The crankshaft at the bottom of your car’s engine translates the vertical movement of the pistons into the rotation that drives the clutch, gearbox, and ultimately the wheels.
The crankshaft has evenly spaced ‘throws’ (e.g., there are four in a four-cylinder engine, shown below), which connect to the bottom of the pistons by connecting rods. These ‘throws’ are offset from the axis of the crankshaft, hence creating a rotational energy.
The crankshaft is supported in the engine by large bearings on each end that tie to the flywheel, and through that, the clutch.
When the clutch is engaged, the rotational energy is then moved through the gearbox, then the differential, and finally through driveshafts to the wheels. That is how the car moves!
What is Crankshaft?
A crankshaft is a crucial element of the power transmission unit. It converts the reciprocating motion of the piston to a rotating movement through its connecting rod.
A crankshaft is made from crankpins, crank webs (also known as crank arms or cheeks), balancing weights, and main journals. The crankpin of the crankshaft is controlled by the large end of the connecting rod.
During any stroke, the center-to-center distance from the crankpin to the crankshaft is half of the piston displacement. A full revolution of the crankshaft is two strokes of the piston.
Parts of Crankshaft
The Main parts of a crankshaft with its diagram:
- Crankpin
- Main journals
- Crank web
- Counterweights
- Thrust washers
- Oil passage and oil seals
- Flywheel mounting flange
#1. Crankpin.
A crankpin is a part of an engine that connects a connecting rod to the crankshaft.
The crankpin is actually a cylindrical surface that provides rotational force to the large end of the connecting rod. The crankpin can also be referred to as a connecting rod journal.
#2. Main Journals.
Main journals are attached to the engine block. These bearings hold the crankshaft and allow it to rotate in the engine.
Main journals are commonly plain bearings, or journal bearings. The main journals vary from engine to engine, usually according to the forces exerted by the engine as it turns.
#3. Crank web.
The crank web is the most important part of the crankshaft. The crank web connects the crankshaft to the main journals.
#4. Counterweights.
Counterweights are a type of weight that provides the opposite force. Counterweights can balance and stabilize the crankshaft. The counterweights are attached to the crank web.
Counterweights are installed on the crankshaft because they will cancel out the reactions caused by rotation. The counterweights help achieve a higher RPM and enable the engine to run easily.
#5. Thrust Washers.
It is generally the case that there are two or more thrust washers to prevent the crankshaft from moving in the length direction of the crankshaft. The thrust washers assemble between the machined surfaces in the web and the saddle of the crankshaft.
It is easy to keep the gap between the thrust washers and crankshaft and to minimize the crankshaft’s lateral movement. In many engines, this is incorporated into the main bearings, however, with the older types, a separate thrust washer is found.
#6. Oil Passage and Oil Seals.
The crankshaft oil passage takes oil from the main bearing journals to the big end journal. Normally the oil passage hole will be through drilled on the crank web.
When the crankpin is up, the combustion force pushes the connecting rod down and oil has the ability to flow between the journal and bearing.
The crankshaft has some length beyond the crankcase on both ends. This will allow oil to leak from these ends. Therefore, in order to contain oil in the crankcase, oil seals are used. There are two oil seals, one at the front end and one at the rear end.
- Front end oil seals are the same range but the failure is not as catastrophic and more accessible. The crankshaft front oil seal will be set behind the pulley and timing gear.
- A rear end oil seal is internally located within the main journals and flywheels. The oil seal is press fitted into the hole between the engine block and the oil pan. The oil seal uses a shaped lip that is held tightly to the crankshaft using a spring called a garter spring.
#7. Flywheel Mounting Flange.
In most cases, the crankshaft is attached to the flywheel by means of flanges. The crankshaft wheel end will usually have a larger diameter compared to the other end. Generally, this is done to create flanges to mount the flywheels.
Construction of Crankshaft
Main bearings, sometimes referred to as main journals, are components of the crankshaft. The crankshaft is supported by the main bearing on the main journals, while a balanced load is provided in opposition to the crankarm for equilibrium.
