Lego Racer, Post #1

For our lego car, we began by experimenting with how gear ratios functioned and influenced the relationship of speed and torque.

We started by connecting gears and seeing how they worked – or didn’t work. We became aware of the direction of spin and the impact of friction created by the gears. We also calculated gear ratios and discovered when one or more gears were nullifying the effect of others and thus just taking up space and creating more friction.

Once we began to understand how gear ratios worked and the tradeoff between speed and torque, we created our first model.

We started off with a box configuration – two stacks of legos through which gears could be suspended by axles. Driving our gear train was an old lego motor – one with low torque but high speed. This influenced us to choose an 8-tooth gear as the input gear. Knowing that we had to move upwards along the torque/speed curve, we then attempted to increase torque. To increase torque, we needed a higher gear ratio. We then connected a 40-tooth gear (ratio 5:1) to increase torque, reducing speed. We then began to add gears, increasing torque. However, the final gears decreased torque and increased speed in the hopes that the wheels might turn faster with the already increased torque. Our gear ratio was as follows:

(5/1)(5/1)(3/1)(1/3) = 25:1. This means that for every 25 turns of the input gear, the output shaft – where we attached the wheels – turned once.

We then attached two wheels in the front to the output shaft, and a third at the opposite end of the car for stabilization purposes. We placed the motor at the back and created a flat platform across the gear box to support the 1 kilogram weight. At the front of the platform was a vertical support to keep the weight from falling forward off the vehicle.

The car moved well enough without the weight, but ultimately did not create enough torque to move the vehicle. We also had overlooked the effect of the 8-16-8 gear combination, which effectively nullified any advancements and only increased friction.

We then experimented with many different gear ratios to find one that produced the fastest car.

Our second iteration involved a 25/3 gear ratio. We had concluded from our first prototype that putting the input 8-tooth gear before the 40-tooth gear was not desirable. From our test, we found that the 8-tooth gear would occasionally skip over some of the 40-tooth gear’s teeth and decrease the whole gear train’s performance. We then decided to place an intermediate gear between the 8 and 40 teeth gears. We put a 24-tooth gear in between with the intention of slowly translating the large speed of the motor and 8-tooth gear to torque of the 40-tooth gear. Our gear ratio was as such:

(3/1)(5/3)(5/3)(3/1)(5/3)(1/5) = 8.33:1

    

With the weight on it, this iteration did not move, and you could hear the motor stalling. We then concluded that this model did not have enough torque to move forward.

Our third iteration was the first one that worked. The gear ratio was very high, 41.6:1. As a result, this model moved forward at a slow rate with little acceleration. The average time it took to complete the 4-meter course was approximately 30 seconds.  The breakdown of our gear ratio shows:

(3/1)(5/3)(5/3)(1/1)(3/1)(5/3) = 125:3

We added a 1/1 gear ratio between two 8-tooth gears to reverse direction of the axle rotation.

Our fourth prototype featured a gear ratio of 13.9. As 8.3 was too low and 41.6 was high enough for it to work, we decided to find a ratio in between the two,  especially underneath 25:1 (from our first model). We got this model to work, and it crossed the finish line in 18 seconds. Our gear ratio:

(3/1)(5/3)(5/1)(1/3)(5/3) = 125:9

Pictures of #4:

We tested this Lego car on the floor:

When we decided to test our car, we needed to make some adjustments which delayed our performance time. We ran our lego car at the end of class, which performed its job in under 20 seconds. However, we forgot to record the racer on the 4m course. I went back on Saturday to recreate our lego car. I rebuilt the car so that everything was the same – except for the gear-driven wheels. The wheels we had used during the race were not available, so I thought it would be a good opportunity to see how wheels of bigger diameter would affect the car. We had hoped to use wheels of a bigger diameter in our actual car, but they all had been in use by other teams.

The rebuilt lego car in action on the course (with the bigger wheels)

The lego car moved noticeably faster with larger wheels, as we had predicted.