High-Torque Transmission

Summary

This project was completed for a competition in my university's section of the American Society of Mechanical Engineers (ASME). I had the cool opportunity of leading a team of four fellow students in the project.  The simple goal was to lift the heaviest weight using a high-speed BLDC motor, and we had about one month to complete the project from initial idea to final prototype.  

Outcome

We won first place! Our final gearbox had a maximum ratio of 19,683 to 1, although our ratio used for the competition was lowered to 2,200:1 due to time constraints. Failure occurred while lifting 40 lbs at around 10,000 RPM.


Details

The answer to win the challenge at first seemed obvious: use the largest gear ratio possible and make the gears out of a strong material. However, we also needed to make sure that the challenge wouldn't take all day, so it was important to find the optimal ratio. We also needed to find other small advantages because every other team would be doing this anyway.

We opted to use herringbone gears, which reduce friction losses by contacting on two points instead of on an entire plane—the case for common spur gears. Herringbones are very difficult to manufacture unless you are 3D printing, which is what we ended up doing. Modeling them was the same as modeling a spur gear (except with a double-extrusion twist) and I used the same parametric sketch as my Planetary Gearset project, modifying global variables. 

Using a pairwise ratio of 3:1, figuring out the total gears needed was pretty simple:

With a total GR of 570:1,  we needed 7 pairs of gears but we designed our transmission to add/subtract gears in the case that we wanted more speed or more torque. Due to the exponential nature of gears, even adding or subtracting one gear when already using a few can make a drastic difference in mechanical advantage.

The gears were sadly 3D printed in PLA due to budget constraints, but we made the teeth as dense as possible. We used steel ball bearings and shafts to mount the gears to the 3D printed box that we designed. 

Gallery