A crankshaft is generally made of alloy steel forged or cast. It is machined and grounded to provide suitable journals for the connecting rod and main bearing.
It should be strong enough to carry the entire thrust of the pistons during the power strokes without undue distortion. Also, it should be carefully balanced to eliminate undue vibration that results from the weight of the offset cranks.
The crankshaft has oil passages drilled in this amount for oil to travel from the main bearing to the connecting rod bearing.
Function of Crankshaft
The crankshaft front end has a gear or sprocket, a vibration damper, and a fan belt pulley. The gear or sprocket drives the camshaft, while the vibration damper reduces torsional vibration.
The fan belt pulley drives the engine fan, water pump, and the generator with a V-belt. The rear end of the crankshaft has a flywheel. The flywheel inertia helps keep the crankshaft rotating at a fairly constant speed.
Main bearing numbers depend on engine design and cylinder numbers. Generally, the more these bearings are, the less chance of crankshaft vibration and distortion of the crankshaft of a given size.
The bearing must support the shaft and give a high degree of stiffness between each crankpin. The compromise is 3 main bearings for a 4-cylinder engine and 4 for a 6-cylinder.
To limit engine vibration, the crankshafts and flywheels are balanced both separately and while mounted. Balance reduces the likeliness of damaging the engine, especially the bearings.
Types of Crankshafts
The following are the types of crankshafts:
- Fully built shaft
- Semi built shaft
- Welded shafts
- Solid single-piece shaft
- Forged crankshaft
- Cast crankshaft
- Billet crankshaft
#1. Fully Built Shaft.
These types of crankshafts are built by shrink-fitting parts together. All components are made separately, and then heated with the crank web heated separately and then shrink-fitted after fabrication. The crank web is usually the crankshaft material that is usually used in older style engines.
The crank pin and main journals are separate components that are machined and bored in the crank web, as they are made thinner in diameter.
The crank web is heat treated and fit on top of the crank pins, and journals are heated along with the crank web (which expands due to the heat). The crank web then cools, and while the crank web is cooling, the diameter of the borehole tries to shrink back to its original diameter.
#2. Semi-Built Shaft.
This type of crankshaft is made from a fully built shaft rather than an assembly of parts. The crank webs are forged solid, and are then shrink-fitted onto the main bearings.
The crankpins undergo additional machining to provide a smooth surface finish. The advantage of using shrink fitting is that you can forge two webs and crankpins in one piece.
The thickness of the crank webs is reduced. A hole (bored through the crankpin) reduces the weight while maintaining strength.
The semi-built crankshaft is lighter than the fully built crankshaft. The crankshaft will carry high loads thus allowing it to endure significant shear and bending stresses and is a type found in some high speed engines.
#3. Welded Shaft.
These welded shafts are made by putting the crank web, crankpin, and main journals together on both sides. These parts are forged and welded together using the submerged arc welding process.
Once welded the journals are free of stress and mechanized. Since continuous grain flow is used, the webs can be thinner. That might bring a smaller crankshaft.
A welded crankshaft saves a good deal of weight. The web thickness and width is reduced. The length of the bearings provides more length.
#4. Solid Single Piece Shaft.
Crankshafts are usually made by forgings or castings as single piece shafts, they can be utilized in low-speed engines and high-speed engines.
It is made in more than one part and bolted together with bolts on integral flanges. This is designed to withstand the load during firing and cyclic tension. The crankshaft is subjected to stresses imposed by misalignment and torsional vibration of the main bearings, including axial vibration.
#5. Forged Crankshaft.
These are much stronger than cast cranks. Forged cranks are used in high-stress engines and even some 16-valve engines. The processes to manufacture them are completely different.
A set of dies is made that is close to the final crank shape. The dies are placed in a hydraulic press that has clamping forces in the tons. Once the die closes, it tightens up a piece of metal to allow it to be pressed.
