A flywheel is made safe by four independent layers, in this order of importance: a rim speed calculated from tested material strength with a safety factor of 4 to 8, a limit on the drive that stops it exceeding that speed, a pit or housing that catches fragments if the rim does fail, and a rule that nobody stands in the plane of the wheel while it turns. A 500 kg steel wheel at 1,500 rpm holds 2.2 MJ, takes minutes to hours to coast to rest, and cannot be stopped by the person standing beside it.
Part of Flywheel Energy Storage
What can go wrong with a spinning wheel, the layers of protection, and the pre-spin checklist, with pointers to the pages that do the arithmetic.
What Can Go Wrong
| Failure | Cause | Defence | Page |
|---|---|---|---|
| Rim burst | Tip speed beyond what the material can bear, or a hidden flaw | Speed from tested strength, safety factor 4 to 8, inspection | Burst Energy Calculation |
| Runaway | Drive or prime mover keeps pushing after the load is lost | A drive that cannot exceed the speed, plus a trip | Speed Limiting |
| Unbalance | Centre of mass off axis, force grows with speed squared | Static balance, staged run-up | Balancing |
| Bearing seizure | Dry or overloaded bearing | Lubrication schedule, temperature checks | Bearings & Friction |
| Stored energy after “off” | Wheel keeps turning after drive is disconnected | A named stop and a lockout | this page |
| Fragments | Any of the above | A pit and a cover | Housing |
The Stop, Named
For this series’ 500 kg, 180 kg·m² wheel at 157.1 rad/s, the time to stop depends on the braking torque: T = I × ω ÷ t.
- In 10 s: T = 180 × 157.1 ÷ 10 = 2,827 N·m, a mean power of 222 kW. This is a violent stop and not a practical one.
- In 60 s: T = 471 N·m, 37 kW.
- In 600 s (10 min): T = 47 N·m, 3.7 kW.
- Without a brake: bearings and air alone take hours; on good ball bearings, more than a day (see Bearings & Friction).
Every installation must say in writing: where the stop is (a brake lever, a dump-load switch, a drive disconnect), who can reach it from outside the plane of the wheel, and how long the machine takes to coast down with the stop applied. If the answer to the last is “hours”, the wheel is a hazard for hours after every use.
Pre-Spin Checklist
- Rim speed at the intended rpm is below the speed from burst strength divided by √4 to √8.
- The wheel is balanced, with all weights bolted.
- No cracks in the rim, spokes or hub (look and tap-test a cast wheel; see Cast Iron & Steel Disc).
- Bearings greased, housings clean, temperature check in place.
- The overspeed device has been tested at its trip speed.
- The cover is in place and the path to the stop is clear.
- Nobody is in the plane of the wheel, and nobody can walk into it.
- The first spin-up is staged, as in Balancing.
Common Mistakes
- Counting one layer as enough. A good speed limit with no pit, or a pit with no speed limit, is one failure from injury.
- Taking “off” to mean “stopped”. The drive being off says nothing about the wheel.
- Treating a lid as a stop. The lid keeps fragments in; it does not stop the wheel.
Safety
Stored energy, not volume, is what injures
The 2.2 MJ in a half-tonne wheel at 1,500 rpm is the energy equivalent of 0.53 kg of TNT (using 4.184 MJ/kg), released as large fast pieces, not as a blast. One eighth of the rim carries 277 kJ at 94 m/s. Never stand in the plane of a spinning wheel, never enter the pit until the wheel is stopped and pinned, and never let a drive that has lost its load keep pushing the wheel.
What’s Next
- Learn next: Burst Energy Calculation — the arithmetic behind the speed limit
- Combine with: Housing — the pit and the cover