Kobe Steel 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 fatigue strength in comparison to the conventional ‘bending forging’ method of forging. Using the new method, Kobe Steel can make near-net-shape forgings for large crank throws.
Ship operators are looking to improve fuel consumption stemming from stricter environmental regulations and the high price of ship fuel oil resulting in the need for ‘eco-ships’ rising. Two approaches are important in the development of eco-ships: reduced drag on the hull and improved engine fuel economy and so lower exhaust emissions.
To improve engine efficiency, ships are using larger propellers rotating at slower speeds. To meet this trend, long-stroke engines are growing in popularity. When long-stroke engines are used, the crank throws have to be longer and heavier than for conventional crank shafts 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, the company has successfully increased the fatigue strength of the crankshaft material by 20%, 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 has developed automatic ultrasonic inspection and numerical inspection to digitally evaluate the area where the crankshaft is in contact with the bearing. These technologies provide substantial improvement on the reliability of the crankshafts, which face higher load stress and bearing loads when used in long-stroke engines.
The International Maritime Organisation, a special agency under the United Nations, adopted measures in 2011 to reduce the amount of CO2 gas emitted 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 of 400 gross tonnage and above.
The targets for CO2 emission reductions are:
2015-2019 -10% or more;
2020-2024-20% or more;
2025 and beyond -30% or more.
Kobe Steel’s new forging method makes near-net-shape forgings for long-stroke crank throws of higher reliability and lighter weight.