To optimize, analyze

November 2, 1998

15 Min Read
To optimize, analyze

FEA drives motorcycle design

Software: MSC/NASTRAN for Windows

Major objective: Increase stiffness

Related CAD packages: SolidWorks, Pro/ENGINEER, I-DEAS

Milwaukee, WI--"We have always been an analysis-driven design company."

That's the way mechanical engineer Chris Vasiliotis describes the culture at Buell Motorcycle Co. (BMC). It's no accident. Despite their apparent simplicity, motorcycles are complex machines. "Failure of any system in the bike would be critical, and since we want our products to offer the highest in safety standards, we do a great deal of sophisticated design and analysis," he says.

Additionally, BMC whips through its design cycle in one year vs. the typical three years for competitors. That timetable eliminates the possibility of checking multiple prototypes. FEA has enabled engineers to cut the number of prototypes in half.

The analysis software they use: MSC/NASTRAN for Windows. The most recent application: system integration, and static and dynamic analysis of components in the 1999 Lightning X1, a "Hooligan," or sport bike.

Among the new features on the bike is a swingarm that carries the rear wheel, and a new frame designed to incorporate changes the company has been pioneering with structural analysis. The rear subframe, which holds the seat, rider, passenger, and luggage, is now a bare, exposed aluminum casting with a brushed finish covered by a clear coat.

Analysis drove several other frame modifications, such as the addition of a fourth tie-bar location near the front engine rubber isolator. Complementing this is a new frame member on the left side of the bike to reinforce the v-bracket, a key structural item for lateral and torsional stiffness that supports the lower front tie-bar. Previous analysis indicated that engineers could increase stiffness by incorporating both these frame changes, and test riding and racing confirmed the analysis.

The bike's Harley-Davidson engine has a natural vibration due to its single crankpin, unbalanced 45-deg V-twin configuration. BMC's patented uniplanar engine-mount system isolates the rider and passenger from the vibration, but engineers still had to design components near or on the engine to withstand fatigue.

"The MSC software has increased the performance characteristics of our design," says Vasiliotis. He adds that besides increased stiffness, benefits have included increased frame responsiveness, lower weight, higher strength, and increased safety.

Lesson learned. Stiffness analysis has provided some unexpected results for the engineers. "The engine is a large aluminum structure with many internal components that increase stiffness, so the way we model the frame for analysis assumes an infinitely rigid engine," says Vasiliotis. Up until now, that was a good assumption. But, now that engineers have been able to attain stiffer frames, they're re-thinking their original beliefs on engine stiffness. "The actual stiffness of the engine might not be as high as we originally thought," admits Vasiliotis.

Engineers say they are pleased with MSC/NASTRAN's modeler interface that allows them to bring in trimmed surface information through IGES from CAD packages such as SolidWorks, Pro/ENGINEER, and I-DEAS. They also like the software's performance on the NT platform. "With dual Pentium II processors, we can initiate an analysis run and continue modeling, increasing efficiency," Vasiliotis asserts.

Regarding the reduction in prototypes: Engineers make several passes on a design before they create prototypes. "You can determine where the main deflections are, where the major issues are with the overall design, and start correcting them," Vasiliotis says. "You can go miles beyond what an initial prototype would indicate just by doing a few days of analysis runs."

Telescope design focuses on controlling vibrations

Software: ANSYS

Major objectives: Optimize and verify design that couldn't prototype

Related CAD packages: AutoCAD, Euclid

Cerro Paranal, Chile--With cosmic understatement, engineers from the European Southern Observatory (ESO) dubbed the telescope system they are designing to be placed atop a mountain in northern Chile the VLT (Very Large Telescope). Very large indeed. The system includes four telescopes with 27-ft (8.2m)-diameter, 7-inch-thick, 26-ton mirrors. ESO says the telescopes will give astronomers a view three-fourths of the way to the edge of the universe.

Engineers used ANSYS FEA and ANSYS/FLOTRAN computational fluid dynamics software in the project. Among the most important tasks with the software: optimize and verify design of the mounting cells for the primary mirrors, called the M1 cells.

Constraining the mirrors' shapes are 150 computerized actuators with 150 hydraulic mounting pads in six concentric rings. The axial and lateral supports automatically adapt to changing forces generated as the mirrors' orientations change during observations of the heavens. Sixty-four actuators also constrain each mirror passively. Engineers used ANSYS to optimize design of the active and passive hydraulic support systems.

