Smooth Operator

smooth operator

Finite element analysis helped Boeing eliminate oiled bearings on its Chinook helicopter.

by Jean Thilmany, Associate Editor

The military Chinook helicopter now in use in Afghanistan was originally designed in the late 1950s and, as Frank A. Smith, Jr., said, the lubricated roller bearings used in the pitch-hinge assembly were there from the get-go.

Smith performs nonlinear finite element analysis at Boeing Military Aircraft and Missiles Systems' Philadelphia rotorcraft division. He estimates that his group saved Boeing about $1 million by using FEA software when redesigning the assembly to do away with the outdated bearing system.
"When they were first put in, roller bearings were good technology," Smith said.

The pitch-hinge assembly on the helicopter permits the blade to rotate on its longitudinal axis and to control thrust, which determines where the helicopter is going. In other words, it dictates the pitch of the rotor blades and is one of the most important hinges on the craft, Smith said. About two years ago, Boeing sought to redesign the pitch-hinge assembly to replace the bearings with a self-lubricated hinge.

"The lubricated bearings need oil and they have to be sealed, but sometimes the oil leaked out," Smith said. "That was one of the biggest payoffs for this project—to eliminate the old bearings and to make a bearing that would last until regularly scheduled maintenance periods. It would severely reduce maintenance costs."

The bearings had to be manually lubricated and maintained, which was an expensive proposition. By designing a bearing system that would last longer than roller bearings and would be easier to replace, Boeing could reduce maintenance costs significantly on the Chinook, Smith said.

The Boeing rotorcraft division recently redesigned the pitch-hinge assembly on its Chinook helicopter in order to replace oil-lubricated bearings with a self-lubricated hinge for maintenance purposes.

Smith works for what's called lean enterprise at Boeing, the name given to a companywide effort to make manufacturing and design processes more efficient and less expensive. As such, Smith focuses on using FEA to bring down the cost of designing, analyzing, and manufacturing new parts. Cost savings come from using the analysis program to reduce the amount of testing and redesign needed before production, Smith said.

FEA is the use of a complex system of points, called nodes, that form a grid, or mesh, across a computer-aided design model. The mesh contains the material and structural properties that define how the part will react to certain load conditions. In essence, FEA is a numerical method used to solve a variety of engineering problems that involve stress, heat transfer, electromagnetism, and fluid flow.

Once the purview of specialists and run only on mainframe computers, the analysis method has been working its way down the corporate engineering chain over the past decade, with the advent of easier-to-use software. Many engineering technology vendors are now marketing software that walks users through a series of steps that let them define the analysis they want to run and then interpret the results.

Boeing uses any of several sophisticated FEA software packages, depending on the project. For the pitch-hinge redesign, Smith used Abaqus software from Hibbitt, Karlsson, and Sorensen Inc. of Pawtucket, R.I.

"It worked out really, really well," he said. "It's a good example of how you can benefit your design process by using more sophisticated FEA technology. People are used to doing things as they've been done for decades. They say, 'Well, it worked in 1950, so it'll work now.' That's a valid argument as well, but this project was my reply.

"That's why I wanted to do this," he added. "Not just for fun, not just for academic interest, but to show real-life savings and benefit to the project."

Foreseeing Flaws

Boeing maintains a variety of FEA programs in-house, including Ansys and Nastran. Smith said the advanced FEA programs on the market today foresee potential design flaws that would be impossible to predict without their use.

"In the past, we'd make some conservative estimates and the designs came out fine, but they could have been even less expensive," he said.

Boeing has seen a dramatic boost in computing power over the past two years, which allows a much fuller and more complete mesh than in the past, Smith said. The mesh detail allows him to solve for more information, including material stresses and contact-surface methods. That ability slashed analysis time because it allows separate parts, already created in a computer-aided design system, to be readily combined into a complete model of the entire assembly and then meshed.

"In this part, the pitch hinge, we have a housing, a shaft, and the bearings. All sorts of different parts are interacting," Smith said. "Before, you'd bring in all those parts from your CAD package and you'd have to mesh each of those separate parts, then get the meshes to line up exactly.

"Now, you bring them in as separate parts, you mesh each part individually, set up contact interaction between the parts, and you're done," he said. "You don't have to line up nodes or anything. There it is. That's how those two parts are going to interact."

The pitch-hinge assembly permits the Chinook helicopter blade to rotate on its longitudinal axis and to control thrust.

