27 March 2011

Jack the hub stripper

Yesterday I stripped my first hub (a Sturmey Archer 3-gear with back-pedal brake) and put the photos of the parts I found inside in a flickr set.

Let me just show you the most interesting pictures here:  

On top of the first picture are all the parts I took out from the brake side of the gear (including the main shaft which comes out on this side) and on the bottom is just the drive side bearing cone with its nuts. I named all the parts using part names I found in technical descriptions of similar gear systems. I think my names make sense, but other description you might find might use different names for the parts.

both sides opened and main shaft taken out on brake side

Note especially the main shaft on the top left, with the planet carrier and its four planet wheels (two shown) and the sun wheel, which is hidden, but can be felt when turn the planet carrier on the main shaft, because the sun makes the planets turn. Planets, sun, and ring gear are permanently engaged with each other, no matter what drive speed is chosen.
As for the changing of speeds, you can see a little piece sitting in a slot of the main shaft. This piece is pulled by the actuating cable (thus, the speed-change lever) and pushed back by the spring shown at the bottom. The little piece then moves a larger piece which on the photo is shifted leftwards on the shaft. I call it the “clutch element” and it normally sits right on top of the little piece.

The second picture shows the remaining parts, which are the most interesting, because those parts take part in switching the gears.

ring element, drive side cover, and driver taken out

The part on the left with the two big pawls is the “ring element” on its inside is the ring gear (interior toothing) of the planetary set. The pawls on the outside engage with the hub shell to drive it, but in some gears the metal ring shown above the ring element will cover the pawls, so that the wheel can be driven by another part at a different speed.
The part in the middle screws into the hub shell and has teeth with which pawls can engage (I don't yet know which pawls tho). The part on the right is the “driver” because it carries the chain sprocket and brings the drive input into the hub. It has two pairs of smaller pawls (hard to see because of the grease, here's a close-up). Depending on gear, the driver will drive the ring element, the planet carrier, or the hub shell directly. I haven't figured out the details of this yet, but I think that the “clutch element” shown in the first picture is playing an important part in it. What's making things more complicated is that the driver also needs to actuate the back-pedal brake which I think is why the pawls on the driver are bi-directional.

On the output side, the hub shell will be driven by the planet carrier, the ring element, or the driver directly. I have heard that the brake element (first picture, top middle) is part of the transmission path in some gears, which I think explains why it also has a pair of free-wheel pawls on it.

I'll post more details about all this when I have figured it out better. 


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