These crankshafts are stronger and more durable. Forged crankshafts respond to heat treatment and therefore provide better dimensional stability.
#6. Cast Crankshaft.
These crankshafts have been used for a long time and are common in diesel and petrol engines. Generally, they are made of malleable iron by the casting process.
They are inexpensive to design and manufacture, making them a popular choice of manufacturers. Cast crankshafts can withstand loads from all directions because they have an uniform random metal grain structure.
#7. Billet Crankshaft.
A billet crank is perhaps your engine’s best type of crank, most commonly made from steel, which can include nickel, chromium, aluminium, molybdenum and a few other elements.
Billet cranks are popular because they have the shortest machining time of the crankshaft, they also need the least amount of balance because of the evenness of the material design.
Faults of Crankshaft
Crankshaft problems happen rarely, but usually under extreme engine conditions. The engine parts are strong, but it has some fundamental faults.
- Worn journals
- Fatigue
#1. Worn Journals.
This usually commonly occurs when there is insufficient oil pressure available. If the crankshaft journal-bearings surfaces start touching the crankshaft, it worsens the oil pressure, increases the clearance and worsens.
If not cared for, the damaged journals will affect the reliability of the engine. This damages the bearings and creates serious damage in the engine.
#2. Fatigue.
This shows that the internal force on the crankshaft has caused the crankshaft to shear. Usually this occurs on the fillet where the journal and the web both involved.
The fillet should have a smooth surface to help relieve any points of weakness and to help avoid fatigue cracks. The cracks can be determined by Magna-fluxing the crankshaft.
Materials Used To Make Crankshaft
High strength and hardness make steel ideal for crankshafts. Steel has a much higher fatigue strength than all other materials of similar cost.
Commonly used crankshaft steels include:
#1. Manganese-molybdenum steel.
Manganese-molybdenum steels are cheap crankshafts forged from low-carbon steel. They are used for medium-duty petrol engine applications.
It is 97.82% iron, 0.3% molybdenum, 0.38 carbon, and 1.5% manganese. The Brinell hardness of manganese-molybdenum steel is 250 BHN.
#2. Nickel-chromium-molybdenum Steel.
Because of the extremely high fatigue strength, these steels are used in diesel engines. Nickel-chromium-molybdenum steel consists of 2.5% nickel, 0.31% carbon, 0.65% chromium, 0.5% molybdenum, and the remaining material is iron.
The Brinell hardness of nickel-chromium-molybdenum is 300 BHN.
#3. Chromium-molybdenum Steel.
It is used for design of the crankshafts for medium- and heavy-duty petrol and diesel engines.
Chromium-molybdenum steel consists of 1.2% chromium, 0.4% molybdenum, 0.3% carbon; and the rest is iron. The Brinell hardness of chrome-molybdenum steel is 280 BHN.
#4. Nodular Cast Iron.
This cast iron has perlite-filled grey cast iron with graphite nodules. Because of their high ductility, these materials are often referred to as ductile cast iron.
This cast iron consists of approximately 3-4% carbon and 1.8-2.8% silicon. The Brinell hardness for nodular cast iron is 200–300BHN.
Advantages of Crankshaft
- It is more effective.
- The crankshaft ensures the smooth operation of an engine.
- It offers more torque.
- By using this, engine power is increased.
Disadvantages of Crankshaft
- Bearings can fail.
- The overall cost of production is high.
- The crankshaft heats up as a result of the increased friction.
Applications of Crankshaft
The crankshaft is an engine component that efficiently converts reciprocating into circular and is used to gain power or energy.
The crankshaft is the heart of the internal combustion engine, bringing about proper operation of a four-stoke cycle engine by moving pistons up or down and also generating energy for suction, compression and exhaust strokes.
Wrapping it up
A balanced crankshaft will provide your engine with smooth operation, increased power and efficiency, and lower engine vibration, The crankshaft primarily generates engine rotation, or rotational speed, as from the pistons linear/influence motion.