A crankshaft is a rotating shaft that converts reciprocating motion of a piston into a rotational motion. It’s commonly used in internal combustion engines to perform such operation. Crankshafts consist of series of cranks and crankpins to which the connecting rods are attached.
A crankshaft with at least one shaft rotates within the engine block. It rotates using the main bearings. The crankpins rotate within the connecting rods using rod bearings.
How crankshafts are made
Crankshafts are usually made from metal like cast iron. Molten metal is poured in a mold during the process (Casting).
Modern crankshafts are made from forged steel which are used in some performance engines. It is made by heating a block of steel until red hot. It’s then form into shape using an extreme high pressure.
Crankshafts are forged to withstand wear and the strain of rotary movement. Alloyed heat-treatment steel or nitride steel materials are used. The crankshaft journals are also surface-hardened.
The functions of crankshafts are to give a smoother drive to the vast motors with multi-cylinders. Linear motion of the pistons which is changed into rotational motion.
In the combustion of the fuel-air mixture, power is produced. This power is transformed into rotary movement of the crankshaft. The linear motion of the pistons is converted via the connecting rod into torque. It’s then passed to the flywheel
The crankshaft’s shaft is bored with some holes that feeds the motor with oil. This oil smoothens the movement. The counterweights aid the adjustment of the framework and the heaviness of the connecting rod.
Crankshafts also function as load bearing as some load is withstand during the process. One of the load is the severe bending and torsional stress.
As the rotary movement of the crankshaft is constantly being accelerated and decelerated, further loads from torsional vibration is added. Bearings also experience high degree of wear.
Crankshaft Components & Design
The componential parts of crankshaft include:
- Main Journals
- Crank pins
- Crank webs
Main journals carry the main bearings and declare the axis of rotation of the shaft.
Crankpins allows the connecting rod to be attached to it.
Crank webs connect the crankpins to the main journals.
Counterweights offers balancing and are mounted to the webs.
The design of a crankshaft is based on the firing ability of the engine and number of cylinders. It is also determined by the design of the engine, numbers of crankshaft bearing and stroke size.
Lubrication plays important role in engine’s efficiency as its working mechanism involve the robbing of two metal parts. Avoiding unnecessary wear on crankshaft, main journals and rod journals both ride on a film of oil. This film of oil sits on the bearing surface.
The oil is supply to the main bearing through the oil galleries from the engine block. It leads to each crankshaft saddle, and the matching hole in the bearing shell collect the oil to the journal.
How crankshaft work
The working of crankshaft is quite interesting and easy. There is a distance between the center of the main journal and the center of the crankshaft pin. This distance is known as crank radius or crank throw. Its measurement determines the range of piston travel as the crankshaft rotates.
The distance from the top to the bottom is known as stroke. The piston stroke is twice of the crank radius.
The rear end of the crankshaft extends outside the crankcase and supported with a flywheel flange. This flange is a precision machined part that is bolted to the flywheel. Its heavy mass allows smooth pulsation of the pistons firing at different times.
Flywheel rotation makes its way through the flywheel and transmission and final drive to the wheels. Crankshafts are bolted to the ring gear in an automatic drive. It carries the torque converter and pass it to the automatic transmission.
Common crankshaft faults
Issues on crankshaft are rare, only if the engine is experience extreme conditions. The engine component is reliable and sturdy but some common fault includes:
Worn journals: is caused when there is no enough oil pressure. The crankshaft journals make contact with the bearing surfaces. This gradually increases the clearance and worsen the oil pressure.
Worn journals can cause serious problem to the engine if care is not taken. It leads to destroyed bearings and cause huge damage to the engine.
Fatigue: is when constant forces on the crankshaft lead to fractures. This problem usually occurs on the fillet where the journals and the be web joined.
A smooth radius of fillet is critical to avoid weak spots which leads to fatigue cracks. Cracks can be inspected on crankshaft using a magnafluxing.