Rube Goldberg Machine Subsystem
Presenting and Justifying a Problem and its Solutions
In this course’s Rube Goldberg Machine (RGM), my system is positioned after a rotating hammer device. This device consists of a 1 lb hammer attached to a 1 ft lever arm, released from the horizontal. The hammer reaches approximately 2.5 m/s at the bottom of its swing, delivering roughly 1.41 J of energy into my device. My system must absorb this impact and convert it into an electrical signal that releases a motorized gate, which in turn releases a marble down the track at a predictable velocity to be received by the device proceeding mine. The main problem my design addresses is reliably transferring the mechanical energy input into my system into a controlled release of the potential energy stored within the marble.
Stakeholder analysis highlighted several primary groups for this project: other students’ designs, the course instructor, the student upstream, and the student downstream from my design. The students require a system that can be repeatedly used and that supplies consistent results upon each iteration. The course instructor has indicated that their needs include comprehension of the source material from this course, a fundamental understanding of designing a system to be 3D printed, and the ability to become a strong engineering communicator. The final stakeholder group to consider is the predecessor and successor within the wider RGM assembly. These students require predictable and reliable input and output behavior from my system. The pressure plate must trigger within the expected hammer input, and the track must deliver a consistent exit velocity for the marble each time. These needs were translated into a set of engineering specifications to properly constrain allowable shear stresses within the pressure plate, gate mass and release timing, as well as marble exit velocity limits for the system.
To address this problem accurately, material property datasheets for PLA and aluminum were used in conjunction with vendor specifications for the motor and switch to assess material limitations and allowable shear stresses within the system. Upon analysis of the pressure plate, it was shown that a PLA stem with a factor of safety of 2 will withstand the impact of the hammer, provided the stem length exceeds 5 mm. This length reduces the transmitted shear stress to 12.5 MPa, which is below the allowable ~14.5 MPa for PLA. Analysis of the gate torque indicated that production of a PLA gate would be too lightweight, causing the marble to pinch the gate in place and prevent it from falling. Manufacturing this component from 6061 aluminum alloy and increasing its dimensions allows the gate to fall upon motor activation. Analysis of the motor speed, performed using a supplied 12 mm gear motor and a 4.5 mm gear, indicated that the servo arm and gate would activate and retract within 0.6 seconds. This speed allows the gate to safely fall while keeping the servo arm operating within the tensile strength limits of PLA.
Together, these analyses and stakeholder needs define the problem statement at hand: to design a system that can safely and reliably absorb a 1.41 J impact, convert that force into an electrical signal, and release a mechanical gate that allows a marble to descend down the guided track at a consistent and controlled velocity.
Analysis of Products with Similar Functions
Several existing products and components were benchmarked and researched to satisfy the functional requirements of my subsystem, including sensing the hammer impact, actuating the gate, and guiding the marble down the track.
The design of this motor system is primarily intended for locomotion and robotics applications. The gearbox’s intended use case involves driving wheels or moving treads, which requires continuous rotation and relatively high torque values to overcome the weight of these systems. This design does not include a built-in mechanism to limit or precisely control small displacements. Additionally, the motor subsystem is constrained by a compact plastic frame, which would restrict placement and require larger modifications to the surrounding assembly to integrate it into the system.
Brus
This motor does not include an integrated gearbox and operates at a speed of approximately 11,500 RPM with a low torque of 75 oz-in at the shaft. Without several additional components, including a gearbox, coupling, and output shaft, this motor is not suitable for the design specifications of my system. It lacks sufficient torque at a usable speed, requires multiple additional components for integration, and does not provide small, predictable displacements required for controlled gate actuation.
This motor is compact, with a 12 mm diameter and a 34 mm length, allowing it to be mounted in tight spaces. The lightweight 13 g motor is well suited for the system, and its built-in gearbox provides high torque at a controlled speed. The included mounting points allow for straightforward integration into the 3D-printed PLA frame. Overall, this motor is an ideal candidate for actuating the gate, allowing the marble to begin its descent in a controlled and reliable manner.
