This week we met with a professor in the mechanical engineering department and asked him to review our gear hub design. Upon first look, he commented that our design was reminiscent of a manual automotive transmission. Automotive transmissions use a piece called a synchronizer (pictured below) to slide back and forth along an axle and engage different gears. Our design utilizes a similar concept. In our most recent prototype we decided to use 4 knob protrusions instead of the many teeth shown on the synchronizer. We learned this week that the teeth would actually be just as durable as the knobs, and would make the shifting much smoother. 

Our meeting also opened our minds to the possibility of utilizing automotive components in our design. These components are much more common and available than bike parts. In the upcoming week, we are going to evaluate the feasibility of using a dual shaft gear box, similar to a manual car transmission. Our goal in doing this is to reduce the cost of our hub by reducing the number of custom components. Meanwhile, we are continuing to search for and contact potential manufacturers.

In our previous oral report, we discussed the modification of the clutch/gear system to use clutch plates that engage laterally along the face of the gears instead of vertically. We have been woking to incorporate this design into our SolidWorks prototype and run a quantitative torque analysis.

In the meantime we have also begun brainstorming for our next prototype: the 3 gear system that adds a direct 1:1 drive. Our current plan is to use the same prototype as our 2 gear system but to add in a lever that moves the entire input hub closer to the output. This forces both the input hub and the output hub into connection via the ring gear and allows the sun and planetary gears to spin freely. The hub could then be moved back into the reverse drive position and the user will be able to shift between high and low gears as previously explained.

We were able to borrow a tool from a local bike mechanic that we thought would be able to remove the inner piece (mentioned in previous update). However, upon attempting to use the tool, we found that it was sized for a slightly smaller hub (picture below). As a result, we were still unable to completely disassemble the hub. We are in the process of contacting more local shops in order to find the right tool for our hub.

In the meantime, we have begun to model the hub using Creo Parametric 3D Modeling Software. Our hope is that by modeling the entire hub virtually, we will be able to virtually modify and test our designs without the need for heavy equipment and without damaging our current hub. Modeling the hub in Creo will be slow because we have to teach ourselves how to use the software along the way. However, we believe that this is an important investment of our time at this stage in the project.

Rowheels Project Update #1 Presentation Slides

Our final design needs to utilize existing commercial parts as much as possible. We decided to order and evaluate a Sturmey Archer 3-Speed Internal AW Hub in order to better understand how a gear hub functions. After taking apart and examining the hub, we plan to evaluate the feasibility of modifying the hub to reverse the first and third gears.

First, we connected the 3-speed shifter and cable to the clutch. We observed the mechanism of the clutch response to movement of the shifter. Next we tested each gear ratio by spinning the power input for one full rotation and observing the rotation of the output. This was to verify that the hub functioned properly before we began the process of reverse engineering.

Next we utilized the instruction manuals and online forums to take the hub apart. We removed the main shaft from the outer hub shell. The Sturmey-Archer hub utilizes a planetary gear concept. The ring gear and planetary gears are located within the hub, while the hub itself functions as the power output. The sun gear is attached to the main shaft that also functions to move the clutch. We were able to remove the main shaft and observed the mechanism of the clutch and driver. As the shaft is in the low gear, the position of the clutch keeps the prongs of the ring gear retracted so that it does not engage the outer shell. The driver powers the ring to produce a 1:3 low gear ratio. The shaft is then moved laterally to the 2nd gear “middle” position. The clutch shifts so that the prongs of the ring gear move outwards and engage the outer shell. The driver still powers the ring gear in this position, which in turn powers the outer shell at a 1:1 ratio. In third gear, the divots at the end of the clutch engage the knobs on the back wall which are attached to the planet carrier. This allows the driver to power the planet carrier with the ring gear still attached to the outer shell which results in a 3:1 high gear ratio.

One challenge we are facing is whether it is possible to completely disassemble the hub to give us complete access to the gear systems. As of now we have only been able to take out the sun gear, clutch, and driver. We have not been able to remove the planet gear carrier from the external hub. Sturmey-Archer recently re-designed their hub and made it slightly more complicated and difficult to disassemble. We are working to address this difficulty.