Flywheel

Part of Steam Engine

The heavy rotating disc that stores rotational energy to smooth out the power pulses of the steam engine and maintain steady speed.

Why This Matters

A piston steam engine produces power in pulses — one or two per revolution — not in a smooth continuous flow. Without a flywheel, engine speed surges during the power stroke and drops between strokes. This speed variation is unacceptable for most industrial purposes. A lathe running with fluctuating speed produces rough cuts. A mill with uneven grinding action produces uneven flour. Any precision machine requires smooth, consistent rotation.

The flywheel solves this by storing kinetic energy during the power stroke and releasing it between strokes. A heavy wheel spinning fast stores enormous energy. When the piston provides a power pulse, the flywheel absorbs extra energy and accelerates only slightly. When the piston is not pushing, the flywheel gives back energy and decelerates only slightly. The result is nearly constant speed despite the pulsed power input.

The flywheel is also the primary energy interface to your belt drive system. Belt pulleys for workshop machines are often mounted on or near the flywheel shaft. The flywheel’s steady rotation provides the clean, even drive that machines require.

Flywheel Physics

Moment of inertia (I): The flywheel’s resistance to speed change. For a solid disc: I = (1/2) × m × r². For a rim-type flywheel (most of mass at rim): I ≈ m × r². A rim flywheel stores roughly twice the energy per pound compared to a solid disc of the same diameter — this is why most large flywheels are rim type.

Kinetic energy stored: E = (1/2) × I × ω² where ω = angular velocity in radians/second

Speed variation (coefficient of fluctuation): The allowable percentage variation in speed during one revolution. For most machinery: 1/20 to 1/30 is acceptable. For precision work: 1/100. The flywheel must be sized to maintain speed within this range given the energy pulse of each power stroke.

Sizing rule of thumb: For a simple single-cylinder engine running at 60–120 RPM with light to moderate loading, a flywheel weighing 20–40 times the engine’s theoretical HP in pounds gives reasonable smoothness. A 5 HP engine needs a 100–200 pound flywheel at minimum. Larger is always better.

Rim-Type Flywheel Design

The rim flywheel consists of:

• Rim: Heavy iron ring forming most of the mass — mounted at maximum radius for best inertia per pound
• Spokes: Six to eight cast iron or wrought iron spokes connecting rim to hub
• Hub: Central boss bored to fit the crankshaft, with keyway

Proportions:

• Rim width: roughly 1/8 to 1/6 of rim diameter
• Rim thickness (radial): roughly equal to rim width
• Spoke cross-section: oval or I-shaped, wide at the hub end, narrowing toward rim
• Number of spokes: 6 for wheels up to 3 feet diameter, 8 for larger

Common sizes:

Engine HP	Typical flywheel diameter	Approximate weight
2–5 HP	24–36 inches	100–250 lb
5–15 HP	36–48 inches	250–600 lb
15–50 HP	48–72 inches	600–2,000 lb

Casting a Flywheel

Large flywheels are usually cast in iron. The casting process:

Pattern making: Build a wooden pattern in two halves, split along the plane through the shaft axis. The hub, spokes, and rim are all formed in the pattern. Include slight taper (draft angle) on all vertical faces to allow the pattern to be drawn from the sand without crumbling the mold.

Molding: The two-part pattern is packed in green sand in a two-part flask (cope and drag). Remove the pattern carefully, cut the pouring gate and risers, and close the mold.

Casting: Pour molten gray iron (cast iron) into the mold. Flywheels benefit from slow, even pouring to avoid inclusions and porosity. Fill the mold completely; the risers overflow when the mold is full.

Cooling: Allow to cool in the mold for at least 12–24 hours. Rapid cooling causes residual stresses and possible cracking.

Quality check: After removing from mold, inspect for:

• Cold shuts: where two metal streams met and did not fully fuse (shows as a line on the surface)
• Porosity: visible holes or rough porous texture in the cross-section
• Cracks: especially around the hub where stress concentrates

Machining: Turn the hub bore to exact shaft size on a lathe. Machine or file the rim outer surface smooth and round. Drill and broach the keyway in the hub.

Balancing the Flywheel

An unbalanced flywheel vibrates, wears bearings rapidly, and at high speed can become dangerous. Balance is essential.

Static balance check:

1. Support the flywheel shaft on two parallel knife-edge rails (or V-blocks)
2. Release the flywheel and let it rotate freely
3. The heavy side will rotate to the bottom and come to rest
4. Mark the position of the heavy side
5. Add material opposite (drill holes in the rim and fill with lead, or attach balance weights) or remove material from the heavy side (drill holes in the rim)
6. Repeat until the flywheel stops at random positions — no consistent tendency to rest with one side down

Dynamic balance (for high-speed flywheels): Static balance is sufficient for flywheels running below 200–300 RPM. For higher speeds, dynamic balancing on a special machine is required to remove couples (opposing imbalances at different axial positions). For most steam engine flywheels running at 60–150 RPM, static balance is adequate.

Typical balance weight: A flywheel with 1 pound of excess weight at the rim radius will be noticeably out of balance. Start small — add small weights, recheck.

Mounting and Keyway

Key selection: A flywheel driven by a key must transmit the full engine torque through the key. Calculate the required key size:

Torque (in-lb) = HP × 63,025 / RPM

Key shear area = key width × key engagement length (half depth of key in shaft × length of hub bore)

For a 10 HP engine at 90 RPM: Torque = 10 × 63,025 / 90 = 7,003 in-lb. A 1-inch × 1-inch × 4-inch long key (2 sq in shear area) at allowable shear stress of 6,000 PSI gives 12,000 lb force × 0.5 inch radius = 6,000 in-lb. This key is marginal — use a 1.5-inch-wide key or longer engagement.

Mounting procedure:

1. Fit key to both shaft and hub keyway — should be a sliding fit with no slop
2. Warm the flywheel hub in hot coals (do not get red hot) to expand the bore slightly for easier assembly
3. Drive the flywheel onto the shaft with the key in place
4. Secure with a nut or taper pin through the shaft end

Safety

A rotating flywheel stores enormous energy. A large flywheel at speed contains more energy than a rifle bullet. If the flywheel cracks or a spoke breaks at speed, the fragments are lethal.

Safety rules:

• Never exceed the design speed — check that governor keeps engine within safe range
• Inspect the flywheel for cracks before each operational period — especially around the hub and spoke roots
• Keep personnel away from the plane of rotation during startup and any speed changes
• Cast iron flywheels are brittle — a sharp impact or thermal shock can crack them without warning
• If the flywheel develops a visible crack, immediately shut down the engine and do not restart until the flywheel is replaced

Speed limit calculation: Maximum safe rim speed for cast iron is approximately 5,000–6,000 feet per minute. Calculate: rim speed = π × diameter (ft) × RPM. A 48-inch (4-foot) flywheel at 120 RPM: rim speed = 3.14 × 4 × 120 = 1,508 ft/min. Well within limits.