With these factors considered and stakeholder needs assessed for functionality, ease of operation, and product availability, the N20 micro DC gear motor was selected as the optimal choice for the system. Its compact size, integrated gearbox, controllable motion, and high torque output make it well suited to actuate the gate upon receiving the electrical signal generated after the hammer impact.
Presentation and Justification of Solution Design Requirements
The solution requirements for this subsystem were derived from the problem statement: to absorb a 1.41 J impact from the hammer, convert that force into an electrical signal located behind the pressure plate, and then use that signal to actuate a gate that releases the marble at a controlled velocity. Listed below are the primary requirements for this device, with stakeholder needs prioritized. The requirements are divided into subsystem functions; however, the primary solution requirements include: (1) impact detection, (2) gate actuation, and (3) controlled marble motion.
1.1
Pressure plate strength
The pressure plate must withstand a 1.41 J impact with a factor of safety ≥ 2.
1.2
Switch Actuation
The switch must reliably actuate within the total travel range experienced by the pressure plate.
2.1
Gate Release Torque
The gate must produce a greater gravitational torque than the opposing torque generated by the marble resting on it, ensuring the gate consistently falls when released.
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Motor Timing
The motor must retract the servo arm by 3 mm within 0.10 seconds in order to clear the gate’s path before it begins to fall.
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Material strength of servo arm
The Servo arm must remain below ~30% of its tensile strength under peak motor torque.
3.1
Track Geometry
The track must maintain a slope of 15° ± 2° to ensure a stable and reliable rolling motion for the marble.
Marble Exit Velocity
3.2
The marble must exit the track with a velocity between 0.5 to 1.0 m/s. this ensures reliable and consistent results for the downstream subsystem.
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Manufacturability
The subsystem must be 3D printable in PLA and capable of being assembled in ≤ 10 minutes. The subsystem must also use only standard hardware and components, except where specifically noted.
Generalizing and Analyzing an Original Solution
The subsystem being designed must transform a hammer-driven mechanical energy input into an electrical signal, which then supplies a controlled mechanical release of a marble. This marble exits the subsystem and activates the next stage of the Rube Goldberg Machine (RGM). A function structure is presented below that details the required inputs and outputs of the design. This function structure enables continuation of the design process without being constrained by specific component names or detailed implementations.
Design Concept Generation, Analysis and Selection
Using the function structure shown above, several design concepts were iteratively developed for this subsystem. From this process, three viable design paths were sketched that incorporate the required design functions while prioritizing stakeholder needs.
In this concept, a piston is primed and positioned behind the marble. Upon receiving an electrical signal, a switch activates the piston, which releases and accelerates the marble down the guided track. This design is mechanically simple and avoids overly complicated moving parts, allowing for high-volume production at low cost.
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However, this concept does not account for the geometric changes required to compensate for the marble’s exit velocity. Secondary stops or guiderails would be required to ensure the marble remains on the track. Additionally, this design does not account for the forces applied by the upstream RGM subsystem. As a result, the marble could be shifted out of position prior to piston activation, leading to inconsistent behavior.
1.1 Switch activated Marble
1.2 Gravity Gate
This concept builds upon the guided track approach by removing the horizontal resting surface and instead placing the marble directly on the track, where it is restrained by a gate actuated by a motor. Upon receiving the hammer impact, a pressure plate activates a pushbutton switch, which sends an electrical signal to the motor. The motor is connected to a servo arm linked to the track gate. Upon motor activation, the servo arm retracts, releasing the gravity gate and allowing the marble to begin rolling.
This concept minimizes user intervention during reset and reduces the number of potential failure points by limiting the number of internal components. As a result, it offers improved reliability and repeatability compared to alternative concepts.
1.3 Weighted Ramp
In this concept, the marble is positioned on one end of a fulcrum, with a weighted block attached to the opposite end via a lever arm. Upon receiving the hammer impact, the pressure plate converts the mechanical energy into an electrical signal, which releases the weighted block. As the block falls to the ground, the fulcrum rotates, positioning the ramp at a 15° angle relative to the horizontal axis and allowing the marble to accelerate down the track.
