The Pencil Sharpener: A Motorized Amusement Park Ride
The Brief
Our team of three was tasked with designing, building, and demonstrating a safe but entertaining amusement park ride for two Lego minifigures, under a set of hard engineering constraints: at least two degrees of freedom, dual-redundant safety systems with fast response, a power draw under 2.5W, and full construction and testing within a single semester. Our design, nicknamed “The Pencil Sharpener,” uses two arms that swing in a fixed 90° phase offset, driven internally by a gearbox and bevel gear system so the mechanism stays visible for demonstration.
My Role
I led the drivetrain design: the 3-stage gearbox that reduces the DC motor's output from roughly 6,600 RPM down to a safe 25 RPM, and the shaft sizing that carries that load through to the arms.
Engineering Highlights
- Gear train design: Designed a 3-stage spur gear reduction (184.8:1 overall ratio) from first principles — module and tooth-count selection, pressure angle, undercutting checks — to hit a 25 RPM target output from a 6,600 RPM motor.
- Shaft analysis: Performed torsional and bending stress analysis (von Mises criterion) on the primary drive shaft, achieving a safety factor above 3 while balancing shaft diameter against bearing and gear-bore tolerances.
- Bevel & reverse gearing: Designed 3D-printed bevel gears to split rotation into two perpendicular axes, and reverse gears to keep the arms from colliding mid-swing.
- Embedded safety system: Contributed to the Arduino-based control system using dual photoresistors for redundant overspeed detection, a manual e-stop, and an L298N motor driver with fast electronic braking.
- Manufacturing: Iterated the design through CAD (OnShape) and built the final structure in MDF and PLA via laser cutting and 3D printing.
Results
