The crankshaft converts the pistons’ straight-line thrust into rotation and carries every power pulse, so it must be strong where its journals meet its cheeks and well balanced, because the force from an unbalanced mass grows with the square of the speed. A 100 g imbalance at a 50 mm radius pulls on the bearings with about 494 N at 3,000 rpm and about 1,974 N at 6,000 rpm.
Part of Internal Combustion
What the crankshaft does, why forging and balancing matter, how its geometry sets the stroke and piston speed, and how a salvaged one is checked.
The Crank Throw Sets The Stroke, And The Fillets Set The Life
Each piston is driven through a rod from a crank pin, offset from the shaft’s centre line by the crank throw radius. The piston moves twice that distance in one stroke, so the stroke equals twice the throw. The corpus engine’s 86 mm stroke implies a 43 mm throw. The shaft turns in main bearings in the block and carries the flywheel on one end; the power pulses arrive one at a time, so a single-cylinder engine relies on the flywheel to carry it between pulses, as Flywheel and Flywheel Design describe.
Forging aligns the metal’s grain along the shape of the part, which is why crankshafts are forged where they can be: a cast crank can have porosity at the very places that carry the highest stress. The sharp internal corners where a journal meets a cheek (the fillets) concentrate stress, and cyclic loading starts fatigue cracks there; Failure Modes explains the fatigue mark. Forging steel in a village is covered in Forging, but a crank forged and machined from scratch is among the hardest jobs in this tree. The honest recommendation is to salvage a crankshaft and check it, and to make one only when no salvage is possible and a lathe, a means of heat treatment and balancing equipment exist.
What You Need
- A salvaged crankshaft with intact journals, or the materials and machine tools to make one.
- Bearings or bearing shells matched to the journals; see Bearing Materials and Plain Bearings.
- V-blocks or centres and a dial indicator to check straightness.
- A balancing stand with knife edges; see Static Balancing and Dynamic Balancing.
Method: Check A Salvaged Crankshaft
Step 1: Clean it and look for cracks, especially at the fillets and around oil holes. A crack is a reason to reject the shaft.
Step 2: Check straightness. Support the shaft on the end journals in V-blocks and turn it with an indicator on the middle journals. Any measurable runout is a sign of bending.
Step 3: Measure the journals. Measure each in several places and in two directions for taper and out-of-round, as for a bore. Compare the sizes against each other and, if possible, against the maker’s data.
Step 4: Check the bearings. Scoring, a blue tint from overheating or flaking of the lining means the shaft ran dry or hot.
Step 5: Balance. For a single-plane shaft, place it on knife edges and see if it rolls to a rest in any position; add or remove weight until it does. For a multi-throw crank, balance in more than one plane; the method is in Dynamic Balancing.
Worked Example: Balance Force And Piston Speed
Imbalance force. A mass m at radius r rotating at angular speed ω pulls with F = m × r × ω². Take m = 0.1 kg (100 g) and r = 0.05 m (50 mm), assumed values.
- At 3,000 rpm: ω = 3,000 × 2π ÷ 60 = 314.2 rad/s, and ω² = 98,696.
- F = 0.1 × 0.05 × 98,696 = 493.5 N, about 494 N.
- At 6,000 rpm: ω doubles and ω² quadruples, so F = 4 × 493.5 = 1,974 N.
So doubling the speed quadruples the force. A shaft that is acceptable at an idling speed can shake an engine apart near its limit.
Piston speed. The piston travels twice the stroke per revolution, so the mean piston speed is 2 × stroke × rpm ÷ 60. For an 86 mm stroke at 3,000 rpm: 2 × 0.086 × 3,000 ÷ 60 = 8.6 m/s. That speed sets how fast the rings slide on the bore and how much heat friction adds, one reason the corpus engines are run at modest rpm.
Common Mistakes
- Straightening a bent crank cold. It may crack; replace it.
- Polishing away the fillets. Sharpening the corner shortens fatigue life.
- Mixing bearing sizes or materials. A soft shell on a hard journal and the reverse have different clearances.
- Balancing only the shaft. The flywheel and pulley must be balanced with it.
Safety
A broken crankshaft or loose flywheel is a flying-metal hazard
Keep the engine to its rated speed, bolt the flywheel with the maker’s method and never stand in the plane of the flywheel when starting. Guard belts and couplings on the output end. See Balancing for the arithmetic of what an off-balance wheel does to its bearings.
What’s Next
- Learn next: Valve Train — the camshaft driven from the crankshaft
- Combine with: Piston Assembly — what pushes on the throws