Stress Analysis Design Project
The goal of this project was to create a structure that could life a 1 pound weight a total of 2 inches, while following the constraints of the playing field, and weighing less than 20 ounces. The challenge was using the tools and materials we had available, and then creating a design that could not only lift the weight once, but then set it back down and lift it again within a 30 second time frame. The playing field is shown below:
The frame was designed from the triangular arm out. The trussed triangular arm was designed with both bending and torsion in mind so that when we started to lift the weight, the entire frame wouldn’t twist which would cause the frame to move instead of the weight. The connection between the base and the arm were built with the same idea, however with a square shape to reduce the difficulty of attaching the three parts. Note that the main aluminum beams are bent into an ‘L’ shape in order to reduce bending in both directions, not just one.
Our lifting mechanism simply consisted of a strip of Delrin with a v-shaped notch to keep the weight from shifting between iterations. The Delrin was used with the longer side in the vertical direction because we were more concerned with the Delrin bending as opposed to twisting. In order to prevent the servo from shifting and to transfer any twisting of the servo to the arm, we fabricated a small cradle for the servo in order to lock it in place. The cradle was made of aluminum bent into an L-shape in order to prevent bending in both directions, and it was placed as close to the triangular arm as possible without being inside the arm. In order to provide enough power to lift the weight, we cut off a section of the provided aluminum U-channel, and attached it to the Delrin via l-shaped aluminum strips. However, in order to give the arm full mobility, we had to create a bracket to attach the counterweight arm to allowing it to swing free of the arm as the servo is directly in line with the arm.
Going into the design process, we knew that the servo we were provided is capable of 72 oz-in of torque. Initially going for an entirely horizontal design so that the initial vertical force would be entirely upward, we did some basic math to show that 72 oz-in/16 oz = 4 in, meaning that the servo could be placed 4 inches from the attachment point on the weight, and still provide enough upward force in order to lift the weight the necessary 2 inches, because the servo had about 90 degrees of rotation.
The (above pictured) first iteration of our design did have an entirely horizontal lifting mechanism; however the base was not wide enough to prevent unreasonable beam deflection, and coupled with the shortness of our design, did not achieve necessary lift before the beam touched the wall. Therefore in our second iteration, we both increased the width of the base, and its height, so that we could better prevent beam deflection, and prevent the arm from touching the wall. However, in doing this we changed the geometry of our lifting mechanism, which called for the inclusion of the counterweight system we previously described. The counter weight provided approximately 11.2 oz-in of additional torque since the counterweight was placed 7 inches away and weighed 1.6 oz.
The two features that we felt were unique, and that we are most proud of, are both the servo mounting bracket we fabricated, as well as the counterweight arm mounting bracket we fabricated. Both of these features were instrumental in achieving our design goals in an efficient manner. We also included some of the following inspirational quotes, courtesy of Alex Clement, that were written on the trusses of the triangular arm:
“Oh don’t worry about it. Grades don’t matter anyways.” –Paul Steif
“There’s a new morning after pill for men, it changes your DNA”
“Einstein’s eyeballs are in a safe-deposit box in New York City”
“In Utah, it is illegal to swear in front of a dead person”
“The early bird may get the worm, but the second mouse always gets the cheese”
“’Yo Mamma’ jokes were first invented by Shakespeare”
“He who must perfect his work must first sharpen his tools.” -Confucius
Our final design weighed 16.4 oz, and had a final lift of 2.5 inches, which was able to be repeated. This fulfilled all of our design goals, in addition to following all playing field constraints.