Choose a flywheel material by the rim speed it can survive, because the stress in the rim equals the density times the tip speed squared. At a safety factor of 8 on the lowest tensile strengths this wiki gives, cast iron is limited to about 42 m/s at the rim, mild steel to about 74 m/s and plain or reinforced concrete to about 10 m/s, and the wheel’s rpm is then whatever puts the rim at that speed for its radius.
Part of Flywheel Energy Storage
The materials a village can make a flywheel from, the speed each can safely reach, and what building one involves, from the pattern to the balance check.
The Material Sets The Speed Before You Draw A Wheel
Every flywheel rim sees a tensile stress of ρ × v² (density times rim speed squared, derived in Burst Energy Calculation). The wheel survives only while that stress stays well below the material’s tensile strength. So the first design decision is the material and its allowable tip speed; radius and rpm follow from it.
| Material | Tensile strength (corpus) | Density | Tip speed at safety factor 8 |
|---|---|---|---|
| Cast iron | 103 to 207 MPa | 7,200 kg/m³ | 42 m/s (using 103 MPa) |
| Mild steel | 345 to 483 MPa | 7,850 kg/m³ | 74 m/s (using 345 MPa) |
| Plain concrete | 1.7 to 2.8 MPa | 2,400 kg/m³ | 10 m/s on a bored disc (using 1.7 MPa) |
Working for cast iron: allowable stress 103 ÷ 8 = 12.9 MPa; v = √(12.9e6 ÷ 7,200) = 42 m/s. The strengths come from Tensile Strength (converted from psi), the safety factor from the 4 to 8 that Safety Factors gives for machine parts under repeated loading, and the concrete row uses the bored-disc stress factor explained in Rim Mass vs Solid Disc.
Timber and stone are not in the table. Stone is brittle and the corpus Flywheel Design limits a stone flywheel to under 100 rpm. For timber the corpus gives a tensile strength along the grain but no strength for the joints between rim segments, which is where a wooden rim would let go, so no safe tip speed can be stated. Treat a timber wheel as a slow smoothing wheel, not a store.
Four Ways To Get A Wheel
- Salvaged engine flywheel. Already balanced and already proven to its engine’s top speed, which is also its limit. A flywheel of 0.15 m radius at 6,000 rpm has a rim speed of 94 m/s, so a salvaged one is a high-stress part even though it is small. Check for cracks and do not exceed the speed it saw in service. See Cast Iron & Steel Disc.
- Cast or cut iron or steel disc. Heavy, robust, simple. The metal has the best strength-to-density of the three.
- Concrete disc. Where metal is scarce. Very low speed, very little energy. See Concrete Flywheel.
- Rim-and-spoke wheel. A heavy ring on light spokes puts the mass where it counts. See Rim-Weighted Design.
Steps That Apply To Every Type
- Fix the energy you need and the material’s tip speed; compute mass and radius with Speed vs Mass Trade-offs.
- Choose the shaft and bearings for the wheel’s weight, which for 500 kg is 4,905 N, before you cast or cut.
- Make the wheel, then balance it (Balancing).
- Spin it for the first time slowly, from a position outside its plane, with the containment in place.
- Raise the speed in steps and stop at each to listen and feel for vibration. Do not go past the speed you calculated.
Common Mistakes
- Choosing the material by what is lying around. A strong, ductile steel scrap is not the same as brake rotors, which are cast iron and brittle.
- Bolting several discs together. Each bolt hole is a stress concentration and the joint is a flaw that the base-metal strengths do not cover.
- Skipping the bore. Remember that the shaft hole itself doubles the peak stress at the bore; see the rim-mass article.
Safety
A cracked or mis-cast wheel is an unexploded bomb
A flaw in a cast wheel cannot be seen from outside. Other corpus pages give 100 m/s for cast iron and 150 m/s for steel as maximum speeds; a reader who takes those as working speeds is at the edge of failure, because at 100 m/s a cast iron ring sees 7,200 × 100² = 72 MPa, only 1.4 times the weakest strength in the table. Use the speeds in the table above, run the first spin-up behind the containment described in Housing, and stay out of the plane of the wheel.
What’s Next
- Learn next: Cast Iron & Steel Disc — the common case in detail
- Combine with: Balancing — the step every type needs before it spins