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Designing Better Buses

Here's why a German bus manufacturer is implementing computer-aided engineering (CAE) capabilities in its product development cycle.

Optimizing and verifying the body structures of urban buses presents on-going challenges to the designers at MAN AG in Munich, Germany. There are various specifications from public transit authorities, styling issues, and serious product liability issues to contend with. MAN's customers, the public transit authorities in Germany, demand and get 10- and even 12-year warranties. "If something happens which is not covered by the guarantee, we have to fix it anyway or the agencies tell us they won't buy any more of our buses," said Dr. Ulrich Breitling, head of MAN's analysis operations in computer-aided engineering (CAE).

MAN, which is Germany's second-biggest bus builder, shipped about 2,000 buses in chassis in 1996. Half of them were exported. Most of the buses are assembled in a plant in Salzgitter with stamped components coming from a facility in Gustavburg.

One thing that makes buses similar to cars is that even though their service lives are becoming longer (in the case of buses, 15 years or longer), the life span of any given model is getting shorter due to rapid technical progress in design. Customers—in the case of buses, the urban transit officials—are also demanding modifications to standard vehicles. "These modifications and constant design changes pose difficulties for us because every bus MAN builds is different."

High product variability is always a challenge when management must rely on standardization and simplification to maintain high quality in manu-facturing operations. Ensuring the integrity, durability and reliability of each new bus body structure means the CAE environment is a demanding one.

What's more, there is plenty of competition, which is driving MAN management to insist that the time it takes for a bus design to get into production—currently four years—be minimized.

"Schedules for development are very compressed, and there is actually very little time and money for analysis," Dr. Breitling observed. "So it is not possible to develop the structural integrity required in modern vehicles without CAE tools." A key CAE tool employed at MAN is ANSYS from ANSYS, Inc. (Houston, PA). Due to time and cost constraints, not everything is fully analyzed: "We always have to be careful to match the correct analysis effort to the problem. To decide how much analysis must be done on each part, we look at how complex it is and how close to its limits it will be in service."

Most of the analysis is linear statics (less than 10% is nonlinear static); all of it is 3D. Linear statics is done with ANSYS superelements. ANSYS nonlinear is used for pure large deformation cases: wheel and axle movement, for example.

To simulate nonlinear behavior of a bus body in a rollover crash test, MAN uses ANSYS/LS-DYNA software. That package, which is an explicit solver, is also used for material nonlinearities and large deformations. This means that not only does it work for simulating crashes, but also for the metalforming processes involved in bus manufacture.

MAN also uses a multibody, dynamic simulation system called SIMPACK that was developed by MAN Technologie Corp. and the German aerospace agency DLR.

 

Development

To help assure that the vehicle structure is developed in a proper manner, MAN utilizes a simultaneous engineering approach. Early on, designers and analysts discuss what functions should be integrated for satisfying strength and stiffness requirements. Weight and cost reductions are two elements that play important roles in vehicle development. Although reducing manufacturing costs is a goal, it is second to reducing the risk associated with new designs and in cutting time to market. According to Dr. Christoph Schoettl, responsible for MAN's bus analysis, "The variety of specifications, liability, and unforeseeable service conditions requires that MAN make its bus structures insensitive to variations within the manufacturing process."

Much of the bus structure geometry is generated with CATIA from Dassault Systemes CAD software; I-DEAS from SDRC is also used. Geometry is transferred to the CAE system with IGES and the finite element mesh (generated by CATIA) is transferred directly. IBM RS/6000 and Silicon Graphics workstations are the hardware platforms.

To meet tight schedules, many of the designs are done in 2D rather than 3D, which means that it is necessary to recreate the geometries in ANSYS or in the I-DEAS geometry preprocessor. Each new bus body is analyzed as a whole using shell and solid representations. Depending on the amount of geometric information in the model, the first-pass analysis of a 12-m bus structure takes 500 to 1,000 man-hours—and that time can be doubled if severe problems are found in the structure design.

"With analysis we often find things we didn't expect," Dr. Breitling noted.

"The really important questions—such as whether all the major components out of about 10,000 parts are interacting correctly within this mechanical system—must be answered by theoretical analysis," Dr. Breitling said.

Yes, they do perform physical tests of the entire bus body, "But by then it is too late," Dr. Breitling remarked. AD&P