24-262 Stress Analysis Project: Group 4

Eric Myers, Christian Schultz, Fred Bolio

How the Mechanism Works

For our mechanism we used the counter weight approach. We bent a 1 inch wide aluminum strip along its vertical axis to create an "L" shape that increased its bending resistance. We then cut a semicircle notch at the end of the moment arm for the screw on the weight to rest in. This notch prevented the weight from shifting and falling off the moment arm, which occurred occasionally at the apex of lift. At the opposite end of the moment arm we placed our counterweight which eased the reactive torque and stress put on the servo motor. With the motor firmly attached as close to the center axis of the I-beam as possible and enough clearance for the moment arm, our crane consistently lifted the 1 lb weight around 3 inches.

Theoretical Analysis

From the analysis of the crane arm, it was estimated that about 53.125% of the motors maximum torque is used to lift the 1 pound weight. Also, assuming a linear relationship between the angle of rotation and the amount of torque supplied by the motor, the height of the lift was approximated to be about 2.593. These results were fairly consistently with the observed results during testing as our crane was able to lift the weight a little over 3 inches on most test runs. The performance of the crane arm is most dependent on the weight and moment arm of the counterweight which we approximated to be 4 oz. and applied at the center of the U-block.

Unique Features

Our crane is a combination of a truss support structure and an I-beam arm to support the motor. Originally, we decided upon an all truss construction, but it failed to give enough support to the moment arm and deflected too much. To resolve the issue we kept the truss structure, but replaced the motor support with an I-beam and strut combination. This combination allowed for the counterweight style moment arm to lift the weight a full 2 with minimal deflection. This I-beam and strut combination allowed for us to complete the goal of the project after initially being below the standards of a final design. We are especially proud of the strength and resistance to deflection that our I-beam provides. Although it added a fair amount of weight to the overall system, it drastically increased the height and consistency of lift.