Several factors invalidate this concept for further iteration. The primary concern is the connection between the weighted block and the fulcrum; any elongation of the connecting string would alter the final ramp position, making this a clear failure point over repeated use. A secondary concern is the high sensitivity of the system to alignment. Precise positioning would be required, and cumulative wear could cause the final ramp angle to vary over time, reducing consistency.
Application of STEM Principles and Practices through Analysis
This analysis was conducted to calculate the work required from the piston to set the marble in motion and guide it down the track. Given the horizontal surface on which the marble is initially placed, there was concern that the hammer impact could potentially knock the marble out of position before the piston had the opportunity to activate. Upon completing these calculations, it became clear how small the work requirement from the piston actually is. For this design concept to function reliably, the starting position of the marble would need to be redesigned to avoid the likely influence of external forces interfering with the subsystem.
The second analysis performed was straightforward but critical for the successor in the RGM assembly. This analysis calculated the expected velocity of the marble after reaching the end of the guided track. Initial estimates were approximately 0.72 m/s, and the calculation confirmed confidence in the selected track length. Friction was not factored into this analysis and must therefore be considered further in the design process. If the resulting velocity were to decrease, the slope of the ramp could be increased to compensate. Additionally, since the marble begins from rest, it is likely that the actual exit velocity will be slightly lower.
The final analysis performed for these concepts was intended to calculate the required weight of the weighted block needed to lower the track and release the marble. Upon reviewing this analysis, it became apparent that the weight of the guided track itself had not been included; instead, only the weight of the marble and the applied torque were considered. If this concept were to be pursued further, the analysis would need to be revised to include the additional torque contribution from the track mass and the fulcrum weight required to overcome it. This concept was ultimately determined to be the weakest of the three, as it contains multiple variables that could fail and render the subsystem unusable. The likelihood of string elongation is high, and any misalignment at the exit of the subsystem could significantly alter the trajectory and exit velocity of the marble.
After evaluating the above design concepts, Concept 1.2: Gravity Gate was determined to be the most reliable and consistent design solution. This concept was selected because it minimizes mechanical complexity while securely containing the marble in the event of a higher-than-expected hammer force. The gravity gate design satisfies the stakeholder needs identified earlier and incorporates features that enable high-volume PLA production with minimal defects. By relying on gravitational actuation, the gate produces predictable marble motion, allowing the downstream RGM subsystem to accurately anticipate the output behavior. Reducing the number of mechanical functions within the system also decreases the likelihood of failure, making this the most defensible and robust concept overall.
Detailed Design, Prototyping and Simulation
Refining the gravity gate concept led to the development of the subsystem model in Siemens NX. The initial model includes all required components to be 3D printed, as well as links to any components that will be purchased and used as-is. A rendered image of the full assembly is included below, along with a hyperlink to the complete PDF. The function of each component is listed below, along with a description of its relationship to other components within the subsystem.
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Concept Modeling.pdf
Part Modeling
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Frame
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Main enclosure for the subsystem footprint, this component is the housing for the pushbutton switch, motor, guided track and pressure plate. the wall thickness was considered and made uniform to allow more consistent prints ​
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Guided track
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The track profile was made to 15° to match the STEM analysis done last week. this component houses the marble and the track gate will slide vertically into the slot on the left side.​
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Track Gate
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This component is made to keep the marble in position until activation. once activated, the motor will retract the servo arm and the track gate will begin to fall, allowing the marble to accelerate down the track.​
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Switch / Motor Housing
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These are simple rectangular structures design to house the electronic components of my subsystem. the switch housing will be placed behind the pressure plate, and the motor housing will encase the servo arm and motor.​
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Pressure Plate / Pin
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This component will be receiving the mechanical force of the predecessor in the RGM assembly. its roll will be to transfer this mechanical force into a pushbutton switch that will activate the motor and turn my subsystem on. the pin will be located behind the pressure plate and act as a force consolidator to ensure the pushbutton switch is activated.
