Fully Built Crankshafts: In fully built crankshafts, various components such as crank pins, webs, and main journals are fabricated separately and then assembled using the shrink-fitting method. This method involves machining the crank pins and journals, boring matching holes in the webs, heating the webs to expand the holes, and then fitting the crank pins and journals into these holes. As the webs cool, they contract and grip the crank pins and journals firmly, ensuring a tight fit that prevents slippage during engine operation. Fully built crankshafts were commonly used in older engines and are advantageous for allowing the replacement of individual parts.
Materials: Typically made from unalloyed carbon steel (normalized).
Applications: Historically used in large, slow-speed marine engines. However, they are less common today due to lower fatigue strength and the massive size required for secure shrink fits.
Semi-Built Crankshafts: Semi-built crankshafts are constructed by forging the crank throws, which include two webs and a crank pin, as a single piece. The main journals, which are forged and machined separately, are then fitted into the webs using the shrink-fitting method. This construction method provides excellent grain flow, following the web around the crank pin and back down the other web, enhancing the strength and durability of the crankshaft. Semi-built crankshafts are often used in large two-stroke crosshead engines. This method also allows for reduced web thickness and overall weight while maintaining strength.
Materials: Often use low-alloyed Chrome-Molybdenum (Cr-Mo) steel for throws and unalloyed carbon steel for journals.
Applications: The industry standard for large two-stroke slow-speed main propulsion engines.
Solid Forged (Single-Piece) Crankshaft
Welded Crankshafts: Welded crankshafts are composed of individual forgings that include the webs, crank pins, and parts of the main journals. These forgings are welded together using processes such as submerged arc welding. This method allows for continuous grain flow and the production of thinner webs, resulting in a lighter and shorter crankshaft. Despite the high capital costs, welded crankshafts provide significant advantages in terms of strength and weight reduction, and they have been successfully used in some marine engines.
Materials: Hardened and tempered low-alloy Cr-Mo steel.
Applications: Modern high-power engines where weight reduction and high natural frequency are critical.
Solid Single-Piece Crankshafts: Solid single-piece crankshafts are forged or cast as a single unit and are typically used in small to medium-speed and high-speed engines. This construction method ensures the highest strength and durability, with continuous grain flow throughout the entire crankshaft. However, the size and weight limitations make this type less suitable for large marine engines, where the crankshaft components need to be individually forged and assembled due to the immense size and weight.
Materials: High-tensile alloy steels like Nickel-Chrome or Chrome-Molybdenum steel.
Applications: Preferred for medium and high-speed four-stroke engines, such as auxiliary generators and smaller propulsion units.
Each type of crankshaft is chosen based on the specific requirements of the engine, including size, speed, and load conditions. The materials used for these crankshafts vary, with slow-speed engines typically using plain carbon steel, while medium and high-speed engines often utilise alloy steels for enhanced performance and durability.