Design and Technology: Product Design (9252) | Mechanical devices

Welcome to Core Technical Principles: Mechanical Devices!

Hello future designer! This chapter is incredibly exciting because we are moving inside the products themselves—to the "guts" that make things move and function exactly how we want them to. Mechanical devices are the hidden heroes of product design!

In simple terms, a mechanical device is a system used to manage force and control movement. They help us change the direction, speed, or type of motion, often making difficult tasks much easier. Ready to dive in? Let's go!

Section 1: The Four Basic Types of Movement

Every mechanical device starts with movement. Understanding the four fundamental types of motion is the first step in designing a functional product. Think of this as the basic language of mechanics.

Don’t worry if these terms sound technical; we use these movements every day!

1. Reciprocating Movement

• Definition: Movement that goes back and forth in a straight line.
• Analogy: Imagine sawing a piece of wood. The saw blade moves in and out repeatedly.
• Key Example: The piston in a car engine, or the needle in a sewing machine.

2. Rotary Movement

• Definition: Movement in a circular path around a central point (rotation).
• Analogy: A bicycle wheel spinning, or the hands on a clock.
• Key Example: Electric fan blades, gears in a clock, drills.

3. Oscillating Movement

• Definition: Movement that swings back and forth in an arc (a curved line). It’s like rotary movement that stops before completing a full circle.
• Analogy: A swing set moving back and forth, or a pendulum on a grandfather clock.
• Key Example: The arm on a screen wiper on a car, or the movement of a doorbell chime hammer.

4. Linear Movement

• Definition: Movement that goes straight from one point to another without changing direction (a pure straight line).
• Analogy: Pushing a box across the floor, or opening a drawer.
• Key Example: The movement of a train on a straight track, or the sliding mechanism on a cupboard door.

Section 2: Levers – The Power of Leverage

Levers are perhaps the oldest and most common mechanical device. They allow us to move a large load using only a small effort. This ability to multiply force is called Mechanical Advantage.

A lever always consists of three parts:

1. Fulcrum (F): The pivot point the lever rotates around.
2. Load (L): The weight or resistance you are trying to move.
3. Effort (E): The force you apply to make the movement happen.

The position of these three parts determines the Class of the lever.

The Three Classes of Levers (FLE is the Key!)

A great way to remember the classes is to think about what is in the middle.

1. Class 1 Lever (F is in the Middle)

• Order: Effort – Fulcrum – Load (E-F-L)
• Action: The load moves in the opposite direction to the effort.
• Key Example: Scissors, a see-saw, crowbars.
• Did you know? Class 1 levers can be used to gain high mechanical advantage (if the fulcrum is closer to the load) or to increase speed (if the fulcrum is closer to the effort).

2. Class 2 Lever (L is in the Middle)

• Order: Effort – Load – Fulcrum (E-L-F)
• Action: The load moves in the same direction as the effort.
• Key Example: Wheelbarrows, nutcrackers, bottle openers.
• Advantage: These levers always provide mechanical advantage (you use less force than the load weighs).

3. Class 3 Lever (E is in the Middle)

• Order: Fulcrum – Effort – Load (F-E-L)
• Action: The load moves in the same direction as the effort.
• Key Example: Tweezers, fishing rods, the human forearm when lifting a weight.
• Trade-off: Class 3 levers never provide mechanical advantage (you always use more effort than the load). Instead, they are used to achieve speed or range of movement.

Section 3: Linkages – Controlling and Changing Motion

A linkage is simply a system of rigid pieces (links) connected by pivot joints. Their main purpose is to transmit force or change the type or direction of movement.

How Linkages Work

Imagine you have a mechanism that is moving left and right (reciprocating), but you need the final output to push up and down. A linkage can solve this problem.

1. The Bell Crank Linkage

• Function: Used to change the direction of force or movement, usually by 90 degrees.
• Appearance: Often shaped like an 'L' or a 'V' connected to a central pivot.
• Example: Used in control systems, often found in brake systems on bicycles or older cars. If you pull horizontally, the linkage converts that force to a vertical push.

2. Push/Pull Linkages (Simple Mechanisms)

• Parallel Linkages: These linkages ensure that the input and output move in the same direction, keeping them parallel. Example: The mechanism that keeps the steps of a step ladder parallel to the floor as they unfold.
• Reverse Motion Linkages: These cause the output to move in the opposite direction to the input. If you push the input link right, the output link moves left.

