Welcome to Mechanical Systems and Structures!
In this chapter, we are going to explore how things are built and how they move. Whether you are designing a sleek new chair, a mechanical toy, or a bridge, you need to understand two things: Structures (the "skeleton" that holds things up) and Mechanical Systems (the "muscles" that make things move). This is a vital part of the Materials, Mechanisms and Systems section of your Design course.
Because MYP Design is all about your ePortfolio, think of this chapter as a toolkit. You will use these concepts to explain your choices in Criterion A (Inquiring and analysing) and to plan your amazing inventions in Criterion B (Developing ideas).
1. Structures: The Strength of Your Design
A structure is any object that provides support or encloses a space. It has to be able to carry a "load" (weight) without breaking or bending too much.
Types of Structures
Most things you design will fall into one of these three categories:
- Frame Structures: These use a "skeleton" of parts joined together. Think of a bicycle frame, a skyscraper's steel beams, or a tent. They are usually light but very strong.
- Shell Structures: These are "hollow" and use a thin outer layer to hold their shape and protect what is inside. Think of an egg, a helmet, or a plastic water bottle.
- Mass (Solid) Structures: These are made of a solid piece of material. Think of a brick wall or a wooden sculpture. They are heavy and stay in place by their own weight.
Forces: The "Push and Pull"
When you design a product, different forces will act on it. If you understand these, you can choose the right materials! Don't worry if these names sound fancy—you see them every day:
Tension: This is a pulling force. Imagine playing tug-of-war with a rope. The rope is in tension.
Compression: This is a squashing force. When you sit on a chair, the legs are in compression because your weight is pushing down on them.
Bending: This happens when a force is applied to the middle of a structure. Think of a shelf with too many books on it—it starts to curve.
Torsion: This is a twisting force. Think of wringing out a wet towel or turning a screwdriver.
Shear: This is a sliding or cutting force. Imagine two parts of a material being pushed in opposite directions, like how scissors cut paper.
Quick Review: To make a design successful in Criterion C (Creating the solution), you must ensure your structure can handle these forces without failing!
2. Mechanical Systems: Making Things Move
A system is a group of parts that work together to do a job. Mechanical systems usually take an input (the force you put in), process it using a mechanism, and create an output (the motion or force that comes out).
The Four Basic Types of Motion
In your design sketches (Criterion B), you might need to show how a part moves. There are four main types:
- Linear Motion: Moving in a straight line in one direction (like a train on a track).
- Reciprocating Motion: Moving back and forth in a straight line (like the needle on a sewing machine).
- Rotary Motion: Moving in a circle (like a wheel or a spinning fan).
- Oscillating Motion: Moving back and forth in an arc (like a playground swing or a pendulum).
Simple Mechanisms
Mechanisms are used to change one type of motion into another, or to make work easier (this is called Mechanical Advantage).
- Levers: A rigid bar that turns around a fixed point called a pivot or fulcrum. (Think of a see-saw or a pair of pliers).
- Gears: Toothed wheels that lock together. They can change the speed or direction of a movement.
Example: If a large gear turns a small gear, the small gear will spin much faster! - Pulleys: Wheels with a groove for a rope or belt. They are great for lifting heavy loads.
- Linkages: Strips of material (like wood or metal) joined together to change the direction of a force. Think of the folding mechanism on a deck chair.
Did you know? You can calculate the "Gear Ratio" to see how much faster a wheel spins. If Gear A has \(20\) teeth and Gear B has \(10\) teeth, the ratio is:
\(Ratio = \frac{20}{10} = 2\)
This means for every one turn of the big gear, the small one turns twice!
3. How to Use This in Your MYP ePortfolio
Since you are assessed on the Design Cycle, here is how you apply "Mechanical Systems and Structures" to get those top marks:
Criterion A: Inquiring and Analysing
When you look at "existing products," don't just say they look cool. Analyse them! What forces are acting on them? Is the chair legs in compression? Does the toy use a linkage system to move its arms? This shows you truly understand the "Materials, Mechanisms and Systems" section.
Criterion B: Developing Ideas
When you draw your designs, use annotations (notes). Instead of just drawing a line, label it: "Steel frame used here to resist tension forces." Use arrows to show the type of motion (rotary, linear, etc.) your mechanism will use.
Criterion C: Creating the Solution
This is where you demonstrate your technical skills. If your plan says you will use a gear system, you need to build it accurately so it functions. If your structure is weak, you might need to "justify changes" to your plan by adding a brace for extra strength.
Criterion D: Evaluating
When you test your product, you are checking if the mechanical system works. If the gears jam, you explain how it could be improved. Maybe the material had too much friction, or the structure wasn't rigid enough?
Summary Key Takeaways
1. Forces: Know your tension (pull), compression (squash), bending, torsion (twist), and shear (slide).
2. Structures: Frames are skeletons, shells are hollow, and mass structures are solid.
3. Motion: Everything moves in either a line (linear/reciprocating) or a curve (rotary/oscillating).
4. Mechanisms: Use levers, gears, and pulleys to make your product more efficient and functional.
Note: For more information on how these parts are powered, see the chapter on Electronic Systems. To see how humans interact with these machines, check out Ergonomics and Human Factors.