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Angle Brackets
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These were design to affix the guided track to the device frame. they are angled 15° to adjust for the track slope, and will be positioned at the bottom of the track and fastened to the frame.​
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Motor / Push Button Switch
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These devices are used to transmit the mechanical energy delivered to my subsystem into an mechanical operation that releasing the marble.
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Plastic Gear
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This gear will be purchased online and used to convert the motors RPMs into a mechanical motion that retracts the servo arm, releasing the gravity gate. ​
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Wiring, AA batteries, Adhesive, Battery Holder, fasteners
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All of these will be purchases as is and used to fully assemble the subsystem.​
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Assembly Process
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Position frame on flat surface and affix it to the surface using adhesive
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Slide guided track into position and fasten in place with angle brackets and fasteners
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Affix motor and pushbutton switch into housings and apply adhesive to outside edges, slide housing into position
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Position gravity gate into track slot and lock in place with servo arm
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Connect battery holder and wires to switch/motor system
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Place marble in resting position behind track gate
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Secure pin and pressure plate to slot in front of pushbutton switch.
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Activate push button switch to release marble
Physics and Engineering Analysis
For this portion of the analysis, three separate approaches were used to evaluate the validity of the proposed concept design. The first analysis examined the shear force experienced by the pressure plate due to the hammer impact. The second analysis evaluated the gate and track mass to confirm that the intended gravity-driven release mechanism would function as designed. The third analysis focused on calculating the required motor speed to ensure the servo arm clears the track gate efficiently and allows it to fall without interference.
The objective of this analysis was to calculate the force experienced by the pressure plate after receiving the output from the upstream RGM subsystem, consisting of a 1 lb hammer attached to a 1 ft arm. While the energy output of this system had been previously calculated, the resulting shear stress on the pressure plate had not yet been considered in the original design.
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Initial calculations revealed that the design was significantly underestimating the shear stress that the pressure plate would experience. PLA was found to have an approximate shear strength of 50 MPa. Using a factor of safety of 2, the maximum allowable shear stress for the pressure plate was determined to be 14.5 MPa. The original design allowed only 1.2 mm of horizontal travel after impact, which resulted in transmitted stresses exceeding this allowable limit.
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By increasing the allowable horizontal travel distance to 5 mm, the transmitted force into the pressure plate stem was reduced, resulting in a calculated shear stress of 12.8 MPa. This value falls safely below the allowable limit and yields a factor of safety of approximately 2.27. Based on this analysis, the pressure plate was redesigned to incorporate the pin directly into the plate geometry, reducing part count while ensuring that the pressure plate will not fail after repeated impacts.
This analysis was conducted to verify that the selected motor would retract the servo arm quickly enough to fully clear the track gate before gravitational motion begins. Calculations showed that the selected motor retracts the servo arm in approximately 0.065 seconds, which is faster than the required gate release time of 0.10 seconds.
While the motor is capable of higher speeds than strictly necessary, the forces generated during retraction remain well within the tensile strength limits of PLA (~50 MPa). Although higher speeds could introduce momentum-related concerns, the required travel distance is only 3 mm, and the resulting forces are minimal. Therefore, the selected motor speed is more than adequate for the intended function and does not pose a structural risk to the subsystem.
The goal of this analysis was to determine whether the weight of the marble resting against the gate would prevent the gate from falling once released. Initial calculations revealed that the original PLA gate design would not fall reliably due to the mass difference between the marble (5 g) and the gate (1.06 g). The torque applied by the marble exceeded the self-weight torque of the gate, preventing consistent release.
To resolve this issue, the gate material was changed to aluminum, and its dimensions were increased. These changes increased the gate mass to approximately 4.21 g, allowing the gravitational torque generated by the gate to exceed the opposing torque applied by the marble. With the servo arm retracted, the gate now falls reliably, releasing the marble as intended.
In addition to the engineering analyses, a manufacturing review was conducted to identify opportunities to reduce part count and fasteners, thereby improving reliability and streamlining high-volume production.
