Spring 2012 Stress Analysis: Design Project
Ben Bamberger, Nathan Cheek, Ross Farquharson
Our first decision was whether we wanted to attempt to achieve the biggest lift or lightest project. We decided to try for the lightest weight as we believed we had a sleek design. The goal for us was to get our motor to the weight using the shortest distance possible between the two points, while still creating a solid structure. The shortest distance was a straight line between the two pieces through the smallest opening. This also reduced the axes for moments that could occur, as it took away the ninety degree turns that were the alternative to a diagonal piece.
In order to ensure that enough torque would be applied by the motor to move the weight, we needed to make sure that when the motor was triggered, our structure would move as little as possible. In our initial design, we used two L-shaped pieces stacked on top of each other as our extending arm. The torque of the motor caused this first design to twist. We then switched to a design with the motor supported by two L-shaped pieces placed back to back, creating a T-shaped bar, with additional support pieces placed near the motor. This reduced the torsion and the bend on the arm.
Since we were not trying to lift the arm very high, we used a short L-bar as the lever arm from the motor. The closer to the motor that the weight was applied on the lever arm, the less torque it took to lift the weight (T=Fd). We needed to calculate the degrees that the motor turned in order to determine how long to make our lever arm. The movement of the weight is a function based on the length of the lever arm and the rotation of the motor. With the motor placed about three inches away from the weight, the torque and distance it lifted created a reasonable balance. It was far enough away that it had the angular capability to lift the weight two inches, but was also close enough to the weight that it did not produce too much torque too lift.
Our next step was to begin trying to make it lighter. We had to figure out where pieces could be reduced or exchanged for smaller pieces. Overall we believed our design was strong, but there was definitely room for improvement.
The biggest issues with our first design were that it was heavier than we wanted and only lifted the weight 1.5 inches. The T beam reaching out to the weight would often flex and twist, reducing the effect of the lift arm. Our first change was to completely redo the long arm that extends out to hold the motor. Our original arm required a four piece + beam and our new design was able to achieve better results with only two pieces in a better made “T” shape. We were able to accomplish this by using two thicker pieces to replace the 4 smaller pieces. We also removed some of the supports that we saw as unnecessary in order to cut down weight.
Now that we were able to lift the weight with less supports, our next step was to cut down the weight of our remaining pieces. While we were able to reduce the weight some, we would have had to reconstruct our entire project in order to weigh less than the current lowest group. Because we were able to lift the weight 2 inches each time, we opted to settle for a successful lift at 11.2 ounces.