In this article, you’ll learn what is Crankshaft? how does it work? Its types, parts, function and more explained in detail with diagrams. And also, you can download the PDF file of this article at the end of it.
What is Crankshaft?
A crankshaft is an essential part of the power transmission system. In which, the reciprocating movement of the piston is converted into the rotating movement by using the connecting rod.
A crankshaft consists of crankpins, crank webs (crank arms or cheeks), balancing weights, and main journals. The large end of the connecting rod is attached to the crankpin of the crankshaft.
During one stroke, the center-to-center distance between the crankpin and the crankshaft is half of the piston displacement. Thus, one complete revolution of the crankshaft makes two strokes of the piston.
Parts of Crankshaft
Following are the main parts of crankshaft with its diagram:
- Main journals
- Crank web
- Thrust washers
- Oil passage and oil seals
- Flywheel mounting flange
1. Crank Pin
The crankpin is a mechanical part of an engine. Which allows the connecting rod to be attached to the crankshaft very firmly.
The surface of the crankpin is cylindrical, to give the rotative force to the large end of the connecting rod. These are also known as connecting rod journals.
2. Main Journals
Journals have attached to the engine block. These bearings hold the crankshaft and provide it to rotate inside the engine block. This bearing is such as a plain bearing or journal bearings. The main bearings vary from engine to engine, often according to the forces given by the engine.
3. Crank Web
The Crank web is the most essential part of the crankshaft. Crank web connects the crankshaft to the main bearing journals.
The counterweights are a type of weight, that applies opposite force, which provides balance and stability to the crankshaft. These are mounted on the crank web.
The reason for installing counterweights in the crankshaft is, they can eliminate the reaction caused by rotation. And it is very helpful for achieving the higher RPM and makes the engine run easily.
5. Thrust Washers
At some points, two or more thrust washers are provided to stop the crankshaft from moving lengthways. These thrust washers assemble among the machined surfaces in the web and the crankshaft saddle.
With the help of thrust washers, it can be easily maintained the gap and helps to reduce the lateral movement of the crankshaft. In many engines, these are made as part of the main bearings, usually, older types, use separate washers.
6. Oil Passage and Oil Seals
Crankshaft oil passage passes oil from main bearing journals to the big end journals. Normally the hole is drilled on the crank web. When the crankpin is in an upward position and combustion forces push the connecting rod to a downward position, it allows oil to enter between journal and bearing.
The crankshaft has some extend beyond the crankcase on both ends. This causes oil to leak from these ends. So, preventing oil from these openings, oil seals are provided. There are two main oil seals are connected at the front end and rear end.
- Front End Oil Seals: They are very similar to the rear end oil seals. However, its failure is less destructive and it is more easily accessible. The front oil seal will be set behind the pulley and timing gear.
- Rear End Oil Seals: They are placed within the main journals and flywheels. This is pushed into a hole between the engine block and the oil pan. The oil seal has a shaped lip that is kept tightly into the crankshaft by a spring called a garter spring.
7. Flywheel Mounting Flange
In most cases, the crankshaft attaches to the flywheel through the flanges. The diameter of the crankshaft wheel end is larger than the other end. This gives a flange face to mount the flywheel.
Construction of Crankshaft
Inside the crankshaft, it is consists of main bearings are also called the main journals. The crankshaft is supported by the main bearing on the main journals. A balanced load is provided in the opposite direction of the crankarm for equilibrium.
A crankshaft is usually manufactured of alloy steel by casting or forging process and is machined and grounded to give suitable journals for the connecting rod and main bearing.
It must be strong enough to take the thrust of the pistons during the power strokes without excessive distortion. Also, it must be carefully balanced to eliminate undue vibration resulting from the weight of the offset cranks.
The crankshaft has drilled oil passages through which oil can flow from the main bearing to the connecting rod bearing.
Function of Crankshaft
The front end of the crankshaft carries gear or sprocket, vibration damper, and fan belt pulley. A gear or sprocket operates the camshaft, and the vibration damper provided to control torsional vibration.
The fan belt pulley drives the engine fan, water pump, and generator with V-belt. Rear-end of the crankshaft carries the flywheel. The inertia of the flywheel tends to keep the crankshaft turning at a constant speed.
The number of the main bearings varies with the design of the engine and cylinders. More main bearing, the less the possibility of vibration and distortion of the crankshaft of a given size.
The bearing shall be sufficient to support the shaft and give one of the greatest stiffness between each crankpin. A compromise is to have 3 main bearings on a 4 cylinder engine and 4 on a 6 cylinder engine.
