Mechanical components are the physical building blocks used to transmit, transform, or control motion and force in engineering systems. Understanding their principles of operation — how each component works — and their applications — where and why they are used — is fundamental to VCE Systems Engineering.
KEY TAKEAWAY: Every mechanical component has a specific function: changing direction of force, changing speed, changing torque, storing energy, or enabling specific motion types.
A lever is a rigid beam pivoting on a fulcrum. It transmits and modifies force and motion.
Classes of levers:
| Class | Fulcrum position | Example | Effect |
|---|---|---|---|
| 1st class | Between effort and load | Seesaw, crowbar | Can increase force or speed |
| 2nd class | Load between fulcrum and effort | Wheelbarrow | Always increases force (MA > 1) |
| 3rd class | Effort between fulcrum and load | Tweezers, forearm | Always increases speed/distance (MA < 1) |
Principle: The lever multiplies force according to the ratio of effort arm to load arm:
$$MA = \frac{\text{Effort arm length}}{\text{Load arm length}}$$
EXAM TIP: Be able to identify lever class from a diagram and state whether it provides a mechanical advantage or velocity advantage.
A linkage is a series of rigid links connected by pivot joints to transmit motion or force.
APPLICATION: Linkages allow complex motion paths to be achieved with simple components. A bicycle brake uses a parallel linkage to ensure even pad contact.
Gears are toothed wheels that mesh together to transmit rotary motion and torque.
Key types:
- Spur gears: Teeth parallel to axis; simple, high efficiency
- Bevel gears: Angled teeth; change axis of rotation (e.g. 90°)
- Worm gear: Screw-like gear drives a wheel; very high reduction ratios, self-locking
- Rack and pinion: Converts rotary to linear motion (e.g. steering systems)
Gear ratio:
$$GR = \frac{N_{driven}}{N_{driver}} = \frac{T_{driven}}{T_{driver}}$$
where $N$ = rotational speed (rpm) and $T$ = number of teeth.
Worked example: Driver gear has 20 teeth at 100 rpm; driven gear has 40 teeth.
$$GR = \frac{40}{20} = 2 \quad \Rightarrow \quad N_{driven} = \frac{100}{2} = 50 \text{ rpm}$$
The driven gear rotates at half the speed but with double the torque.
VCAA FOCUS: Gear problems frequently appear in exams. Always identify driver vs. driven gear and apply the gear ratio correctly.
A pulley is a wheel with a grooved rim over which a rope or belt runs.
Belt-and-pulley systems transmit rotary motion between shafts and can change speed:
$$\frac{N_1}{N_2} = \frac{D_2}{D_1}$$
where $D$ is pulley diameter. A small driving pulley connected to a large driven pulley reduces speed.
REMEMBER: In a belt-and-pulley system, the belt speed is the same on both pulleys — the smaller pulley rotates faster.
A cam is a rotating or sliding piece that converts rotary motion into a specific pattern of reciprocating or oscillating motion in a follower.
Common cam profiles:
- Pear-shaped (egg cam): Follower dwells (pauses), then rises, dwells, then falls
- Eccentric (off-centre circle): Smooth up-and-down motion
- Snail cam: Rises gradually then drops suddenly
Application: Engine valve operation — as the camshaft rotates, cams push open valves at precisely timed intervals.
STUDY HINT: Draw the cam profile and trace the follower movement to understand the output motion pattern.
A crank is an arm attached at a right angle to a rotating shaft, used to convert rotary motion to reciprocating linear motion (or vice versa) via a connecting rod.
A ratchet consists of a toothed wheel and a pawl (catch) that allows rotation in one direction only, preventing reverse motion.
APPLICATION: Ratchets are safety-critical components when holding loads — they prevent back-driving under load.
Springs store and release elastic potential energy. They provide restoring force proportional to displacement (Hooke’s Law):
$$F = kx$$
where $F$ = force (N), $k$ = spring constant (N/m), $x$ = extension or compression (m).
Types:
- Compression springs: Resist being compressed (e.g. car suspension, pen click mechanism)
- Extension springs: Resist being stretched (e.g. trampoline, garage door)
- Torsion springs: Resist twisting (e.g. clothespeg, mouse trap)
- Leaf springs: Flat stacked plates; heavy vehicle suspensions
Worked example: A spring with $k = 500$ N/m is compressed by 0.04 m.
$$F = 500 \times 0.04 = 20 \text{ N}$$
Bearings reduce friction between rotating or sliding parts by providing a low-friction interface.
| Type | Operation | Application |
|---|---|---|
| Ball bearing | Balls roll between inner and outer races | Electric motors, bicycle wheels |
| Roller bearing | Cylindrical rollers; higher load capacity | Gearboxes, conveyor rollers |
| Thrust bearing | Handles axial (along shaft) loads | Propeller shafts, steering columns |
| Plain bearing (bush) | Sliding contact with lubrication | Crankshaft journals, door hinges |
COMMON MISTAKE: Students often confuse the function of bearings (reduce friction and support loads) with the function of seals (prevent contamination and retain lubricant). They are separate components that often work together.
| Component | Primary function | Motion type |
|---|---|---|
| Lever | Force/distance trade-off | Linear |
| Linkage | Transmit/transform motion | Various |
| Gear | Change speed/torque | Rotary |
| Pulley | Change force direction/magnitude | Linear/rotary |
| Cam | Convert rotary → specific pattern | Rotary → reciprocating |
| Crank | Convert rotary ↔ linear | Rotary ↔ linear |
| Ratchet | One-direction motion control | Rotary (intermittent) |
| Spring | Store/release energy | Linear or torsional |
| Bearing | Reduce friction, support loads | Rotary or linear |
