2.4 Case Study in Conceptual Design: Mousetrap-Powered Vehicles
57
For instance, an electron beam melts metal powder in a vacuum chamber,
creating very strong parts that can withstand high temperatures. Customized
production is giving engineers the ability to manufacture a product as soon
as someone orders it to one-of-a-kind specifications by taking advantage of
rapid manufacturing technologies.
2.4 CASE STUDY IN CONCEPTUAL DESIGN:
MOUSETRAP-POWERED VEHICLES
Requirements
development
In this case study, we trace the progress of a hypothetical team of engineering
students as they generate concepts for designing a mousetrap-powered
vehicle. Readily visualized and built, these vehicles are a useful means for
experiencing part of a design process and for gaining an appreciation of the
trade-offs that must be made for a design to satisfy all its requirements.
As described in Section 2.2, the first stage in a design process is developing
a set of comprehensive requirements. In our illustrative case study of designing
and building the vehicle, the system requirements are essentially provided to
the teams of engineering students by the instructor, as follows:
• Vehicles must travel 10 m as quickly as possible
• The vehicle must be powered by only a standard household mousetrap.
Energy that is incidentally stored by other elastic elements or obtained
from a change in elevation of the vehicle’s center of mass must be
negligible
• Each vehicle is to be designed, built, refined, and operated by a team of
three students
• Teams will compete against one another in head-to-head races during a
tournament; so the vehicles must be both durable and reusable
• The mass of the vehicle cannot exceed 500 g. The vehicle must fit
completely within a 0.1-m3 box at the start of each race. Each vehicle
will race in a lane that is 10 m long but only 1 m wide. The vehicle must
remain in contact with the surface of the lane during the entire race
• Tape cannot be used as a fastener in the vehicle’s construction
Each of these requirements constrains, in different ways, the hardware that
the teams will ultimately produce. If any single requirement is not met, the
entire design will be inadequate, regardless of how well the vehicle might
perform relative to the other requirements. For instance, because the racing
lane is ten times longer than it is wide, the vehicle must be capable of traveling
in a reasonably straight line. If a particular vehicle is fast, but it sometimes
veers outside the lane, then it could be defeated by a slower vehicle in a headto-head race. The design teams recognize that the vehicles should not be
optimized with respect to only one specification, but rather balanced to meet
all of the requirements.
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Chapter 2
Conceptual design
Mechanical Design
We next follow the thought process of a hypothetical team as it begins
to create several design concepts. The students document their ideas
in a bound design notebook, and they use written comments and hand
drawings to describe each concept. Subsequently, the team will record
progress as prototypes are constructed and tested in order to document
the outcome of their iteration efforts. In short, the notebook serves as
a log to chronicle the team’s entire design experience. As described in
Section 2.2, such notebooks are often dated, signed, and even witnessed to
formally document a product’s development. With an eye toward your own
professional career, you should also begin the practice of systematically
recording your original ideas.
First Concept: String and Lever Arm
An idea that emerges from the team’s first brainstorming session is based
on using the mousetrap’s snap arm to pull and unwrap string from a drive
axle. Together, the team members sketch the concept shown in Figure 2.19.
As the trap snaps closed, string is unwound from a spool that is attached
to the rear axle, and the vehicle is propelled forward. The concept vehicle
incorporates a lever arm that lengthens the snap arm supplied with the
mousetrap, pulls more string from the axle, and changes the velocity ratio
between the mousetrap and the drive wheels.
Although this concept has the positive attribute of being simple and
straightforward to construct, the team raises a number of questions and lists
them in their notebook:
• What should be the length of the lever arm’s extension and the radius of
the spool that is attached to the drive axle? With a long-enough string,
the vehicle would be powered steadily by the mousetrap over the entire
course. On the other hand, if the string is shorter, the mousetrap will
close sooner, and the vehicle would coast after being powered only along
the first part of the course. The team’s discussion of this issue prompts
the idea for a tapered spool, as sketched in Figure 2.19(c), which would
enable the velocity ratio between the mousetrap and the drive axle to
change as the mousetrap closes.
• Should the mousetrap be positioned behind, above, or in front of the
drive axle? In their concept sketch, the students drew the mousetrap
directly between the front and rear wheels. At this stage, however, that
placement is arbitrary, and the team has no reason to expect that choice
to be better than any other. This question could be resolved in the future
by building a prototype and conducting some tests.
• What should be the radius of the wheels? Like the length of the lever
arm’s extension and the radius of the spool, the radius of the drive
wheels influences the vehicle’s velocity. The team noted on its sketch
that computer compact discs could be used as the wheels, but the vehicle
might post a better race time with wheels having a smaller or larger
diameter.
Another question is how to place the two mouse traps, whether
to put them next to eachother or one behind the other one.
