B3.3 — Mechanical systems application and selection (HL)
Key concepts
B3.3 takes the mechanism theory from A3.3 and applies it to real product design — choosing, specifying, and integrating mechanisms so that products actually move, lift, turn, and transform motion as intended.
Designing with mechanisms
Mechanism design starts with the input and output requirements:
- What input is available? (hand force, motor torque, pedal, spring)
- What output motion is needed? (rotation speed, linear force, specific path)
- What is the required mechanical advantage or velocity ratio?
- What packaging constraints apply? (space, weight, cost)
From these, the designer chooses a mechanism type (gears, levers, cams, linkages) and specifies its geometry.
Mechanism specification and selection
Specifying a mechanism means defining:
- Gear ratios or lever arm lengths
- Torque transmitted
- Speed (RPM for rotary, m/s for linear)
- Efficiency expected
- Backlash tolerance (play between mating parts)
- Load capacity and safety factor
A designer selects from standard components (stock gears, off-the-shelf linkages) where possible. Custom components are expensive to tool, so designers avoid reinventing parts.
Performance calculations
Mechanical calculations verify the mechanism will do its job before manufacture:
Torque calculation: T = F × r (force × radius)
Power transmission: P = T × ω (torque × angular velocity in rad/s) P = F × v (force × linear velocity)
Gear train output speed: ω_out = ω_in / gear ratio
Output force through a lever: F_out = F_in × (effort arm / load arm)
These calculations tell the designer whether the chosen motor, material, and geometry will deliver the required performance.
Prototyping mechanical systems
Mechanisms are notoriously hard to visualise — they must be physically built to verify. Prototyping approaches:
- Cardboard linkages with brass paper fasteners as pivots — near-free proof-of-concept
- Laser-cut plywood or acrylic — rapid higher-fidelity versions
- 3D-printed gears and cams — quickly explore custom geometries
- LEGO Technic — rapid assembly of standard mechanisms for conceptual testing
The first physical prototype always reveals issues invisible in CAD: binding, excessive friction, slop, interference.
Testing and refining mechanisms
Mechanism testing checks:
- Does it move? No binding or jamming through full travel
- Is it efficient enough? Measured torque out vs torque in
- Does it meet backlash requirements? Measurable play
- Does it wear acceptably? Cycle testing over expected product life
- Is it reliable? Random failure rate
Refinement typically reduces friction (better bearings, lubrication), tightens tolerances (less slop), or strengthens weak members that deflect under load.
Integration
A mechanism rarely exists alone — it sits inside a product housing, connected to other subsystems. Integration challenges:
- Mounting — attaching the mechanism securely without distortion
- Clearances — ensuring moving parts don't contact housings
- Access — servicing and assembly
- Sealing — protecting from dust, moisture, lubricant escape
- Noise — mechanisms often generate unacceptable noise without damping
The mechanism's designer must coordinate with the product's industrial designer and structural designer throughout.
Case studies
Umbrella mechanism — complex multi-bar linkage converts a sliding handle motion into spreading canopy. Deceptively simple appearance disguises hundreds of design iterations balancing strength, weight, and cost.
Tape dispenser cutter and friction brake — a simple rotary tape dispenser hides a serrated-blade cutter (shear force perpendicular to pull direction) and a friction spring that prevents the tape from over-rotating. Small details, months of testing.
Rolling luggage retractable handle — telescoping tube with spring detents. Must lock firmly at full extension, release with light touch, retract fully without jamming. Users perform this action thousands of times per product — reliability matters.
Car window winder (manual) — worm gear driving a sector gear driving a scissor linkage lifting the glass. Four linked mechanisms delivering significant mechanical advantage with quiet, smooth operation. A textbook example of mechanism integration.
Glossary
- Mechanism — an assembly of moving parts that transmits or transforms motion.
- Torque — rotational force (N·m).
- Backlash — free play between mating mechanical parts.
- Efficiency — ratio of useful output to input energy.
- Gear train — two or more gears meshed to transmit motion.
- Linkage — pivoting bars producing complex motion paths.
- Integration — combining a mechanism with the surrounding product systems.
- Clearance — free space between moving and stationary parts.
Check your understanding
1. A designer needs a mechanism that converts a motor's rotation (6000 RPM) into much slower, higher-torque rotation for a cordless drill chuck. Describe the mechanism they might use.
A reduction gearbox: a series of gear stages where each meshes a small gear driving a larger gear. For example, a 3-stage planetary gearbox with each stage giving 5:1 reduction achieves 125:1 total reduction, dropping 6000 RPM to 48 RPM while multiplying torque by 125 (ignoring losses). Planetary gears are compact, efficient, and common in cordless drills. The designer specifies stage ratios based on the required chuck torque and motor characteristics.
2. Calculate the torque delivered by a 0.2 m lever when 40 N effort is applied perpendicular to the end.
T = F × r = 40 × 0.2 = 8 N·m. This is the torque delivered at the pivot. If the load were at a 0.05 m radius from the pivot, the force at the load would be T / r = 8 / 0.05 = 160 N — a mechanical advantage of 4.
3. Explain why backlash matters and give a product example where it must be minimised.
Backlash is free play between mating parts (e.g. gears). In a gear train, backlash causes the output to lag the input when direction reverses, creating imprecision and noise. In CNC machine positioning, backlash causes tool misplacement — the cutter lags the command signal at each direction change. CNC machines use preloaded ballscrews and anti-backlash gears to eliminate this error. Cheap children's toys can tolerate backlash; precision machines cannot.
4. Identify three considerations when integrating a mechanism into a product housing.
Mounting: the mechanism must be secured so that operational loads do not distort the housing or shift alignment. Clearance: moving parts require free-space margins to avoid contact with the housing through their full travel. Serviceability: the mechanism must be accessible for assembly, lubrication, and repair — either through removable housing panels or designed assembly sequences. Neglecting any one produces products that cannot be manufactured or maintained.