the handling is not fun with both of them (9.4kg total).

 

[Tiberius]

I stripped my second set of gears at 80v and under 20A,

If that was 20A at the battery, then it could be much more at the motor. High voltage has the effect of raising the top speed, but it will also allow higher motor currents at low speed. Ironically, going easy on the throttle may not be enough to save you, as the worst damage could happen at partial throttle settings.

Its part of the peculiarity of the way the controller/motor combination works. 80V / 20A in could translate to 20 V / 80 A out to the motor at low speed, partial throttle. And as Richard says, its the (motor) current that sets the torque.

What we really need is a new type of controller, or a new type of limiter.

Nick

 

[voicecoils]

Its part of the peculiarity of the way the controller/motor combination works. 80V / 20A in could translate to 20 V / 80 A out to the motor at low speed, partial throttle. And as Richard says, its the (motor) current that sets the torque.

Very interesting, thanks for that! My understanding of these things is developing but fairly slowly. I'm used to thinking of just power (and usually in kW and MW) and energy (kWh, MWh, TWh) but with EV's I'm learning that it's necessary to drill down farther.

20V / 80 A when amps = thrust is clearly a danger to the gears. What you've said about partial throttles agrees with my on-road experience. This I suppose is the benefit of gearing, that the motor can spin up faster to the max RPM's for a given voltage faster.

 

[Kurt]

Its part of the peculiarity of the way the controller/motor combination works. 80V / 20A in could translate to 20 V / 80 A out to the motor at low speed, partial throttle. And as Richard says, its the (motor) current that sets the torque

So in this example your saying that in some low throttle situations the controller will take The 80v with a limited current flow of 20A and cut the voltage output down to 20v but allow a current flow of 80A to the motor.

My understanding was that you use your max torque/Amp flow at start up from a stand still for a fraction of a second though until you have some momentum.Im sure once your moving if you current limit to the controller you can restrict the forces on your gear train.I remember when I setup current limiting through CA the first time I set it at 1A just to test it in my garage this was when I was running a dual geared motors.I think my 2yo daughter could have held the bike back. So the CA was restricting torque. :?
Kurt.

 

[docnjoj]

Mine stripped shortly after hitting a large bump that surprised me. Damage could have occured on reload of suspension on way down? It was on a downhill run, but when I reached level street, Skippy crunchy awaited me! Knuckles metal gear sounds good! What about drilling it out and trying oilite (cheaper that ball bearings)?
otherDoc

 

[Tiberius]

Its part of the peculiarity of the way the controller/motor combination works. 80V / 20A in could translate to 20 V / 80 A out to the motor at low speed, partial throttle. And as Richard says, its the (motor) current that sets the torque

So in this example your saying that in some low throttle situations the controller will take The 80v with a limited current flow of 20A and cut the voltage output down to 20v but allow a current flow of 80A to the motor.

That's the essence of it. The point is that limiting the battery current isn't the same as limiting the motor current.

To take the example further. Suppose the motor is turning at 1/4 of the max speed you get with 80 V. The back emf of the motor is 20 V. So power taken from the battery at 80 V is delivered to the motor at 20 V (The powers in and out of the controller have to balance, apart from minor losses). But you have set a 20 A limit, so the controller limits the throttle opening to the setting that gives 20A in, but that means 20 V / 80 A out.

OK, those numbers were plucked out of the air, and I expect the Bafang resistance is too high to put 80 A through it, but you see the point. High battery voltage and low motor speed are the worst combination, even with a battery current limit. In those conditions, you can get current multiplication, even at low throttle openings.

Nick

 

[fechter]

At very low throttle settings, the math starts to change as the controller goes into discontinuous operation. My measurements indicate you could get 3-4 times the battery current to the motor at max. It could get a bit higher with some motor/controller combinations.

The Kelly controller limits motor current as well as battery current. Most of the smaller controllers only work off battery current. A CA also works off battery current. It would be possible to make a current limiter that measures motor current, but it gets tricky for a brushless motor. It seems like it ought to be possible with a current sensing transformer like a clamp-on meter.

Of course, stronger gears would just be a whole lot better.

 

[Knuckles]

Caught Keywin in Hong Kong on Skype this morning.

I asked him to please contact KHK as they have a Hong Kong Factory ... http://www.yuenfatgears.com/index_e.htm
(Send emails to Keywin asking for metal gears for Bafang ... ecrazyman@gmail.com ... )
(The squeaky gear gets the oil! - Gotta Love China! )

Their website says they do make special gear orders in small quantities (and China MUST be cheaper than New York or Japan).

