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Computerized Handbook Optimizes Cutting Machine Design

Closed form solutions and Finite Element analysis Enables Weight Reduction

Joseph O'Hara consulted the Desktop Engineer computerized handbook to help him remanufacture the company's 34,000-pound PVB cutting machine.
A computerized engineering handbook has enabled DuPont Consulting Mechanical Engineer Joseph S. O'Hara PE, to reduce the weight of a machine used to cut tough plastic.

The 34,000-pound machine cuts polyvinyl butyral (PVB), the material used as an interlayer in automotive windshields to prevent shattering. An earlier version of the machine experienced weld failures due to the high forces generated when cutting this material. To help relocate the welds to lower stress areas, O'Hara used a computerized engineering handbook to perform closed-form solutions and finite element analysis. The plant mezzanine where the machine was to be located, however, was unable to support the 40,000-pound weight of the initial design. So O'Hara performed multiple iterations of the analysis to optimize the cross-sections of structural members and reduce the weight of the machine.

PVB film has been used in windshields since 1938 and has applications in residential and commercial construction for windows, skylights, atriums, partitions, curtain walls, doors and roofs. Glass laminated with PVB will not shatter; even if the glass is broken, the opening will not be penetrated because glass fragments adhere to the interlayer. PVB is a pliable but very tough material that also has a very high strain rate sensitivity; this means that the faster it is cut, the more strongly it resists. O'Hara says he experienced the unique properties of PVB firsthand when a chunk of concrete fell off a truck and was thrown right at his windshield. The windshield broke but the PVB interlayer prevented the concrete from penetrating the interior of the vehicle, thereby saving his life.

Current processes for producing PVB sheet include recycling of some finished material as production input. In order to be re-used, these blocks must be cut up into slices that are small enough to go into a shredder. The block guillotine has a large blade that is driven through the PVB blocks by a hydraulic cylinder driving a modified four-bar linkage. These cutting forces generate reaction forces many times higher on various places on the machine frame.

O'Hara considered using a computer-aided design (CAD) and associated finite element analysis (FEA) program. But, he says, he knew that a considerable amount of time would be involved. He would have to define the geometry of the machine to a high level of detail in the CAD program and then build a finite element model of the machine which would require estimating stress transients in each area of the machine in order to size elements.

Computerized engineering handbook

Instead, O'Hara says he decided to turn to a computerized engineering handbook that is designed to reduce the time required to solve more than 5,000 common engineering problems typically referenced in more than 100 engineering reference books.

The Desktop Engineer, from Desktop Engineering International Inc., reportedly makes it possible for engineers to select the type of problem using a graphical user interface, then enter the required parameters in response to prompts. Input is automatically verified by the program. The program then generates documentation that shows all intermediate calculations.

The Desktop Engineer includes more than 50 modules grouped into the following categories: geometric analysis, static analysis, dynamic analysis and buckling analysis. These categories are used to analyze structures including straight beams, curved beams, cables, circular arches, circular rings, columns, discrete systems, disks, foundations, frames, grillages, helical springs, plates, shafts, shells and solids.

O'Hara says he began by modeling the existing machine. Next, using the frames module of The Desktop Engineer, O'Hara says he constructed a simple stick-model of the machine, input the section properties determined previously and the forces exerted by the linkage. In less than five minutes, the program automatically generated a simple finite element model, ran a finite element analysis and provided output that showed the stress on each node of the simple model. O'Hara says he then identified the loads in each area of the structure and confirmed that the welds that had failed were located in high-stress areas. The deflection output was used to optimize the cutting area of the machine, enhancing performance.


O'Hara relocated welds to areas of low stress and provided increased support in areas of high stress. However, these changes raised the weight of the machine about 30 percent above acceptable levels. He took advantage of the redo function on the computerized engineering handbook to perform a parametric analysis on the machine frame. He reduced the cross-sectional properties of the members in the frames module of the program and then re-ran the analysis.

O'Hara says he then switched to the thin-walled cross-sections module of the handbook. He says his goal was to find a cross-section that would provide the required strength without exceeding the weight limitation. He settled on a 12-inch by 12-inch square tubing with one inch thick walls for the base and top of the structure and a unique configuration consisting of three vertical and two horizontal members made of two-inch thick plate for the cross-members. Both of these members had to be specially formed. O'Hara also used the shaft section of the module to design the torque tube in the four-bar linkage and the beam section for the dead shaft. The final machine has a footprint of 10 feet by 12 feet and is about 6 feet tall.

O'Hara says his design met the requirements of the application. The machine has been in operation for over a year at DuPont's Parkersburg, WV, plant and has not experienced any failures nor any significant downtime.

O'Hara credits the computerized engineering handbook with the fact that the machine went from initial concept to production in only eight months, about half the time required for the previous machine. "The program can be learned in a day so it's perfect for an engineer that doesn't perform analysis on a regular basis," O'Hara says. "Yet it is powerful enough to analyze hundreds of design alternatives and iterate to an optimized solution in well under a day" w

For more information, contact Desktop Engineering International Inc., (800) 888-8680 or (201) 818-9700 or Fax (201) 818-9707


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