Here are some of our first iterations of our gear trains:
Once we had a few options, we decided to use the two big wheels as the driven wheels since they would pull the car a further distance per rotation. We also decided since the two driving wheels would be the ones driving, we only need one wheel at the back to make the car level and rotate along with the driving wheels. We decided to use the mid-sized wheels since it held the gear train more to the level of the big wheels.
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| 24:40 | 8:40 | 40:24 | 8:40 1:25 |
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| 8:24 | 8:40 | 40:24 1:9 |
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| 8:40 | 8:40 | 8:40 | 40:24 1:75 |
We decided to build up from our 1:75 gear ratio train since it seemed to provide enough torque that it would be able to carry the weight and still move. Since the weight we got did not have a flat bottom or top, we thought that we should build a structure that would hold it. Magnolia got to work on that as I figured out where to attach the wheels, PicoCricket, and motor. We decided that having the motor in the inside would make the weight of the car more even rather than have the motor on the side. We then thought that the PicoCricket should just be on top of the motor as well since it would keep all the weight in the center. The actual 1 kg weight and its structure was also then put right next to the PicoCricket to keep all the weight together.
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| Magnolia's awesome structure that did not allow the weight from rolling around. It was really sturdy and if you held it upside down, the weight would not fall out of the structure. |
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| Our tank from the front |
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| the tank from the back |
Our first iteration that was able to carry the weight and move was a mammoth of a car hence the name the tank. It had a 1:75 gear ratio which at that time we thought was pretty good. We were pretty ecstatic when we turned it on and it was able to move with the weight but it was so slow! It took a little over 40 seconds for it to go across the 4 meter course.
Amy let us know that we should experiment with gear trains with less gears and to make sure that everything was perpendicular which was something that we had not really kept in mind as we made the first few iterations. She also suggested to try different gear trains and record how fast they were.
Magnolia and I decided we needed a much smaller ratio so we first played around with the current 1:75 gear train we had. We removed the last 40s which made the gears 8:40, 8:40, and then 8:24 which we calculated to be the same as the one we currently had but at least had fewer gears. We created more gear trains with much smaller ratios.
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| 8:40 | 40:24 | 8:40 1:15 |
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| 8:24 | 8:24 | 8:24 1:9 |
Xi Xi suggested that we make our car much thinner and less bulky. She helped us realize that having a wide car made the wheels much further which then placed more weight on the axles which created more friction. Xi Xi also let us know that our weight should be able to just sit on the car without the additional stabilization from a structure holding it.
In order to make our car much smaller, we decided to try putting some of the gears together vertically and have the initial 8-tooth gear right on the axle jutting out from the motor and then place the motor slightly above the 40-tooth gear attached to the 8-tooth gear. We decided to place the motor slightly above because in order to have the 8-tooth gear attached to the motor, and then have a 40-tooth gear attach to the 8-tooth, another 8-tooth gear had to go in the middle because the motor would touch the 40-tooth gear. So by having the motor slightly above the 40-tooth gear, the 8-tooth and the 40-tooth gears were able to be attached with out any additional gears. The rest of the gears in the train were attached on the same level. By having the motor higher up, we were also able to make the car much thinner. We then just attached a long thin Lego across the car for the weight to lean against and it was enough to keep the weight from falling off.
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| Final iteration 8:40 | 40:24 | 8:40 1:15 |
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| top view |
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| side view |
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| back view |
We found that if any Lego was attached across from one side of the car to the other on top of the motor, the gears would not turn. We realized that this was because having any force pushing down on the motor would cause the 8-tooth gear attached to it to push down on the 40-tooth gear causing them to not move. So we then decided to have the weight closer to the back of the car so that it would not exert a downward force on the motor and the PicoCricket was attached to the front of the motor for the same reason. Our final iteration was able to cross the course in about 11 - 12 seconds. During race day, it finished the course in a little over 12 seconds.
Reflection:
Given more time, I would try to change the gear train to have less gears since it would decrease the amount of friction. I would also try to cut down on the number of Lego pieces used and also try to make the car much smaller. Having a much smaller car, I think would make it much faster. I would also love to try to figure out a different way to attach the motor. I think the main reason why our car was not as fast as the other 1:15 cars was because of the way out motor was attached. I believe how we currently have it does not allow the motor to turn its axle freely which caused the gears to turn slower.
Engineering Analysis:
Our final gear ratio was 1:15 (8/40 * 40/24 * 8/40). We decided this gear ratio by trying different ratios out and finding which one was right in the middle and gave us the perfect amount of torque and speed since finding this sweet spot would give us the most amount of power. When we made the different iterations of gear trains, we found that the 1:75 (8/40 * 8/40 * 8/40 * 40/24) we initially tried provided so much torque that it was way too slow at 40 seconds. We also tried a ratio that was 1:25 (24/40 * 8/40 * 40/24 * 8/40) and that was at around 20 seconds which was much faster so we decided that we needed to lower our ratio even more. We found that a 1:9 (8/24 * 8/24 * 8/24) was very fast but it would not move with the weight. The 1:15 (8/40 * 40/24 * 8/40) was not as fast but it at least moved with the weight.
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