ME360: PRODUCT DESIGN
ME360 Product Design is a project-based course completed during my third-year while in undergrad. The following menu presents the sections taught throughout the course, intended to aid in the development of various practical skills. Under each section, you will find a series of design exercises, completed projects, and design analyses.
STRUCTURAL COMPONENT DESIGN
FUNCTION
&
SHAPE
STRESS & STRAIN ANALYSIS
MATERIALS SELECTION
PROTOTYPING
UNDERSTANDING HOW TO DESIGN
STRUCTURAL COMPONENTS
Assignment #2: Skateboard Desk Design
As designed and written by Isabella R. Reyes
March 3, 2021
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MAIN GOAL: Create a lightweight skateboard deck design.
CONSTRAINTS:
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Supports a 180lb user
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Supports shoe sizes up to size 12
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Cannot cause a vertical displacement of more than 0.375" across the span of the board
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Assume that the board is supported by revolute joints (hinges) above the front and back truck axles (which hold the wheels)
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Design the deck to a factor of safety of 3 (maximum stress should not exceed 1/3 of the yield stress of the chosen material) to account for load variations
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Aim to produce the lightest deck that will satisfy the design requirements even in the worst-case loading scenario
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Can be any shape
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Can be of any material
The Intial Design Process...
The deck is the main component that supports the user and allows the user to perform various tricks. In its simplest form, the board is nothing more than a singular, cohesive piece. While most produced boards have a degree of concavity, the deck for this analysis is modeled as a flat deck. Both the curvature and rigidity of the deck play a role in the board's performance during tricks and general use. Therefore these factors are chosen based on user preference. The board is raised above the ground by a truck-and-axel component that connects each pair of wheels. This component is fixed to the deck at its base-plate. The truck-and-axel acts as a revolute joint as the wheels, adjoined by the axel, can rotate freely. Since each truck is fixed and themselves do not rotate, I will assume that the area where each base plate is attached to the deck is analyzed as a fixed geometry. Figure 1 depicts the placements for the fixed geometries in relation to the underside of the board.
Figure 1. Placement of Fixtures
REFERENCES:
The deck bears the direct weight of the user and distributes the force throughout the board while it is supported by the truck and wheels. The weight of the user is typically distributed across two areas, otherwise referred to as their foot placement. Therefore, I am choosing to analyze the resultant force applied to the board as areas of pressure. In doing so, I am trying to simulate how a user's weight is distributed as realistically as possible. The board has to not only spacially accommodate a user with a shoe size up to size 12 but support them as well. Therefore, I will assume that the force of 180lbs is applied to the deck across two areas equivalent to the surface area of a size 12 foot [see reference for "Shoe Size Surface Areas"]. Figure 2 depicts this assumption visually.
REFERENCES:
Figure 2. Placement of loads.
Both the fixture of the trucks and pressure placements are defined as restraints and loads, respectfully, within the static analysis in SolidWorks. Before any displacement analyses can take place board, must be modeled first.
I began by determining the measurements of my board. With an applied force of 180lbs, I designed the deck to accommodate the average rider, meaning that the board is 8.00 inches in width, 31.38 inches in length, and 0.51 inches thick. To determine these measurements, I utilized the resources of Autonomy, a skateboarder-run business, and "How A Skateboard is Made" from the site How Products are Made. I went to SolidWorks to extrude a 31.38" x 8" x 0.51" rectangular prism. To which I added 3.94"-radius fillets to curve the ends of the board. Doing so gives my board the iconic and conventional "popsicle" shape.
REFERENCES:
Material selection...
Before a static analysis can take place, the material of the model must be chosen first. Bounded by the constraints of designing a lightweight board, given an applied force and a maximum deformation, I needed to choose a material that satisfies a particular performance index. The specific performance index is extracted formulaically through a derivation of the functional and geometric properties. In its simplest state, the board is equivalent to that of a beam, simply supported with two loads equidistant from supports. I elected on distributing the load across two areas so that my analysis would closely reflect the real-life condition if a user had been standing on the board with both feet positioned equidistant from the ends. Below is my sketch of the board and analysis for obtaining the performance index.
REFERENCES:
From the derivation (the above work to the right), I obtained a performance index of Young Modulus to density with a slope of 1. After this, I utilized the GRANTA EduPack Materials Database to hone down the best material choices for my board. On GRANTA, I created a chart to compare the Young Modulus to the density of all materials. Using the line function and dragging the slope over areas of different materials, the goal is to maximize E to ρ. Therefore, any material that lies above the line was taken into consideration.
The figure above shows the chart comparing Young's Modulus to Density for all materials within GRANTA's database.
While the uppermost areas of materials would be exceptional to choose from, they would need to be further analyzed in regards to their yield strength, cost, and other factors. I decided to focus on the upper-middle range of materials which happens to be the wood family.
The figure above shows types of woods with a relatively high Young's Modulus.
