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Fixing N360 guide slot wear.
#1
I consider this to be the most serious issue with the NuVinci N360, well with my kind of usage it is, 2 – 3k miles is not a long life. Can this be fixed and if so how?

Why is it happening?
Well I don’t think it’s a mismatch in metal hardness issue. If an axle slides over a surface and rotates in the opposite direction to that which it wants to roll the surface is going to wear no matter what. So why is the axel sliding? If a ball on an axle is going to pivot on its centre when guided by slots at each end, the ends of the axle must rotate in opposite directions. If the two ends are fixed to each other only one end will rotate in the right direction. Whichever end has the highest load will dictate the direction of rotation.

How could this be fixed?
One way to fix it is to isolate the two ends of the axle by putting a bearing on one end. The potential problem is that this friction is a necessary part of the functioning of the shifting mechanism. It may be that it holds the axles and avoids the need for a large resistance in the shifter to prevent auto shifting. Adding friction to the shifter would put extra strain on the cables.

It seems like too simple a fix. I feel there’s a reason Fallbrook do it the way they do and it’s probably because fixing it will require a complete redesign and a much more complicated mechanism.

What I’m considering doing.
The centre of the axle is 5mm and enlarged to 8 – 8.3mm on each end. Also on each end is a 9.7mm diameter washer and a 9mm OD O-ring. The ends need to retain the planets roller bearing so cannot be completely removed. A suitable bearing such as one with a 5mm ID and 8mm OD has a width of 2 – 2.5mm. The enlarged end of the axle is 4.5mm long giving sufficient room for such a bearing.

[Image: faoo.jpg]

Will such a small bearing take the loads?
A bearing of the required size has a load rating of 300 – 400N. A very rough estimate of the load on one of these bearings in a worst case scenario is about 300N. This assumes a power transfer of 1000W to a wheel rotating at 60rpm (26 inch wheel road speed 4-5 mph) and a traction ring diameter of 84.5mm. The bearing will be on the output side so this is why wheel torque is important. So it looks plausible, all I need is some kind of lathe to turn down the axle ends.
- Oran
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#2
My gues is that the balls in such a small bearing would get crushed fast with this kind of pressure... Maybe a simple changeable brass sleeve would be a better idea... It will take the wear and then, when it starts to act up again, you simply change the sleeve... My two cents Smile
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#3
That’s a rather good idea. Bronze bushings seem to be reasonably easy to find in the right size.

As long as the steel axle rotates against the bronze and the bronze doesn’t slide against the aluminium stator it should work. The bronze will wear out instead of the steel and the aluminium will be protected because the bronze will roll over it.

The question is will this happen or will the bronze slide on the aluminium? Will it be the difference in the coefficient of friction between steel - bronze and bronze – aluminium that determines it? Or will it simply be the smaller diameter surface that will slide?

I suppose if I can fit a bushing without damaging the ball axle it would be worth a try. I reckon that one end or maybe both of the axles are pushed on and riveted as the ball and roller bearing must have been assembled first.
- Oran
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#4
Hardened Aluminium is much harder than brass sleves wich are porous to let oil sip in. (brass sleeves bushing are not just plain brass... they are made like "Aero chocolate bars" on the microscopic scale Smile) I don't think it could possibly eat througs the groove and same goes with the axle.

I think that as you say, since the diameter of the axle is smaller the friction should be less than the one between the bushing and the aluminium groove. Plus, the surface of the axle is ultra hardened and polished so the friction is already less important than the faced aluminium slot.

As for the axle, I suspect that they are simply "press fitted" inside the ball. An hydraulic press for bearing disassembly would seperate both part easily...in theory!
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#5
Oh wait... the ball and axle are "solidary" aren't they? The ball does not spin around the axle?
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#6
The ball is on a needle bearing so the axle only rotates during shifting.

What about this bushing it’s made of bronze not brass but has all the right dimensions.
- Oran
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#7
Oh.. that material is Oilite! it's perfect! Look at the picture, it is super porous.. I always thought Oillite was brass... it seems like it is bronze!
We were talking about the same thing, I was just wrong about the name!
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#8
I’m getting to a point where I need to decide if I’m going to attempt to fix the guide slot wear issue. The hub I’m currently using is a bit worn and I have a brand new hub waiting to be used. I could modify the planets and there axles from an old hub and install them in the new hub with its unworn stator.

First however I want to say a bit more about the slot wear. I measured the extent of the damage and it is not as bad as I had initially thought because it’s concentrated at the underdrive end. Photos always show the worst because the underdrive end of the slot is on the outside of the stator. The following graph plots the amount of wear against the hubs ratio.

[Image: Statorslotwear1500milesforum_zps63ef1cb6.jpg]
The amount of wear in tenths of a millimetre is for employees of Fallbrook only at the moment.

The overall trend of the graph is what I would have expected considering that the lower the ratio the higher the torque. So the tendency towards less wear at the overdrive end makes sense. The graph is complicated by the fact that my power output and the percentage of time spent in a given ratio varies over the ratio range. The graph demonstrates that I spend more time and transmit more power in ratios below 1:1 then above. This has a lot to do with the hill climbing I do on my commute to and from work. At least 1/3rd of my journey time is spent climbing in or near full underdrive. The total annual height gain just from commuting is approx 25,000m (80,000ft).

So what role does shifting under load play in the wearing process? I think it has a significant contribution but not in the way I initially thought. Using the shifter to change the ratio under load is not the problem. The hub I measured had been ridden 650 miles with an almost seized hub interface making shifting under load almost impossible. The amount of wear had continued to increase since it was last checked at 900 miles.

The shifting under load that I think is the problem is that which the hub does naturally in response to changes in torque. Have a look at the topic “Inner gear ratios of the Nuvinci 360” for more about this. What this means is that with every pedal stroke the hubs ratio shifts a little bit and back again. The higher the power and torque is the larger the shift in ratio will be.

The conclusion from this is that it takes a lot of hill climbing in full underdrive with a high power output, low cadence and close to minimum sprocket ratio to wear out a hub. If you’re wondering why hubs powered by motors aren’t getting worn out even quicker I think my theory explains that as well. The steady torque from a motor shouldn’t make the hub continually shift and so won’t contribute much to the wearing process. It takes human power to do the damage. Looks like I need to learn how to smooth out my pedal stroke.
- Oran
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#9
It's been a while since I last posted, I've had a few woodworking projects outside my normal job that have kept me busy. At this time of year I give my bike an annual service, it gets striped down, bearings regreased and parts replaced. I thought now would be a good time to have a go at fixing the guide slot wear.
The plan is to take the planets from the hub I'm currently using, which is leaking and has a worn stator. I can modify them and transplant them in to a brand new hub. This way l will be starting with new traction fluid which will hopefully remain clean if the bushings work. I have ordered some bronze bushings to replace one end of each planet axle and they should arrive soon.
- Oran
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#10
Don't forget to take some pictures for us!
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