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Reduce part count
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I made two separate housings for the switch and the motor for my system, both of these can be removed. The switch housing can simply be a modification to the frame. I will redesign it so there is a cutout in place which allows the user to slide the switch in and the built-in tension bars on the switch will keep it in place.​
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The motor housing is not needed either as it only adds an extra step of adhesive and makes it difficult to mount the motor inside. My redesign will remove the motor housing and instead modify the frame to have a raised area with three mounting points for the motor fasteners. This will allow the user to easily mount the motor in place with any adequately sized screwdriver.
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There is a pin that I made to be installed between the pressure plate and the switch, this can be removed and the redesign of the pressure plate analysis will incorporate the pin into the new design. This will create less points of failure in the design and allow the new system to only have two parts (switch, Plate) instead of four (housing, pin, plate, switch)
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Reduce fasteners
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Remove the L bracket and fasteners that are installed on the bottom of the guided track, this will be replaced with a modified track that has extension added on to both sides of the vertical wall from the frame. This change will allow the guided track to be slid in and out of place freely but prevents CCW or CW rotations of the guided track and allows gravity to assist in locking the track in place.​
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With the removal of the motor housing the guided track will now be levered into place and not firmly secured. I want to avoid the use of fasteners on this end. I will modify the frame to extend out with a wedge to allow the guided track to rest on top of it. With the above modifications to the L brackets this will allow the user to lay the device on its back and slide the guided track into place and will lock it in once its upright with the assistance of gravity. I may reiterate this as I feel I overcomplicated the wedge. But this configuration allows the majority of this system to be printed at once as the frame is now integrated with the rest of the components​
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A revised model with these considerations made can be found here ---> Model Redesign.pdf
Simulation/Computational testing and Data Collection
Simulations of the gravity gate subsystem were completed prior to prototyping to ensure potential failure modes could be identified before printing began. Several motion tests were performed in Siemens NX using joint definitions, motion analysis, and analytical predictions to evaluate the interaction between connected components.
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The first simulation focused on the motion of the track gate and servo arm to understand how these components interact during actuation. Using the revolute joint function in Siemens NX, the servo arm was retracted from the engaged position to the activated position. This simulation confirmed that the gate had sufficient clearance to fall, that the marble would not be impeded by the gate becoming stuck, and that the alignment between the servo arm and track gate was correct. From this simulation, the required angular displacement to release the gate was calculated to be 76°. No interference between components was observed, and once the servo arm was fully retracted, the gravity gate had adequate clearance on all sides to begin falling.
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The second analysis examined the timing and actuation speed of the motor. By inputting the motor RPM values determined during earlier calculations into Siemens NX, the retraction time was simulated and confirmed. NX indicated a full retraction time of 0.042 seconds, which is approximately twice as fast as the initial estimate. Despite the increased speed, the forces experienced by the servo arm remain well within material limits. The expected stress is approximately 12 MPa, while PLA has a tensile strength of approximately 50 MPa.
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The third analysis evaluated the modified track gate design. The gate width and thickness were increased to 8 mm and 4 mm, respectively, and the material was changed from PLA to 6061 aluminum. These changes allowed updated calculations to be performed, confirming that the aluminum gate generates approximately four times the gravitational force of the original design. This verified that the gate mass now exceeds that of the marble and will freely fall upon activation.
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An additional simulation opportunity would be the evaluation of print quality under high-volume manufacturing conditions. Analyzing temperature distribution and cooling behavior during printing could help identify potential warping and further validate manufacturability.
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The finalized Gravity gate subsystem design for the RGM assembly can be found here --> Final Subsystem Design.pdf​​​​
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Evaluation, Reflection, and Recommendations
For the purposes of external evaluation, the feedback and peer evaluations received throughout the term are summarized below, along with how each was addressed within this portfolio.​
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In module three, I incorrectly completed the assignment regarding a function structure diagram. my instructor provided insight in to how this could be completely correctly and I have included my revised function structure diagram in this portfolio.
