Of all an engine’s moving parts, the crankshaft is the biggest and heaviest. There’s no doubt about it, the crankshaft is the foundation on which the combustion engine is built.
No part of the process – intake, compression, combustion, or exhaust – occurs without a crankshaft to put it into motion.
But for most people, understanding how the crankshaft operates isn’t quite clear. Here’s how it works, what its purpose is, and what can go wrong with the crankshaft.
What is Crankshaft?
An engine crankshaft runs inside the bottom end of a car’s engine and converts the vertical movement of the pistons into horizontal rotational movement, which ultimately drives the wheels via the gearbox.
Crankshafts have existed for hundreds of years, having been used in water mills and sawmills. They eventually became prominent in paddle boats, turning the energy from steam-powered pistons into rotational energy for the paddle wheels.
In today’s cars, the crankshaft consists of evenly spaced ‘throws’ (there are four in a four-cylinder engine, as shown below), which are attached to the bottom of the pistons by connecting rods. These ‘throws’ are offset from the axis of the crankshaft, which is what creates the rotational energy.
The crankshaft is attached to the engine by large bearings at either end. It connects to the flywheel, and through that, the clutch.
When the clutch is engaged, the rotational energy of the crankshaft is transmitted through the gearbox and on through the differential to the driveshafts, which are attached to the wheels, hence creating the car’s ability to move.
Camshaft vs crankshaft
A camshaft is positioned near the top of engine and is driven by the crankshaft via a timing chain or timing belt.
The camshaft (there can be up to four per engine, although the crankshaft diagram below shows two) features cam lobes along its length that work with the valve train (which comprises the pushrod, valve lifter, valve spring, valve and tappet or rocker arm) to facilitate the entry of air and fuel into the combustion chamber and the egress of exhaust gases following the power stroke part of the internal combustion engine cycle.
The camshaft’s rotating cam lobes open and close the engine valves. The lobes vary in size and shape from engine to engine to govern the duration a valve opens for and by how much. The more valves there are, the more air and fuel and can be taken in and the more exhaust gases can escape, which increases engine power.
The engine crankshaft is positioned near the bottom end of the engine (as shown in the diagram below) and is attached to the pistons via connecting roads – the downward movement of the pistons as a result of the combustion process causes the crankshaft to rotate.
The crankshaft is essentially the backbone of the internal combustion engine.
Parts of Crankshaft
Following are the main parts of the crankshaft with its diagram:
- Crankpin
- Main journals
- Crank web
- Counterweights
- Thrust washers
- Oil passage and oil seals
- Flywheel mounting flange
1. Crank Pin
The crankpin is a mechanical part of an engine. This allows the connecting rod to be attached very firmly to the crankshaft.
The surface of the crankpin is cylindrical in order to transmit the torque to the large end of the connecting rod. These are also known as connecting rod bearings.
2. Main Journals
Journals are attached to the engine block. These bearings hold the crankshaft and keep it rotating in the engine block. This bearing is, for example, a plain bearing or journal bearing. The main bearings vary from engine to engine, often according to the forces exerted by the engine.
3. Crank Web
The Crank web is the most essential part of the crankshaft. The Crank web connects the crankshaft to the main bearing journals.
4. Counterweights
The counterweights are a type of weight that applies an opposing force that gives balance and stability to the crankshaft. These are mounted on the crank web.
The reason for adding counterweights to the crankshaft is so that they can eliminate the reaction caused by the rotation. And it is very helpful to get the higher RPM and keep the engine running 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
The crankshaft oil passage passes oil from the main bearing journals to the large end journals. Usually, the hole is drilled in the crank web. When the crankpin is in an up position and combustion forces push the connecting rod down, oil can penetrate between the journal and the bearing.
The crankshaft extends slightly beyond the crankcase at both ends. This will cause oil to leak from these ends. Oil seals are provided to keep oil out of these openings. There are two main oil seals connected at the front end and the rear end.
- Front-end oil seals: They are very similar to the rear-end oil seals. However, their failure is less destructive and more accessible. The front oil seal is installed behind the pulley and timing gear.
- Rear end oil seals: They are placed in the main journals and flywheels. This is pushed into a hole between the engine block and the oil pan. The oil seal has a molded 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.
Types of Crankshafts
There are different types of crankshafts depending on the mode of formation. The different types of crankshaft include a fully built shaft, billet, cast, forged, single solid piece, welded piece, and semi-built shaft.
Let us examine these types of crankshafts in more detail.
Fully Built Shaft
This type of crankshaft comes in individual parts. In other words, the crankpin, main journals, and crank web are apart. For this type, the individual crankshaft parts require assembling in a fully built shaft. The assembling involves machining and boring the journals and crankpin into the crank web.
After fabrication, shrink fitting of components ensures the crankshaft stays together. This type of crankshaft is often slightly smaller in diameter.
Welded Shaft
These are crankshafts made by assembling the main journals on both sides of the crankpin and web. After assembly, the next thing is to use the submerged arc welding process to weld these components together. Crank webs used in this type of crankshaft are often thinner, and the journals are usually stress-free.
Forged Crankshaft
Often used in some 16-valve engines and high-stress ones, forged crankshafts are one of the strongest shafts. Made using a set of dies with a shape and size approximately that of the crank, this type of crankshaft is durable. Besides, it has better dimensional stability due to its ability to react to heat treatment.
