Stress Project Group 3

Steven Oetjen          Joshua Eickmeier          Edward Burns

The truss structure, held down at four points by the clamps, goes through the larger hole in the obstacle and supports the servo within range of the weight. The arm, supported by the servo, has a counterweight on one end to increase the effective lifting torque, and a hook on the other end to catch the weight. The aluminum strips that were to undergo compression, or significant torsion and bending were bent along their length to resist these loads. The counterweight was made of a cut aluminum U-channel and delrin strips, bolted together. The total weight of the structure, servo, arm, and counterweight is 18.5 oz.

We tested our servomotor using a bottle of water which we filled to different amounts of weight. After lifting a maximum of 9.3 ounces at a distance of 4.75 inches, we determined that our motor’s practical maximum torque was 44.2 oz-inches. As a rough estimate, we figured that our motor needed to lift 16 ounces at a distance of 5 inches, meaning it roughly needed to have a total torque of 80 oz-inches. To meet this goal and have a reasonable factor of safety, we added a counterweight of 4.2 ounces at a distance of ten inches back from the servomotor. This additional torque gave us a total theoretical torque of roughly 86 oz-inches.

The use of a counterweight produced an increased effective lifting torque, which allowed the use of a longer lever arm, ultimately resulting in a larger lift distance.
The low weight of the structure itself is notable. Having such a lightweight structure gave the group the flexibility to add a fairly heavy counterweight (4.2 oz) to the arm without exceeding the maximum total weight of 20 oz.
The hook on the end of the lifting arm allowed the weight to be held at the right location of the arm without slipping off, no matter the angle to which the arm was raised.