ME Design

The final ME Design project requires students to design and verify the components and connections of a mechanical structure. Further, the structure must contain a distinct frame and power train. My group chose to design a coffee bean size-sorter for micro-mills in Costa Rica. See the final report and presentation for detailed calculations.

Design Overview

The size-sorter contains four distinct parts. A motor box is used to drive the main power screw. Beans located in between the pushers move forward as the power screw rotates. The drum is fixed in place with slits of increasing size to sort the beans as they fall through. The scale of this machine allows the micro-mill to sort their entire 450 kg harvest in 6 cycles.

Frame Design

The frame was analyzed under static loading, cyclic loading, and buckling. Using AISI 1006 HR Steel, the size-sorter frame has safety factors of 7.28 against static failure and 45.5 against fatigue failure. The top beams have a safety factor of 6.95 against buckling, and the side columns have a safety factor of 13.28 against buckling. Again using AISI 1006 HR Steel, the power screw has a safety factor of 1217 against buckling.

Connections

M6x1, Class 9.8, 18mm bolts are used to secure the truss joint, yielding safety factors of 1.45 against shear failure and 3.11 against fatigue failure. 6mm E60xx welds are used to connect the top beams to the power screw bearing, yielding safety factors of 6.3 against static failure and 3.79 against fatigue failure. 65mm M5 bolts are used to secure the spacers, yielding safety factors of 16.3 against static failure, 20.3 against fatigue failure, and 208.1 against loosening.

Power Train

For the machine to push the beans at 1 in/s, a motor with 1/8 hp and 475 rpm was chosen from McMaster-Carr. A gearbox was carefully designed to transfer this input to an output of 66.4 lb-in of torque to the power screw at 120 rpm. Assuming moderate shock and average mounting, using AGMA Cast Iron Grade 40 gears yields a safety factor of 5.32