MIDWEST CONVEYOR/DYNAMIC ANALYSIS TAP ALGOR FEA FOR CHRYSLER CAR CARRIER DESIGN

The new carrier, designed and constructed by Midwest Conveyor, tilts a Jeep Grand Cherokee to a 37 degree angle to reduce worker stress and fatigue. Photo courtesy of Chrysler Corporation.

Component for newest Chrysler assembly plant helps reduce worker injuries and increase productivity. Use of FEA reduces prototype requirements and saves Midwest Conveyor time and money.

Automobile assembly plants are not what they used to be. That's because assembly workers are not what they used to be. Chrysler's new plant in Detroit, which began production of the new 1993 Jeep Grand Cherokee in January, is an ergonomics showplace. The Jefferson North Assembly Plant has a variety of innovations designed to make workers safer and more productive.

Because most of the Jefferson North workers are being rehired from the ranks of laid-off personnel, they are much older than typical auto workers. According to Aaron Taylor, President of United Auto Workers Local 7, "Ergonomics are very important because the average age of the plant's workforce is 51. Their years of experience are a great asset, so we've planned ways to help them work smarter, not harder."

One of the toughest jobs in the old Plant, built in 1907, was working under the vehicle. It meant standing in a pit and working directly overhead. This led to injuries and lost time. In designing the new plant, Chrysler called upon Midwest Conveyor, Inc. of Kansas City, Kansas, to design a totally new carrier that would, in effect, bring the vehicle to the worker.

Critical Constraints

The carrier, called a "Trim Carrier", had to meet a large number of critical design criteria dictated primarily by the access requirements of robots used in the assembly process. This design "envelope" resulted in a geometric shape that was quite complex. Adding to the complexity was the fact that the carrier was required to tilt 20 degrees fore-and-aft and 37.5 degrees side-to-side to allow access to the underside of the vehicle.

The carrier, with the vehicle mounted on it, moves along the assembly line on a trolley-like attachment at its top. The side-to-side tilting is accomplished by a rail, similar to a roller coaster, which comes into contact with wheels on the carrier's bottom edge.

This enhanced version of the actual FEA model used in the analyses was prepared for presentation purposes by Mike Porter of Dynamic Analysis, Inc.

Initial Design

According to Robert Jensen, V.P., Engineering for Midwest Conveyor, "We began by doing a preliminary design for the carrier using conventional 'pencil and paper' methods. The resulting geometry included primary structural members with asymmetric bending and torsion loads. Our design engineers suspected that this complex geometry might create higher than acceptable stresses in several areas. "

Due to these suspicions," continues Mr. Jensen, "and because our customer required a fast turnaround, we decided to perform a finite element analysis prior to constructing a costly, time-consuming prototype."

Robert Jenson, V.P., Engineering for Midwest Conveyor Company (right) and Dynamic Analysis Principal Engineer Mike Porter with FEA model of new car carrier.

FEA by Dynamic Analysis

Midwest Conveyor is a longtime Algor customer. However, in this case, the company decided to get an "outside" review of the design. Mike Porter, Principal Engineer for Dynamic Analysis of Leawood, Kansas was asked to construct an Algor FEA model and perform a series of analyses.

The initial model, constructed using beam and plate elements, was analyzed for both static and dynamic loading. In keeping with the time constraints, Mr. Porter had analysis results in just five days. The analyses confirmed the suspicions of the engineers at Midwest Conveyor. The deflections at several points would be too high and stresses on several of the structural members were above acceptable levels. Revisions to the original design were obviously needed. "

One of the difficulties in the analysis was the multiplicity of loading conditions," says Mr. Porter. "The loads imposed by the tilting mechanism and the weight of the vehicle added a significant degree of complexity. Without the superb modeling environment of Superdraw II, setting up these loading conditions would have proved an even more formidable task."

Changes Required

Based upon the results of Mr. Porter's initial analyses, several design changes were made, primarily in the shape of major structural components. The Algor FEA model was revised and a new series of analyses were performed. According to Mr. Porter, "Analysis of the new model indicated that the design would meet established deflection and stress criteria. A prototype was constructed based on the design changes. Tests performed on the prototype confirmed the analysis results."

FEA Saves Time/Money

According to Midwest Conveyor's Robert Jensen, "The use of FEA as a design tool resulted in savings in both time and money. The elimination of the first prototype, which would not have been adequate, offset the cost of the analyses."

In addition," continues Mr. Jensen, "several areas of high stress that were identified on the model might not have been so apparent in the prototype and could have slipped through into the final product. Retrofitting the carriers to fix such a problem would have been considerably more expensive than the analysis. Finally, the analysis resulted in the shortening of the design process by several weeks."