Fundamentals of Mechanical Engineering

24-101

Mousetrap Car Design Project

Group 39: The Thirty Shirts

Anton Galkin

Lluis Penalver-Aguila

Jordyn Melino

Recitation C

As part of the introductory Mechanical Engineering course at Carnegie Mellon University, we were involved in a design project that coupled the aspects of design, analysis, and construction. The project task was to design and build a car that would travel twenty feet, cross a line, and come back using only the energy stored within a maximum of two mousetraps and six rubber bands. With two design reviews, our project underwent two significant iterations to streamline our final product. This webpage includes the design concepts, a description of how the vehicle works, and the most interesting and innovative aspects of our final mousetrap car.

Design Concepts

Our initial concept for the mousetrap car involved a vehicle that would move down the track under one type of power source, come to a stop, and then move backwards using the other type of power source. The idea was based around traveling the first 20 feet powered solely by rubber bands, and then reversing its direction using a trigger mechanism on the mousetraps. Wound up rubber bands would make the car travel twenty feet to the midpoint mark, and then a pair of preset mousetraps would stop the car and make it return back to the start/finish line.

Description of How it Works

The rubber band power design includes four rubber bands connected to two strings (two bands to a string) which wrap around the front axle and hook onto nails sticking out of the rear axle. When the wheels are rotated backwards, the string winds up, and the rubber bands are tensioned so that they store more potential energy with every turn of the axle. When released, the rubber bands return to their slack lengths, turning the wheels and moving the car forward. Initially, the axles turn together because the strings wrap around both axles which produces four-wheel drive. To make it return, we use a lever arm connected to two mousetraps that are initially set up so that they remain cocked during the first twenty feet. The arm has been connected to the metal bars on the traps with zip ties, paper clips, and wire. The lever arm has a ribbon attached to its end which is also connected to the front axle. By traveling the first twenty feet under rubber band power, the ribbon wraps up around the front axle.

The firing mechanism is set off by way of a string that runs from a calculated point on the return driveline. Attached to the driveline at one end, the trigger string runs inside the rectangular tube lever arm, to a piece of metal serving as a wedge below the “cheese” flaps of the mousetraps. When the return driveline is pulled taut, the trigger string pulls the metal wedge out from below the mousetrap flaps. The mousetraps then flip forward releasing the lever arm and unwinding the ribbon. The unwinding ribbon therefore serves as a brake at the twenty foot mark and provides the return power.

Innovations and Interesting Features

The four-wheel drive that is the product of the rubber bands and the string is a very innovative idea. Our group is proud of the fact that our car has four-wheel drive. This feature allows us to power the car from four wheels instead of two and thus initial traction problems have been overcome. We get power from both axles instead of just one like most other groups have.

Another interesting feature to notice is the wheels. We were disappointed with the CD wheels we had originally used and decided to go with something thicker, sturdier, yet still light. We cut wheels out of balsa and glued two pieces together in a cross-grained fashion. Then, to get a perfectly round wheel, we stuck the glued pieces onto a drill which in turn spun them as the outsides were held against sandpaper. The wheels turned out exceptionally well and when we tested them they ran straight and smoothly. We also put two rubber bands on the outside of each wheel for more friction.

A third and truly essential innovation is our driveline tensioner that allows the initially slack return driveline to wrap around the front axle in a neat fashion. The driveline tensioner is set up on a bracket just rear of the front driveline and sandwiches the slack driveline ribbon between a rubber washer and a smooth piece of balsa. The rubber washer retains the slack closer to the fixed end while the ribbon that is being fed out can easily slip by the smooth sanded balsa.

Performance

Our final car weighs 1 lb, 3.9 oz. The overall dimensions are 13.125”x 22”x 4.7”. The final car works rather well although it has encountered several issues from the beginning. The car moves very fast the first twenty feet under the rubber band power. The firing mechanism has been tweaked so that is does indeed fire when the car has travel at least twenty feet, yet it is secure enough that it does not misfire. The return driveline works well and brings the car back to the start/finish line quickly, but it has problems in winding up around the front axle. To counteract this issue, we introduced the driveline tensioner, but this tensioner is only effective to a certain point. Once a majority of slack has been played out under tension, the final bit of slack pops out of the tensioner and it is here where the driveline fails to wind up straight. An effective way to fix this problem would be to place guide pieces on the axle itself which we have not done on this vehicle. As far as times are concerned, we do not know the exact times, but we are most likely completing the whole course in 13-15 seconds. The car moves considerably faster on the first part of the course than on the return.

The car itself is easy to set up. We had to make it a quick setup due to the fact that there are sixty seconds in which we are able to prepare our car before sending it down the lane. Some features help to promote its swiftness in preparation. These features include the firing string running inside the lever arm, the easy-to-use driveline tensioner, and the longer axis that serves as a handle to stretch the rubber bands in the beginning.

Hardest part

Our group overcame a few obstacles throughout this project. One, which was inevitable for most groups, was the lack of proper supplies/materials. We had to buy our own balsa, glue, aluminum, etc. and spent valuable time walking to hardware stores around town.

Another very important aspect that we had to be careful with was making the front and back axles parallel. If they were a little off from one another then the car would not go straight. There is a two foot boundary in which the car has to stay in and with the first prototype we had issues getting the car to travel in this lane. We also had to make sure that the wheels wouldn’t be wobbly for the same reason.

We also had to put a lot of craftsmanship into the car to make it sturdy. Nothing could be poorly made or else it would reflect on the performance of the car. Time was a hard thing to manage because each of us had our own hectic schedules.