They already have the spec since KHK made the two metal gears I bought.
Just bore it out and press in a 608 bearing. Not rocket science.

-K

 

[TPA]

I am expecting tooling for the gears anytime, in the meantime I have 2 more questions.

first one is probably a no brainer

1. Would the majority of you rather have a mild steel gear or a 6061 aluminum gear. I balk at anything harder than 1018 steel since the gear hob is only tool steel and is a little expensive (would wear out quickly on say chromoly).

2. There are 3 types of bearings (sealed, shielded on 2 sides, and shielded on 1 side). The picture that viocecoils posted is shielded on one side. I'm leaning toward getting that same kind to assure it all assembles in the motor correctly, unless someone can confirm that a double shielded bearing will fit without issue.

I anticipate some free time in the next week to work on these and expect a complete drop in replacement (gear with bearing installed) to cost around $10 to those who want them.

If I can get them made soon I will send 3 to Knuckles to torture test and report on them.

 

[lawsonuw]

I'd skip the aluminum. It's just not well suited to applications with high pressure sliding. Alloy steels are generally quite soft annealed (even 4140 aka "cromoly") and machine as well as 1018 steel. People also make steel alloys specifically for machinability. (say a 1118, which machines like butter) My two cents on material is to ask your machinist what his favorite "free machining" alloys are, then pick one with 30-40 points of carbon. The extra carbon will make the gears respond well to heat and surface treatments for those really crazy power nuts and give the gears a good initial strength.

Marty

P.S. http://www.matweb.com/ and a particular manufacturer web site is a good place to get background info on a material. Past that it's best to talk to someone.

 

[TPA]

I'm my machinist. Yes 4140 cuts well, but I'm having to buy the HSS tool myself, and I would like to get a lot of parts out of it.

If the majority here request 4140, I'll use that. It is pretty versatile and can be heat treated to be very hard. However 4140 is prone to a lot of internal stresses that can ruin the gear when heat treated, so I'll leave that part to whoever thinks they need to do it.

Edit: Also 4140 forms a surface "crust" when heat treated, so the heat treating would have to be done before machining, or the crust could be polished off, if one was very careful about it.

 

[johnrobholmes]

I would prefer a softer gear to save the other gears in the hub.


If you can get them done quick I would surely be up for a set or three.

 

[Knuckles]

This is the spec for the "test" metal gear I used ... http://www.qtcgears.com/KHK/newgears/KHK074.html

The SSAY1.25-28


Cool 3D viewer of the gear here ... http://catalog.catalogds.com/catalog/view.html?ms=http%3A//ms4.catalogds.com/ModSvr/makepart.asp%3Fname%3Dssay.prt%26manid%3DQTC%26filename%3Dkssay1_25_28%26instance%3DSSAY1_25-28%26output%3DPView%26paramcount%3D0


I have been checking for this gear like crazy. TPA you are THE MAN! :wink:
Double check you cost but I will start with 12 gears. I can also send you the metal one I have and the original Nylon gear.

btw ... This is the bearing here ... http://www.vxb.com/page/bearings/PROD/Scooter/608ZZ10

-Knuckles

 

[TPA]

Knuckles:

I'm ordering those bearings.

Also: What metal would you prefer the gear be made out of?

 

[johnrobholmes]

Softer steel. You are the machinist, I would trust your judgment on what would be soft enough to keep most wear on the planetaries but hard enough to keep from mushing.

 

[TPA]

Problem is, I don't have a bafang motor. I have no idea what it is made of. My best guess would be just plain mild 1018 steel to make the gears out of as long as the sun and ring gear of the motor are steel. If the motor is Chinese it is extremely unlikely that it is harder material than 4140, and so 4140 gears would wear the motor components instead of the other way around.

If wear on 1018 occurred, is should be slow enough to get plenty of use before replacement was required, and less likely to wear the motor gears.

Still waiting on tooling.......

 

[johnrobholmes]

Internals look like mild steel to me, but I don't know how to properly verify.

 

[TPA]

OK then we will start with 1018 mild steel for gears according to Knuckles specs and work from there. Looks like it will involve trial and error, hopefully without a lot of error.

I suspect the 1018 will work fine.

 

[Knuckles]

Knuckles:

I'm ordering those bearings.

Also: What metal would you prefer the gear be made out of?

TPA ...

What ever is easy for you.
Does 12L14 steel work for you? Just got a (very expensive) quote for that.

Or these specs maybe?


But any steel grade is far better than Nylon.
Heat treating is way overkill IMHO.
Heck ... It's only a 3kg motor!

PS ... I sent you a PM. You WILL have a Bafang soon enough! Laced and ready to go!
Gotta love TRADE within the sphere