The Fabricator - Sample Issue

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 Co., www.precisionsheetmetal.com.

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 asked.

“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 standard reply.

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 of aluminum!”

“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 suggested.

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 is it?”

“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 softball.”

“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 ^C^FIGURE ²was 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 ^anFOS of less than 1.3 inch.” We stud- ^C^FIGURE ³ied the image (see Figure 4). “The mass is up to 1.76 ounces. Still pretty lightweight.”