Stress Analysis Design Project

Our Mechanism's Interesting and Original Features

Our mechanism had a few interesting and original features built into it. Although we tried to create a project that was unique, we found that it was at times not wise to stray from common designs that have been used in previous years or by our peers. When building the base of our mechanism we considered how the aluminum plate would be clamped to the ground and as a result if we built our project correctly we could use this to our advantage.  We built a base that protruded from the aluminum plate diagonally. For the most part we noticed that other groups chose to build a platform with a horizontal beam-like structure sitting on top of it. We also built an arm-like structure in such a way as to induce twisting in a desired way. This twisting due to lifting the weight would occur such that the base plate would be affected the most. Since we took advantage of the fact that the base plate was clamped down, our mechanism did not end up deflecting very much and the weight lifted up to the calculated height of two and one-eighths inches.

The Design Team

Daniel Benvegnu is a sophomore mechanical engineer at Carnegie Mellon University. He is originally from Anaheim, California and graduated from Canyon High School. He likes to play basketball and video games. After completing his undergraduate Daniel wants to work overseas for Bechtel Inc. as an engineer.

Gabrielle Page is a sophomore at Carnegie Mellon University double majoring in biomedical and mechanical engineering. She is originally from St. Croix, Virgin Islands and graduated from Country Day School. She likes to play soccer. After completing her undergraduate degree Gabrielle is still unsure as to what it is she would like to do.

First Design Review

When creating our prototype we had to decide which window to build our structure through. If we made the structure go through the smaller window, we thought that the force of the weight on the servo would overcome the strength of our structure. As a result, this force would cause the structure to deflect downward across its length. Therefore we opted to build our structure through the larger window. Since the window does not lie directly in front of the clamps, we had to figure out how to build our base diagonally out to the hole. We decided on using a rectangular base protruding from the aluminum plate, angled towards the window, with supports to keep it steady under stress. We found that this particular geometry worked well considering that it had been our original idea to create a triangular arm protruding out from our rectangular base, and this geometry allowed us to easily assemble such an arm. After creating the arm and attaching the servo to the arm we concluded that it was pertinent that we include some kind of counter-weight to assist the servo in lifting the weight as the servo was not able to lift the weight on its own during preliminary trials. We then drew a free body diagram of the servo with an attached lifting mechanism as shown below. Analysis of this mechanism showed that the force created by the torque due to the servo combined with moment created by the counterweight about the servo is sufficient enough to lift a one pound weight. The lifting mechanism extends out towards the weight at one end of the servo and back towards the window at the other end of the servo. The placement of the servo relative to the weight was crucial in that the reduction of this distance “a” could greatly increase the lifting potential of the servo as shown in the FBD.

In order to achieve maximum height, we designed the arm such that when attaching the servo, the arm would be level with the weight. Since the maximum angle of rotation of the servo is 100 degrees, the arm should have no problem reaching a height of 2 inches given that the portion of the arm in contact with the weight is long enough. The end of the arm extended towards the window contains a counterweight to help raise the 1 lb weight 2 inches. In order to provide only downward force acting at the end of the arm, the counterweight was hung from a cylindrical rod so that the force was only applied at the point where it was attached to the arm (acting like a pivot) versus laying the counterweight along the length of the arm. Laying the counterweight along the length of the arm provides a distributive force along the arm which we felt would have less of an impact on the overall lifting potential of the lifting mechanism (arm + servo). As of now we cannot find a tool to cut the “U” –shaped aluminum piece we were provided with. As a result, the piece is too long and restricts the lifting mechanism from rotating fully. After careful analysis we determined that if we were able to cut the piece at a certain point the lifting mechanism would be able to rotate enough to lift the weight to the 2 inch mark.

Final Design Review

After the first design review we decided to stick with our original plan which involved using a counterweight on the arm to assist the servo in lifting the weight. We cut the “U”-shaped aluminum piece into thirds because if we cut the aluminum into larger pieces the counterweight would touch the bottom of the playing field when the servo was turned on and the arm attempted to lift the weight. The three pieces alone were not enough to create a moment that, when combined with the torque of the servo, would lift the weight.  We were able to obtain a second “U”-shaped aluminum piece and cut that into thirds also. We used four of the pieces of the aluminum piece for our counterweight. Two pieces were placed together with the U’s fitting inside each other forming a box and the same was done with the other two pieces. The two boxes were screwed together with one slightly higher than the other. Originally the counterweight was placed so that it was parallel to the arm. When the servo was turned on and the arm was put in motion the counterweight touched the bottom of the playing field preventing full rotation of the arm. We adjusted the counterweight so that it was no longer parallel to the arm but now was tilted upward at angle where the arm could rotate as much as possible before the piece touched the playing field. We tested the mechanism at this point in our design and the arm slipped on the weight when in motion preventing maximum force from the arm on to the weight. In order to prevent slipping we screwed a small piece of aluminum in the shape of a hook perpendicular to the arm. After testing the new arm on the mechanism the hook corrected the slipping and the arm was able to lift the weight two inches.

Images of Our Mechanism

Figure 1. Mechanism while lifting weight.

Figure 2. Lifting mechanism raising weight

Figure 3. Lifting mechanism.

We would like to thank Professor Paul Steif and the teaching assistant Sara Elizabeth Whitby for their assistance and guidance in completing the design project.