In our design of mechanical elements class, groups of four were tasked to design a gear box, taking into account the failure criterion of each component. In this class, we learned how to calculate safety factors, determine factors for infinite life of components. We mostly used Von Mises stress in these calculations. Our textbook was Machine Design by Norton.

This gearbox is intended for use in a run-of-the-river hydroelectric generator, where you take the relatively high torque of water turning a water turbine, and translate it into a low torque but high speed shaft using the gearbox. This high speed shaft generates electricity by running magnets past coils of wire, making use of Lenz’s law. This process of generating electricity is encapsulated in machines called “dynamos”, which is a good term to search for if you’d like to learn more about the process. The electricity generated through this process would be used to power a house or cabin in the area.

After determining a few “fluff” items such as where this generator will be located on the stream, what the head of the water was (change in height), and ultimately the gear ratio we wanted, we started the design of the gearbox. We decided on a 12.7:1 gear ratio with two helical gearsets, since it is desirable to avoid integer gear ratios; it helps the teeth wear our slower. Look up “hunting gear ratio” if you want to know more about the phenomenon. Helical gears were chosen as they allow smoother turning, transfer torque more effectively, and are quieter (than straight/spur gears). See below for a sneak peak of the finished product.

We designed our gearbox so that the input and output shafts were concentric, which would allow the manufacturer to make one single through-hole in the outer casing. Also, the two larger gears and two smaller gears were the exact same, and all bearings and shafts were the same diameter. This would decrease manufacturing costs. Technically, one could have made the output shaft much thinner than the input shaft (since it is carrying 12.7 times less torque), but we chose not to in order to have a smaller number of unique parts.

This project took the entirety of the semester, and we learned about more factors we needed to consider as the semester progressed. We learned that it was desirable to have the keys that connect the shafts to the gears be the first point of failure, since they are simple, cheap, and easily replaceable. With this in mind, we designed our other components to have a slightly higher safety factor. In this analysis, we had to consider every change in diameter, every notch, the gear teeth, and many other factors to ensure that we knew the true safety factor of each component. Unfortunately, as much of the values were calculated by referencing tables from the textbook, we did miss one that should have been updated. We believe that if we increased the diameter of the input shaft and increased the face width of the gears (and did the proceeding iteration), our design would be perfect. This work would have been far too high to do in the time remaining, so we left it for “future work” at the end of the report.

If you’re interested, check out our final report, spreadsheet, (or pdf version of the spreadsheet).