Recently, Kobe Steel Ltd. got a boost in its efforts to expand die forging for near-net-shape parts for large crankshafts, as Nippon Kaiji Kyokai (ClassNK) authorized the steelmaker to use a factor of 1.05 for calculating the design fatigue strength of crank throws. Kobe Steel uses die forging to produce “ultra large” crank throws with cylinder diameters of 80 cm or more, with a fatigue strength that is 20% greater than that of conventionally forged cranks.
The “throw” is the offset arm on a crankshaft, to which the engine connecting rods attach. Larger crankshafts, and thus larger, stronger throws, are increasingly important in powering ocean-going cargo ships and tankers. Larger diesel engines are being adopted to improve fuel consumption and lower exhaust gas emissions, along with large propellers that rotate at slower speeds. A large crankshaft is needed to link the engine pistons to the propeller.
ClassNK is a certification agency that establishes and verifies maritime safety and performance standards. It previously approved Kobe Steel to produce small and medium-sized throws, and Kobe Steel is the world’s only producer certified for the full range of throws for two-cycle engines.
Kobe Steel is one of the world’s largest manufacturers of crankshafts, but it developed the new die-forging method specifically for near-net-shape crankshaft parts for marine engines. It first announced the development in 2014.
Die forging allows Kobe Steel to produce high-strength, lighter weight components with an approved, 5% margin in fatigue strength in the design phase. Having already received the same ClassNK approval for small and medium-sized throws, Kobe Steel is the only company in the world to receive certification for the full range of throws, ranging from small to large, for use with two-cycle engines.
Kobe Steel is developing other products and processes to serve the large crankshaft market, including stronger and cleaner steel grades for the crank throws, journals, and shafts, as well as high-strength materials for the rudder parts.