We began by making a simple four-bar mechanism and turning it by hand as shown before. We made the links out of ¼” wood and 3D printed the hinges. This demonstrated the idea and showed us physically where the fastest point of the mechanism was. We then took our motor specs and designed a gear ratio that matched the necessary angular velocity and torque. This consisted of a modular gearbox that could be any ratio between 4:1 and 1:4 which output to a 10:1 planetary gear set. Every gear was cut out of two ¼” pieces of wood and glued together. This reduced the gear stress and improved meshing. For axles, we used bolts with lock nuts so we could constrain axial motion while minimizing friction. Washers were placed between every rotating component. We screwed a 2x4 into the baseplate to raise up the entire four-bar mechanism so that it could rotate and also fit the planetary gears on top of the table. After our first assembly attempt, we were able to drive the mechanism from the input shaft but it would stick sometimes. This was because the housing of the planetary gears had too much extra tolerancing (in an attempt to reduce friction) so the gear would slightly twist and get stuck. At Steve’s recommendation (a TIW overseer) we cut discs out of 1/32” acrylic and glued them to both sides of the three planetary gears. This reduced the extra room in the planetary housing and greatly reduced contact friction because of the new acrylic contact surface. After this modification and some small tuning of bolt tightness, our mechanism was so well assembled that it was backdrivable. In addition, if the arm was left in an upward position, gravity alone was able to pull it down while driving the rest of the gears backwards. The physical assembly of our mechanism went very well and performed beyond our expectations.
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