Stress Analysis Design Project
24-262 Spring 2012
Group 29: Felix Chiu, Jacob Helmers, Lenny Rodriguez
Our design consists of three main areas: the servomotor and its arm, the beam that holds the servo assembly at one end and is held by a truss configuration at the other, and the truss configuration itself and its base. We used a flat square of aluminum as a mount for a frame that extends upwards and to one side in order to permit a single straight beam to reach through the smaller of the two holes to the weight. The frame consists of a main cube with diagonal trusses for stability, and a triangular extension out above one clamp to hold the long beam in place. The beam itself consists of two long U-shaped aluminum member bolted together sideways to form an I-beam. We initially chose the I-beam configuration for resistance to bending, and although it did exhibit some twisting in testing, the twisting was not so severe as to prevent a successful lift. At the far end of the beam, we mounted the servo underneath, and attached a roughly 8-inch long member to serve as lifting arm and counterweight arm. Lifting was carried out by a loop of aluminum that attached to the counterweight’s screw.
Assuming torque output of 72 oz-in and factoring in the 3.5-inch distance between the servomotor and the lifting point, the servomotor alone would ideally utilize not even 2/3 of its maximum torque to lift the 1-pound weight. However, in practice, our servo alone was unable to lift the weight more than a half an inch or so, and thus a counterweight was added to the lifting arm to increase upward force. This counterweight, weighing 12 ounces and mounted with its center of mass located roughly 2 inches from the servomotor’s connection to the lifting arm, contributed a maximum 24 oz-in of torque (when the arm is level). This proved to be enough to consistently lift the weight 2.5 inches.
As the lifting point was located 3.5 inches from the servomotor, our set-up’s geometry and a 90-degree turn from the servo would permit an ideal lift of almost 3.75 inches. However, due to decreasing torque from the counterweight as the arm rotates farther away from level, as well as a variety of other factors including some deflection in the long beam and imperfect output from the servo, this ideal maximum was not reached.