Make a high-speed flywheel from steel, and use cast iron only below about 42 m/s at the rim, because cast iron is brittle, variable and holds hidden casting flaws. A mild steel disc of 0.4 m radius and 50 mm thickness weighs 197 kg and, run at its 74 m/s limit (1,767 rpm), holds about 270 kJ or 75 Wh, while the same speed on a cast iron wheel would load the rim to 39.4 MPa against a weakest strength of 103 MPa.
Part of Flywheel Energy Storage
Solid metal flywheels: when to use cast iron or steel, how to size and limit them, and how to make, check and mount the disc.
Steel Is The High-Speed Choice, Cast Iron The Low-Speed One
Tensile Strength gives cast iron 15,000 to 30,000 psi (103 to 207 MPa) and mild steel 50,000 to 70,000 psi (345 to 483 MPa), and calls cast iron brittle and unreliable in tension: internal flaws and surface notches start fractures at stresses well below what a clean test piece suggests. Steel gives warning (it necks and deforms); cast iron does not.
Rim stress is ρ × v² with ρ = 7,200 kg/m³ for cast iron and 7,850 for steel:
| Rim speed | Cast iron ring | Steel ring | Margin, cast iron (103 MPa) | Margin, steel (345 MPa) |
|---|---|---|---|---|
| 30 m/s | 6.5 MPa | 7.1 MPa | 16 | 49 |
| 42 m/s | 12.7 MPa | 13.9 MPa | 8.1 | 25 |
| 74 m/s | 39.4 MPa | 43.0 MPa | 2.6 | 8.0 |
| 100 m/s | 72 MPa | 78.5 MPa | 1.4 | 4.4 |
| 150 m/s | 162 MPa | 177 MPa | 0.64, fails | 1.95 |
The history is consistent with the first rows. The corpus Flywheel page for steam engines puts the maximum safe rim speed of cast iron at 5,000 to 6,000 feet a minute, 25 to 30 m/s, where the ring stress is 4.6 to 6.7 MPa. Other corpus pages give 100 m/s for cast iron and 150 m/s for steel as the maximum peripheral speed. Those are close to the burst speeds of the weakest metal in the table, not working speeds; divided by 2 to 3, as the carrier page advises, they give 33 to 50 m/s for cast iron and 50 to 75 m/s for steel, which agrees with the 42 and 74 m/s here. A working speed on this page is one that keeps the margin at 8 or more.
Worked Example: A Steel Disc For A Small Store
Inputs: mild steel, 7,850 kg/m³; radius 0.4 m; thickness 0.05 m; design tip speed 74 m/s (345 MPa ÷ 8, from Speed vs Mass Trade-offs).
- Mass = 7,850 × π × 0.4² × 0.05 = 197 kg.
- ω = 74 ÷ 0.4 = 185 rad/s, which is 1,767 rpm.
- I = ½ × 197 × 0.4² = 15.8 kg·m².
- E = ½ × 15.8 × 185² = 270 kJ = 75 Wh. About 56 Wh can be drawn out between full and half speed.
- Bore stress, with a small shaft hole: 0.825 × 7,850 × 74² = 35.5 MPa, a margin of 345 ÷ 35.5 = 9.7 against the weakest steel.
Seventy-five watt-hours from 197 kg of steel shows why this is a smoothing device. The same-size disc in cast iron, held to 42 m/s, has a mass of 7,200 × π × 0.4² × 0.05 = 181 kg, ω = 42 ÷ 0.4 = 105 rad/s, I = ½ × 181 × 0.4² = 14.5 kg·m² and E = ½ × 14.5 × 105² = 80 kJ = 22 Wh. Energy per kilogram falls with the square of the speed limit, so the weaker metal stores about 30% as much for the same size.
Making A Cast Iron Disc
Follow Sand Casting and the wheel-specific notes in Flywheel:
- Make the pattern with draft on all faces and build in a riser; a casting without one traps air and shrinks hollow.
- Pour slowly and evenly so that inclusions float out.
- Cool in the mould for at least 12 to 24 hours. Fast cooling locks in residual stress and can crack the wheel.
- Inspect the surface for cold shuts (a line where two streams met without fusing), rough porous patches and cracks, especially at the hub where stress concentrates.
- Hang the wheel from a rope and strike it with a wooden mallet. A sound disc rings; a dull note means a crack or a major void. This is a craft rejection test, not an acceptance test: a clear ring does not prove the iron is free of internal porosity, so it never raises the speed limit above the table.
Steel Discs
Cut the disc from a single plate, not from welded segments. Weld metal is a flaw line, and this page has no sourced joint efficiency for hand welds. A bolted stack of plates is not a solid disc: each plate carries its own hoop stress and the bolts do not share it, and every bolt hole is a stress concentration. Keep bolt holes well inside the rim.
Hub, Shaft And Keyway
Warm the hub in coals to expand the bore (do not get it red hot) and drive it onto the shaft with its key, as the corpus flywheel page does. The keyway is a notch in the highest-stress region of the wheel, so keep its corners rounded and its edges away from the bore surface.
Common Mistakes
- Choosing rpm from the shaft or bearing rating. The rim speed, not the bearing, is the limit.
- Using a cast iron result for steel or the reverse. Their strengths differ by a factor of 3 or more.
- Trusting a weld or a bolted stack as a solid disc.
Safety
Cast iron breaks without warning
A cast iron wheel at its limit gives no sign before it breaks, and a 100 kg wheel at its safe speed already carries about 40 kJ (see 2 × I × omega² for the arithmetic). Keep a margin of 8, put the wheel in a pit, do not spin a wheel that has any visible crack, and never stand in the plane of the rim.