A crankshaft is a mechanical part that is used to convert the reciprocating motion of a piston into rotational motion. This rotary motion further reaches the flywheel and generates the necessary torque required for its operation. It is connected to a piston with the help of a connecting rod. A crankshaft is a prominent part of the power transmission system of an engine and is mainly used to drive a variety of parts of a machine such as a camshaft, radiator fan, etc. A crankshaft is responsible to transmit motion to a camshaft, which in turn controls the opening and closing of the inlet and exhaust valves of an engine. A crankshaft mechanism typically consists of crank and crankpins attached to connecting rods. A crankshaft rotates at a high speed; therefore, centrifugal force acts on it. Also, the piston and connecting rod exert a significant amount of impact force on its surface.
Construction of a Crankshaft
A crankshaft is typically made up of cast iron. Usually, the process of casting is used to make the frame of the crankshaft. A crankshaft should be lightweight and must possess a high tensile strength, which is why some of the crankshafts are also manufactured by the process of forging steel. For this purpose, a block of steel is heated until it gets red hot. Later, this forged steel is kept under high pressure to transform it into the desired shape. Crankshafts are also subjected to surface hardening and heat treatment to improve strength and decrease the rate of wear and tear. The most preferred heat treatment processes include nitriding, carburising, chromium plating, etc. Nitride steel, chrome vanadium steel, nickel steel, nickel-chromium steel, nickel chromium-vanadium steel, or other alloyed steel are the materials that are generally preferred to construct a crankshaft.
Parts of a Crankshaft
A crankshaft consists of eight important parts as given below:
1. Main journals
Journals are the part of a crankshaft that keeps it attached to the engine block. The journals of a crankshaft are arranged along a straight line, i.e., they are placed linear to each other.
2. Crank Pin
Throws or crankpin is the part of a crankshaft that helps the connecting rod stay attached to the crankshaft firmly. A crankpin is cylindrical in shape. The main purpose of a crankpin is to supply rotative force to the larger end of the connecting rod with maximum efficiency.
3. Crank Arm
The crank arm is also known as web or crank web. The Crank web is one of the most important parts of a crankshaft. The main purpose of a crank arm or crank web is to connect the crankshaft to the main bearing journals. A crank web is positioned opposite to the crank pin.
4. Crank Weight
Another name for crank weight is balancing weight. As the name suggests, it is used to balance the crankshaft. Crank weights are mounted on the crank web. They tend to apply an equal magnitude force in the opposite direction to neutralize or balance the applied rotatory force. Crank weights typically improve the stability of the system, thereby ensuring the smooth running of the system at high rotation per minute (rpm).
5. Oil Passage
The crankshaft oil passage passes the oil from the main bearing journals to the connecting rod bearings for the purpose of proper lubrication. For the purpose of lubrication, holes are drilled on the crank web. When the crank pin moves in an upward direction, the connecting rods tend to move downwards, thereby allowing oil to enter between the journal and the bearing.
6. Thrust Washers
Thrust washers are used to prevent the crankshaft from moving along the length. They are necessary to maintain the gap between two parts of a crankshaft and are assembled between the web and the crankshaft saddle. Earlier, the crankshaft mechanism employed thrust washers separately; however, the modern engines employ camshafts in which thrust washers are integrated as a part of the main bearing.
7. Flywheel Mounting Flange
A crankshaft is attached to the flywheel with the help of flanges. The diameter of the crankshaft on the flywheel side is greater than the other end, thereby allowing the flange to mount the flywheel with ease.
8. Oil Seals
The oil poured into the holes drilled into the crank web spills or gets leaked out of both ends of the crankcase. To prevent this leakage from the ends, oil seals are used. There are two oil seals connected to the front and rear ends of the crank frame given below:
1. Front End Oil Seal
The front end oil seals are placed behind the pulley and the timing gear. The failure of front end oil seals is comparatively less destructive and can be traced easily.
2. Rear End Oil Seal
The rear end oil seals are placed within the main journals and the flywheels. The oil seals are held in place with the help of a lid held tightly between the crankshaft by means of a spring known as a garter spring. The destruction caused due to the failure of a rear-end oil seal is difficult to access and repair.
Working of a Crankshaft
The front end of a crankshaft is attached to a gear, sprocket, pulley, and vibration damper. The gear or sprocket is used to drive the camshaft. The pulley attached to the face of the crankshaft is responsible to actuate a water pump, an engine fan, or an alternator. The purpose of the vibration damper connected to a crankshaft is to attenuate the unnecessary crank twist and to suppress vibrations in the engine. A flywheel and an oil seal is attached to the back end of the crankshaft. The flywheel helps the crankshaft exhibit rotation continuously at a constant speed, while the oil seal avoids oil spillage and leakage. To avoid further vibrations in the system, the crankshaft and flywheel are required to be balanced individually. This helps improve and maintain the machine health and reduce the risk of damage or failure of the engine.
