This article illustrates tests in a digital laboratory that helped engineers ensure that a critical bracket would support the plastic-molded models increasingly favored by consumers. A case in point is Coachmen Recreational Vehicle Co. of Middlebury, IN. Coachmen engineers have used finite element analysis software from Pittsburgh-based Algor Inc. to test an innovative new dashboard under loading conditions experienced by a recreational vehicle (RV) while on the road. Coachmen Recreational Vehicle Co. designs and manufactures RVs, including motorhomes, travel trailers, fifth wheels, and truck campers for sale primarily in the United States and Canada. Coachmen recently introduced its Sportscoach model motorhome, a contemporary, affordable vehicle with an aerodynamic design and more features and ergonomic considerations than any other Coachmen product. The Industrial Designers Society of America awarded its 1998 bronze Industrial Design Excellence Award in the transportation category to the Coachmen Sportscoach, which was judged on its design innovation, user benefits, ecological responsibility, aesthetics, and appeal.
To stay competitive in a growing recreational vehicle (KV) market, RV manufacturers must design safe, aesthetic vehicles that require minimal maintenance. They must road test RVs for safety and durability to verify all components’ proper functioning. At the same time, they need to reduce time to market and cut costs to gain or maintain an edge. As a result, engineers in the industry are identifying new ways to simulate the functioning of components and systems on a computer using finite-element-analysis (FEA) tools.
A case in point is Coachmen Recreational Vehicle Co. of Middlebury, Ind. Coachmen engineers have used FEA software from Pittsburgh-based Algor Inc. to test an innovative new dashboard under loading conditions experienced by an RV while on the road.
Coachmen recently introduced its Sportscoach model motorhome, a contemporary, affordable vehicle with an aerodynamic design and more features and ergonomic considerations than any other Coachmen product. One of the innovations in the new Sportscoach is a plastic-molded, automotive-style dashboard that replaces the outdated, vinyl dashboards of older motorhomes. Coachmen used Algors linear-static stress-analysis software to analyze the new dashboard’s steel support structure after subjecting it to a static load using the computer. The company optimized the structure’s geometry by locating and strengthening its weak areas, and shortened the length of its design cycle in time to introduce the Sportscoach at the Recreational Vehicle Association trade show in Louisville, Ky.
The Comforts of Home
Coachmen Recreational Vehicle Co. designs and manufactures RVs, including motorhomes, travel trailers, fifth wheels, and truck campers for sale primarily in the United States and Canada. Coachmen’s Class A Motorhome Series is marketed to those who wish to explore the great outdoors for months at a time, but who also want to travel with the comforts of home.
Coachmen’s top-of-the-line Sportscoach was constructed more efficiently and ergonomically than previous models. Its new ABS plastic dashboard is tool-molded in one piece rather than assembled from numerous pieces, like the conventional vinyl dashboard. The dashboard is smaller and lighter, increasing cabin space and RV capacity, which enables customers to purchase additional features. It also gives drivers greater road visibility, while its gauges and instrument panel are larger and placed in the drivers line of sight.
Because the dashboard can be up to 96 inches wide, it requires a support structure beneath it. The steel support bracket that Coachmen designed for the plastic dashboard consists of four parallel bars that extend approximately 18 inches into the cabin from the front of the vehicle. They are connected by two perpendicular bars, one mounted to the front bulkhead of the vehicle and the other extending across the width of the dashboard.
Coachmen needed to test the strength of the new dashboard support structure to locate potential stress because the dashboard’s collapse could cause extensive injury or maintenance costs. Coachmen realized that the RVs low, wide dashboard invites people to sit or lean on it. The company determined that simulating the weight of a person sitting on the dashboard’s midsection and the weight of a person leaning on the dashboard’s side section above the glove box would represent the most severe stress profile experienced by an RV dashboard. Designing toward this profile would ensure that the dashboard could withstand other RV experiences, such as vibration during road travel.
“Producing and testing a dashboard prototype can cost $40,000, so the entire dashboard design, including the steel support structure, must be as close to the final design as possible when a prototype is constructed,” said Jim Keough, Coachmen’s engineering design manager. “Using FEA software to test the support bracket allowed us to quickly pinpoint problems with our design, and saved us the time and cost of two prototype tests.”
