3.1
Designing Machines
The can opener has essentially remained the same since it was
patented in 1858 (Figure 1). Why has this machine endured in its
design over so many years? Many attempts at different designs have
shown us that this design is the most efficient to date: no one has yet
found an easier way to open a can (unless you use electricity!).
Machines are designed to help make things easier for us to do.
They meet a specific need or perform a task. A machine can
• transform energy;
• transfer forces from one place to another;
• change the direction of a force;
• change the magnitude of a force; and/or
• increase or decrease speed.
Figure 1
Machines as Systems
Most machines can be thought of as a system
made up of subsystems. Each subsystem
performs a different function; together they
make up a system that performs a certain task
or larger function. The typical can opener has
two subsystems (Figure 2). The first subsystem,
made up of a lever and a wedge, is designed
to pierce and cut the metal of the lid. The
second subsystem, made up of a wheel and
axle, rotates the can.
Many subsystems are made up of
mechanisms. A mechanism is a system of
moving parts that changes an input motion
and force into a desired output motion and
force (Figure 3). Most machines have their
mechanisms hidden from view within the
body of the machine. Although we rarely see
these subsystems, we can be sure that
wherever movement takes place—whether in
a simple device like a can opener or in a
complex technological system like the
Canadarm on the space shuttle (Figure 4)—
mechanisms will be found.
The invention of the can opener was a
response to the use of cans for storing food.
Its function is to remove the lid of a can safely.
second subsystem
wheel and axle
Figure 2
A can opener is a
system made up of
two subsystems.
Figure 3
first subsystem
148
Unit 3
lever and wedge
Making Connections
1. Think about a machine that you
have used within the past 24
hours.
(a) How was the machine useful
to you?
Figure 4
(b) Identify the subsystems in the
machine and their functions.
Machines and Controls
The can opener, and any other system, can only
operate as a system if each subsystem performs its
function, works together with the other subsystems,
and has an energy supply. There is one more
requirement: control. Think about opening a can. A
can can be opened slowly or quickly. Some cans are
harder to open than others—more force is needed—
but sometimes only until the can is “started.”
Sometimes the opener gets “stuck” or the subsystem
that rotates the can hasn’t got a good grip—the
opener must be taken off the can, and reapplied. The
human operator controls both the can opener and the
movement of the can using the grip (the lever) by
changing the magnitude of the force applied and by
changing the point of application of the opener. If
there is no human operator controlling every force, a
machine must have its own internal controls: a way of
varying the force applied, the speed of the operation,
and so on. To work efficiently and reliably without
human intervention, a machine’s controls must be
self-correcting—they must adjust to the situation.
More complicated machines usually require more
complicated controls.
SKILLS HANDBOOK: 8B Creating a Design Folder
8D Exploring an Issue
(c) Summarize how the
subsystems contribute to the
overall purpose of the
machine.
2. When a can opener is being
operated, what are the input
motion and force? The output
motion and force?
Exploring
3. Using electronic and print sources,
research what types of materials
have been added to make can
openers easier or more
comfortable to operate. Prepare a
poster with a diagram showing
how different parts of the basic
structure of the can opener have
been modified over time.
8B
Reflecting
4. Write a short story describing
what your life might be like if
society had not developed
machines.
8D
Mechanical Advantage and Efficiency
149