2.4 Case Study in Conceptual Design: Mousetrap-Powered Vehicles
Lever arm's
extension
Figure 2.19
First design concept
that is based on a
lever arm for pulling
and unwrapping
string from the drive
axle. (a) Side view
with two wheels
removed for clarity.
(b) Top view of the
vehicle. (c) Concepts
for straight and
tapered unwinding
spools.
59
Compact disc
wheels
Rotation
String
Unwinding
spool
(a)
Mousetrap
Spring
Snap arm
(b)
String's
tension
(c)
The team records these questions and discussion topics in their notebook, but
they leave them for future consideration. At this early point in the conceptual
design stage, no decisions need to be made on dimensions or materials.
However, if this concept eventually emerges as a promising candidate, the team
will need to resolve these issues before constructing a viable prototype.
Second Concept: Compound Geartrain
As the discussions continue, the team next devises the option shown in
Figure 2.20 (see on page 60), in which a compound geartrain transfers power
from the mousetrap to the drive axle. This vehicle has only three wheels,
and a portion of the body has been removed to reduce weight. The concept
60
Chapter 2
Figure 2.20
Mechanical Design
Rear axle
Compound geartrain
Second concept
that is based on a
compound geartrain between the
mousetrap’s snap
arm (which rotates
through one-half
turn) and the drive
axle (which is
powered over the
race course’s full distance). (a) Top view
of the vehicle.
(b) Layout of the
two-stage geartrain.
Cut out body
to reduce weight
Spring
Snap arm
(a)
Gears #2 and #3
on same shaft
Gear #4 attached
to rear axle
N2
N3
Snap arm attached
to gear #1
N1
Mousetrap
N4
(b)
incorporates a two-stage geartrain, and its velocity ratio is set by the numbers
of teeth on the four gears. The team’s illustration of a two-stage geartrain
is arbitrary; a system with only one stage or more than two stages might be
preferable. However, the students accept such ambiguity, and they realize
that a decision for the geartrain’s velocity ratio is not yet necessary.
During the give-and-take of the meetings, the team identifies additional
constraints that are common to their first and second concepts. For instance,
the students agree that the vehicle should be designed so that the drive wheels
do not spin and slip as the vehicle accelerates. Otherwise, some portion of the
limited energy that is available from the mousetrap’s spring would be wasted.
To prevent slippage, weight could be added to the vehicle to improve contact
between the drive wheels and the ground. On the other hand, a heavier vehicle
would be slower because the potential energy of the mousetrap spring is
converted into the vehicle’s kinetic energy. As they investigate the project in
more detail, the students see that the technical issues at hand are interrelated.
Even in the context of this seemingly straightforward exercise, the designers
must grapple with competing constraints and requirements.
2.4 Case Study in Conceptual Design: Mousetrap-Powered Vehicles
61
Third Concept: Sector-Shaped Gear
The team’s third concept combines and extends certain ideas that arose during
the earlier discussions. The design of the concept in Figure 2.21 incorporates
a geartrain between the mousetrap and the drive wheels, but it enables the
vehicle to coast once the trap closes. The students envision such a vehicle as
accelerating quickly over the first few meters of the race course, reaching peak
velocity, and then coasting at that speed over the remaining distance. In their
concept, a sector-shaped gear, instead of a full circular one, is attached to
the snap arm of the mousetrap. A small notch at one end of the gear enables
the mousetrap to disengage from the simple geartrain once the snap arm
has closed, as shown in the Figure 2.21(c). The sector-shaped gear serves
as the input to the geartrain, and the output gear is directly attached to the
front drive axle. The idler gear (Section 8.5) is included to increase the offset
between the mousetrap and the front axle.
With several concepts in mind, the students can now begin making trade-offs,
considering various materials, narrowing down their options, and experimenting
with prototypes. Although selecting specific components would be premature, the
students use their imaginations to list some materials that could be used: foamcore
poster board, balsa and poplar wood, aluminum and brass tubing, threaded
rods, plexiglass, ball bearings, oil and graphite lubricants, wire, and epoxy. After
addressing some of the technical issues that have been raised and performing
some order-of-magnitude calculations, the team might decide to build and test a
few prototypes before selecting the one concept to be refined in detail.
Sector-shaped
gear
Figure 2.21
Third concept that
is based on a simple
geartrain and a
sector-shaped
gear. The vehicle is
powered over the
first portion of the
race course and then
coasts at top speed
over the remaining
distance. Side views
of the geartrain (a) as
the mousetrap begins
to close, (b) during
the powered phase,
and (c) during the
coasting phase where
the notch disengages
the sector-shaped
gear from the drive
wheels.
Front drive
wheel
Cut out
notch for
coasting
phase
Gear
attached
to wheel
Idler
gear
Body
(a)
Mousetrap
N2
N3
No meshing while
vehicle is coasting
N1
(b)
(c)