To store energy in a flywheel, cast or bolt together a heavy disk and spin it using a motor or mechanical drive; stored energy is E = ½Iω², where I = ½mr² for a disk, so doubling the radius stores 16× more energy and doubling the speed stores 4×. A worked example from the body: a cast iron disk 60 cm diameter and 10 cm thick has a mass of 204 kg, moment of inertia of 9.18 kg·m², and stores 4,527 J (≈ 1.26 Wh) at 300 RPM — modest in capacity but deliverable as very high instantaneous power in a fraction of a second. Maximum safe peripheral velocity for cast iron is about 100 m/s and for steel about 150 m/s; exceeding these limits causes catastrophic bursting. Enclose the flywheel in a protective case — even at low speed, a flywheel can grab clothing or limbs.
Part of Energy Storage & Batteries
A spinning flywheel stores kinetic energy that can be released as mechanical or electrical power — ideal for smoothing intermittent power sources and delivering short bursts of high power.
Flywheels Store Kinetic Energy and Deliver Power Bursts
The flywheel is one of the oldest energy storage devices in human history — the potter’s wheel and the millstone both use flywheel principles. A heavy wheel spinning at speed stores significant kinetic energy and can release it as useful mechanical work. Connected to a generator, a flywheel converts mechanical storage to electrical storage and back with high efficiency and zero degradation.
For a rebuilding civilization, flywheels solve a specific problem that chemical batteries handle poorly: rapid power bursts. Starting a large electric motor, operating a forge blower for a brief intense heat, or running a welding arc for seconds at a time — all these demand power faster than a battery can safely deliver. A flywheel charged by a modest continuous generator can discharge through these loads without battery damage.
Flywheels also smooth the output of intermittent mechanical power sources. A water wheel whose rotation varies with current speed, or a windmill that surges and lulls, produces uneven torque. Adding a heavy flywheel to the shaft averages out this variation, delivering steady rotation to a connected machine or generator.
Physics of Flywheel Storage
Stored energy: E = ½Iω², where I is the moment of inertia (kg·m²) and ω is angular velocity (radians per second).
Moment of inertia for a disk: I = ½mr², where m is mass and r is radius.
Practical calculation: A cast iron disk flywheel, 60 cm diameter, 10 cm thick:
- Volume = π × 0.3² × 0.1 = 0.0283 m³
- Mass = 0.0283 × 7,200 (iron density) = 204 kg
- I = ½ × 204 × 0.3² = 9.18 kg·m²
- At 300 RPM (31.4 rad/s): E = ½ × 9.18 × 31.4² = 4,527 J ≈ 1.26 Wh
This is modest — about equal to a small AA battery — but the power can be delivered in a fraction of a second if needed, enabling very high instantaneous power output.
Scaling up: Energy scales with mass and the square of both radius and speed. Doubling the radius (to 120 cm) with the same speed stores 16× more energy. Doubling the speed stores 4× more energy. High-speed flywheels are far more energy-dense but require better bearings and stronger materials to avoid catastrophic failure.
Speed limits: The stress in a spinning disk is proportional to ω²r². The maximum safe speed is limited by material tensile strength. For cast iron: maximum peripheral velocity ~100 m/s. For steel: ~150 m/s. Exceeding this causes the flywheel to burst with catastrophic energy release.
Construction
Casting a flywheel:
- Make a sand mold from a wooden pattern — a flat disk with a central hub
- Cast from gray cast iron (melts at ~1200°C, achievable in a cupola furnace)
- Machine the bore of the hub to fit the shaft, and turn the outer rim to balance
- Static balance test: mount on a horizontal shaft and let it rest — the heavy side goes to the bottom; remove material from heavy side or add to light side until it rests at any angle
Alternative fabrication:
- Stack steel plates and bolt them together — easier than casting
- Concrete flywheel with embedded steel reinforcement for low-speed applications (potter’s wheel, grinding stone)
- Lead-filled steel drum for very dense, compact storage
Shaft and bearings:
- Heavy flywheels require robust bearings — journal bearings (shaft turning in a lubricated sleeve bore) or ball bearings
- Self-aligning journal bearings machined from bronze or cast iron, lubricated with tallow or oil, can support hundreds of kilograms of shaft weight
- Bearing alignment is critical — misalignment causes rapid wear and vibration
Housing: Enclose the flywheel in a protective case. A flywheel failure at high speed throws debris with lethal energy. Even at low speed, a flywheel can grab clothing or limbs.
Integration with Generators
Motor-generator flywheel set: An electric motor drives the flywheel during periods of surplus power. When power is needed, the now-spinning flywheel drives a generator. Efficiency 80–90% per cycle (motor efficiency × generator efficiency, less bearing losses).
Direct mechanical coupling: A flywheel on a water mill shaft smooths torque variations. A belt drive connects the flywheel to the mill machinery. The flywheel acts as a low-pass filter — absorbing brief torque surges and filling torque gaps.
Clutch and engagement: For controlled power delivery from flywheel to load, a friction clutch allows engagement at any speed. Engage gradually to avoid shock loading. A dog clutch (positive engagement) is simpler but requires speed matching before engagement.
Practical Applications
Motor starting: Charge flywheel with a small motor over 30–60 seconds, then engage clutch to spin up a large motor quickly. Avoids the current surge that would otherwise damage the power supply.
Forge blower: A hand-cranked flywheel stores energy between crank strokes, delivering steady airflow to the forge rather than pulsed. Large millstone-style flywheels were traditional on lever-action bellows for this purpose.
Short-run machinery: Lathes, grinders, and drill presses used intermittently. Spin up flywheel, engage work, disengage when done. The flywheel provides smooth, sustained power through the cut.
Emergency power pulse: Charge flywheel slowly from any available source; discharge through generator in seconds to provide brief electrical power (starting a radio, charging a capacitor bank, powering lights).