A3.3 Introduction to mechanical systems (HL only)

A3.3 Introduction to mechanical systems (HL only)

Welcome to the "muscles" of Design Technology! While structural systems (A3.2) are about keeping things still and stable, mechanical systems are all about movement. In this chapter, we explore the theoretical principles of how machines take an input, process it, and create a useful output. Whether it is a simple pair of scissors or a complex car engine, the same basic rules of physics apply.

1. What is a Mechanical System?

A mechanical system is a group of components that work together to control or change forces and motion to perform a task. We often describe these using a simple "systems" model:

• Input: The energy or force you put into the system (e.g., pushing a bicycle pedal).
• Process: The mechanism that changes the input (e.g., the chain and gears).
• Output: The resulting action or movement (e.g., the wheel spinning).

Design Tip: Remember that machines don't "create" energy; they just transform it or make it easier to use.

2. Types of Motion

Before designing a system, you need to understand the four ways things can move. Most mechanical systems convert one type of motion into another.

Linear Motion: Movement in a straight line in one direction (e.g., a paper trimmer cutting a sheet).
Reciprocating Motion: Back-and-forth movement in a straight line (e.g., the needle on a sewing machine).
Rotary Motion: Movement in a circle around a fixed center or axis (e.g., a bicycle wheel or a cooling fan).
Oscillating Motion: Back-and-forth movement in an arc or curved path (e.g., a playground swing or a pendulum clock).

Quick Mnemonic: Think "LRRO" (Linear, Reciprocating, Rotary, Oscillating) to remember the four types!

3. Mechanical Advantage (MA) and Velocity Ratio (VR)

The main reason we use machines is to gain a mechanical advantage. This allows us to move a large load using a much smaller effort.

Mechanical Advantage (MA)

This is the ratio of the Load (the weight/force being moved) to the Effort (the force you apply).
Formula: \( \text{Mechanical Advantage (MA)} = \frac{\text{Load}}{\text{Effort}} \)

If \(MA > 1\), the machine makes the job easier by magnifying your force. If \(MA < 1\), the machine requires more force but might move the load faster or further.

Velocity Ratio (VR)

This compares the distances moved. In a perfect world, if you move your hand 10cm to move a load 2cm, your VR is 5.
Formula: \( \text{Velocity Ratio (VR)} = \frac{\text{Distance moved by effort}}{\text{Distance moved by load}} \)

Efficiency

In the real world, friction gets in the way. Some energy is always lost as heat. Efficiency tells us how much energy is actually doing useful work.
Formula: \( \text{Efficiency} = \frac{MA}{VR} \times 100\% \)

Common Mistake: Students often mix up Load and Effort. Just remember: Effort is what YOU do; the Load is what the OBJECT does.

4. Simple Machines: The Building Blocks

Mechanical systems are built from several classic "simple machines." You should be able to identify these and understand how they work.

Levers

A lever consists of a rigid bar that pivots on a Fulcrum. There are three classes of levers, depending on where the Fulcrum, Load, and Effort are located.

First Class: Fulcrum is in the middle (e.g., a seesaw or crowbar).
Second Class: Load is in the middle (e.g., a wheelbarrow).
Third Class: Effort is in the middle (e.g., a pair of tweezers or a fishing rod).

Memory Aid: "FLE 123"
• Fulcrum in middle = 1st Class
• Load in middle = 2nd Class
• Effort in middle = 3rd Class

Pulleys

Pulleys use ropes and wheels to redirect force or provide MA.
• A Fixed Pulley only changes the direction of the force (\(VR = 1\)).
• A Movable Pulley moves with the load and halves the effort needed (\(VR = 2\)).
• A Block and Tackle system uses multiple pulleys together to greatly increase MA.

Gears

Gears are toothed wheels that lock together to transmit rotary motion. They can change the speed, direction, or torque (turning force) of a system.

Formula: \( \text{Gear Ratio} = \frac{\text{Number of teeth on driven gear}}{\text{Number of teeth on driver gear}} \)

If a small driver gear turns a large driven gear, the speed decreases but the torque increases. This is like putting a bicycle in a low gear to go uphill!

Cams and Linkages

• Cams: These convert Rotary motion into Reciprocating motion. An egg-shaped cam spins, pushing a "follower" up and down.
• Linkages: These are bars connected by pivots. They can change the direction of motion or the distance a force travels (e.g., a toolbox hinge or a folding chair).

5. Key Takeaways for the IB Exam

• Motion Conversion: Be ready to describe how a system might change rotary motion (a motor) into linear motion (a sliding gate).
• The Trade-off: You never get something for nothing! If a machine gives you a high Mechanical Advantage, you will have to move the Effort a much longer distance to move the Load a short distance.
• Calculations: Practice simple rearrangements of the MA and VR formulas. Calculators are allowed, so focus on setting up the equation correctly.
• HL Context: While this chapter (A3.3) covers the theory, you will apply these choices in Strand B (B3.3) when selecting actual parts for a design.

Did you know? Even the most advanced robots in the world are still fundamentally based on these simple principles of levers, gears, and cams!

Quick Review Box

1. Mechanical Advantage (MA) = Load / Effort.
2. Four motions: Linear, Rotary, Reciprocating, Oscillating.
3. Efficiency is always less than 100% because of friction.
4. Driver gear is the one connected to the power source; the driven gear is the one being moved.