24-262 Stress Analysis Design Project

Spring 2012

Group 9:

Mike Pitz

Tim Kitka

How it Works:

Our structure was designed to be a large stationary truss with a lot of strength that allows a small moving contraption to reach the weight and lift it.  To get the strength we needed out of the flimsy aluminum strips, we bent all of the strips into a 90-degree angle lengthwise.  We then used these strips to create our main truss, which reached out to the weight (See Photo).  This truss had to stay sturdy under the force of the weight.  With the challenge of reaching the target weight accomplished, we next had to design a mechanism to lift the weight.  To do this we used a basic lever system in conjunction with the given motor.  With a hook under the weight, and a counterweight on the backside of the lever, we were able to maximize the limited amount of torque provide by the motor to lift the weight. Further breakdown of the lifting mechanism, along with the calculated lifted capacity can be found in the Performance section.

Performance:

For this project we recognized that the maximum torque of the motor, as well as the range of motion of the motor were not sufficient to meet the project requirements by itself.  To account for this we utilized the concepts of the mechanical advantage provided by a lever system.  The ideas behind our projected performance is as follows.

Torque:

With only 72 oz-in of torque provided by the motor, we came up with a design that used counterweights, along with the motor to supply the appropriate amount of torch to lift the 1lb weight. To do this we used moments about the point where the motor is connected.  With the target 1lb weight set 4 inches away from the motor, we calculated that a counterweight of  7 ounces set 9 inches from the motor would put the action arm in equilibrium about the motor shaft.  However, the motor shaft can also provide a moment, so we calculated that only a 4 ounce counterweight set at the same 9 inches would be necessary to lift the weight with respect to the torque proved by the motor.

Range of Motion:

With only 90 degrees of motion provide by the motor, our design utilizes yet another mechanical advantage provided by a lever mechanism.  This mechanical advantage is the amplification of motion by extending the motion arm radius.  While the motor only moves 90 degrees, by extending the motor wheel radius from ½ inch, which yields about 0.71 inches of motion, to 4 inches, which yields 5.66 inches of motion, we gained enough motion to fulfill the project requirement.

What part of your contraption is most interesting and original?

I think that our hooking mechanism was the most original design on our mechanism. Most other people used a simple bar with a notch cut out of it to grab the bar protruding from the weight. What we did was bend a piece of circular aluminum into a hook that allowed us to gain a little over a half inch when the arm was in its full upright position. This was wobbly on our second design review; however we were able to add screws in the side of the lever arm that solidified the hook in its “groove.”

What part are we most proud of?

What we were most proud of in our design was the very small defection that the truss as a whole experienced. Our design weighed in at 20 oz exactly and was extremely sturdy. We built the truss in a three-piece design and then were able to connect the three to make a very sturdy structure. This design included a base, a horizontal section, and then the final truss that placed our lever arm near the weight, we then added a support bar the stretched from the base to the truss section that added a significant amount of support. We were able to put the entire thing together in a very clean fashion, with no open “squares” in our trusses that would have taken away from the strength of the design.