To reduce vibration in the engine, the crankshaft and flywheel are balanced separately. They are often tested for balance when mounted together. Balancing is necessary to prevent damaging the engine, especially the bearings.
Types of Crankshafts
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 made by assembling parts by parts together. In this, all the components are shrink-fitted after the fabrication process. This type of crankshaft used in old engines.
It is consists of a separated crank web, crankpin, and main journals. This crankpin and journals are machined and bored in the crank web. They are slightly smaller in diameter.
The crank webs were heated and fit into the crank pins and journal holes (which expanded in size due to heat). As the crank web cools, the diameter of the borehole will try to shrink back to its original size.
2. Semi Built Shaft
These types of crankshafts are not made by assembling various parts compared to a fully built shaft. Crank webs are completely forged and are shrink-fitted to the general bearings.
The crankpins are further machined to provide a smooth surface finish. The benefit of using this shrink-fitting method is to make two webs and crankpins with a single forging. The crank webs are reduced in thickness. A hole is bored through the crankpin, reducing the weight without compromising its strength.
The semi-built crankshaft is lighter than the fully built crankshaft. The crankshaft is capable of carrying high loads and can therefore withstand high shear and bending stresses. These types are also found in some high-speed engines.
3. Welded Shaft
These welded shafts are manufactured by assembling crank web, crankpin, and main journals on both sides. These are forged and then welded together by the submerged arc welding process.
After welding, the journals were stress-free and mechanized. Due to the use of continuous grain flow, the webs can be made thinner. That may lead to a smaller crankshaft.
The welded crankshaft allows considerable savings in weight. The width and thickness of the webs are reduced. Thus, providing longer bearing lengths.
4. Solid Single Piece Shaft
These types of crankshafts are made by forging or casting process as forming single piece shaft. They are utilized both in low-speed engines and high-speed engines.
It is built in more than one part and bottled together with bolts on integral flanges. This is designed to endure the load during firing and cyclic tension. The crankshaft is stressed due to misalignment, torsional vibration, and axial vibration of the main bearings.
5. Forged Crankshaft
These are much stronger than cast cranks. Forged cranks are typically used in high-stress engines and some 16 valve engines. They are designed in completely different ways.
A set of dies is formed to the approximate size of the crank. These die rest in a large hydraulic press which has a clamping force of several tons. Once the die is closed, the metal is pressed very tightly.
These types of crankshafts are stronger and more durable. Forged crankshafts react to heat treatment and thus give better dimensional stability.
6. Cast Crankshaft
These types of crankshafts have been used for a long time. There are commonly found in various diesel and petrol engines. Generally, they are made up of malleable iron by casting process.
They are very cheap to make and work well, so they are a common choice for manufacturers. Cast crankshaft withstands loads from all directions because the metal grain structure is uniform and random.
7. Billet Crankshaft
A billet crank may be the best type of crank in your engine. Steel is commonly used to make such cranks. This includes nickel, chromium, aluminum, molybdenum, and other elements.
Billet cranks are familiar because of the shortest machining time of the crankshaft. They further need minimum balance due to the uniform design of the material.
Faults of Crankshaft
Problems on the crankshaft appear very rarely. It occurs when the engine is undergoing extreme conditions. The engine parts are strong and robust. But they involve some basic faults:
- Worn journals
1. Worn Journals
It usually occurs when there is not enough oil pressure available. If the crankshaft makes contact with the journal bearing surfaces, it gradually increases clearance and worsens the oil pressure.
If not taken care of it, worn journals can cause serious problems for the engine. This destroys the bearings and causes heavy damage to the engine.
This occurs when the constant force on the crankshaft leads to breakage. This issue usually occurs on the fillet where the journal and web are involved.
A smooth surface of the fillet is necessary to avoid weak spots that cause fatigue cracks. The cracks can be inspected using Magna-fluxing on the crankshaft.
Applications of Crankshaft
The crankshaft is commonly used in an engine for converting the reciprocating motion into a circular motion, making the use of energy or power much easier.
A crankshaft is actually the heart of the internal combustion engine. The crankshaft is liable for the proper operation of the engine. In some circumstances, it also saves energy to move the piston for compression, suction, and exhaust strokes.
Wrapping it up
A balanced crankshaft can keep your engine running smoothly, provide more power, less waste of energy, and reduce engine vibration. It is mainly employed to convert linear motion to rotational speed.
I hope that I have covered everything about crankshafts. If you have any doubts about this article on “crankshaft parts and function” you can ask in the comments. If you liked this article, please share it with your friends.
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