Challenges of Turning Designs into Reality: Lifting a Hopper with a Mechanism

Challenges of Turning Designs into Reality: Lifting a Hopper with a Mechanism

Mihan Bandara 
Undergraduate Engineering Student
McMaster University
Hamilton, Canada
bandarm@mcmaster.ca    

Austin Mardon 
Faculty of Graduate Studies & Research
University of Alberta
Edmonton, Canada
mardon@ualberta.ca

Abstract—When an engineer begins to fabricate a part or mechanism that they have designed, there many factors that they must first consider such as clearance, assembly, and material selection. Considering these factors early in the design process is essential to the success of any project.

I.    INTRODUCTION
    Over the last couple of months at McMaster University, I have completed 3 design projects that required me and my team to apply our learning to real-life problems. Our latest project tasked us with creating a mechanism that would be used to unload sorted recyclable materials to their assigned bins. The mechanism would have to use either a linear actuator or stepper motor to lift a hopper mounted on a terrestrial drone. The mechanism was designed conceptually on Autodesk Inventor and then fabricated by our team. This journey has taught me multiple lessons about the challenges that engineers face when taking an idea from design to reality.

II.    DESIGN SUCCESSES
       Our team went through many iterations of our design before we settled on the rack and pinion system shown in our Inventor model. After choosing the rotary actuator, we knew we needed a way to turn rotary motion into an up and down linear translation — a rack and pinion did just that. Because the hopper was tilting and not just going up and down, we created a rail system that allowed the hopper to rotate while keeping the rack stationary. Coupled with our rack braces, this ensured our gears remained in contact with the rack at all hopper heights and angles.  

Fig. 1 Digital model of the rack and pinion mechanism on Autodesk Inventor

       Our team was concerned with the amount of torque the stepper motor could produce so we used a 2:1 gear ratio that doubled torque and halved speed. This proved to be effective during our demonstration as the mechanism easily lifted the hopper. The lower speed lessened the likelihood of gear slippage and vibrations, so it was a good design decision. Our final mechanism was fabricated using parts made of 3D printed PLA, laser-cut acrylic, and a hand-cut steel rod. PLA was used to create complex parts that took minimal load like stands, rails, and braces while acrylic was used for load-bearing parts with 2D designs like gears and racks as the acrylic was less likely to warp under compression or elasticity. Steel was used for the axle due to its high shear modulus of ~80 GPa [1] which is around 20 times higher than PLA [2]. Material selection contributed to the eventual success of our design as the mechanism showed no wear and tear when it was tested.  

       When designing a mechanism, it is important to determine what the input is, what the output needs to be, and how you can use different parts like gears and pantographs to get there. It is equally important to consider what materials you will fabricate those parts out of by considering material properties indices (MPIs) like cost, strength and machinability. We learned that considering these factors in your design process is essential to creating a successful mechanism.

III.    LESSONS LEARNED
Our design looked great in Inventor but that was way less than half the battle. Because our design was somewhat complex, it required large intricate 3D printed parts. These parts took up over an hour of printing time each, meaning we could only get one part printed per class. Occasionally our prints failed, and we did not have enough time to try them again. Because these parts took so long to create, we had to get it right the first time, except we didn't. When creating assembly using CAD software you often forget to consider the subtle differences between what you see on your screen and how it will work. Unfortunately for our team, we only noticed those differences after the parts were in our hands. For example, the rack of our rack and pinion system was laser cut from 6 mm acrylic. The brace the rack went through was also 6 mm, which meant 2 hours of sanding each of the racks. The racks also had a knob at the end that prevented it from getting out of the brace. While this was a good idea, the same thing that stopped it from going out of the brace also stopped it from ever going in it in the first place. Cutting the knobs off with a hacksaw was risky because the material can break, but fortunately, it held together.  

Our assembly transferred all the rotational motion through an axel, so we needed to make it out of something strong. I managed to repurpose an old steel rod, but it was a loose fit through the gears. We were forced to hot glue the gears to the axel rod, which meant the entire weight of the hopper was essentially lifted by a hot glue seal. 

IV.    CLOSING REMARKS
Although our design worked perfectly in the end, these issues taught us some valuable lessons. First, always consider clearance, an M4 fastener will not fit in a 4mm hole, you must make it larger. Second, understand how your components will come together. In real life parts do not go through each other, so make sure you can assemble your parts the way you intend to. Lastly, consider how forces travel through your mechanism. We used acrylic gears and a steel axle for maximum strength, but all that torque had to go through hot glue — your mechanism is only as strong as its weakest part. We were able to overcome all the issues we had with some long hours and elbow grease, but it would have been way easier if we did it right the first time. Make sure you consider all the factors of your physical mechanism before you start making it and you are sure to produce better results. 

REFERENCES
[1]    X. J. Gu, S. J. Poon, G. J. Shiflet, and M. Widom, “Ductility improvement of amorphous steels: Roles of shear modulus and electronic structure,” Acta Materialia, vol. 56, no. 1, pp. 88–94, Jan. 2008, doi: 10.1016/J.ACTAMAT.2007.09.011.
[2]    V. C. Pinto et al., “ScienceDirect Comparative failure analysis of PLA, PLA/GNP and PLA/CNT-COOH biodegradable nanocomposites thin films,” Procedia Engineering, vol. 114, pp. 635–642, 2015, doi: 10.1016/j.proeng.2015.08.004.