Solid Single Piece Crankshaft
Made through either forging or casting, solid single-piece shafts have been used in both high-speed and low-speed engines. The crankshaft design makes it able to endure load incurred during cyclic tension and firing.
However, it is important to note that this type of crankshaft undergoes stress due to the torsional and axial vibration of the main bearings and misalignment.
Billet Crankshaft
Commonly made using steel, billet crankshafts are one of the best. Aside from steel, this crankshaft also contains other elements like molybdenum, aluminum, chromium, and nickel. Billet crankshafts have the shortest machining time and only require a minimum balance.
Cast Crankshaft
Made with iron fabricated through the casting process, this type of crankshaft is present in various petrol and diesel engines.
Cast crankshafts are quite popular among machinists and manufacturers because they are pocket friendly and machinable. Furthermore, their random and uniform metal grain structure allows them to withstand loads from every direction.
Semi-Built shaft
The first step in making this crankshaft is forging and shrink-fitting the crank webs to the general bearings. The next step involves providing the crankpins with a smooth surface finish through the machining process.
Semi-built crankshafts are lighter in weight when compared to fully-built crankshafts. They can withstand bending and high shear stresses, which gives them application in the engine of some high-speed vehicles.
Construction of Crankshaft
The following materials were used to build the crankshaft:
- Cast Iron
- Carbon Steel
- Vanadium Micro-Alloyed Steel
- Forged Steel
The cranks can be assembled from different parts or made in one piece (monolithic).
The monolithic version is the most popular crank worldwide. However, some large and small internal combustion engines have assembled crankshafts.
These shafts can also be cast from malleable cast iron, modular or ductile steel. Welded assemblies are cast in steel. This inexpensive method is suitable for inexpensive production engines with acceptable loads. The forging process has excellent strength. Hence, forging is known as the preferred method for building crankshafts.
Signs Of A Bad Crankshaft
Because the crankshaft is such an integral part of your engine, a bad crankshaft will usually be noticed immediately. Some of the underlying causes of a bad crankshaft are:
- A forging or casting flaw
- Insufficient lubrication
- Bearing wear
- Loose-bearing main caps
- Distorted bearing bores
- Improper bearing clearance
- Bearing failure
The results of these conditions can be a cracked or even broken crankshaft. You may hear a knocking noise from the engine, or notice that the engine’s oil pressure is low (from a gauge or warning light). In severe cases, the engine will simply stop running.
If you notice any of these signs, pull your car over immediately, and shut off the engine. Do not try to drive it any farther. Arrange to have your car towed to your mechanic for diagnosis and repairs.
How Do You Fix A Bad Crankshaft?
Unless you have extensive experience in repairing and rebuilding engines, this is definitely a job for your mechanic. The first step for the mechanic, once your car arrives in the repair shop, is to disassemble your engine and see exactly what the problem is. Depending on the severity of the damage, repairs may be possible and cost-effective, or may not be worth the expense.
Here are a few possible scenarios:
The crankshaft has been damaged, but nothing else inside the engine is affected: The crankshaft may be repairable, or it may need a replacement. Remanufactured crankshafts are available, which are a cost-saving solution, compared to buying a new crankshaft.
If your car is very old with high mileage but otherwise runs fine, you might be able to find a cheap used crankshaft in a junkyard, to save money and keep your car running a bit longer. The crankshaft bearings should also be thoroughly checked, to confirm that they are still in good condition. Bad bearings will quickly destroy your new crankshaft.
The crankshaft and its bearings have been damaged: This type of damage is more extensive, because the bearings must also be repaired. The forces that caused the crankshaft damage have also damaged the bearings, causing excess wear, or making their formerly round surfaces into an irregular shape.
A new or remanufactured crankshaft and bearings may solve the problem, but if the damage is too extensive, you might consider getting a remanufactured replacement engine (if the car is fairly new). If your car is old and worn out, it may be time to say goodbye and send it to the junkyard.
The crankshaft, the bearings, and the top end of the engine have been damaged: This is the most catastrophic type of crankshaft-related damage. In addition to damage to the crankshaft and its bearings, the connecting rods and the pistons attached to the crankshaft have also been damaged.
And if the pistons hit the tops of the cylinders, they can cause serious damage to the intake and exhaust valves. Now you have a huge and expensive mess that involves repairs to your entire engine. A replacement engine may be your best option if the car has some value left in it.
Otherwise, unless you are covered by some type of mechanical warranty, it may be best to cut your losses and junk it — or as we say today, “recycle” it.
How Much Does Crankshaft Replacement Cost?
As mentioned, crankshaft replacement is always a major repair.
In case you need a crankshaft replacement, be prepared because the crankshaft alone can take $100-$1000 with an additional labor cost of around $300 to $900.
The crankshaft replacement is going to be expensive as discussed above and also very much time-consuming. When your mechanic starts working on your car you’d expect when it goes out, is going to take some other car parts with it.
In most cases, you will have to replace the engine along with the crankshaft. This is one of the most expensive and time-consuming problems you can have done, so be prepared for it take a few days, at least. As your mechanic begins work on the car, expect more problems to be found.
