Kobe Steel breakthrough points way to more compact engines

Built up crankshaft Built up crankshaft

AUGUST 30, 2017 — Opening the way for more compact engines, classification society ClassNK has given Kobe Steel approval to use a K factor of 1.15 as the design margin in the manufacturing process for crank throws, a main component of built-up type crankshafts.

The certification — a world first — is a result of combining Kobe Steel's super clean steelmaking process with a die-forging method developed by the company utilizing its strengths as an integrated forging manufacturer.

The approval of the 1.15 as the K factor for the calculation of the fatigue strength exceeds the 1.05 certification that Kobe Steel received in 2014.

The 1.15 K factor recognizes a margin of 15 percent to design engines using high-strength crankshafts supplied by Kobe Steel and opens the possibility to design compact ships' engines with high output.

Non-metallic inclusions in steel are a factor in reducing fatigue strength, and reducing these impurities is important to maintain the cleanliness of steel. Conventional clean steel contains a reduced amount of large sulfur inclusions. However, through its proprietary refining equipment and reaction control technology, Kobe Steel is able to minimize sulfur as well as all other inclusions to achieve steel of higher cleanliness.

The trend toward eco-ships has seen ships using larger propellers that rotate at slower speeds, calling for engines with higher output at the lower rotating speed region. Engines, as a whole, are therefore tending to increase in size due to the lengthening of the engine stroke. 

Crankshafts that can bear higher loads make it possible to increase an engine's output per cylinder, making it possible to achieve a required engine output with a more compact engine with fewer cylinders.

Kobe Steel now plans to explore possibilities for weight savings and compactness together with engine makers and shipbuilders.

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