Humans have been using tools to make farming easier since the dawn of civilization. Since the turn of the 20th century, the growing use of mechanized power to till and sow fields has enabled farmers to realize incredible efficiency gains in the production of low-cost, safe, and nutritious foods and other crops, such as fibers. Today, bigger machines require less manpower to plant the crops that feed and clothe a swelling world population.
Humans have been using tools to make farming easier since the dawn of civilization. Since the turn of the 20th century, the growing use of mechanized power to till and sow fields has enabled farmers to realize incredible efficiency gains in the production of low-cost, safe, and nutritious foods and other crops, such as fibers. Today, bigger machines require less manpower to plant the crops that feed and clothe a swelling world population.
Unverferth Manufacturing has been a manufacturer and supplier of innovative agricultural equipment since 1948. In order to design a more robust product and meet a tight deadline, Unverferth recently used ALGOR finite element analysis (FEA) software to speed up the design cycle for their new 12-row, folding- frame Ripper-Stripper® strip-till subsoiler, which prepares 10"-wide seed beds spaced 40" apart, a width that is commonly used in cotton production.
Through 2001 and 2002, engineers at Unverferth used the software on a number of projects, most of which involved the analysis of single parts or small assemblies. At the end of 2002, the much larger Ripper-Stripper project presented itself. The Ripper-Stripper subsoilers are designed to prepare seed beds for planting by cutting through growth from the prior year. The subsoiler’s frame attaches to a tractor using a standard three-point hitch. Pointed, curved shanks extend below the frame to break up soil as deep as 18". By breaking up the soil, roots, moisture, and nutrients can move freely through the subsoil to promote plant growth. The new addition to this product line would save cotton farmers time by preparing more rows simultaneously.
Work on the project began in late autumn of 2002. Since cotton planting begins in southern U.S. states in February, Unverferth had less than three months to design and produce prototypes of the new product and get them into the fields for testing. The new design would not only accommodate up to 12 40" rows, but would need to incorporate a folding frame, which increased the complexity of the design and introduced additional loading scenarios. The folding frame had to consider the loads in the hinges as well as the cylinder loads needed to fold the frame. Engineers started with a few ideas that would not completely change the design from the smaller version and would be made of parts that were already in stock for existing products.
The design was modeled in Autodesk’s Mechanical Desktop, and the assembly was then brought into ALGOR. InCAD technology was then used to directly capture the 177-part assembly for a series of linear static stress analyses. Various mesh sizes were tried before finding one that was suitable. The design uses a lot of tubing with thin walls, so the mesh had to be large enough so the analysis would run quickly, but small enough to accurately capture the detail around the weld joints.
The design was then put through its paces with a series of linear static stress analyses that simulated conditions the subsoiler would experience while pulling through hardpan soil, lifting out of the ground, folding up its 10' wings, and being transported. For each analysis iteration, displacement and von Mises stress results were examined. Stresses were then compared to the material yield point with a factor of safety applied.
The overall strategy was to first optimize the geometry, including the thickness and shape of the components, to distribute the loads as much as possible while minimizing the weight, and then consider stronger steel alloys for high-stress areas. The size of the parts that used more expensive alloys was minimized to control material costs. Making changes was complicated by the fact that the subsoiler has a number of possible configurations to accommodate different widths of rows and allow for a variety of attachments.
A calculated "pull" load simulated the stresses the subsoiler would experience when the shanks pull through approximately 18 inches of soil. A "lift”" load simulated the stresses that would be produced when the subsoiler was lifted out of the soil. Based on the simulation results, the shape of the large mast plate in the center of the subsoiler was optimized to reduce stresses to an acceptable range without adding weight. After optimizing this part, the pull test was rerun to verify that these changes did not affect the performance of the subsoiler.
Another simulation was a worst-case scenario involving an attempt to fold the frame under full tractor hydraulic power. With the frame in its flat operating position, constraints were applied to the hitch and the two ends of the frame. The full hydraulic load was applied as forces to the wings. Analysis results revealed high stresses in the hinge area. After a half-dozen iterations to optimize the design for the folding load, the team repeated the first two loading scenarios and found that the new design did not pass the pull test. Further alterations had to be made to the hinge to accommodate all three load cases.
The final scenario that the team considered was road transport, in which the fully constrained hitch must bear the dead weight of the assembly. The least demanding of the four scenarios, the analysis verified that the design would withstand road transport.
Overall, there were nearly three dozen iterations performed. Using the FEA software, Unverferth was able to increase the capacity of the frame tenfold with a total weight increase of only about 60 pounds of steel — less than 1% of the total weight.
The Ripper-Stripper subsoiler exceeded field-test expectations during the 2003 planting season and is available for this year’s planting season.
This work was done by Unverferth Manufacturing Co. (www.unverferth.com) in cooperation with ALGOR, Inc. For more information, contact Julie Halapchuk of ALGOR at Tel: 412-967-2700, ext. 3029; e-mail:
