Forged Crankshafts

A crankshaft is the component that turns the up-and-down motion of pistons into the rotation that drives a vehicle, generator, pump or compressor. Each connecting rod attaches to a crankpin offset from the shaft's axis; the shaft itself turns in main journals supported by bearings; and webs join the pins to the journals. Counterweights, usually formed as part of the webs, balance the rotating and reciprocating masses.

Every firing stroke in an engine, or every compression stroke in a compressor, pushes hard on a crankpin. That force bends the shaft between its bearings and twists it along its length. The load then reverses and repeats — thousands of times a minute, for the entire life of the machine. A crankshaft therefore has to be strong enough for peak loads, stiff enough to limit vibration and, above all, able to survive a very large number of fatigue cycles.

The geometry problem

Few components combine so many difficulties in one part: offset pins, deep webs, large counterweights, long slender sections and several sharp changes of section — all of which must be forged soundly and then machined to fine tolerances on precisely positioned axes. The shape is also hard to fill in a die, because metal has to flow sideways into webs and counterweights while also forming long journals.

Understanding crankshaft failure explains nearly every decision about how crankshafts are designed and made. Crankshafts rarely break in the middle of a journal. They break at the fillet radius where a journal or crankpin joins a web, because the sudden change of section concentrates stress there. A fatigue crack starts at that fillet and grows across the web until the remaining metal can no longer carry the load.

Everything that improves crankshaft life does so by strengthening that fillet, or by reducing the stress that reaches it:

• Grain flow that bends around the fillet instead of being cut through it
• A generous, smooth fillet radius, free of tool marks
• Compressive residual stress at the fillet surface, introduced by fillet rolling or induction hardening
• Clean, sound material with no inclusions or porosity near the surface
• Controlled vibration, so torsional resonance does not multiply the stress

Our crankshaft failure analysis explains how to read a failed crankshaft and trace the cause.

Crankshafts are made three ways, and each has a legitimate place.

Cast ductile iron crankshafts are cheaper at volume and perform reliably in moderately stressed passenger car petrol engines. Billet crankshafts machined from solid bar allow total design freedom for racing and prototypes, although machining cuts through the grain. Forged steel crankshafts are the standard wherever cylinder pressures are high, duty is heavy or service life is long — diesel engines, commercial vehicles, tractors, compressors and industrial engines. The comparison is covered in more depth in forged versus cast crankshafts.

Why hardenability matters here

Crankshaft webs and journals can be substantial sections, and the core must develop real strength, not just the surface. That is why chromium-molybdenum grades such as 42CrMo4 dominate: they harden uniformly through sections where plain carbon steel would remain soft at the core. See EN8 versus EN19 for why section size decides the grade.

Micro-alloyed steels

Micro-alloyed grades containing small additions of vanadium develop their strength during controlled cooling straight from forging, removing the separate quench and temper cycle. For high-volume automotive crankshafts this saves energy and lead time and avoids quench distortion, although impact toughness is generally lower than that of quenched and tempered alloy steel.

Forging

Crankshafts are closed die forged, usually on presses where the heavy sections benefit from deformation reaching the core. The sequence normally includes preforming to distribute metal along the length and into the webs, followed by blocking and finishing impressions. Some multi-throw designs are forged with their throws in a single plane and then twisted while hot to set the throws at their final angles. Flash is trimmed hot, and the forging is checked for straightness.

Heat treatment

Alloy steel crankshafts are quenched and tempered to develop core strength; micro-alloyed crankshafts are cooled at a controlled rate. Because long, slender forgings can distort during heat treatment, they are straightened afterwards and stress relieved where required.

Machining

Machining is extensive. Centres are established, then main journals, crankpins, flanges and ends are turned or milled. Oil holes are drilled through the webs to carry lubricant from main journals to crankpins, and their edges are chamfered and polished — a sharp oil hole edge is itself a crack initiation site. Crankpin positions must be held accurately relative to the main axis, because throw and angular errors upset engine balance and timing.

