PHONOGRAPH GEARBOX

My objective was to design a compact gearbox capable of driving a phonograph with an input speed of 3–4 rpm and an output of 78 rpm. Working within strict spatial constraints (a 12” × 12” × 4” housing, a horizontally oriented hand-crank input shaft, and a vertically oriented output shaft), I determined that a four-stage gear train was required to achieve the necessary speed increase while maintaining a maximum gear ratio of 3.5 per stage. I began the layout with a bevel gear pair to change the shaft orientation, followed by three spur-gear stages with ratios selected through calculation and trial-and-error to match standard gear sizes. Minimum pinion tooth requirements were verified using interference equations, and final gear ratios and shaft spacing were established using preliminary “Crayola CAD” sketches.

Beyond ratio selection, I considered key aspects of practical gear-train design, including transmitted loads, power output, shaft material selection, keyways, bearing placement, and AGMA-based stress analysis. Because the phonograph operates under relatively low loads, I chose cost-effective, mass-produced gears that meet performance requirements without unnecessary expense. While the scope of this project focused primarily on the gear train rather than full housing and mounting details, the project reinforced my understanding of how speed, torque, gear geometry, and spatial constraints interact in a real engineering system.