1. Planning

1.1 Reflections on Engineering Design

Introduction

Students will work in groups of 3 or 4 to design a vehicle powered by a basic mousetrap. Students will be assigned “jobs / titles” in their groups and will employ the Engineering Design Process (EDP) in order to optimize vehicle performance to meet specified constraints and criteria. The project is designed to reinforce concepts related to Force, Energy, Work, Power, Friction, Moment, Rotation and Kinematics. The design process is introduced as an organized approach to facilitate reaching the best solution.

Key Skills / Knowledge 

ENGINEERING

1. Understand components of EDP
2. Techniques for useful brainstorming
3. Use of decision matrices to define criteria
4. Optimization
5. Organized building, testing and modification based on physics principles

PHYSICS

1. Basics of energy transfer through drive train design
2. Understanding of the lever and wheel and axle as machines that do work
3. Determine factors that influence friction
4. Determine factors that influence air resistance
5. Connection of force to acceleration
6. Relate torque (moments) to rotation
7. Apply concepts of inertia, center of gravity, stability and weight distribution

1.2 Assignment of roles

Assignment of Roles

Project Manager

Looi Chi Han

Drivetrain Engineer

Looi Chi Han

Wheel Engineer

Arjun Appavoo

Chasis Engineer

Bryan Loh

1.3 Engineering Goals and Brainstorming

1st Idea

The first idea that our team has generated is based on a very basic concept that is used by many others. The mousetrap will be placed on the top of the car with a long stick attached to the spring mechanism as shown in the picture below. One end of the stick should be attached to the mouse trap and the other attached to a string. The string will be use to coil around the axle of the wheels of the mouse trap. When the mouse trap is released, the elastic potential energy of the mouse trap will cause the mouse trap to snap back, causing the stick attached to the mouse trap to pull the string forward, thus spinning the axle of the car, causing it to spin and thus, the car moves forward.

2nd Idea

The second idea that out team has generated is an improvement of the very first idea which will be elaborated on later. First of all holes will be cut on the mouse trap, allowing it to be bent back further, causing it to have a wider range to turn. and thus, allowing the string to be pulled further than the second design. In addition to the addition of the holes for a wider turn of the mouse trap, a compound gear will be added. Instead of turning the axle connected to the wheel directly, The string attached to another axle that is connected to a 40-tooth gear. This 40 tooth gear will turn a 16-tooth gear which is attached to a 40-tooth gear. The second 40 tooth gear will turn another 16-tooth gear which is attached to a 24 tooth gear that turns the final 16-tooth gear. In total, the gear ratio will be 1:225.  This will spin the back wheels at 225 times the speed the string turns the axle. To obtain speed, we will have to ensure the strength of the mouse trap is sufficient to turn the axle.

3rd Idea

The third idea that our team has generated is a completely different method compared to the first two ideas. Instead of using the string, we will use the energy of snapping the mouse trap to hit a board protruding from the axle. This will make the wheels accelerate quick and move at a constant deceleration thereafter.

1.4 Decision Making Matrix

Criteria	Weight	Size	Appearance	Time to produce	Cost to produce	Ease of use	Availability of materials	Environmental Impact	Safety	Row Total	Normalised Value
Weight		2	3	2	2	1	1	3	2	16	16
Size	1		3	1	0	1	1	3	0	10	10
Appearance	0	0		0	0	0	0	0	0	0	0
Time to produce	1	2	3		2	3	2	3	0	16	16
Cost to produce	1	3	3	1		0	0	0	0	8	8
Ease of use	2	2	3	0	3		2	3	0	15	15
Availability of materials	2	2	3	1	3	1		3	0	15	15
Environmental Impact	1	0	3	0	3	0	0		0	7	7
Safety	1	3	3	3	3	3	3	3		22	22
Column Total										109	109

Criteria	Normal Priority Value	Design Idea #2		Design Idea #1		Design Idea #3
weight	16	1	16	2	32	2	32
size	10	2	20	1	10	3	30
appearance	0	3	0	2	0	1	0
time to produce	16	3	48	2	32	3	48
cost to produce	8	1	8	2	16	3	24
ease of use	15	3	45	2	30	1	15
availability of materials	15	3	45	3	45	3	45
environmental impact	7	3	21	3	21	3	21
safety	22	3	66	2	44	1	22
	Total		269		230		237

  

1.5 Design rationale and notes

Wheels

The wheels will be made of CDs. This is due to the fact that the CD is a easily available material that has a large diameter. This will allow a greater distance covered with one spin of the axle that we are using.
On top of using the CDs, the 2 back wheels will be covered in Anti-Slip mat. This will allow the 2 back wheels to have a greater grip of the ground that the car will be travelling on to ensure that there is an increase in friction in order to maximise the full circumference turn of the wheel on the floor, hence allowing the car to move the furthest distance possible. The two back wheels discussed are the wheels which are powered by the mousetrap.
However, the two front wheels are merely boogie wheels used to keep the car balanced so that the car would be very stable when driving. As these are merely boogie, hence as much friction as possible is reduced on the wheels (no anti slip mat is placed).
Furthermore, we used two boogie wheels instead of one so as to ensure that the car would not drift and turn, which may result in the car turning, hence reducing the effective distance of the car travelled.
Also, we have decided that both boogie wheels should be of the same size. This is to ensure that the entire chassis is leveled. This is to prevent the weight of the car from pressing down on the boogie wheel, if the chassis was slanted at the front. This is because, as the car slants when the size of the bogie wheel decreases, the free body diagram shows that more and more weight is placed on the boogie wheel. This would then cause more friction between the boogie wheel's axle and the axle hole which the axle is slotted through.

