Stress Analysis Design Project 2012

Christopher Wysocki, Scott Gerfen, and Kevin Quintana

Basic Function

This variant of the design project functions in a manner suspiciously similar to (most) projects submitted by other teams: A support tower is centered on a square base, which clamps securely to the playing field at two points. Attached to this tower is a beam (hereafter referred to as the “main arm”) extending from the tower to a point near the target weight. The servo motor is affixed at the far end of the main arm, and a lever arm is in turn affixed to the motor; in the case of this particular project, a loop on the end of the lever arm engages and lifts the weight.

Support tower

Main arm

Servo and lever arm

Distinguishing Characteristics

This design takes the slightly unconventional route of using only a single beam (or, at the least, a construct equivalent to a single beam) for the main arm, instead of parallel beams connected crosswise, or some construction with a rectangular or triangular cross section. This single-beam main arm primarily serves to minimize the weight of the entire structure; in addition, the elimination of braces and supports on the arm reduces bending due to the inherent weakness at connectors.

Also of some note is the cross section of the main arm—by moving mass away from the center, the cross-shaped cross section decreases the tendency of the member to twist, while remaining relatively easy to assemble. As two arms of the cross are parallel to the plane of bending, an appreciable resistance to that manner of deformation is also maintained.

Theoretical Performance

During testing, the main arm was not found to bend or twist a significant amount, so for the purposes of theoretical performance calculations only the actions of the lever arm are really relevant. Further, the weight of the lever arm itself can more or less be neglected, and the servo is assumed to operate at the advertised 72 oz.-in., 90 degree rotation. Under these conditions, the arm (just shy of 4.4 inches in length from the servo axle to the leading edge, and at an initial angle of approximately 50 degrees below the x-axis) could be expected to lift a 16 ounce weight using 70.4 oz.-in. of the available 72 oz.-in. of torque. Again, disregarding losses due to deformation, this setup could theoretically hoist the weight to a position 40 degrees above the x-axis, or 2.83 inches above the weight’s starting position. (It is worth noting that in practice, the setup could consistently achieve lifts of approximately 2 inches, with an [unofficially recorded] maximum of 2.3 inches.)

Fun Trivia Fact

The final version of this project has absolutely nothing in common with the first iteration save for some similarities in the design of the base. As the first iteration was a rickety, abject failure in every sense of those terms, it has since been dismantled and all records of its existence have been destroyed.