Partial redesign. The analysis of the M1 cell structures led engineers to a partial redesign when they found the natural eigenfrequencies of the cells to be too low. They rearranged base frames and reinforced beam cross sections to meet stiffness and mass requirements.

Thanks to the precision of the finite element analysis, engineers eliminated physical prototypes from the design cycle for the M1 cells.

Controlling vibrations is critical in telescope design because they can reduce picture quality. Astronomical observations require long exposure times, and the telescope has to follow a star as smoothly as possible. Among the disturbance sources are wind, bearing friction, encoder inaccuracies, and tachometer and motor torque ripple.

To avoid resonance distortions in the telescope structure during celestial observations, engineers specified the important eigenfrequencies far away from each other. The higher the lowest mechanical eigenfrequency, the higher the bandwidth of the position loop and the shorter the time required for system transients to decay.

Beyond analyzing the cells, engineers used ANSYS/FLOTRAN to optimize design of the partially open structures that house the telescopes. Nearly 100 ft high and 100 ft in diameter, the structures have to withstand wind that averages 22-35 mph, with gusts as high as 120 mph. Engineers also used ANSYS/FLOTRAN to analyze air turbulence and pressure distribution over the surface of the mirrors.

Though they won't provide a cost estimate due to project complexity, engineers say that without FEA they would have had to use hand calculations, which would be less accurate, and physical testing. Therefore, they say, FEA helped them cut costs, improve telescope-system performance, and shorten the design cycle.

Event simulation makes doors shipshape

Software: ALGOR

Major objective: Design optimization and shock testing

Related CAD packages: AutoCAD

Pascagoula, MS--Among the least conspicuous yet most important components of any ship are the doors. There can be more than 100 of them within a ship or on its exterior in direct contact with water and wave splash. They have to close tightly and form a watertight seal to guard the ship's compartments from being flooded if damage causes a leak. They also block the spread of noxious fumes and slow the spread of fires.

The Ingalls Shipbuilding division of Litton Industries recently developed a new quick-acting watertight door for military use. To optimize the design and meet the U.S. Navy's water-pressure requirements, Ingalls engineers used stress analysis and Mechanical Event Simulation from ALGOR. They also used ALGOR's Mechanical Event Simulation software to simulate a shock test of the door.

Ingalls' doors are lighter, require less maintenance, have a more advanced sealing mechanism, and are less detectable by enemy radar than traditional watertight ship doors, the company says. The balsa wood and fiberglass composite door panel is about half as light as traditional carbon steel doors. The new door also has a bulkhead support built into its frame that separates compartments.

When turbulent waters cause the ship's decks to move in opposite directions, the bulkhead experiences shearing loads that concentrate around the doors. Existing doors distort and then fail prematurely as a result of shearing loads, engineers say, causing the bulkhead to experience plastic deformation. Ingalls' door has a stiff ring of material around its frame that absorbs the shearing path to reinforce the door.

The door has a metal, corrosion-resistant sealing mechanism with six to eight latches that open and close the door, forming a watertight seal. Engineers say the seal is stronger than traditional seals because its soft, spongy, neoprene gasket compresses against the bulkhead.

Laboratory hydrostatic tests at Ingalls call for doors to go into a tank filled with enough water to produce the Navy's required water-pressure level, then mount on a test wall to see if they open and close as designed. Each test costs about $3,000, engineers say. To cut that cost, Ingalls used ALGOR's linear and nonlinear stress analysis capabilities.

First, they designed a model of the door in AutoCAD, then used ALGOR's HoudiniTM program to convert it to a 3D solid brick finite element model. Next, they added a solid brick model of the sealing mechanism--created by Hartwell Corp., who tested it with ALGOR's linear stress analysis software--to the door model.

Subsequent linear stress analyses with ALGOR's composite processor revealed excessive stress on the door's composite surface under water pressure that would cause cracking and peeling. To uncover specific deflections in the door under varying pressure loads over time, engineers used ALGOR's Accupak/VE Mechanical Event Simulation for Virtual Prototyping with Linear and Nonlinear Analysis. The nonlinear analysis indicated excessive stress and deflection in the core portion of the door panel. To get the added flexibility the panel needed, engineers reduced its thickness.