It's not uncommon to have a skateboard made of wood [see reference for "Skateboard Deck Materials"]. This is because woods are durable, lightweight, and allow for some flexibility during use. Flexibility coincides with rigidity. Rigidity is measured by determining a material's Young's Modulus. Since woods have relatively high elasticity and a relatively low density, it understandable why wood is an ideal material choice for a skateboard deck [see reference for "Strength, Rigidity, and Hardness"]. Preferring to compare analyses within the woods family, I chose pine (pinus monticola) as my initial material of study.
REFERENCES:
The figure above shows a snippet of GRANTA's data for pine (pinus monticola).
Finite element analysis
To complete my CAD, I applied the material-of-choice to my model in SolidWorks. Any non-generated variables were manually filled in using the information supplied from GRANTA.
With all necessary information known, a static study simulation within SolidWorks could now be completed. For the study, I set the following parameters:
Defined Restrictions
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Row 1 of images below: The reference geometries for each the placement of a base plate is set as a fixed geometry (notice the two equidistant, dark squares on the bottom of the board).
Defined External Loads
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Row 2 of images below: The 180lb applied force was applied as pressures, distributed over the approximate surface area of a size 12 foot, placed equidistant from the ends.
Generated a Mesh
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Row 3 of images below: A mesh was applied as a "blended-curvature mesh" to accommodate the transitions between edges and fillets. (The mesh density slider was left at the middle/neutral.)
After running the study, the first set of images below displays the vertical displacement simulation for the model. The second set of images display the Von Mises Stress and the strain results. The maximum displacement or deformation endured by the board occurs at the middle region and is estimated to deform 0.02676" from the neutral axis. The goal was to endure a deformation no larger than 0.375"; assuming the simulation is accurate, a pine board of these dimensions would be able to support a 180lb user.
Design Optimization
It successfully passes the displacement check, but is it lightweight?
Yes! Using Solidworks's evaluation and mass properties tool, a board of this size and make would weigh 1.50lbs.
What about the Factor of Safety?
According to the static analysis from SolidWorks, this pine deck would result in an overall Factor of Safety (FoS) of 3.8. Meaning that this design would fail at 3.8 times the design load.
Now, having designed a deck that supports an 180lb user, endures a vertical displacement less than 0.375”, has a factor of safety of 3, I wondered how a different material would affect my results. To properly compare each analysis, the only factor changing is the material set within SolidWorks. All fixed geometries, external loads, and measurements remain unaltered. In doing so, I intended to see how the overall vertical displacement, the factor of safety, and the weight change based on the material selected.
Returning to GRANTA, I chose three materials, all within the woods family: maple (Acer macrophyllum), bamboo (longitudinal), and birch (Betula alleghaniensis). I decided to choose these three as the performance factor is comparable for each and because skateboards are commonly made of these types of woods [See in "Additional References" the thesis entitled Analyzing the Flexural Properties of Self-Constructed Bamboo Laminates]. The analysis for each wood is summarized below, displaying images of the results and a brief overview.
Maple
Static Analysis 2: Maple
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Overall Vertical Displacement
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0.02701"
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Factor of Safety (FoS)
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4.4
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Weight of Board
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2.15 lbs
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Maximum Stress
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1384 psi
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Minimum Stress
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0.9091 psi
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Bamboo
Static Analysis 2: Bamboo
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Overall Vertical Displacement
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0.01684"
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Factor of Safety (FoS)
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4.2
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Weight of Board
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2.46 lbs
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Maximum Stress
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1381 psi
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Minimum Stress
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1.014psi
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Birch
Static Analysis 2: Birch
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Overall Vertical Displacement
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0.01945"
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Factor of Safety (FoS)
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5.9
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Weight of Board
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3.04 lbs
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Maximum Stress
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1384 psi
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Minimum Stress
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0.0901 psi
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Finite Element Analysis (FEA) is a method used to analyze how a part or assembly will behave when imposed under particular conditions. The static analysis feature through SolidWorks Simulation utilizes FEA for any given model and produces scores of studies relating to displacements, stresses endured, safety checks, only to name a few. FEA can be used to optimize any design by changing a variable, running subsequent studies, and comparing the results to each other to determine which is most feasible. For this design, I used FEA to compare the behavior of the deck based on each material change. Each variation of the deck was analyzed similarly, while the only difference between studies was the material selection. As a result, each study or each material is adequately compared to one another. The studies showed that each design would be safe to produce since each Factor of Safety resulted in a value greater than 1. Understanding how each design behaves and noting that each design can sufficiently support the applied load, any of the materials analyzed can be chosen for the final design.
Final Remarks
If the shape and dimensions of the design don't change, the weight of the board can only be reduced through the selection of its material. In each of the analyses, all materials were able to withstand each constraint. Yet, pine proved to be the lightest board. Therefore, the material with the lowest density will result in the lightest board. It is no surprise that each wood was able to satisfy all design constraints. The materials chart generated a slope to rule out the areas of average-to-high ratios of Young's Modulus to density, otherwise known as the performance factor. Narrowing down the chart to the woods region meant that any material from this family of materials would prove to be an acceptable choice. As the main goal of this assignment was to design a lightweight skateboard deck, I would choose pine as my material of choice.