A “Toolbox” for Forensic Engineers

Intellectual Property Cases 403
so metals are excluded. The polymer options are also favored from
another viewpoint: vibration damping or attenuation. Vibrations
from the motor and blade could cause resonance in the body, thus
making the mower noisy in use. It could also cause fatigue cracking
in susceptible areas of the body. Polymers are favored here because
they tend to absorb or damp vibrations, while metals tend to transmit
them freely (like a bell).
5. Thermal resistance is important when the mower is in use: the motor
will develop moderate temperatures of up to about 70°C. Metals will
easily resist such temperatures, but polymers could be affected. If a
polymer is chosen it should be able to remain rigid above this temperature
to give a safe margin.
6. The loading criteria are critical, and a stiff material is demanded.
Polymers are generally less rigid than metals, so care would be needed
in choosing the wall thickness. The greater the wall thickness, the
greater resistance to bending and torsion. The importance of torque
and bending stiffness are directly related to the safety of the user; if
it were too low, the part of the shell next to the blade could be
distorted and make contact with the blade, causing catastrophic failure.
Monocoque construction is favored because the overall product
stiffness is much greater than that of individual shells. In addition,
an external flange could increase the local stiffness of the critical outer
edge of the mower to resist external blows. The options thus become
as shown in Figure 13.13; stage 2. The favored option would be the
monocoque mower because it provides a cover for the motor, shielding
it from the user.
7. The final set of criteria deal with the manufacture of the device: it
must be capable of mass manufacture with few assembly steps. Injection
molding was well known before the priority date of the B&D
patent, and tough thermoplastics such as ABS can be molded easily
into very complex shapes. Some complexity is needed because wheels
may be attached, a handle is needed to connect the user to the body
in order to steer the device, the motor needs to be fixed securely to
the body, the cable feeding electricity to the motor needs to be fixed
securely and bosses for screw fixtures will be needed during assembly.
The material is tough (impact resistant) and resistant to the environment,
is thermally stable to about 100°C and can be made stiff enough
for the function of the mower.
8. If injection molded, a single, closed monocoque is impossible to make
by molding since the method involves injection of hot polymer into
a metal tool with a shaped cavity. The cavity is produced by two
mating halves that must be capable of separation at the end of the