Rolling bearings are the right choice for a high-speed flywheel because they cut friction 10 to 50 times against plain bearings, but they must be sized for more than the wheel’s weight. A 500 kg wheel on two bearings loads each with 2.45 kN at rest and up to 8.6 kN if its centre of mass is 1 mm off the axis at 1,500 rpm, so use salvaged bearings only after cleaning, inspecting and repacking them, fit one of the pair as the thrust location, and treat a bearing running above 70 °C as failing.
Part of Flywheel Energy Storage
Choosing, loading, mounting and monitoring ball and roller bearings on a flywheel shaft, mainly from salvaged stock.
Why Rolling Bearings Suit This Job
Roller Bearings gives rolling friction as roughly 10 to 50 times lower than sliding friction for equal loads, which is why rolling bearings run cool and need little lubricant. They also start from rest without wear. Both are what a flywheel needs: it spends its life at high speed and is stopped and started rarely. That page also says to reserve scarce salvaged roller bearings for fast, precise shafts such as motors and generators, and a flywheel shaft belongs on that list.
Ball bearings contact the races at points, so they suit high speed and moderate load. Cylindrical rollers contact along a line, so they carry more radial load at moderate speed. Tapered rollers take radial and axial load together.
Worked Example: Loads On Each Bearing
Inputs: 500 kg wheel centred between two bearings; weight 500 × 9.81 = 4,905 N; unbalance force m × e × ω² = 12,337 N for e = 1 mm at 1,500 rpm (see Balancing).
- Static, per bearing: 4,905 ÷ 2 = 2,452 N (2.45 kN).
- Worst case with the unbalance pointing down: (4,905 + 12,337) ÷ 2 = 8,621 N (8.6 kN) per bearing, 3.5 times the static load, and swinging between that and (12,337 − 4,905) ÷ 2 = 3,716 N upward once every revolution.
- Good balance (the G6.3 case, 494 N): (4,905 + 494) ÷ 2 = 2,700 N.
This page has no catalogue rating to compare these with. A salvaged bearing’s own part number identifies its dynamic rating; look it up if a catalogue survives, and if none does, choose a bearing physically larger than the load seems to need and reserve the wheel’s slowest speeds for it. Never size a flywheel bearing on the weight alone.
Method: Fitting Salvaged Bearings
- Clean in solvent. Check for roughness, catching or looseness by turning by hand. Any roughness means progressive failure; put a rough bearing on a non-critical slow job, not on the wheel.
- Repack with grease, filling about 30 to 50% of the free space; overfilling churns and heats.
- Fit the inner ring to the shaft with a firm press fit and the outer ring in the housing with a light fit. Warm the inner ring gently in oil to expand it. Never press on the outer ring to seat the inner, and never hammer a bearing.
- Align the two housings on one axis. If the shaft span is long or the frame can flex, make one of the pair self-aligning.
- Fix the shaft axially with one bearing only, as Thrust Bearings advises, so thermal growth cannot jam it. The other bearing floats.
- Seal and shield against dirt and damp. A pit is humid: dirt and water destroy a bearing before overload does.
Monitor By Coast-Down
The coast-down test in Bearings & Friction is the cheapest bearing monitor. A fall in the time to lose 10% of the speed means the drag is rising; log it every month. A bearing running above 70 °C is failing, so add a thermometer or a hand-check (outside the plane of rotation, on the housing, with the wheel stopped) for heat, and listen for rumble at low speed.
Common Mistakes
- Sizing on the dead weight. The unbalance force can triple the load.
- Fitting both bearings axially fixed. Heating then loads the bearings against each other.
- Reusing a bearing that feels slightly rough. It will fail, usually at speed.
- Using a bearing whose speed rating you don’t know on a tip-speed-limited wheel. Bearing bore is small, so the bearing’s own rolling-element speed is low, but check the stamped limit when one exists.
Safety
A seized bearing turns the shaft into a brake
A bearing that seizes at 1,500 rpm stops the shaft but not the 2.2 MJ in the wheel, which keeps turning and can twist the shaft off or break the housing free. Put an oil, grease or temperature check on a fixed schedule, keep people out of the plane, and if a bearing runs hot or loud, stop the drive and let the wheel coast down with nobody near it; see Speed Limiting for the stop.
What’s Next
- Learn next: Reducing Windage Losses — with good bearings, the air is the main loss
- Combine with: Journal Bearings (Low-Tech) — the fallback when no rolling bearing survives