Smith said that, had he used gap-element technology—that is, fitting together the mesh of various parts—rather than contact-surface analysis, it would have taken him more than three weeks to build the model for analysis. Instead, he spent three days on the project. And that included constant design changes.

"I'd start building my model and then, a day later, a designer would give me something and say, 'Oops, sorry, this was off by an inch,' and I'd have to restart again."

Boeing saves money by using FEA software because the final design is more efficient, and it weighs and costs less to make than a design without extensive analysis. Engineers also can spend less time testing and changing the design. The analysis tells them what to change, so fewer tests are necessary to find design flaws.

Smith calls the bearing redesign a landmark project, in that it marked a change in philosophy of how analysis is performed at the Boeing rotorcraft plant. FEA was used in the past on other projects, even on past designs of the pitch-hinge assembly. But that analysis was generally used to support hand calculations of stresses on the parts. Plant engineers hope to use FEA software primarily to replace hand calculations in the near future.

From Oil to Teflon

The latest version of the pitch-hinge assembly called for using dry lubricated bearings. This, of course, would do away with the need for oil-lubricated roller bearings. Because the dry-lubricated bearings made of Teflon don't have a high-compressive strength—that is, they had a hard time withstanding the stress put on them—the engineers used wide bearings to spread the contact stresses.

"And, normally, the wide bearings would mean we'd be good to go," Smith said.

But the FEA analysis run by Smith showed that most of the bearing area was not actually being used to absorb the stress. The analysis showed contact stresses on the bearings at 90,000 pounds per square inch. The Teflon from which the dry-lubricated bearings were made could take only 30,000 psi. Those numbers added up to an obvious problem, Smith said.

"Imagine the housing rotates back-and-forth around the shaft," he said. "And this happens every time the shaft goes around. Every time the rotor spins around, you've got high-speed relative motion. The bearing material was initially yielding, but who knows how long that would last. There was no life to the bearing in that configuration."

After studying the analysis, Smith announced to designers: We have a problem here. He also called for them to come up with some ways to solve it. They brainstormed new ideas and Smith picked the ones to analyze, he said.

To redesign the assembly, Boeing engineers used finite element analysis software, which is a system of points, called nodes, that form a grid, or mesh, across a model.

He said that, instead of rebuilding the entire part assembly and reconfiguring the mesh—the tedious, time-consuming, three-week job the software had shortened—he could quickly change a part configuration and have that reflected in the mesh and, thus, in the analysis.

Engineers wanted to avoid a major project redesign. They'd thought they were done with the project and had budgeted accordingly. Smith noted that catching the problem before making a prototype, let alone the actual part, saved a huge expense because at that late stage in the project, redesigning the produced part would have carried a considerable cost. Still, instead of testing the part, as they logically would have done without the ability to quickly analyze designs, engineers used the analysis results as a guide and came up with another idea.

The designers, with Smith's help, decided to split the liners between bearing and shaft with a layer of rubber. They used a ring of rubber sandwiched between rings of steel, which allowed the shaft to deflect more stress and relieve some of the severe contact stresses made with the Teflon bearings. Contact stresses fell from 90,000 psi to about 18,000 psi.

The rubber and steel rings were original enough that Smith's department applied for a patent. If another company licenses the concept, the analysis might not only have saved Boeing money, it could also make money for the company, Smith said.

Analyze and Test

The analysis and redesign portion of the pitch-hinge assembly project took one year. Smith said he built so many models for analysis during that time that he got sick of looking at a particular version.

"I'd nearly complete the model and just then, it would go through a drastic design change," he said.

"Some people might say, 'A year, that's a long time for an analysis,' " he said. "But if you consider the amount of detail that went into it and the fact that it wasn't simply a matter of doing hand calculations and being done with it—there were a number of iterations—then, a year isn't too bad. Especially when you consider it saved us a million dollars."

The pitch-hinge assembly is now in the prototype and testing stage. Engineers recently ran a thermal-friction test to se the long-term wear pattern on the Teflon bearings. The pattern almost exactly matched the wear pattern predicted by FEA simulation. "That was the icing on the cake," Smith said. "The results were pretty darn close."

In the future, Smith's department plans to use FEA to model even more details to improve analysis accuracy. There are many more applications for detailed FEA that can benefit, he said.

The rotorcraft division's use of FEA was at the growing-pains stage before the pitch-hinge assembly project, Smith said. But the project proved that the benefits of improved design accuracy outweigh any of the growing pains.