Design Process.

The initial design intent wat to make a mechanism that looked like the mechanism shown in figure 1 (from the introduction section).  To map out the dimensions of the linkages and make sure they would all fit together correctly, the mechanism was designed in SolidWorks, as shown below in figure 2. After the linkages were designed in SolidWorks, they were turned made from laser cutting a 1/8^th inch thick sheet of acrylic. Acrylic was chosen over wood so that the finish would be as smooth as possible, to make the sliding components work well.

Figure 2. SolidWorks assembly of initial mechanism design

The initial concept for the slider was to make it a piece that wrapped around a vertically oriented pvc pipe. However, the hole in the slider made for the initial prototype was too small and did not fit the PVC pipe, so a metal rod that was too small was used instead. Another issue with this initial prototype, as shown in figure 3, was the pin connection between the 4^th link and the slider.

Figure 3. First prototype of mechanism

The plan was to cut a piece of scrap wood that would fit on top of the slider drill a hole through the wood, and insert the pin through the hole. However, given the small dimensions of the slider, cutting out this wood piece proved more difficult than anticipated. Thus, for the initial prototype, the pin was simply glued onto the slider, which was far from ideal. There was also an issue with the slotted link colliding with the head of the post screw that connected the crank arm to the ground. A couple washers were used in an attempt to space out the slotted link from the crank arm, but this was not enough. This also caused the slotted link to be positioned at an awkward angle, with the side of the link connected to the crank arm being further away from the block of wood than on the side connected to the 4^th link. This caused the side connected to the 4^th link to sometimes collide with the wood block.

              In the final design and prototype, the issues with the vertical rod and the slider were resolved by switching to a slider that slides horizontally along the ground. This allowed for the slider to be connected to the 4^th link by a steel post screw, and negated the need for a vertical member, as shown in figure 4.

Figure 4. Final prototype

The post screws making up the joints connecting the ground to the slotted link and the crank arm to the slotted link were replaced by hex bolts, which allowed the slotted link to be spaced apart an appropriate amount from the crank arm using hex nuts. This is shown in figure 5.

Figure 5. Top view of mechanism