For this week’s assignment, my partner (Keer) and I had to create a Lego race car capable of travelling across a 4 m track whilst being powered by a PicoCricket. The car would have to be able to carry a 1 kg weight and travel the track in the least amount of time possible.
Visual Summary:
Our first design consisted of 16 and 24 tooth gears, holding a ratio of 1:243. This design accounted for the weight and took 16 seconds to travel across the racetrack. However, we were determined on doing better. Because we wanted to create a system capable of high amounts of torque, we increased the gear ratio to see its effects on the car’s speed.
Once gain our second design included 16, 24 and 40 tooth gears and held a gear ratio of 1:405. When tested, the racecar was significantly slower, taking 25 seconds to travel across the track. From this design, we realized that although we were successful in increasing the torque to account for the 1 kg weight, we did so at the cost of the speed of the car.
We realized that increasing the gear ratio would only prove to further slow down the car. Therefore, our 3rd iteration consisted of 8 and 24 tooth gears with a gear ratio of 1:81. The design seemed promising as the car now travelled the track at 13 seconds whilst carrying the weight. Although this was our fastest time yet, we were adamant on further increasing the speed.
3rd Iteration (1:81)
Since decreasing the gear ratio proved to increase the speed of the car, our 4th iteration had a gear ratio of 1:27. With the current gear ratio, the car was able to complete the track in 12 seconds whilst carrying the weight. Given the time restriction we had on the assignment, we decided to use this ratio as our final design. Had we had more time we would have like to experiment with more gear ratios and see how further lowering the ratio effected the speed of the car.
4th Iteration (1:27)
In the end, our final design (pictured below) made use of 8 and 24 tooth gears. Fundamentally, our whole iteration process focused on finding the optimum gear ratio for which the power of the vehicle can be maximized — a concept revolving around rotational motion and torque.
Gear Ratio Calculation: (8:24)^4= (1:3)^3= 1:27
Engineering Analysis:
Mechanisms are defined as the devices that transform input forces and movement into a desired set of output forces and movement. For example: gears and gear trains, linkages, belts and chain drives, cams and followers etc.
Torque and rotational motion were two large underlying physics concepts stressed on the racecar.
- The product of the tangent force and distance from the point of application is defined as torque (τ= Fr(sinθ)).
- Angular velocity is the rotation rate about an axis (ω= V/r)
- Power is the energy per unit time, also calculated by the product of torque and angular velocity
It is important to note that torque and angular speed, are two quantities inversely proportional to one another. Therefore to attain a low torque and high rotational velocity, a large gear would drive a small gear and vice versa.
The graph below helps illustrate the relationship between torque and rotational speed. The maximum denotes the stall torque and the minimum represents the no-load speed. The point equidistant from both extremes holds the maximum power. Hence, throughout this project Keer and I aimed to find the respective torque and rotational speed values that will provide sufficient torque while exerting maximum power.
Reflection:
Overall, we were extremely proud of our racecar. In hindsight, I would say that we could have attempted to approach the project in a more scientific/methodological manner versus the trial an error basis we largely worked with. Personally, it took me some time to understand the relationship between torque and angular velocity. Through playing around with the gears and continuously trying to make a highly efficient car, I was able to better grasp the relationship between the two variables.
During the first couple of iterations, we didn’t necessarily consider the role friction played. Often we would find it hard to create a direct connection between the PicoCricket, motor and gears. Therefore, we would end up adding additional gears of a 1:1 ratio, to maintain the gear ratio but connect the generator. The implementation of additional gears would have increased friction within the mechanism and thus decrease overall efficiency.
Furthermore, making sure everything is perpendicular to one another and that the bushings had an appropriate amount of leeway was a challenged we faced. If either of the mentioned factors were slightly less/more than ideal, the racecar would noticeably slow down.
Had we had more time, I would have liked to further analyze the most sensible way to place the weight on the car. Although air resistance would not play a huge role in this race, I would like to evaluate the ways in which the orientation of the weight would affect the speed of the car. Similarly, I would have liked to see how different types of wheels would have affected the functionality of our racecar.
