In my last post, we decided that we needed a gear train to turn the BLDC motor shaft quickly enough to generate useful voltages. Using the datasheet for the motor, I generated a 3D model. Then, I started to design the gears. The final design is shown below:
Turning the crank causes the cyan gear to rotate. The cyan gear will then rotate the magenta gear next to it, and this increases the rotational speed by a factor of two. This is because the magenta gear has half the radius as the cyan gear; in other words, the gear ratio is two to one. Then, the magenta gear spins the yellow gear, which again increases the rotational speed by a factor of two. Then, the yellow gear turns the orange gear, which increases the rotational speed by a factor of three. All together, the shaft of the motor (and the orange gear) will rotate 12 times (2 x 2 x 3) each time the crank is rotated once.
The dark green rods are 4mm thick rods, which are held in place by ball bearings and pillow blocks. The pillow blocks must be carefully placed in the assembly to make sure all the gears mesh properly.
Here are a couple of nuances to note:
- All gears have a module of 2. This means the gear pitch diameter (the effective diameter of the gear) is 2mm x # of teeth. The cyan gear has 24 teeth, so it has a pitch diameter of 48 mm. The module must be chosen carefully. The smaller the module, the smaller the gear, which means the gear train takes up less space. However, it also means smaller teeth, so the gears won’t be able to bear much torque. Since I’ll also be 3D printing these gears, a smaller module also makes the gears harder to print. A larger module, on the other hand, means the gears are stronger and easier to print, but it also makes each individual gear bigger, making them more expensive (takes more filament to make). Note that all the gears have the same module.
- The magenta and yellow gears are actually two gears fused together. This allows for the gear train to be compact: the cyan gear spins the small magenta gear with a gear ratio of 2:1. Then, because the small and large magenta gears are physically connected, they effectively have a gear ratio of 1:1. This approach is very common as it allows for compact and high ratio gear trains.
- The gear thickness is pretty arbitrary. Thicker gear faces make the gears stronger, at the cost of price (filament) and space. The thicker the gears are, the longer rods you’ll need as well. In this case, since this is more of a proof of concept than a work horse, I chose the widths without much thought.
- When printing the gears, play around with the shell thickness. The shell should be thick enough to make the teeth solid; the body of the gear can have infill. The teeth take the most beating and wear, so you want them to be as strong as possible.
- The cyan gear is unique, as it has a hub. The cyan gear is attached to a cylinder, which extends past the pillow block, and is connected to the crank. This is system’s weakest link: the 3D printed cylinder is press fit into the 3D printed crank, and this is also where the most torque will occur. Making the overlap of the cyclinder and crank thicker, and reinforcing it with glue would be a good idea.
- The pillow block near the crank has a different orientation than all the other pillow blocks. This was done to minimize the distance between the pillow block and the crank. As the person turns the crank, they’ll exert a lot of force on the shaft, so the pillow block should be as close as possible to the crank to support the shaft.
- As shown in the 3D models, there is a lot of play. The yellow and magenta gear, for example, can move along the shaft. I’ll need to put in spacers and washers to fill the voids to avoid that.
Once the gear train is assembled, I’ll hook up the 3 phase power output of the generator (power input for the motor) up to a hex bridge and a bulk capacitor. I ordered the bridge off of Amazon, and had a 25 V, 6800 µF electrolytic capacitor laying around. Next time, I’ll build the system, and then we can give it a test spin!
EE Diary 