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In module five, I was instructed to include a mechatronic component to my system as it was a requirement for the final project. this was originally a misunderstanding on my part, I was under the impression that for module five the assignment was to only model the portion of your subsystem that was interacting with the student after me. it was never my intention to not include a mechatronic function in this design, just that I misunderstood the instructions of the assignment.
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In module six discussion post, Ryan, Ethan and Morgan all provided feedback on my original design concepts, Both Morgan and Ryan commented a suggestion on adding a curvature to the tracks to act as guardrails for the marble. This is a great idea and one that I had already planned to implement. I included a blown up view of the track as well as making note of the curvature I intended to apply to it, but I should have been clearer in my description as they weren't able to spot it. Ethan suggested I add a pivot point to the gate mechanism to ensure its release, and although I like this idea and tried to find a way to implement it, I was not able to find a cohesive design path that incorporated this idea. in the future, if I had more space inside my subsystem I think this design idea would be perfect, as it would allow the operator to reset the system with the press of a button instead of physically resetting the gate.​
Reflection on the Design Project
Throughout this project I've had the opportunity to expand my knowledge on engineering design, modeling and mechanical analysis substantially. I was quite terrified to be honest, as my only prior experience with this kind of work was a single introductory course regarding CAD design. As the weeks progressed I slowly started to understand the larger picture at play and that this wasn't "really" about building and designing a RGM assembly, but rather a creative way to expand our knowledge on the various methods and systems that are at play with an engineering assembly. This design project has increased my understanding of component and functional design by not only expanding my vocabulary, but by integrating the deliberate design protocols that engineers must adhered to.
During module one I really didn't have a grasp on what was being instructed of us and view this course as 10 separate modules with work to be completed in each one. As the weeks progressed and we started iterating on our design concepts, I started to build some confidence in the idea of prototyping and brainstorming ideas. At the beginning I was scared of the kind of failure without realizing its one of the most crucial aspects of the design process. As we progressed through the modules and we starting designing in CAD I started to enjoy the process of failing. This course allowed me to build confidence in my design process that I wouldn't have had otherwise. I think the weakest point in my design process would be the simulations and mechanical analysis done in Siemens NX, this was my first exposure to this process and it took quite a few YouTube videos and late nights to understand what I was actually doing in the software. In the future this process will be easier as I have this knowledge to look back on, but I would have like to have dedicated more time to this Modules nine and ten rounded out the design process and informed me on the processes involved in 3d printing prototypes. these modules let to me apply fillets and redesigning the thickness of my frame walls to apply the rules learned in these modules. these modifications increased the overall strength of my subsystem and would likely prevent failure cases regarding warping or impact forces.
Presentation of Designer's Recommendations
The biggest recommendation I could offer in thoroughly read the project modules for this course. I know this doesn't directly pertain to the project construction, but every time I ran into a problem or got confused I was able to revisit modules each week and it helped me get back on track.
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My gate redesign is an area that I would reconsider in a future design. the introduction of a new material adds a new set of variables to consider such as thermal expansion, friction between the materials and wear over time. these issues could be addressed in a redesign, my recommendation would be to instead use a high density plastic such as PETG or nylon to increase the weight of the object without having to made significant modifications to the subsystem. This change would cut out the metal fabrication from the process and streamline high volume production.
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The pressure plate assembly operates and is within shear tolerances, but its closer than I would like, and is most likely the first part that would fail. My recommendation for this would be to redesign the pushbutton housing so that a spring-damper system could be installed. This spring would alleviate the forces applied by the hammer and extend the life of my system.
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The motor I chose has more than enough torque to complete its task within the subsystem, and future designs could include a timing circuit or calibration control to reduce the speed of the motor. this could extend the life of the motor and reduce any excess forces the subsystem may experience when the motor activates.
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The guided track design works and does the task required, but for long extended use cases I didn't consider frictional wear from the marble moving down the track. I would recommend printing multiple tracks with different surface densities to calculate the expected wear the track would experience.​