Shock tests. Having optimized the door's geometry, engineers now turned to testing it for shock. Military ship doors have to withstand the intense vibration caused by the accelerated movement of a ship responding to a severe disturbance like a storm or mine explosion.

Traditional shock tests at Ingalls take place in a laboratory, where engineers drop a large weight on a shock table where the component rests. The vibration causes the component to bounce. Engineers then mount it on a test wall to see if it will open and close as designed. Each shock test costs between $5,000 and $15,000 and can take two weeks.

Mechanical Event Simulation, says ALGOR, doesn't require engineers to define input loads because the physics define them during the event.

The virtual shock test performed with Accupak/VE software was identical to the laboratory test, Ingalls says. The simulation calculated stresses in the door over time caused by the dropped weight, and engineers compared the stress values to the material property's yield values. They were only concerned with the effect of the initial bounce because it represented the maximum acceleration force from a ship's thrusting movement. Result: no catastrophic deflection in the door or any of its components.

Ingalls engineers say that analyzing the door on the computer vs the lab saved months of time and thousands of dollars.

Finally, they can mesh the model

By  Laurie Peach, Associate Editor

Software: COSMOS/Works

Major Objective: mesh complex parts

Related CAD package: SolidWorks

Nichols Aircraft Div. of Parker Hannifin Corp. (Ayer, MA) issued this challenge to finite element analysis software vendors:

Analyze the lubrication housing of an airplane engine--a labyrinth of input and output connections, valves, tubes, debris monitors, and filters responsible for breathing life through the veins and arteries of the machine.

Initially, most accepted. After receiving the IGES computer file from Nichols, most politely declined.

So the story goes, says Richard Friedman, principle engineer for Nichols.

Structural Research & Analysis Consulting Group (SRAC, Los Angeles, CA), however, didn't cower. Instead, they unleashed COSMOS/Works on the problem.

"The software is amazing," says Friedman. "What you think of today, you can do tomorrow. Before, if I had been asked to analyze a pump housing such as the one we designed for the new Rolls Royce Trent 500 Engine, I would have asked, what year am I retiring?"

Nichols designs lubrication and scavenge pumps for the global aircraft, marine, and defense markets. In cars, oil recycles from the pan to oil pump via gravity. But gas-turbine airplane engines aren't gravity drained. A pump housing incorporates several pumping elements that either lubricate or "scavenge" spent oil for recirculation, sometimes drawing the black gold from as far away as 30 ft.

The customer, Rolls Royce, specifies the requirements such as the lubrication needs for gearbox bearings and the engine. The lubricating is the easy part. Only one section of the pumping system serves as a lubrication device for the plane, the rest are for scavenging, says Friedman.

The Rolls Royce Trent cast aluminum pump housing is a complicated maze of interior tubes and curved surfaces made from an investment casting. It contains seven pumps in all. Because space is tight, engineers stack them on top of one another. "We need to consider the location of the pumping element in relation to the interface connection locations from the customer," says Friedman.

With COSMOS, Friedman took less than 24 hours to mesh the complex housing model. "This is the real break through," he says. "We've been making solid models since solid modeling first came out. Because of the complex topography of the surfaces and the geometry inside, we weren't able to obtain a quick mesh until COSMOS."

Older software meshes would fail or result in extreme simplification of the model. If Nichols sent designs out, software companies would say, "this is the most complex geometry we've ever seen" and admit they couldn't analyze it.

Such was the case when Nichols worked on a similar project prior to the Trent 500. When SRAC saw the housing, they agreed it was the most complicated they had ever seen and meshed the part before accepting the consultation. The analysis was completed without suppression of any of the features. Nichols purchased the software and it arrived just in time to support the Trent 500 analysis.

In addition to the analysis itself, one of the biggest problems Friedman encountered was transferring the geometry of the SolidWorks model to the analysis programs he tried. "We had IGES transfer problems, and then even bigger problems with the meshing capabilities of the analysis systems. We just couldn't get to first base with any of the programs we tried, and they were very well known systems."

Nichols developed propriety software to design the pump housing. Because time to market is so critical, their customer can't take a long time getting to market, says Friedman. In the case of the Trent 500, engineers took only 4 1/2 months from contract reward to the critical design review, setting a record for such a complicated part.

"Ten years ago, we couldn't have done this kind of project at all," says Friedman. Nichols expects the pump to be in full operation by 2003.

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