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- Supports assume infinite stiffness in specific degrees of freedom. Some supports (like fixed) have reactionary forces and moments in all DOF, some like (roller and pinned) have reactionary forces and moments in only specific DOF
- All that matters is that different types of supports resist different types of forces/moments, which you can use to solve/isolate forces and understand the reactions happening in a system
- Supports are integral to setting up a simulation right. If you have too many constraints, your sim will be too stiff and you will have inaccurate results. Too little constraints, same problem, inaccurate results, that can cause you to falsely presume your part is safe/effective.
Friction is also present in some capacities within the Ergonomics system, with the main part being the braking system.
- Friction is the force that resists or opposes either
- a) relative motion → Kinetic Friction (although relative motion can be at rest so potentially Static Friction)
- b) impending motion → Static Friction
- Friction is made up of the coefficient of friction which is determined by the surfaces and materials you are working with and the normal force, the force pressing the contacting surfaces together.
- Friction is also most often associated with heat energy, meaning that when friction occurs, heat dissipation is likely to follow. Think of this as when you rub your hands together → friction occurs → heat follows shortly after
- As an example, friction is present in the braking system as when the driver pushes the brake pedal and shifts the braking fluid, which sends hydraulic pressure to the brake caliper, which cause the brake pads to contact the brake rotor, thereby causing friction, which then slows down the rotor and wheels, while also releasing some of the transferring kinetic energy as heat to the atmosphere.
Two Force Members
- A two force member in our case would be a tube member with forces only acting in two locations, with the forces being equal opposite and colinear.
- This is mainly useful when analyzing the loading of a part (say in a simulation setting) where you can pick out two force members of a structure, which simplifies the force layout of the structure and limits the amount of unknowns
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Materials
Start here → Materials Lecture.pdf
Every material is made up of a crystal structure (crystal defects, a unique organization of atoms, etc.) which leads to materials with unique/different properties (i.e. this is how you get alloys, a mixture of chemical elements with at least one metal).
- You can look more into this on your own, but essentially, we pick a tube material based on its makeup and its properties (weight, density, yield strength, Young’s modulus, etc.)
- Material selection is a lot more in depth with Ergo as compared to Frame, as you the specifications for each part may vary. While the entire frame needs to be one rigid piece, welded together and uniform, a single material will be used like 6361 aluminum or 4130 chromoly steel. However, with Ergo, you may need a light weight brake and accelerator pedals that are subject to a lighter load, but require a more rigid, easily weldable metal for the belly pan, that must hold high amounts of weight and therefore be exposed to high stress.
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Lastly, the Factor of Safety, at least in our case is measured with the yield strength of the material, as well as the max von Mises equivalent stress from the given simulation/reaction (the equation is Max eq stress/yield strength).
- This indicates when a part can fail
- FOS of 1 means the current simulation/reaction will begin to fail at 1 times the max equivalent stress, 2 means the structure begins to fail at 2 times the max equivalent stress, etc.
- FOS of 1 is the absolute bare minimum a structure should have before it is deemed manufacturable, but some parts of the frame must have a 1.1 minimum, due to high safety goals/concerns (although you should always shoot for something >1.6 FOS)
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A big point as it pertains to the braking system is the difference of compressible vs. non-compressible substances/fluids. In the braking system, we are working with brake fluid, which like water, is an incompressible fluid, meaning that when a pressure is applied, the fluid will move and transfer that pressure as opposed to taking it and becoming compressed.
- However, when setting up a braking system, it's important to bleed your brakes, which just means purging the system of air bubbles. When you install brakes and therefore brake fluid, bubbles are present in the lines before bleeding. Because air IS compressible as opposed to your brake fluid, the air will compress and take energy and pressure away from the brake fluid as it's trying to move and activate your brakes. This is what causes a brake to become squishy and unresponsive, as when air bubbles are present you are basically just compressing and decompressing the air with minimal brake fluid movement.
- On the same type of note, using Thermodynamics, we know that when this pressure is applied, some of that energy flowing through the fluid will transfer to heat energy, which lead to brake fluid boiling. This is more so a concern of the driver (as excessive/improper braking is the cause of brake boiling), but this also plays into brake fluid selection. At its core, just like any water, is it is boiled, vapor is released, which contains air bubbles. As previously mentioned, if your brake fluid is boiling, and therefore, you create air bubbles, well I think you know where I am going with this...
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Lastly, piggybacking off of the types of flow, consider the pathing of your tubing. Consistent and smooth fluid streams are naturally better suited to straight, consistent paths, but inevitably you might need to change the directions and pathing of your lines.
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