To prepare for the support structure’s stress analysis, Scott Schrock, product designer at Coachmen, created a model of half the dashboard support bracket, using AutoCAD 14 computer-aided-design (CAD) software from Autodesk Inc., in San Rafael, Calif. He modeled the passenger side in order to analyze stress results for the area above the glove box. Jim Keough imported the model into Algor to prepare for a finite-element analysis that would replicate the application of weight on the support bracket and reveal resulting stress. Keough used Algor’s Supergen automatic meshing tool to create a four-node quadrilateral mesh.
“Supergen quickly made the elements symmetrical in nonuniform areas, which saved me a lot of time,” said Keough. “In the past, I created a mesh around critical features like holes by hand, and made certain that the elements matched.”
Keough chose 3-D plate elements to best represent the steel bracket’s thin- walled material and orientation. “It was convenient for me to quickly alter the material’s thickness to optimize the design when it became necessary,” said Keough.
To determine the steel material’s yield point, Keough chose the Tresca stress yield criterion because of its conservative yield calculation for combined stress. Keough then applied steel material properties to the model based on standard steel purchasing requirements. He applied boundary conditions at the interface between the bracket and the front bulkhead of the vehicle.
Keough applied a 200-lb. static load to the left bar of the support bracket model, or “arm,” that represents the center of the dashboard support and a 100-lb. static load above the glove box. The 200 lbs. represented the average weight of a person sitting on the midsection of the dashboard and the 100 lbs. represented stored material in the glove box, as well as a person leaning on the passenger side of the dashboard. The goal of the analysis was to determine if these heavy cantilevered loads would exceed the yield stress of the steel bracket. If its yield stress was exceeded in the analysis, the bracket would not support the plastic dashboard covering under similar real-world conditions.
In the first analysis, Keough found excessive stress in the crossbar above the glove box. He increased the stiffness of the entire bracket support by increasing its material thickness. The stress remained excessive in the same area in the second analysis, so Keough added a one-inch flange to the crossbar above the glove box in order to increase its strength and reduce stress.
The third analysis revealed a new problem as a result of the thickened crossbar; some of the stress from the crossbar had shifted to the arm of the bracket support model. It also showed that the one-inch flange did not eliminate excessive stress above the crossbar. Keough added another one-inch flange to the top of the arm to compensate for the shifted load, and also added a Z channel to the length of the crossbar to stiffen the entire bracket assembly.
The final analysis showed that stress levels had been reduced below Coachmen’s limit of 25,000 psi except at the tip of the arm, which Keough attributed to point loading over a small area. Keough had successfully reduced stress by strengthening the support bracket in critical areas. He had completed crucial redesign steps before creating one physical prototype.
“The weakest point in the model was identified in each analysis,” Keough said. “I was able to easily work the weak point out of the design in each iteration to arrive at the final configuration.”
Coachmen’s advanced development department proceeded to manufacture a dashboard prototype. They tested the prototype in their laboratory, checking its fit, function, and response to loading. The results correlated closely with Algor’s software results. A final RV prototype was created and subjected to a lifetime durability test at an automotive test ground. The test simulates actual road events, such as railroad track crossings and other rough terrain. The road test also confirmed Algor’s software analysis results.
“The virtual manipulation of the design was much more cost effective and timely than the trial-and-error method of prototyping and testing each concept,” said Keough. “The evaluation of the bracket design would have taken weeks longer without the analysis.”
Keough also appreciated being able to share the visual results with his company with minimal effort. “To locate an area of interest, I don’t have to scroll through tables with elements and node numbers,” he said. “I can obtain a colored stress output, convert it to a bitmap file, and place it in a word-processing document for easy presentation to my colleagues.”
Coachmen did not study the effects of dynamic stress' on- the dashboard, such as vibration, because the company designed it with the most severe static load in mind. The support bracket inherently handles lighter loads like everyday vibration from road conditions. But Coachmen is expanding its design processes in the future to include composites, nonlinear and vibration analysis.