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Kobe Steel develops new forging method for built-up crankshafts used in ship engines

August 2, 2013

Kobe Steel, Ltd. has developed a new die-forging method to manufacture built-up type crankshafts for diesel engines in ships. The new die forging method offers considerable improvements in design fatigue strength in comparison to the conventional “bending forging” method. Using the new method, Kobe Steel can make near-net-shape forgings for crank throws.

Due to requests to improve fuel consumption stemming from stricter environmental regulations and the high price of ship fuel, the need for “eco-ships” is rising. Two important points in the development of eco-ships are ship development to reduce drag on the hull and engine development to improve fuel consumption and lower exhaust gas emissions.

In engine development, ships are using larger propellers that rotate at slower speeds, driving the need for low-rpm propellers with high output. To meet this trend, long-stroke engines are growing in popularity. When long-stroke engines are used, the crank throws are longer and heavier than conventional ones, leading to higher load stress on the crankshaft and higher bearing loads.

Kobe Steel has recently developed a new die-forging method using its experience as an integrated crankshaft manufacturer covering all processes of steelmaking, forging, machining and inspection. Incorporating the new forging method and techniques of clean steelmaking, Kobe Steel has successfully increased the fatigue strength of the crankshaft material by 20 percent, compared with conventional forging. As a result, long-stroke crank throws of higher reliability and lighter weight can now be commercially mass produced.

Along with the development of the new die-forging method, Kobe Steel developed automatic ultrasonic inspection technology and numerical inspection technology that digitally evaluates the part where the crankshaft touches the bearing. These technologies provide substantial improvement on the reliability of the crankshafts, which face higher load stress and bearing loads when used with long-stroke engines.

In addition to the new die-forging method, Kobe Steel conducts research and development of advanced materials for marine parts. Responding to customer needs, Kobe Steel is contributing to the advancement of the shipping and shipbuilding industries.

Background Notes

Stricter environmental regulations

The International Maritime Organization, a special agency under the United Nations, adopted measures in 2011 to reduce the amount of CO2 gas emissions from ships under the International Convention for the Prevention of Pollution from Ships (MARPOL). From 2013, the new regulations affect all construction contracts for new ships with 400 gross tonnage and above.

Construction contract year Reduction target until 2014 for contracted vessel construction using average CO2 exhaust emission volume as the standard
2015-2019 -10% or more
2020-2024 -20% or more
2025 and beyond -30% or more

Long-stroke engines

Refers to the ratio of the piston stroke to the cylinder bore diameter. The longer the bore/stroke ratio, the higher the efficiency.

Crank throw

The throw is the offset arm on a crank to which the connecting rods to the engine are attached.