13.4 Design Process
Brainstorming/Initial Idea
The initial design idea for the egg cracker mechanism is shown below in Figure 1. It is similar to the final design in that it involves a slider-crank connected to a rack and pinion that rotates gears. A key difference is that our initial design involved the use of torsional springs on the gears to keep them from rotating freely, as annotated in Figure 1. While the idea of spring-loaded gears seemed viable, we determined that this represented a significant torque value that would be required from the motor to rotate the mechanism, and our goal was to be able to use the smallest motor possible to accomplish the task. Thus, we decided instead to use an intermittent motion profile to lock the gears in place when the rack was disengaged but allowed free rotation when the rack interfaced with the gears.
The other key difference to note is that this initial idea did not involve four-bar mechanisms to push the eggshells out. Rather, this initial idea only involved a singular arm on either side to push the shells out. After some inspection and analysis, we determined that the use of a singular arm would not have enough range of motion to swing up and around to dislodge the shells.
With this in mind, we designed a four-bar linkage that would achieve the desirable path of motion to eject the eggshells from the TPU holders. We used MotionGen to get an initial idea of the link lengths and configuration needed and then refined them in MATLAB. The result of this was the four-bar mechanism shown in Figure 2, where Point P is the point that makes contact with the shells. Please see the kinematic analysis section for the position profile of Point P.
Design Iterations
The main component of the final design that required iterating was the egg holders. Because eggs come in a variety of shapes and sizes, we needed the holders to be flexible enough to accommodate larger eggs while still holding tight around small eggs. For this reason, we chose TPU as the material instead of PLA because TPU is a much more flexible polymer. See Figure 3 below for the different iterations of the egg holders, with the initial design on the left and the final design on the right.
The other component that required some iteration was the intermittent motion profile, shown below in Figures 4 and 5. The reason that this needed iteration was because it required high precision in order to engage and disengage exactly at the moment we desired. We had to refine the dimensions of the trapezoidal face on the left so that at the moment the teeth engaged with the gear, the face would unlock the gear. Another small detail is to note the rounded corner at the top of the trapezoidal face. It was necessary to cut material off of this corner in order to avoid collision with the side of the rack.
Prototype
Our designed prototype is shown below in Figures 6 and 7. As can be seen, the prototype was one side of the full final design. The purpose of this was to prove that the four-bar linkage was capable of dislodging the egg shell and that the rack and pinion subsystem was effective in transforming the linear motion into the rotary motion of the gear. This prototype was manually actuated by pushing down on the rack to slide it. Note that since the egg holder was still in its early iteration stage, the plastic egg was glued to the holder to imitate being held. Based on this prototype, we knew our concept was viable and would work with some minor adjustments made for the final design.
Final Design
After making the necessary adjustments to the prototype, the final design was fabricated and assembled, shown in Figures 8 and 9. Adding the other half of the design from the prototype proved to be a simple task as it was only a matter of spacing the gears properly.
There are a couple key adjustments to the final design from the prototype that are worth noting. First, notice the TPU egg holders in their final iteration as compared to the prototype. Press fit slots were made for the holders so it was easy to interchange the holders without having to reprint the whole gear face. The other key adjustment that is not so apparent is that small slots were cut into Link 4 of the four-bar mechanism. The reasoning for this was to keep the four-bar mechanism away from its toggle point. We noticed that when the mechanism approached its toggle point, it would lock up. Thus, we included the slots in the link to keep the whole linkage a safe distance away from the toggle point. The next section discusses in further detail how this final design was fabricated and implemented.