Surface strengthening and finishing

Journals and pins are induction hardened, or the shaft is nitrided, then ground to size and surface finish. Where specified, fillets are rolled. The finished crankshaft is dynamically balanced, crack tested and inspected dimensionally before dispatch.

Three processes are used, often in combination, to raise crankshaft fatigue strength beyond what the forged and heat treated steel achieves on its own.

All three work in part by placing the surface into compression. A fatigue crack needs tensile stress to open and grow; residual compressive stress at the fillet surface has to be overcome before the crack can even start. That is why a correctly rolled or hardened fillet can outlast a plain one by a wide margin.

Balancing

Any residual unbalance in a crankshaft generates rotating forces that load the main bearings every revolution and cause vibration. Crankshafts are dynamically balanced after machining, with correction by removing material — typically by drilling counterweights — to the balance quality grade specified by the engine or machine designer, usually under ISO 21940-11 (formerly ISO 1940-1).

Key tolerances

• Journal and pin diameters and roundness, typically ground
• Journal and pin surface finish
• Runout of main journals relative to the shaft axis
• Crankpin throw — the offset radius — and angular position of each throw
• Fillet radius and its blend into the journal
• Flange face runout and pilot diameter

Inspection

When a forged crankshaft fails early, the cause is usually one of a small number of mechanisms.

A crankshaft with a fatigue crack must be replaced. Grinding a cracked shaft undersize does not remove the fatigue damage and invites a sudden failure later.

A crankshaft drawing that omits a single critical requirement leaves that decision to whoever manufactures it. These are the items a complete crankshaft specification should state, and why each one matters.

If some of these are unknown — common when replacing a shaft for an older engine — we establish them from the original part, as described below.

Replacement crankshafts for older diesel engines, compressors and gensets often have no drawing available, and the original manufacturer may no longer exist. A worn or failed crankshaft can still be reproduced accurately, provided the process accounts for wear and captures the properties as well as the geometry.

1. 01Clean and inspect the sample. Magnetic particle inspection shows any existing cracks, which also indicates how the original failed.
2. 02Identify the material. Spectro analysis establishes the actual grade rather than a guess.
3. 03Map hardness. Hardness on journals, pins and core reveals the original heat treatment and surface hardening, and a sectioned sample can show case depth.
4. 04Measure geometry. Journal and pin diameters, throw, angles, fillet radii, oil hole positions and flange features are measured, compensating for wear by referencing bearing and mating dimensions.
5. 05Check for previous regrinding. Undersize journals mean the original standard size must be established from bearing data or unworn features.
6. 06Issue a drawing for approval. The reconstructed geometry, material, hardness and inspection requirements are documented before any tooling or material is committed.
7. 07Manufacture and verify. The crankshaft is forged, heat treated, machined, hardened, ground and balanced, then inspected against the approved drawing.

Crankshafts are commonly reground during engine overhaul, with journals and pins ground to a standard undersize matched to available undersize bearings — often in steps of 0.25 mm, such as 0.25, 0.50, 0.75 and 1.00 mm. Regrinding restores roundness and surface finish, but it removes material from the most carefully engineered surfaces on the shaft, so it must be done with care.

• Stay within the hardened case. An induction hardened journal can only be ground as far as its effective case depth allows. Grinding through the case leaves a soft bearing surface that wears rapidly.
• Nitrided shafts need special attention. A nitrided case is shallow, so even modest regrinding can remove much of it; re-nitriding may be required.
• Preserve the fillet radius. The grinding wheel must be dressed to reproduce the correct fillet. A reduced radius or an undercut concentrates stress at exactly the location where crankshafts crack.
• Crack test after grinding. Grinding can generate heat checking or reveal existing cracks. Magnetic particle inspection should follow regrinding.
• Re-check balance and oil holes. Oil hole edges should be re-chamfered and polished, and balance verified where material removal is significant.

A crankshaft that has reached its final undersize, lost its hardened layer or shows any fatigue crack should be replaced rather than reworked.

We can also produce crankshafts from a worn or obsolete sample through reverse engineering, identifying the steel by spectro analysis and issuing a drawing for approval. Related components include connecting rods, camshafts and flywheel blanks, and we supply compressor and genset manufacturers.