For our mechanism, we decided to have three sections that joined together to form the overall structure. The base was made tapping directly into four solid aluminum rods. We chose this because, they were sturdier in bending and torsion relative to their weight compared to a truss strucuture. Each rod has its own aluminum base which provided a means to clamp the rods to the platform and help stabilize the structure. These four rods were connected to a four-sided truss, which extended laterally until it was colinear with the bigger opening. Each face of this structure had cross beams that helped prevent torsion and bending. Connected to this was a triangular truss that extented perpendicularly through the hole towards the weight This truss had stabilizing beams that ran perpendicular to the three main beams that formed the truss. These again prevented the arm from bending downwards from the weight. Attached at the end of this, was the actual lifting mechanism. This consisted of a servo-motor connected to a T-beam. One end of the beam had two screws that stopped the weight from slipping off the lifting arm at extreme angles. The other end had a four ounce counterweight. This weight was placed to maximize lifting moment while still not touching the ground at the top of the lift. It was also centered so that it did not cause a twisting moment along the axis running along the length of the T-beam. These truss connection, lifting mechanism, and base connection are shown in more detail in the images below.
For the purpose of our analysis we assumed that the servo exerted 42 oz-in of torque and had a range of motion of 100 degrees.
One calculation was done to make sure that the vertical displacement of the arm would indeed be more than two inches. We assumed that the motor had a range of motion that was 50 degrees below the horizontal and 50 degrees above the horizontal.
D1 = Lsin(45) = 2.5sin(45) = 1.77"
D2 = Lsin(55) = 2.5sin(55) = 2.04"
Total Distance = D1 + D2 = 1.77 + 2.04 = 3.81"
The other calculation we had to do was to make sure that the servo could provide enough torque to lift the weight. These were done at the maximum angle (55 degrees) and horizontal.
Calculations for horizontal:
Total Moment = L1*W1 - M - L2*W2 = 2.5*16 - 42 - 5*4 = -22 < 0
Calculations for theta = 55 degrees
Total Moment = D1*W1 - M - D2*W2 = 1.43*16 - 42 - 2.86*4 = -30.56 < 0
Since both of these calculations result in a total negative moment, the servo was assumed to be able to lift the load to the maximum height.
The most original and interesting feature of our project was the use of tapped cylindrical rods for the base. These allowed a strong yet lightweight base for our mechanism. Some other groups used the rods for their base. However, we didn't notice any other groups tapping directly into the aluminum rods like we did. This provided a unique means of mounting the lateral truss onto the rods. The other interesting feature was the transition from a four-sided truss to a three-sided truss. This provided a stable structure for our lifting mechanism.