Alrighty, here’s what we’ve come up with so far…
We wanted something that would:
-Minimze the amout of gears required (especially weird shaped, expensive ones)
-Have gear ratios: 1:1, 1:1.5, and 1:2.25, which on a 24" wheel would give equivalent wheel size of 24",36", and 54"
-Have simple, reliable shifting
-Minimize dead-zone between gears, i.e. no excessive travel by any gear-changing mechanism. This is also tied in to trying to make the device as compact as possible
-Be compatible with most unicycles
-Be mechanically robust, and minimize the chance of seizing or accidental gear changes
I should probably also mention that (for the purposes of our presentation) we’re envisioning this on a 24". That way, the cycle is small enough to do some street, large enough to do muni, and geared to make commuting easier.
Alright, on with the design thus far:
We tried a lot of designs that involved sliding a kick-shifter along the axle to shift, but we found that there were too many gears. The shifter would have a left, middle, and right position which would cause it to jut out from either of the cranks. We also found that it was difficult to find a good way to mesh gears by sliding them into different gear combinations, especially since several combinations were required, and the gears had to be spaced out enough to avoid binding with adjacent gears.
Eventually, we determined that the gear ratios that we wanted could be obtained by having a single planetary gear with two different diameters, driven by two sun gears which could be locked to the frame independently. The larger end of the planet drives the ring gear.
We found that we could get our desired gear ratios of 1:1, 1:1.5, and 1:2.25, by using the following gear diameters (where the smaller sun gear has been arbitrarily set to 1), which can be scaled as needed.
Sun_1 = 1
Planet_1 = 0.5
Sun_2 = 1.25
Planet_2 = 0.25
Ring = 2.0
Figure 1 (which is not labelled well!) is a scale drawing of the planetary hub that will give the desired gear ratios. Note that there is a single planetary gear with two diameters, that the sun gears can be locked independently, and that the larger end of the planet always drives the ring gear.
Figure 1: Planetary gear system drawn to scale for 1:1, 1:1.5, 1:2.25 gear ratios
The method of locking the sun gears is a little more tricky. We wanted it to be reliable, and to minimize dead-zones in between gears. Our design can be seen in Figure 2.
Figure 2: Gear changing mechanism
The axle is powered by the cranks, and is directly connected to the planet arm. Over the axle, there is a sleeve which is attached to the frame on the left side of the drawing. Over this sleeve, there is another sleeve which also does not rotate relative to the frame, but slides left/right over the first sleeve. This left and right sliding controls the gears. The outer slider has several holes it in which house ball bearings. The inner-side of the sun gears are curved inwards, as well as ridged along their circumferece to create ball-shaped “pockets” of low energy for the ball bearings to become trapped in. This stops motion of the sun gear, causing it to become locked relative to the frame.
To control the 1:1 ratio, the slider continues farther, and eventually pushes a portion of the planet arm which is free to slide along the pin. This meshes with the hub, and stops relative motion of the planet arm and ring gear, making the system 1:1. The sliding portion of the arm returns to its normal position via a spring or something similar.
Several of the parts involved are drawn in more detail in Figure 3.
Figure 3: Pictorial views of some of the parts involved in the mechanism
To test the feasibility of the design, I made some measurements of the hub of my Bedford 24" and drew a 1:1 scale drawing of the design, using gear sizes that would provide the required gear ratios. The smallest sun gear was chosen to be 3cm in diameter, which makes the ring gear 6cm in diameter. Figure 4 shows our design so far.
Figure 4: 1:1 scale drawing of planetary hub based on measurements of Bedord 24" unicycle hub
I sectioned the figure down the exact centre of the axle to show the cut-away view on the top half. The ball/slider is shown in the position relating to the highest gear. The ring gear is contacting the larger side of the planet gear. Everything that moves relative to the crank input is hatched down and to the right. The curved inner-surface of the sun gears can be seen, as well as both sliders. The one without hatching slides left/right. I need a mechanism to control the movement of this part, which is why I kept the left part of the diagram somewhat ambiguous (unfinished).
As a side note, with this design there is the potential to have a neutral gear for gliding. If the slider moves left one more time (shifting up from the highest gear), both suns are disengaged and are free to spin as they please. I’m not sure if that’s an idea worth developing, but it’s there.
So, among minor complications, the biggest thing we’re trying to figure out right now is how to control movement of the slider that determines the gear.
A typical bike cable could be used, which would allow gear control at the front of the seat or somewhere simliar. We’re also trying to figure out a ratcheting kick-lever mechanism. This would allow gears to be changed by steping on a small lever which sticks out between the frame and the wheel. Our most current design involves a a small lever sticking forwards to control upshifting, and a small lever sticking backwards to control downshifting. These would thread the sleeve forward a specific amount each time they are stepped on.
Anyways, that’s where we’re at for now.