Lego Racer

For this assignment I worked with Izzy King to design a lego vehicle that is propelled by one PicoCricket motor. The vehicle had to be able to carry a 1 kg weight as fast as possible on a 4 meter straight, carpeted track.

Design Process

Practice

Before we got done to actually building our race car we spent sometime learning about how the materials and motor we were using actually worked. To do this my partner and I created several lego gear trains with various gear ratios and numbers. Building these trains helped us to understand the mathematics behind gear ratios/gear reductions. In class we learned that higher the gear ratios allowed for more torque, but often led to less speed. Lower gear rations, on the other hand, allowed for more speed at the expense of having a lower torque. Due to this tradeoff, one must think carefully about the intended use and possible strain that their motor powered creation will be put under when designing its gear layout. 

The image below is of our final gear train which has four gear pairs and a gear reduction of 27:1. We determined the ratio of this gear train by multiplying the gear ratios together.

Brainstorming

Once we had some understanding of how to work with the gears and motors, Izzy and I started to think about how we might want the overall design of our car to look. To keep the car as light as possible we decided to only use 3 wheels in our design. Less wheels would also decrease the friction between the car and the carpet.  For the actual shape of the car we chose to create a wedge design with lateral axes for the wheels. The angled body of the car would would help to take weight off of the two back wheels that would be connected to the motor, allowing them to turn faster. The lone front wheel would not be connected to the gear train and would primarily be there to provide balance.

Based on our practice with gear trains we decided to use a 15:1 gear reduction (seen below minus the gear attached to the motor). The gear train consisted of an 8 toothed gear attached directly on the motor, which then connected to a 40 toothed gear, which then connected to a 24 toothed gear. This 24 toothed gear controlled the spin speed of a 8 toothed gear connected to a 24 toothed gear, which was then connected to a 40 toothed gear that controlled the speed of the back wheels.  With this gear reduction our motor did not stall, and was able to turn the final gear, where the wheels would be attached, relatively fast.

Building/Testing

After making a gear train that did not stall when attached to the PicoCricket motor, we began to figure out how to build the rest of the car. The challenge was being able to fit the three parts of the motor, the wheels, and the 1kg weight without having them interfere with the movement of the gear train and back wheels wheels. This was a trial and error process in which we had take apart and resemble the car multiple times.

Our first concern was how we would attach the front wheel on. We settled with the design below.

In the image above one can see that the front wheel is attached to a platform located on top of the PicoCricket motor. This allows it to turn freely as the car is propelled forward by the back wheels. Originally we had planned to use a smaller wheel for the front wheel, but chose to keep all three wheels the same for several reasons including to keep the gear train from touching the ground. 

Once we had this structure sorted out, we began building upwards to accommodate the other two parts of the motor, as well as the 1kg wight. Once we had a structure that held everything, we tested our car out on the track.

Something surprising that happened with our first test run was that our car went backwards! Instead of the lone front wheel leading the car, the two motor propelled back wheels took the lead. We tried switching the direction of the motor to what we expected it would be, but found that having the back wheels leading the car was actually more favorable. So, we decided to keep that element of the design.

Regardless of the car's direction, though, our first test run went pretty well with our car's race at about 10 seconds. We wanted our car to go faster, though, so we went back and  cleaned up our design; adding structural supports as needed and rooming any needed weight. In one of our tests the gears went out of alignment, so the car did not move. To fix this we made sure that the bushings holding them in place were secure and numerous. Our final design can be seen below.

Dissected view of car

Height: ~13 cm

Length: ~24 cm

With Weight

The Race

Engineering Analysis

For this assignment we wanted to design a car that would be as fast as possible while still being able to carry a 1kg weight. This meant that we had to take into account both the rotational speed and torque of the gears we were using. The relationship between these two variables can be seen below.

The motor we were using for the car was the PicoCricket motor. This motor is known for having a higher speed, but very little torque. This is why we had to design a gear train with a large enough reduction that it would give just as much torque as needed without reducing the speed too much. The gear train we ended up using had a 15:1 reduction which ended up being very favorable for our car during the race. (Gear ratio in train order: 8:40, 40:24, 8:24, 24:40) 

There were several other facets of our design that contributed to its success. In class we discussed that things such as wheel diameter, material strength, and friction played a large role in the car's speed. 

For our car we chose the largest wheels available, which had a diameter of 8cm. While this choice was primarily to prevent the gear train from touching the ground, we learned that having larger wheels increases their linear output from the angular velocity they receive from the gears. This can be understood through the equation:  (Angular velocity)(wheel radius) = Linear Velocity. 

Material strength also played a factor in how much friction the car experienced. The axles use for the wheels were made of relatively thin plastic, and thus would bend if put under significant weight. This would in turn cause the wheels to sit at an angle, which would reduce their speed. To prevent this we positioned our wheels as close to the car as possible.

We also tried to increase the car's speed by limiting the amount of static friction it would experience. This was achieved by lessening it's weight and only using three wheels. 

Reflection

Over the course of this project I have learned a lot about how gears and motors work. I had not previously know about the relationship between torque and speed and how it relates to linking multiple gears together. I had also never fully considered all the thought and work put into making sure that the gears of larger machinery ran smoothly. Especially when one has to take into account the friction between gears as well. 

As we learned in the process of making our car, so many variables need to be considered when designing a piece of machinery. We had to make many adjustments to not only our gear train, but the placement of our wheels and supports as well. All this was to ensure that the friction experience by the gears in the car and the wheels on the floor was as low as possible.

We could try to improve the speed of our car by testing different support structures to lessen the weight, and thus experienced friction. This, however, is unnecessary as it is time to move on to our next project!

Thanks for stopping by!