Traditional approaches to product development require considerable time and effort to design, build, and test prototypes. But design optimization software programs, coupled with finite element analysis (FEA), are helping engineers to shorten the process.

For example, Engineers at Rockford Powertrain Inc., a leading supplier of drivetrain components to heavy equipment manufacturers such as Caterpillar and John Deere, use design optimization software to substantially reduce the need for prototypes and laboratory tests. The result: better products brought to market in less time, which translates into faster response to customer needs.

Model verification

Before incorporating CAD software into its engineering process, Rockford’s senior product engineer, Gordon Cummings, put a design software package from Algor Inc. to the test. He evaluated the software capabilities by using it to analyze an end yoke, a drivetrain component popular in mining, construction, and agricultural equipment, that must withstand a variety of forces at constantly changing angles.

Some of these yokes are made from steel, not for strength, but to facilitate welding to tubing. Other types, called fitting or slip yokes don’t require welding. Thus, they can be manufactured from less expensive ductile iron. The ductile iron yoke selected for the software evaluation is a U-joint component in the driveshaft of a log skidder, Figure 1. In this application, the U-joints are exposed to considerable abuse as the skidder drags logs over rough terrain to a bark stripper. Because the yokes had been tested in the laboratory and experimental data was available, this data provided a good opportunity to validate results of the FEA program.

Results of the software analysis confirmed the earlier laboratory test results. Mr. Cummings commented, “A comparison to the parts from our fatigue testing machine shows failures along the clusters of finite elements with the highest stress levels in the software model.” Figure 2 illustrates the yoke stress pattern produced by the FEA program and Figure 3 shows a yoke that was deliberately broken in the company’s test laboratory at Rockford, Ill.

How the software works

Based on the satisfactory results of the yoke analysis, Rockford expanded its Algor software package to include several complimentary programs and began using them for both design optimization and failure prediction.

The design process starts with a Superdraw program, which enables users to create models of parts or import them from CAD packages. In either case, the size of the part is established by the scale drawing. At Rockford, an engineer typically imports a drawing from AutoCAD into the Superdraw program, then applies a finite element mesh to the part surfaces, using Supersurf. Then a program, called Hypergen, automatically generates a solid mesh of tetrahedronshaped elements (like a pyramid) based on the surface mesh.

Once the model is ready, the engineer adds boundary conditions and loads. This is where experience and good engineering judgment is essential to obtaining an accurate simulation of the part. Finally, a Stress Analysis program, as the name implies, calculates stresses in the part and pinpoints the probable failure location.

Though it takes 2 days or more to create a model, today’s finite element analysis software performs an analysis in 5 hr or less, depending on model complexity. Prior to using Hypergen, converting the model from a surface mesh to a solid mesh for analysis took a week or more.

Three years ago, a finite element analysis typically took about 8 hr to run on a 286 PC. To avoid a long wait, the engineer often left the PC on overnight, anticipating a solution the next morning. But more often than not, someone turned the power off or an electronic gremlin interrupted the run. Today, a 486-66DX PC with 8 MB of RAM and a 750 MB hard drive completes a comparable solution in 10 to 15 min.

Design optimization

Historically, engineers at the drivetrain manufacturer used traditional methods of calculating product parameters, such as stresses and endurance limits, then tested the product to verify the calculations. Their test facility has several test machines that generate torque and speed, one of which can spin a 50-lb part at up to 12,000 rpm to determine its burst speed.

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