After we tested our windlasses, we began working with gears and motors. We tested the two different types of lego motors and learned how to use the components to make the motor spin. We learned how to attach the pico cricket, motor board, the motor and the gears the create a gear train that would be powered by the motor.
Day 8 (2/19/16)
Today we started by doing a worksheet to learn more about gears and gear ratios. We analyzed different gear ratios and learned how to put the gears together to make a lego gear train. We learned that going from a small gear to a big one increases the torque by the gear ratio factor. The reverse is true for speed, going from big to small gear increases the speed. These concepts became very important to us as we began designing our lego racer.
Once we finished our worksheet, we began designing our lego racer. We started designing our lego racer by using trial and error. We decided to first make a gear train that would allow the wheels to move and then go from there. Here is what our first design for the car looked like:
To this design, we added a gear train and tested all of our wheels. We ultimately decided to go with the largest wheels and the smallest wheels.
Sara and I then decided to divide and conquer to test out different gear ratios to determine the best configuration. We each worked on our own designs, but shared our findings with each other and incorporated each others ideas.
My first iteration had a gear ratio of 1/8.3. I used a 24 tooth gear connected to the motor that turned a 40 tooth gear below it. The axel with the 40 tooth gear had an 8 tooth gear along the outside. That 8 tooth gear turned a 40 tooth gear that was also on the same axel as another 8 tooth gear. I used a 40 tooth gear intermediately connected with an 8 tooth gear on the axel with the wheels. The design looked like this:
Unfortunately this design was not fast enough because the gear ratio was too high. The gear train provided too much torque to the car and therefore minimized its speed.
While I was working on my design, Sara was also working on testing different gear ratios. She tested gear ratios of 0.6, 0.4, 0.167, and 0.11.
By using the data that Sara got from her designs, we determined that we needed a gear ratio somewhere between 0.12 and 0.4.
Outside of class we put together more iterations of the lego car that looked like this:
We ended up testing gear ratios of 0.08, 0.04 and 0.24. It turned out that our best lego racer was the one with the 0.08 gear ratio. When we tested the gear ratio of 0.08 we recorded a time of 10.6 seconds.
Day 9 (2/23/16)
Today was race day. Sara and I decided to go with the 0.08 gear ratio car since, when we tested it, it finished with a time of 10.6 seconds. The gear train ended up being set up the same as the first iteration, but instead of a 24 tooth gear on the motor, we attached a 16 tooth gear. We also changed our design slightly to account for the different height of the 16 tooth gear and we added a platform for the weight and for the pico cricket.
When it came time to race the cars, our car did not perform as we intended, nor as we expected. Instead of completing the distance in around 11 seconds, it finished in 20! That was a huge change compared to our test run. Originally we thought our issue might have been due to the batteries, however when we changed the batteries it only improved performance by a few seconds. We think it might have been due to excess friction in the gears, but we are still not quite sure what happened.
If we had more time, we could have refined our design to make the lego structure lighter. Also, we probably would have been able to improve our gear ratio to make the car go even faster. Most of the teams determined that the ideal gear ratio for this scenario was around 1/15. We had a gear ratio of 1/12.5 so we were close, but we could have made our car even faster.
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