Welcome to the World of Mechanical Devices!
Ever wondered how a tiny squeeze on a bicycle brake lever manages to stop the whole bike? Or how a sewing machine needle moves up and down so fast? That is the magic of mechanical devices!
In this chapter, we are going to explore how we can use different mechanisms to create movement, change the direction of a force, or make a heavy load feel much lighter. These are the "Core Technical Principles" that every designer needs to know to make products work effectively. Don't worry if it seems a bit "physics-heavy" at first—we'll break it down into simple steps!
1. The Four Types of Movement
Before we build machines, we need to understand how things move. In the world of D&T, there are four main types of movement you need to recognize:
Linear Movement
This is movement in a straight line and in one direction.
Example: A drawer sliding out of a desk.
Rotary Movement
This is movement following a circle around a fixed point.
Example: A wheel spinning on a bicycle or a cooling fan.
Reciprocating Movement
This is a back-and-forth movement in a straight line.
Example: The needle on a sewing machine or the blade on a power saw.
Oscillating Movement
This is a back-and-forth movement in a curved path (an arc).
Example: A playground swing or the pendulum in a clock.
Quick Review:
• Linear = Straight line (one way)
• Rotary = Around in a circle
• Reciprocating = Back and forth (straight)
• Oscillating = Back and forth (curved)
Did you know? Most machines involve changing one type of movement into another. For example, a car engine turns reciprocating movement (pistons) into rotary movement (wheels)!
2. Levers: Making Work Easier
A lever is the simplest type of mechanism. It uses a pivot (also called a fulcrum) to help move a load using an effort. Designers use levers to change the magnitude (size) of a force. There are three "orders" or classes of levers.
The "FLE" Memory Trick
To remember which lever is which, just remember the word FLE. The letter in the middle tells you what is in the center of that lever:
1. Fulcrum in the middle = 1st Order
2. Load in the middle = 2nd Order
3. Effort in the middle = 3rd Order
First Order Lever
The Fulcrum is in the middle.
Example: A seesaw or a pair of scissors.
Second Order Lever
The Load is in the middle. This allows you to lift very heavy weights with less effort.
Example: A wheelbarrow or a nutcracker.
Third Order Lever
The Effort is in the middle. This doesn't make the load lighter; instead, it gives you more control or speed.
Example: A pair of tweezers or a fishing rod.
Key Takeaway: Mechanical Advantage (\(MA\)) is when a lever allows you to move a large load with a small effort. It is calculated as:
\(MA = \frac{Load}{Effort}\)
3. Linkages: Changing Direction
Linkages are used to transfer motion or change the direction of a force. They are usually made of rigid parts (like metal or plastic bars) joined together.
Push/Pull Linkages
These keep the direction of movement the same. If you push one end, the other end moves in the same direction.
Example: A sliding bolt on a garden gate.
Bell Cranks
These are clever because they change the direction of motion through an angle, usually 90 degrees.
Example: The brake cables on a bicycle or the flush mechanism in some toilets.
Quick Review: Linkages connect parts of a machine. Bell cranks "turn the corner" with the force!
4. Rotary Systems
These systems involve things that spin. They are used to change speed, direction, or the type of motion.
CAMs and Followers
A CAM is a shaped piece of material attached to a rotating shaft. A follower rests on the CAM. As the CAM spins (rotary motion), it pushes the follower up and down (reciprocating motion).
Example: Controlling the valves in a car engine.
Simple Gear Trains
Gears are toothed wheels that lock together.
• If the gears are different sizes, they change the speed of rotation.
• In a simple gear train, the driver gear (the one you turn) and the driven gear (the one that moves as a result) will spin in opposite directions.
• To make them spin in the same direction, we add a third gear in the middle called an Idler Gear.
Math Tip: Gear Ratios
If the driven gear has 20 teeth and the driver gear has 10 teeth, the ratio is \(2:1\). This means the driver must turn twice to make the driven gear turn once!
Pulleys and Belts
Pulleys are like gears but without teeth. They use a belt to transfer the rotary motion from one shaft to another. They are quieter than gears and can be used to move parts that are further apart.
Example: The belt inside a washing machine or a pillar drill.
Summary Checklist
Before your exam, make sure you can:
• Identify the 4 types of movement (Linear, Rotary, Reciprocating, Oscillating).
• Draw and label the three classes of levers (remember FLE!).
• Explain how a Bell Crank changes direction by 90 degrees.
• Understand that CAMs turn rotary motion into reciprocating motion.
• Calculate a simple gear ratio based on the number of teeth.
Don't worry if this seems tricky at first! Try looking for these devices around your house—the more you see them in real life, the easier they are to remember!