At left, the customer’s casting design produced 100% cracks in several locations in these rib areas. At right, Dalton used a stress simulation to show the customer the crack was built into the design and not caused by poor foundry practice.
gating and design changes and had a pretty good suspicion it was stress- related, we finally went to the cus- tomer and said, based on our experi- ence with the stress simulation, we think we need to run the part in the stress program,” Burita said. The customer agreed and Dal-
ton obtained the software to use through the design iterations of the part. According to Burita, the first simulation run in the stress program using the original design predicted stress in the locations the crack had been appearing perfectly, confirming the origin of the crack was built into the customer’s design and not the result of poor casting practice. “Te big thing was the significance
of the change that had to be made,” Burita said. “We wanted to make sure we were confident the design was going to work because we had metal tooling. Before we cut it, we wanted to make sure we were right.” Te engineers discovered the stress
in the casting was occurring in the original design because the base was so large and voluminous it took much longer to solidify than the rest of the casting. Te bottom was pull- ing down on the ribs that connected to the top rim, while the top was resisting because it was already frozen solid. Tis caused a pull where the rib attached to the sidewall, leading to the stress cracking. “There is nothing you could do
in the process to prevent, change or alter that,” Burita said. “The simulation helped prove that to the
Ten design iterations with modifications to the casting and rib design were modeled and analyzed before a final solution was found.
September 2015 MODERN CASTING | 41
customer. Seeing the actual picture really sold it.” Based on the simulation results,
Dalton engineers, along with the customer’s engineers, began making modifications to the design, even- tually working through 10 design changes before settling on one that indicated zero stress issues in the simulation.
The final, successful gear case housing design featured thin- ner ribs that were more curved in shape with a little more draft angle, which provided a more solidification-friendly transition from the diameter of the casting’s top, around the windows of the rim, and down to the massive base on the bottom.
The modified design (right) featured thinner, curved ribs compared to the original design.
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