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A crankshaft’s primary function is to transform the linear motion of the piston into rotational motion. The pistons are connected to the crankshaft through the connecting rods. The crankshaft is mounted within the engine block through its main journals. The connecting rods are fixed on the conrod journals of the crankshaft. On opposite sides of the conrod journals, the crankshaft has counterweights that compensate outer moments, minimize internal moments, and reduce vibration amplitudes and bearing stresses. At one end of the crankshaft, the flywheel is connected, and on the other end, the valve timing gearing.
The rear end of the crankshaft extends out of the crankcase and ends with a flywheel flange. This precision-machined flange is bolted to the flywheel, the heavyweight of which helps smooth the pulsation of the pistons by firing at different times. Thanks to the flywheel, the rotation makes its way to the wheels through the transmission and the final drive. In the automatic, the crankshaft is bolted to the ring gear, which carries the torque converter, passing the drive on to the automatic transmission. This is essentially the engine’s output and the power is taken where it is needed: the propulsion for ships and aircraft, the induction coils for generators, and the road wheels in the vehicle.
The main bearing logs, or just the main logs, are clamped into the engine block and the engine rotates around these logs. All crankshaft journals are machined perfectly smoothly and roundly, and often hardened. The main journals are secured in stools in which a removable bearing insert is installed. The bearing is softer than the journal and can be replaced as it wears and is designed to absorb small amounts of contaminants, if any, in order to prevent damage to the crankshaft. The main bearing cap is then bolted over the log and tightened to the exact torque specification.
The connecting rod logs are offset from the axis of rotation and attached to the large ends of the pistons’ connecting rods. Confusingly, they are also commonly referred to as crank pins or rod bearing journals. Pressurized oil is fed through an angled oil passage drilled from the main diary.
Metal-to-metal contact is the enemy of an efficient engine, and therefore the main logs and rod logs are both mounted on an oil film that sits on the bearing surface. Supplying oil to the main bearing log is easy: the engine block oil galleries lead to each crankshaft saddle, and the matching hole in the bearing shell allows this oil to reach the log.
The crankshaft is subject to strong rotational forces, and the mass of the connecting rod and piston moving up and down exerts a significant force. Counterweights are cast as part of the crankshaft to balance out these forces. These counterweights allow for a smoother running engine and higher RPM.
A crankshaft will be balanced at the factory. In this process, the flywheel is attached and the entire assembly is spun on a machine that measures where it is out of balance. Balance holes are drilled in the counterweights to reduce weight. If weight needs to be added, a hole is drilled and then filled with heavy metal or mallory. This is repeated until the crankshaft is balanced.
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