32 precision matters | by Gerald Davis, Contributing Writer
A little FEA
There’s a guy, a balloon, and a bracket …
Gerald Davis is a job shop consultant and
chairman of the board of DSM Manufacturing
This is a story about a guy who
needed a bracket. One afternoon he came breathlessly
into my office and panted, “I
need a bracket! Quick!”
“What is the bracket for?” I asked.
“I can’t tell you.”
“OK, what can you tell me?”
“It goes into a balloon.”
“That must be one tough balloon,”
I thought out loud.
He continued this coy cat-and-mouse description of his project, and I
eventually discovered that he was
working on a secret government
research project using a high-altitude
helium balloon. His instrument was
to be carried in the gondola. The
experiment involved some sort of liquid and he needed to suspend his
instrument package like a pendulum.
“Do you care about the material?”
“I don’t know. What are the
options?” he replied.
“Generally speaking, you have two
choices—ferrous and nonferrous
material,” I offered helpfully.
“Which one is cheaper?” was his
Oy! I thought to myself. This is one
low-budget secret government project.
“Well, iron is generally less expensive
than aluminum,” I mused.
“Can’t be iron. This thing is going
to be outdoors, so rusting could be a
problem,” he said, worried.
“Stainless steel doesn’t rust,” I
observed. He looked hopeful. “But, it
is probably one of the most expensive
materials on a per-pound basis,” I continued. He looked downcast.
After a moment’s pause he brightened and said, “Hey, let’s make it out
“Yeah, that makes sense,” I agreed.
“Aluminum is lightweight and reasonably affordable. However, it does
oxidize when it gets wet. We’d have
to coat it with something,” I warned.
“No problem. But could we make
it electrically conductive?” he asked.
“Chromate would get us there,” I
After another labyrinthine dialogue, I learned that his instrument
had a valve that closed to keep the
liquid from spilling if the gizmo tipped
over too far. What he wanted was
basically a pinball machine’s tilt
switch. It just needed to be lightweight.
“I need a little bit more information and then we can get started modeling your bracket. How much does
your instrument weigh? And how big
“That’s classified,” he retorted. I
glared at him until he relented. “Well,
I can tell you it weighs more than 3
pounds and less than 5 pounds,” he
whispered. “It’s about the size of a
“Aha! You want an aluminum
bracket with a conductive coating
that will suspend a 4-pound mass that
is 31⁄ 2 inches in diameter?” I queried.
He nodded happily (see Figure 1).
C FIGURE 1
“That looks pretty cool!” he to 0.080 in. thick. That boosted the
observed. “How much does it weigh?” mass to 2. 24 oz., but we ended up with
It took only a few mouse clicks to an FOS of more than 2. I asked the
assign material (0.050-in. 3003-H14 system to show me where the FOS
aluminum) and check the resulting was less than 2. 5 (see Figure 5).
mass—1.44 ounces. “Well, let’s build I was pleased. This design was now
one then!” he said encouragingly. plenty strong, in my opinion. “It is
“Let’s check to see if this is strong now twice as strong as it needs to be,”
enough to hold up the weight,” I I announced.
advised. Again, with just a few “When this thing does bend,
mouse clicks we had a finite element where does it move the most?” he
analysis (FEA) study of the bracket. wondered.
The stress distribution image (see
Figure 2) let us quickly spot the
parts of the bracket that were working the hardest.
“This might be OK,” I noted. “The
area in red shows the maximum stress.
Let’s check the factor of safety
(FOS),” I suggested.
“What does that mean?” he asked.
“An FOS of less than 1 indicates
that the material has been stressed
past its yield point. Our model has an
FOS of about 0.6. This material is too
flimsy.” I showed him where the FOS CFIGURE 2
was reporting a failure in the design
(see Figure 3).
“We need to change the design.
Let’s try a different alloy with a bit
more thickness.” So I changed the
alloy to 6061-T6 and selected 0.062-
in.-thick material from the gauge
table. I also kicked the bend radius up
to 0.250 in., knowing that 6061-T6 is
tough to bend without cracking.
After rerunning the FEA, the results
looked much better.
“Now the FOS is mostly up over
1.2—and only two zones have an FOS of less than 1.3 inch.” We stud- CFIGURE 3
ied the image (see Figure 4). “The
mass is up to 1.76 ounces. Still pretty
“So are we ready to go to manufacturing?” he asked impatiently.
“Sort of. What happens if the balloon bounces when it hits the ground?
Wouldn’t it be better to add a little
“That depends on how much additional weight we have to add,” he
So we did a little bit more R&D
and kicked the material thickness up CFIGURE 4
The FABRICATOR | An FMA Publication
www.thefabricator.com | March 2007