Types of Crankshaft
1. Single Piece Crankshaft
As the name itself suggests, a single piece crankshaft is built by a solid single piece of material. These types of crankshafts are usually used in the multi-cylinder engine and are compatible with engines working on both moderate speed and high speed. The end of the connecting rod of a single piece crankshaft is built with the help of two pieces. Single piece crankshafts are capable of withstanding load during firing and cylindrical stress that mainly arise due to misalignment of the main bearings or because of axial and torsional vibrations.
2. Built-up Crankshaft
In a built-up crankshaft, all the components of the crankshaft are fabricated and manufactured separately with the help of multiple separate pieces of substances. For instance, the crank webs, crankpins, and journals are machined separately. These parts are then heated and shrink-fitted together artificially. As the fitted parts tend to cool down, they shrink in size and form a tight grip with each other. The big end of the connecting rod of a built-up crankshaft is built with the help of a single solid piece of the element.
3. Semi-built Crankshafts
In semi-built crankshafts, all the parts of the crankshaft are not assembled together, instead, some of the parts are forged and constructed together as a whole. For instance, the crank web and bearing generals of the crankshaft are forged, shaped, and fully built together. These types of crankshafts are generally used in diesel engines used for the main engine of a ship.
4. Forged Crankshafts
Forged crankshafts are built with the help of the forging process. Here, a single piece or block of metal is forged and is given the shape of a crankshaft. Assembling the parts of the crankshaft through processes like welding is not required. Forged camshafts are typically used for engines that operate at a medium speed such as a generator. For heavy and bulky engines, such as a two-stroke engine, the forging process of manufacturing crankshafts is not preferred. Forged crankshafts are comparatively more robust than cast crankshafts. To improve the strength of forged crankshafts, the process of induction hardening is used.
5. Welded Crankshafts
In welded crankshafts, all the parts of the crankshaft, such as crank web, journals, etc., are manufactured separately and are then welded together. Welded crankshafts are stronger as compared to the other types of crankshafts, but the cost of manufacturing is quite high, which is why these types of crankshafts are not preferred for general applications.
6. Cast Crankshafts
Cast crankshafts are manufactured by casting a malleable iron. Cast crankshafts are the most popularly used crankshafts in diesel and petrol engines because they are easy to manufacture and are cheaper than the other types of crankshafts. A cast crankshaft is often subjected to heat treatment to improve its tensile strength and to reduce the rate of wear and tear, thereby increasing the life span of the engine.
7. Billet Crankshafts
Billet crankshafts are made up of 4340 alloy steel that typically consists of nickel, chromium, aluminium, and molybdenum. Billet crankshafts are advantageous because they have the lowest machining time and are capable of transferring maximum power to the load. Also, these types of crankshafts require minimal balancing.
Examples of Crankshaft
1. Diesel Engines
The diesel engine crankshaft is generally manufactured by forging a piece of alloy steel. For this purpose, the steel is firstly heated at a significantly high temperature and moulded in shape with the help of a forging die. Diesel engines typically have a longer stroke than the bore diameter, which is why the crankshafts used in diesel engines are comparatively larger. Billet crankshafts can also be used to improve the performance of a diesel engine because they are stronger than the other types of crankshafts.
2. Electric Generators
Electric generators are yet another example of the machines used in daily life that make use of a crankshaft. The reciprocating motion of the pistons gets converted to a rotary motion with the help of the crankshaft. The rotary motion is further used to drive the flywheel. The resultant power is then used to move an alternator along a circular path, thereby allowing the conversion of mechanical energy into electrical energy. The generated output energy can be stored or used to drive various electrical appliances.
3. Internal Combustion Engines
A crankshaft forms the basis of working of internal combustion engines. It is mounted within the engine block. The crankshaft is connected to the pistons with the help of connecting rods. The main purpose of a crankshaft in combustion engines is to convert the linear motion of the piston into a rotational motion thereby allowing a direct coupling of power to high dynamic loads.
4. Drive Camshafts
One of the prime applications of a crankshaft lies in driving the camshaft. A crankshaft is used to transfer energy to the camshaft with the help of a chain drive mechanism or a belt drive mechanism. The mechanical parts camshaft and crankshaft are usually used in combination. The speed of a system containing both the camshaft and the crankshaft is maintained in such a way that the camshaft rotates one time for every two rotations of the crankshaft.
5. Transmit Power to Parts of an Engine
A crankshaft mainly functions to transmit power to various parts of the engine system such as valve timing, air conditioning compressor, fuel pump, water pump, alternator, and many others.
6. Motorcycles
A crankshaft is a prominent part of the engine of a motorcycle. It is used to provide the driving power required to move various parts of the engine and to control and minimize the engine vibrations. The crankshaft used in motorcycles can be either cast crankshaft or forged crankshaft. For racing bikes, the crankshaft is fabricated using lightweight materials such as aluminium, titanium, an alloy of aluminium and steel, etc. The material used to construct the crankshaft of a motorcycle is required to have a dense mass and must be superior in quality to reduce the vibrations.