Lever Arm

The lever arm will be made by directly attaching the string to the lever arm of the mouse trap. This is because, we are already making use of gearing to maximise the rotational distance of the mousetrap lever. Hence, a lever arm is not needed. Furthermore, as we have cut the mousetrap to allow the lever arm of the mousetrap to bend down further( so as to be able to store more elastic potential energy which is then converted to more kinetic energy which would then allow the car to move further), hence if we include a lever arm, it may either hit the floor or the wheel which would only cause the car to be hindered and move a shorter distance due to an increase in the opposing force which would hence cause a decease in the resultant force, which then leads to a decrease in the acceleration of the car.
Furthermore, many others may feel that due to moment, by increasing the length of the lever arm, it would increase the time the force is supplied, as an increased distance of the stick from the pivot point will increase a wider circumference the stick will pull the string, hence allowing the mouse trap to supply a constant force for a longer period of time. However, they fail to realize that by adding a stick to the mousetrap, the torque provided by the spring of the mousetrap is decreased. This is due to the fact that the turning force applied by the spring is constant, but the moment required to turn the wheel is increased due to the increase of perpendicular distance between line of action of force and pivot by the stick, hence the resultant force will decrease and thus, torque will be reduced.


Chassis Material

The materials used for the chassis will be made of household items like wooden planks, corrugated board and also a little bit of Lego bricks to enable easier construction of the compound gears. This is due to the fact that we want to gear down the axle turned by the string to enable the car to travel further with less turn of the axle that is turned by the string.

Chassis Shape

The chassis shape will be made of a rectangle. This will enable easier construction of the wheels and the compound gear box. We made it into a rectangle with a small height at the front to decrease air resistance to enable the car to move with a lesser amount of force, and also increasing the resultant force which results in a greater acceleration.

Location of Mousetrap

The mouse trap will be placed closer to the back wheel than the front wheel. Due to the fact that the string will be turning the 2 back wheels instead of the back wheels, the added weight at the back will ensure that the 2 back wheels are constantly in contact with the ground ensuring that the car will not loose energy by allowing the wheels to slip, hence ensuring the maximum distance travelled by the car.

Axles

Some of the axles will be made of metal square rods. This decreases the chances for the axles to bend into a shape that disallows our compound gears to work properly. Due to this reality, we decided to use square rods that fit into the regular lego beam holes. Furthermore, metal rods can fit through the holes of the gears, and able to turn with it effectively.

String Type and Attachment

The string type that we are going to use is cotton twine string. This is due to the fact that cotton twine strings are twisted countless number of time hence they are built to not break easily and can withstand a huge amount of pressure. Adding on to that, the cotton twine string is easy to find in any arts and crafts shops and is also very cheap.

Furthermore, the nylon string is more elastic than rope or cotton wine. Due to this added elastic property, when the mouse trap pulls, the rope will be stretched, causing the force applied by the rope on the wheel to decrease, thus making it difficult for the car to move due to its decreased force applied on the wheels of the car to turn.

As for the attachment, we have thought deeply for this particular part. Firstly, we had tried beforehand and knew that by uncoiling a string around a square rod by pulling it requires a lot of force.
This is because, by using a square rod, the perpendicular distance between line of action of force is lower than when using a circular rod. This will cause the force applied by the string on the square rod to be lesser than the force applied by the string to the circular rod.


Furthermore, we also knew that a completely circular and smooth axle (lego axle) would not work. This is because, the string coiled around the axle will simply be pulled off from the axle when the lever is released, without turning the axle much as there is not enough grip.
Or, once again, because there is not enough friction/grip between the axle and the coiled string, when the lever is released, the entire axle will merely be pulled towards the direction where the string is pulling the axle, rather than turning it. This is because, due to the lack of friction/grip between the string and the axle, the string is incapable of turning the axle when the string is pulled. However, as it is tightly bounded to the axle, it will only pull the axle towards the direction the string it being pulled by the mousetrap lever.

Hence, we have decided to place bushings along the square axle to make it round. However, we will place anti-slip and bounded tot he axle by rubber bands to maximise friction between the string and the axle to allow the most effective turning of the axle.

1.6 Materials used

Material	Where is it used?	Reason?
CDs	It is used as wheels in the car.	This is because the CD has a large diameter that will allow the car to travel further per turn.
Anti-Slip Mat	It covers the two back wheels at the point of contact with the floor.	Doing this allows the wheels to have a better grip of the floor, allowing it to maximise the force applied by the mouse trap.
Wooden Blocks	Supports the Mousetrap on the car.	Doing this will prevent bending of the Lego beams, protecting the chassis of the car.
Corrugated Board	Guides the nylon string on the car.	This disallows the nylon string to ledge on to another part of the car when it is unwinding form the axle. The line on the corrugated board also allows the airflow to flow up the board easily and smooth, hence reducing the air resistance.
Cotton Twine String	Turns the axle connected to the first set of gears in the gear box.	Using cotton twine string will lessen the chances of the string breaking due to tension. It also has more friction hence allowing it to grip onto the sides axle more efficiently.
Metal Square Rods	Used as the axle of the gear the nylon string will turn.	This allows the axle to be less flexible, reducing the chances for the axle to bend or break when the axle is turned by the string.
Plastic Lego Beams	Used as the main material for the surrounding box for the gears.	This will allow easier construction of the gears as there are holes pre-drilled into the beams to guide the axles for the gears.
Plastic Lego Gears	Used as the gears to gear down the speed of the axle that is turned by the string.	This will allow the wheels to be turned at a faster rate compared to the first set of gears at the expense of strength.

1.7 Preliminary Sketch