Section 4: Cams and Followers – Turning Circles into Jumps

A Cam and Follower system is a classic mechanical method used to convert Rotary movement (circular spinning) into controlled Reciprocating (up and down) or Oscillating (swinging) movement.

Think of a cam as a specially shaped spinning wheel, and the follower as a stick resting on the edge of that wheel.

The Components

• The Cam: The rotating component, usually attached to a motor shaft. Its shape determines the movement of the follower.
• The Follower: The component that rests on the edge of the cam and moves up and down or swings.

Types of Cams (The Key Shapes)

The shape dictates the follower's motion (how long it lifts, stays high, or drops quickly).

1. Pear Cam (or Heart-shaped Cam):
   • Action: Rises gradually, dwells (stays still) for a long time at the top, and then drops quickly.
   • Example Use: Valve control in car engines.
2. Eccentric Cam (or Circular Cam with Offset Centre):
   • Action: Creates a smooth, consistent rise and fall (harmonic motion).
   • Example Use: Simple pumps or shaker mechanisms.
3. Snail Cam (or Drop Cam):
   • Action: Very gradual rise, then a sudden, dramatic drop. Crucially: This cam only works if the cam rotates in one direction.
   • Example Use: Hammering mechanisms or stamping machines.

Types of Followers

The end shape of the follower affects friction and performance:

• Roller Follower: Reduces friction the most. Good for high-speed operation.
• Knife-Edge Follower: Simple, but creates high friction and wears out quickly.
• Flat/Mushroom Follower: Allows for precise control but can slip if not guided correctly.

Section 5: Gears – Transmitting Power and Changing Speed

Gears are wheels with teeth that mesh together. They are essential for transmitting rotary motion from one shaft to another, and crucially, for changing speed and turning force (torque).

Basic Gear Principles

1. Driver Gear: The gear that is attached to the power source (motor) and makes the system move.
2. Driven Gear (or Follower Gear): The gear that receives the power and moves the load.

Direction Rule: When two external spur gears mesh, they always rotate in opposite directions.

Speed and Torque Rule:

• If the Driver gear is smaller than the Driven gear, the speed decreases, and the turning force (torque) increases. (This is used for high power, like climbing a hill on a bicycle).
• If the Driver gear is larger than the Driven gear, the speed increases, and the turning force (torque) decreases. (This is used for high speed).

Important Types of Gears

1. Spur Gears

• Description: The most common type. They have straight teeth and are mounted on parallel shafts.
• Use: Simple gearboxes, clocks, toys.

2. Bevel Gears

• Description: Have teeth cut on a cone shape.
• Use: Used to transmit power between shafts that are at an angle (usually 90 degrees) to each other.
• Example: The drive system in a car differential.

3. Worm and Worm Wheel

• Description: A screw-like gear (the worm) meshes with a spur gear (the worm wheel).
• Key Feature: Provides extremely large speed reduction in one stage. It is also self-locking, meaning the worm wheel cannot turn the worm, only the worm can turn the wheel.
• Example: Tuning mechanisms on musical instruments, conveyor belt drives where back-driving must be prevented.

4. Rack and Pinion

• Description: Converts rotary movement (the circular pinion gear) into linear movement (the straight rack).
• Example: The steering mechanism in most cars, or the focusing mechanism on a microscope.

Chapter Conclusion: Putting it All Together

Congratulations! You have now mastered the fundamental mechanical devices that form the backbone of almost every designed product. Whether you are designing a high-tech kitchen mixer or a simple desk lamp, you will need to select the right mechanism to control the movement.

Designing the mechanism is about choosing the device that gives you the required movement (Reciprocating, Rotary, etc.) while providing the correct mechanical advantage or speed increase.

Keep practicing identifying these mechanisms in everyday objects—that’s the best way to prepare for your design assessments!

Summary Checklist

• Do I know the difference between Rotary and Oscillating movement?
• Can I identify the Fulcrum, Load, and Effort in a lever?
• Do I know that linkages primarily change the direction or type of movement?
• Do I remember that Cams convert Rotary to Reciprocating/Oscillating?
• Do I understand that smaller gears driving larger gears reduce speed and increase torque?