(removing 3/4 of the material
from the center section),
we can have up to 16 times
the strength of the original
one inch bar, again for the
same weight. This is free
strength through manipulation of material shape.
The equation
triangle that
defines most
engineering
compromises.
For a set length and
width, these sections
of equal weight vary
dramatically in their
vertical strength.
More Triangle
Another way of getting
free weight is to again look
to triangles. If we look at a
cut-away profile of a robot
and we have a rectangular
drive system under a square weapon
system, adding armor can be done
in two ways. One is to cover each
section independently; the other to
triangulate the armor plating so as to
bridge the outer points. This second
option adds strength in several ways.
Firstly, it braces the weapon to
the drive, forming a structural
triangle. Secondly it reduces the
amount of material you need, as the
hypotenuse of the triangle (the
longest side) is less than the total of
the flat vertical plate and the flat
horizontal plate. In the example of
three plates — all identical lengths —
the vertical being three inches high,
the horizontal four inches wide, and
the proposed single piece bridging
both points being five inches wide,
we are saving two inches of material
( 3+ 4= 7). We can either use this
saved weight elsewhere or make our
five inch plate 40% thicker (almost
twice as strong!). Another benefit is
the perceived thickness to incoming
attack. Hitting a two inch thick plate
straight on forces the material to
survive all of the imparted energy.
By inclining the plate 45
degrees, the incoming impact is not
only deflected upwards, reducing the
energy transfer, it is also now dealing
with the hypotenuse of a perceived
triangle. This meaning an effective
thickness of just under three inches,
some 50% more width, and effectively twice the strength. Adding all
of these benefits together, we have
increased material thickness,
increased preserved thickness,
reduced energy transfer, and
increased structural strength, all
from simply changing the armor
shape from vertical and horizontal to inclined. This free strength
stuff is easy, isn’t it?
Reducing Stress
Angled sides
increase effective
thickness of
the armor.
Most cracks, splits, tears, and
rips can be traced to a start point.
Whether a flaw, damage, or a hole,
almost all failure points can be easily
spotted if you simply look for them.
Every hole you drill is a possible
failure point, so drill them where
your forces are lowest, such as in
the center of beams (remember our
lesson on ‘I’ beams?). Visualize the
way the forces are acting in the
components. I have seen
many bolted aluminium
chassis which have been
made in such a way that
the bolts are in simple
shear all the time. A very
easy design change
would remove most of
the sideways forces from
the bolts and allow them
to be used in tension
as designed, thus
maximizing strength.
There are many tips
and tricks to make the
most of your materials,
but it is all common
sense. The more you do
it, the more you get used
to visualizing the forces, their
direction, and magnitudes. Work out
the areas that are under the most
stress and design ways to sidestep
these loads. Creative thinking and
improving your understanding of
engineering principles will allow you
to dramatically improve your strength
for a given weight and cost. SV
James Baker is a member of Team Xbotz,
www.xbotz.com.
Low, highly angled sides
deflect impacts upwards.
Bridging the gaps
with armor adds
strength, reduces
weight, and
deflects hits.
SERVO 09.2008 29
