A crankshaft is a shaft driven by a crank mechanism that converts reciprocating motion into rotational motion. It is primarily used in engine components for automobiles, agricultural machinery, ships, and more. The crankshaft converts the reciprocating motion of pistons, moved by combustion in the engine, into rotational energy, which is then transmitted to the clutch, transmission, and other components. This article explains the structure of crankshafts and discusses the production process and challenges involved in manufacturing them.
Crankshafts consist of several parts, including the journal, crankpin, counterweight, and main shaft.
The journal, also known as the main journal, connects to the bearing on the cylinder block side. It refers to the axis that is an extension of the main shaft held by the engine's bearings.
Located at the position connected from the journal to the crank arm, the crankpin connects to the connecting rod. It is known to rotate more extensively than the journal.
The counterweight, also called a balance weight, is mounted to reduce the inertial forces generated by the motion of the pistons and connecting rods. Depending on the mounting position, it is called a full counter or a semi-counter.
The main shaft is the part of the crankshaft that connects to the outside of the engine. It is equipped with features such as keyways for rotation prevention and screw parts for connecting to the flywheel.
Crankshafts have a complex shape and are used in engine parts that require heat resistance and strength, so their production involves a combination of forging, cutting, heat treatment, and polishing processes. Especially in the cutting process, general-purpose machine tools like lathes and machining centers are used. For mass production requiring more than what general machines can cover, crankshaft-specific machines, such as crankshaft millers, are integrated into the production line.
The complex shape of crankshafts makes them prone to poor machining accuracy, mainly due to chatter vibration and workpiece deformation.
Long and slender workpieces have low rigidity, making them more susceptible to vibration amplification, especially far from the chuck. In crankshafts, chatter vibration is likely to occur in the journal area, potentially causing decreased machining accuracy and tool breakage.
During the turning process, the crankshaft body deforms due to spindle rotation and the pressure from the tailstock. Significant deformation can worsen the journal area's swing accuracy, leading to engine noise, power reduction, and damage to surrounding parts when the finished product is in operation.
For chatter vibration and workpiece deformation in crankshaft cutting, the following measures can be considered:
When roughing, starting from the most vibration-prone areas allows the margin adjusted during finish machining to absorb the impact of chatter vibration. In finish machining, selecting cutting tools with small cutting resistance, such as those with a 55° edge angle or small nose radius, is effective.
To minimize workpiece deformation, it is effective to keep the rotation speed low during turning and set lower pressure for the tailstock. Additionally, heat treating the material to increase rigidity and ensuring the accuracy of center holes in previous processes can be beneficial.
However, in mass production processes, it might be challenging to keep rotation speeds low due to time constraints. Adjusting tailstock pressure has disadvantages, such as the risk of the workpiece coming loose, so calculating the proper grip strength and adjusting with a minimal margin is necessary.
This article explained the structure of crankshafts and discussed their production process and challenges. Easing machining conditions is effective for improving machining accuracy of crankshafts. However, adjusting machining conditions can be difficult in mass production to ensure production efficiency. Solving challenges within various constraints emphasizes the importance of years of experience and cooperation with tool manufacturers to select tools that can address these issues effectively.
