Group #18 Stress Analysis Writeup

Bruno Berry Nia Christian Allen Miller

Our task was to lift a 2 pound weight at least 2 inches with the constraints presented by the playing field. Our solution was to build a rectangular truss with the lever arm acting inside of the truss to negate twisting moments along the axis of the truss. We chose to use the smaller and closer of the two holes to minimize the overall weight and possible deflections of our truss. We increased the area moment of inertia by bending the strips of aluminum we were given into L-bars. This process reduced the deflection of the constituent members and overall truss under load. The maximum torque delivered by the servo motor was 72 oz-in or 4.5 lb-in (specified by the manufacturer). We added a 0.1875 lb counterweight to the opposite side of the lever arm to increase the effective moment created by the servo motor, such that the total arm length was 5 in with the servo motor placed in the middle of the lever arm. In testing, it was found that the 90 degree angle of rotation from the servo would be sufficient to lift the weight at least 2 in. The servo motor was predicted to use 100% of its maximum torque to lift the 2 lb weight:

(Torque of motor) + (L/2)(Weight) (L/2)(Counterweight) = 0

(4.5 lb-in) + (L/2)(2 lb) (L/2)(0.1875 lb) = 0

L/2 = 2.48, Total length of arm = L = 4.96

The truss incorporated several design features our team is especially proud of. As mentioned prior, the lever arm was placed inside of the truss such that the force the lever arm exerts on the truss to lift the weight will not cause a torque along the axis of the truss. The torque the servo motor had to exert was lessened by attaching a counterweight to the opposite end of the lever arm to provide a counteracting moment. Material from the base of the structure was removed to lower the weight of the structure such that the counterweight could be added.

Photo 1 :D3:38 PM 02/05/2013Photo 2 :D