Physical products, machines and mechanisms

Checklists and rough numbers for the working model (step 2) and the feasibility budget (step 4). Every value here is rough: use it for sanity checks, not final decisions.

Function checklist

Work through the ones that apply:

Energy: where it comes from, how it’s converted, where it goes. Include losses and the peak demand.
Material and fluid flow: what goes in and out (water, air, food, fuel, waste), and where it’s stored.
Signals and control: sensing, timing, user input, feedback, and what happens on a fault.
Load paths: how every force travels through the parts to the ground or the hand.
Motion: what moves, and what constrains it (bearing, hinge, slide, guide). Check the range, end stops, clearances and wear.
Sealing: where liquids, gases or dust must be kept in or out.
Heat: what produces heat, where it goes, and the maximum temperatures of the parts.
Power supply: mains, battery or human power, and for how long.
User interface: controls, feedback, reach and force.
Safety: pinch points, sharp edges, electrical shock, hot surfaces, tip-over, stored energy (springs, pressure), children.
Maintenance: access for cleaning, filters, wear parts and repair.

Quick formulas

Quantity	Formula	Note
Rotational speed	ω = rpm × 2π / 60	in rad/s
Centripetal acceleration	a = ω² r	a spin drum at 1000 rpm and r = 0.25 m ≈ 280 g
Rotating power	P = τ ω	τ = torque in N·m
Linear power	P = F v
Hydraulic power	P = Δp × Q	3 bar × 12 L/min = 300 000 Pa × 0.0002 m³/s = 60 W
Heating	Q = m c ΔT	water c ≈ 4.18 kJ/(kg·K); 10 L by 40 K ≈ 1.7 MJ ≈ 0.46 kWh
Kinetic energy	E = ½ m v², rotating: ½ I ω²	the energy to reach a speed is not the power to hold it
Potential energy	E = m g h	1 kg lifted 1 m ≈ 10 J
Stress	σ = F / A	compare with yield strength ÷ safety factor
Pressure force	F = p × A	1 bar on 10 cm² = 100 N
Energy ↔ power	1 kWh = 3.6 MJ; 1 W = 1 J/s

Typical values (rough, verify)

Power sources

Mains: EU 230 V / 16 A ≈ 3.7 kW per circuit (UK plug 13 A ≈ 3 kW); US 120 V / 15 A ≈ 1.8 kW.
Tap water: 2–4 bar static, about 6–15 L/min at a tap, so tens of watts of hydraulic power at most.
A person: pedalling about 75–100 W sustained (fit adult); hand crank about 20–50 W; bursts of a few hundred W.
Sun: about 1000 W/m² at noon on a clear day; panels about 20% efficient, so about 200 W/m² peak and roughly 0.6–1 kWh per m² per day.
Batteries: Li-ion about 150–250 Wh/kg; lead-acid about 30–40 Wh/kg; AA alkaline about 3 Wh.

What things use

Washing machine: spin motor 300–500 W; water heater about 2 kW; about 40–60 L of water per wash.
Fridge: about 1 kWh/day. Kettle: 2–3 kW. Microwave: about 1 kW. LED bulb: 5–10 W. Phone charging: 5–20 W.
Multirotor drone hover: about 150–200 W per kg.

Human factors

Comfortable repeated push or pull with the hand: about 20–50 N. Button press: about 2–10 N. Max grip: about 300–500 N.
Comfortable carry with one hand: under about 10 kg. Standing work surface: about 90–100 cm. Seat height: about 42–48 cm.

Materials (density; yield or tensile strength; notes)

Material	kg/m³	Strength (MPa)	Notes
Mild steel	7850	yield ~250	E ≈ 200 GPa; rusts
Stainless 304	8000	yield ~215	corrosion resistant
Aluminium 6061-T6	2700	yield ~275	E ≈ 69 GPa; 1/3 the stiffness of steel
PLA (printed)	1240	~30–50	stiff and brittle; softens around 55–60 °C
PETG (printed)	1270	~30–45	tougher; softens around 75–80 °C
ABS / ASA (printed)	1050	~25–40	softens around 90–100 °C; ASA is UV resistant
Nylon (PA)	1010–1140	~40–70	tough, low friction, absorbs moisture
Softwood (pine)	~500	bending ~40–80	along the grain; much weaker across it
Hardwood (oak, beech)	~700	bending ~80–110
Concrete	2400	compression 20–40	weak in tension (~10%); needs rebar

3D-printed parts are much weaker between layers than along them. Orient the part so the main load runs along the layers.
Safety factor: about 1.5–2 for known loads and well-understood materials; 3–4 or more for people, uncertain loads, shock, or brittle and printed parts.

Manufacturing

Choose the process for the quantity and the means:

Process	Good for	Key rules
FDM printing	1–100 parts, prototypes	overhangs over ~45° need support; walls ≥ ~1.2 mm; holes print undersize; about 0.2–0.5 mm clearance between mating parts; bed size limits part size
SLA / resin printing	fine detail, small parts	brittle; hollow parts need drain holes
CNC machining	metal and plastic, 1–1000 parts	internal corners have a radius equal to the tool’s; deep narrow pockets are hard; the tool must reach every face
Injection moulding	1000+ parts	tooling costs thousands to tens of thousands; draft 1–2°; even walls about 1.5–3 mm; avoid undercuts; ribs ≤ 60% of wall thickness
Sheet metal	enclosures, brackets	inside bend radius ≥ thickness; holes at least 2× thickness from a bend
Casting	complex metal shapes in volume	draft, even sections, generous fillets
Welding	steel frames	torch access, heat distortion, weaker heat-affected zone
Woodworking	furniture, frames	load along the grain; joints carry the load; wood moves with humidity

Design for assembly: fewer parts; parts that locate themselves; standard fasteners; one assembly direction; parts that can’t go in the wrong way.

Failure modes to check

Fatigue under repeated loads, especially at sharp corners and holes.
Wear at sliding and rotating contacts. Plastic parts creep under constant load.
Corrosion, especially mixed metals in water.
Vibration, resonance and imbalance at speed. Spinning loads amplify any imbalance.
Thermal expansion between different materials.
Leaks at every seal and joint. Water together with mains electricity is a hazard.
Loosening fasteners, and misuse: overload, the wrong user, children.

“Why do existing products do it this way?”

Before replacing a conventional solution, find the reason it exists. The usual reasons are cost, safety regulations, reliability, manufacturing limits or physics. If the new design removes a part, check that the part’s job is still done somewhere.