Put the flywheel in a pit with its axis vertical, so that the plane of the rim is horizontal and fragments thrown outward hit earth and not people, and cover the pit with a heavy lid that carries the shaft through a small hole. This wiki has no sourced wall thickness for stopping a flywheel’s fragments: the 15 to 25 mm steel ring an earlier page recommends could absorb at best about 2.1 MJ if the whole ring stretched to fracture, about as much as the wheel itself holds, and a fragment striking a small patch would do far less, so rely on depth, earth and distance and not on a thin steel shell.
Part of Flywheel Energy Storage
Siting and building the place a flywheel runs in: orientation, the pit, the lid, the lockout, and what can and cannot be stated about wall thickness.
Orientation Decides Where The Fragments Go
A broken rim throws pieces in the plane of the wheel. Two ways to mount it:
- Vertical axis, wheel horizontal. The plane of the rim is horizontal, so fragments travel sideways into the walls of the pit. A lid above the wheel only has to deal with pieces that ricochet. The wheel’s full weight, 4,905 N for 500 kg, sits on a thrust bearing; see the step-bearing example in Journal Bearings (Low-Tech).
- Horizontal axis, wheel vertical. The plane is vertical, so fragments go up and down: the lid has to stop them, which is the harder job, and anything above the wheel is in the line of fire.
Prefer the vertical axis in a pit. This is the engineering reasoning, not a sourced rule.
What The Pit Must Face
Using the 500 kg steel ring of this series:
- Working speed, 94.2 m/s: one eighth of the ring, 62.5 kg, carries ½ × 62.5 × 94.2² = 277 kJ.
- Burst speed (weakest steel), 209.5 m/s: the same fragment carries ½ × 62.5 × 209.5² = 1.37 MJ.
- Upper bound on range, with no drag: v² ÷ g, 905 m and 4,475 m (from Burst Energy Calculation).
Why A Thin Steel Ring Is Not A Proven Answer
An earlier page puts 15 to 25 mm of steel around the wheel with a 10 cm clearance. No source for that thickness exists in this wiki. An upper bound on what such a ring can absorb: the plastic work is at most the strength times the strain to fracture times the volume. Take a ring of 20 mm wall and 0.30 m height around a 1.4 m diameter wheel: volume = π × 1.4 × 0.02 × 0.30 = 0.0264 m³. With assumed values of 400 MPa and 20% strain to fracture (recalled mild-steel properties, not from the corpus):
- Maximum absorbed = 400e6 × 0.20 × 0.0264 = 2.1 MJ.
That is about the entire energy of the working wheel, 2.2 MJ, and it needs the entire ring to stretch to failure at once. A fragment hits a patch, and the patch yields and tears long before the ring far from it contributes. The bound shows only that 20 mm is not obviously adequate; it does not show it is inadequate, and it cannot show it is safe.
What To Build Instead
- Dig deep. Put the wheel so that its whole rim, at its lowest and highest point, is below the original ground level. Earth is cheap, heavy, and absorbs energy by deforming without throwing pieces back.
- Keep the sides firm. Line the pit against collapse, using the practices in Excavation Design for depth and slopes, with a clear gap round the wheel for run-down and inspection. Drain it so it does not flood.
- Back the walls with earth. Make the pit surround thick on every side; the corpus gives no number for how thick, so make it as thick as the site allows and test the layout at a much lower speed first.
- Cover it. A heavy lid of timber or steel over the pit, with a small hole for the shaft or belt, not an open top. The shaft hole should be small enough that a fragment cannot leave through it.
- Keep the zone clear. No path, workplace or dwelling within the possible throw. The range bounds above are hundreds of metres to a few kilometres, so distance cannot be the only layer; but a clear field around the pit is.
- Interlock and lock out. The lid must be closed for the drive to be powered (a limit switch or a physical key that releases the power), and before anyone enters the pit the wheel must be stopped, pinned against rotation, and the drive isolated.
Test At Low Energy First
Run the new pit with the wheel limited to a fraction of the working speed. The energy goes as speed squared: at 25% of the speed the wheel holds 6.25% of the energy, 139 kJ for the example. Check that everything vibrates normally and that no fragment path exists, then step up.
Common Mistakes
- A pit with an open top. Fragments leave upward.
- Believing a thin steel ring is enough. The bound above shows it is not obviously so.
- Entering the pit to check on a coasting wheel. It is still turning.
- A lid that cannot be removed safely. Make the lid openable only after a deliberate lockout step.
Safety
The pit must be assumed to be breached
The most likely cause of injury is a person in the plane of the wheel, or in the pit, when the rim fails or the wheel is still coasting. Keep the pit lid closed while the wheel turns, never enter the pit until the wheel has come to rest and is pinned, and keep dwellings and paths well clear of the throw range. Do not trust a thin steel shell, and do not run the wheel above the speed calculated from tested strength.
What’s Next
- Learn next: Speed Limiting — keeping the drive under the speed the pit was designed for
- Combine with: Safety & Containment — the layers around the pit