A bronze or babbitt journal bearing can be made without any rolling parts, and it works for a flywheel, but at the corpus friction coefficient of 0.02 to 0.06 it bleeds a 500 kg wheel at 385 to 1,156 W and empties it in one to three hours, so use it for short-term smoothing and not for storage. Fit the bearing with about 0.001 inch of clearance per inch of shaft diameter, feed it from a ring or wick oiler, and run the wheel up and down slowly because the oil film only forms once the shaft is turning.
Part of Flywheel Energy Storage
A flywheel shaft carried in plain sleeves: material, clearance, lubrication, the friction penalty, and a step bearing for a vertical shaft.
Making The Bearing
Follow Plain Bearings and Bearing Materials:
- Material. Bronze (the corpus gives 88% copper, 10% tin, 2% zinc as a gunmetal example) running on a steel or iron journal. Iron on iron gall and seizes. Babbitt lining in a cast iron split housing is the alternative.
- Housing. Two matching halves; bore slightly undersized for finishing.
- Clearance. About 0.001 inch per inch of shaft diameter. A 50 mm (about 2 inch) shaft therefore needs about 0.002 inch, or 0.05 mm. Too tight starves the oil film and seizes; too loose lets the shaft wobble and wear.
- Oil grooves. Cut them outside the loaded area. Where the wheel rests on the bearing, the groove must not break the oil film. A helical groove at 30 to 50% of the width is the best the corpus gives.
- Oiler. A ring oiler (a loose ring dipping into a reservoir) or a wick oiler; a cup that is topped up by hand is acceptable only for intermittent use.
The Friction Cost
For a 500 kg wheel on a 50 mm shaft the friction torque is T = μ × W × r = μ × 4,905 N × 0.025 m, so 2.45 N·m at μ = 0.02 and 7.36 N·m at μ = 0.06. Power at 1,500 rpm (157.1 rad/s) is 385 W and 1,156 W. The wheel holds 2,221 kJ, so the stored energy lasts 2,221,000 ÷ 385 ≈ 5,770 s (1.6 h) at the first rate if the power stayed constant, and because the torque is constant while the speed falls, the wheel actually stops in 3.2 h at the lower coefficient and 1.1 h at the higher one. The full table and method are in Bearings & Friction.
Interpretation: the wheel is a short-term buffer. For a belt-driven mill wheel or an engine flywheel, whose job is to smooth strokes within a second, this loss is irrelevant. For a store meant to last hours it is disqualifying.
Starting And Stopping
A plain bearing works on a hydrodynamic film that the turning shaft drags into the gap (Plain Bearings). At rest the oil is squeezed out; at slow speed the film is thin and the metal touches. So the bearing wears mostly at start-up and run-down, and a flywheel that is brought up and down often will wear a bronze bearing faster than one that spins for days. Keep the oiler full before every start and run up slowly through the first revolutions.
A Step Bearing For A Vertical Shaft
A wheel on a vertical shaft in a pit puts its full weight on one thrust bearing. The corpus (Thrust Bearings) gives babbitt a working limit of about 400 to 500 psi (2.8 to 3.4 MPa). For 4,905 N a collar with 60 mm outer and 25 mm inner diameter has an area of π ÷ 4 × (0.06² − 0.025²) = 0.00234 m², so the contact pressure is 4,905 ÷ 0.00234 = 2.1 MPa, about 304 psi, inside the limit. Cut a radial oil groove in the face and feed it by wick. Add the belt pull and any unbalance force before you accept the size.
Common Mistakes
- Letting the oiler run dry. Within an hour a dry metal bearing can seize solid.
- Using lignum vitae or wood. The corpus keeps these to slow loads; a flywheel at speed is not slow.
- Cutting the groove through the loaded zone. The film collapses and the bearing runs hot.
- Treating the friction figure as exact. A running oil film can be better than the table; time your own coast-down.
Safety
A seizing plain bearing does not stop the wheel
A dry bearing heats and welds to the shaft in minutes. The wheel’s 2.2 MJ then twists the shaft or wrenches the housing from its mount. Fit a lubrication check as a standing routine, keep people out of the plane of the wheel, and let a hot-running wheel coast down unattended; see Safety & Containment.
What’s Next
- Learn next: Reducing Windage Losses — the other half of the loss budget
- Combine with: Lubrication — oils and feeds for the bearing