Marine Crankshafts — Builds, Bedplate, Chocks & Alignment

1. Why Big Shafts Come in Pieces

Camshaft timing, firing order and every driven auxiliary hang off correct crankshaft rotation — and its failure stops the ship. Slow-speed shafts run plain carbon steel (0.2–0.4% C), faster engines alloy steel, but no forge on earth makes a 300-tonne crosshead shaft whole. Construction method is therefore a joining method, chosen per size:

2. Four Builds Compared

Design loads are firing plus three cyclic stresses: misalignment bending, torsional vibration and axial vibration — every later measurement chapter exists because of this trio.

3. Deflection Watches the Bearings

Bearing wear or shaft bend opens and closes the webs as each throw turns. A dial gauge between webs at successive crank positions reads that breathing — even-keel ship, trim recorded, positive for opening, negative for closing — against maker maximums. The companion lesson covers the full five-position procedure; the companion MEP lesson covers the curve.

4. Fourteen Roots of Misalignment

When deflection condemns the alignment, hunt in this order. The gauge only tells you to look — each cause below is a different repair:

• Wiped main bearing.
• Slack foundation or main-bearing bolts.
• Cracked saddle or pockets.
• Deformed bedplate or transverse girder.
• Slack or broken tie bolts and chocks.
• Corrosion-weakened structure.
• Hull deformation or grounding.
• Crankcase explosion or fire damage.
• Worn stern-tube or intermediate bearings dragging the line.
• Excessive piston-assembly bending loads.

5. Bedplate, Holding-Down & Epoxy Chocks — the Foundation Holds the Line

Fabricated bedplate. Longitudinal girders carry the main-bearing saddles, transverse girders tie them into a box — welded from 0.23% carbon steel, the low carbon that welds without cracking. The lineage runs plate → box → fabricated: cast-iron boxes first, then longitudinal + transverse weldments, then the fabricated MAN B&W pattern with its bearing pockets and A-frame feet.

1. Tension the holding-down bolts. Torque to maker figure with jacking screws taking the weight first — the wedge-and-hammer test then proves seating: a tight wedge rings, a loose one thuds.
2. Pour the epoxy. Dam the gap, mix resin and hardener to plan, pour from one side only so air walks out ahead, cure 48 hours before loading — chock plans live in the manual, not in memory.
3. Re-check on schedule. Holding-down tightness re-proved at intervals (Workshop schedule: 1k → 4k → 8k → 12k → 16k → 20k hours, then every 4k) — a walking engine eats bearings, then the shaft line.

Warning — loose foundation, lost engine. Slack holding-down shows as fretted chocks, weeping joints, drifting deflections and finally cracked bedplate or girders — the misalignment list in §4 names every downstream victim. Tie-rod tension above and holding-down tension below are one system: the sandwich in Two-Stroke Structure §5.