A concrete flywheel is a slow smoothing wheel, not an energy store: plain concrete holds only 1.7 to 2.8 MPa in tension, which limits the rim of a bored disc to about 10 m/s at a safety factor of 8, and at that speed a 500 kg, 1 m diameter concrete disc turns at 191 rpm and holds only 12.5 kJ (3.5 Wh). Spinning it at the 400 rpm that an earlier page suggests puts its rim at 20.9 m/s, a safety factor of only 2 on the weakest concrete, and steel reinforcement does not fix that.
Part of Flywheel Energy Storage
Casting a flywheel from concrete where metal is scarce: what the tensile limit allows, why reinforcement does not raise it, and how to build and run one.
The Limit Is Tension, And Concrete Has Almost None
Tensile Strength gives plain concrete 250 to 400 psi, which is 1.7 to 2.8 MPa, and calls that “effectively zero for design”. It is the number that matters here, because a spinning disc is in tension. Rim stress for a disc on a shaft, with the bore, is about 0.825 × ρ × v² (see Rim Mass vs Solid Disc), with ρ = 2,400 kg/m³ for ordinary concrete.
| Tensile strength | Burst speed, thin ring | Burst speed, bored disc | Speed at safety factor 8 |
|---|---|---|---|
| 1.72 MPa | 26.8 m/s | 29.5 m/s | 10.4 m/s |
| 2.76 MPa | 33.9 m/s | 37.3 m/s | 13.2 m/s |
Working, bored disc at 1.72 MPa: v = √(1.72e6 ÷ (0.825 × 2,400)) = 29.5 m/s at the point of cracking. Dividing by √8 = 2.83 for a safety factor of 8 on stress gives 10.4 m/s. The wheel’s burst speed is only about 30 m/s, a fraction of any metal’s.
Lime concrete, which a village without cement will make, reaches only 40 to 60% of its final strength at 28 days according to Concrete Making, and the tensile figures above are generic. Test a cast sample of your own mix before trusting any number here.
Worked Example: Why 400 rpm Is Too Fast
The carrier suggests a 1 m, 500 kg disc at 400 rpm. A 500 kg disc of 2,400 kg/m³ is 0.208 m³ in volume, so about 0.265 m thick at 1 m diameter.
- ω = 400 × 2π ÷ 60 = 41.9 rad/s; rim speed 41.9 × 0.5 = 20.9 m/s.
- I = ½ × 500 × 0.5² = 62.5 kg·m²; E = ½ × 62.5 × 41.9² = 54.9 kJ = 15.2 Wh.
- Bore stress = 0.825 × 2,400 × 20.9² = 0.87 MPa, against 1.72 MPa for the weakest concrete: a margin of 2.0.
A margin of 2 on a brittle material with no warning is not a safe wheel. Run the same disc at 10 m/s instead:
- ω = 10 ÷ 0.5 = 20 rad/s, which is 191 rpm.
- E = ½ × 62.5 × 20² = 12.5 kJ = 3.5 Wh.
- Bore stress = 0.825 × 2,400 × 10² = 0.20 MPa, a margin of 8.7.
Half a tonne of concrete for 3.5 Wh is why this wheel smooths a treadle or a millstone drive but cannot bank energy.
Reinforcement Does Not Raise The Speed Limit
Concrete cracks at a small strain. Steel bars cast into it carry hoop tension only as the concrete stretches, so while the concrete is intact it is the concrete that carries almost all the tension, and once it cracks the steel begins to carry the load across a cracked, weakened wheel. Treat the bar as a way to hold the pieces together after a failure, not as a way to run faster. A steel hoop shrunk around the rim, a different construction, is a metal rim holding a concrete core and must be designed as a metal ring.
Building One
- Cast it in a single pour in a level form, with the hub plate and shaft sleeve placed first. A construction joint across the disc is a plane of weakness.
- Vibrate or rod the mix to remove voids and keep it wet for 14 days or more; see Curing for the damp-cure routine.
- Place mild-steel mesh through the whole disc and a closed hoop of bar near the rim, lapped over a long length. Their job is to hold the pieces together after a crack, not to raise the speed limit.
- Build the hub from a steel plate bolted through the disc, not an open bore: the bore is the most stressed point.
- Balance it as in Balancing; concrete is far less uniform than metal and a heavy side is likely.
Common Mistakes
- Taking the carrier’s “below 50 m/s” tip-speed limit. Even a bored disc cracks at 29 to 37 m/s on these strengths.
- Counting on the rebar. It does not carry hoop load until the concrete has already cracked.
- Skipping the cure. Under-cured concrete is weaker than the table.
Safety
A concrete wheel breaks into heavy pieces without warning
When it bursts, a 500 kg disc at 20.9 m/s breaks into large pieces; one-eighth of the mass, 62.5 kg, carries ½ × 62.5 × 20.9² = 13.7 kJ. Keep the rim speed at or below 10 m/s, run the wheel in a pit or behind a thick earth bank, and stay out of its plane. See Safety & Containment.
What’s Next
- Learn next: Rim-Weighted Design — a metal rim on a light frame stores far more per tonne
- Combine with: Speed Limiting — making sure a slow wheel cannot be driven fast