Crankshaft - the basis of a piston engine

Of course, everyone has heard about the crankshaft. But, probably, not every motorist clearly understands what it is and what it is for. And some do not even really know what it looks like and where it is. Meanwhile, this is the most important part, without which the normal operation of a piston internal combustion engine (ICE) is impossible. 

This part, it should be noted, is rather heavy and expensive, and its replacement is a very troublesome business. Therefore, engineers do not stop trying to create alternative lightweight internal combustion engines, in which one could do without a crankshaft. However, the existing options, for example, the Frolov engine, are still too raw, so it is too early to talk about the actual use of such a unit.

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The crankshaft is an integral part of the key assembly of the internal combustion engine - the crank mechanism (KShM). The mechanism also includes connecting rods and parts of the cylinder-piston group. 

When the air-fuel mixture is burned in the engine cylinder, a highly compressed gas is formed, which during the power stroke phase pushes the piston to bottom dead center. 

The connecting rod is connected to the piston at one end with the help of a piston pin, and at the other end to the connecting rod journal of the crankshaft. The possibility of connection with the neck is provided by a removable part of the connecting rod, called a cap. Since the connecting rod journal is offset relative to the longitudinal axis of the shaft, when the connecting rod pushes it, the shaft turns. It turns out something reminiscent of the rotation of the pedals of a bicycle. Thus, the reciprocating motion of the pistons is converted into rotation of the crankshaft. 

At one end of the crankshaft - the shank - a flywheel is mounted, against which it is pressed. Through it, the torque is transmitted to the input shaft of the gearbox and then through the transmission to the wheels. In addition, the massive flywheel, due to its inertia, ensures uniform rotation of the crankshaft in the intervals between the working strokes of the pistons. 

At the other end of the shaft - it is called the toe - they place a gear for, through which the rotation is transmitted to the camshaft, and that, in turn, controls the operation of the gas distribution mechanism. The same drive in many cases also starts the water pump. Here are usually the pulleys for the drive of auxiliary units - power steering pump (), generator, air conditioner. 

Design

Each specific crankshaft may have its own design features. Nevertheless, elements common to all can be distinguished.

Those sections that are on the main longitudinal axis of the shaft are called main journals (10). The crankshaft rests on them when installed in the engine crankcase. Plain bearings (liners) are used for mounting.

The connecting rod journals (6) are parallel to the main axis, but offset relative to it. While the rotation of the main journals occurs strictly along the main axis, the crank journals move in a circle. These are the same knees, thanks to which the part got its name. They serve to connect the connecting rods and through them they receive the reciprocating movements of the pistons. Plain bearings are also used here. The number of connecting rod journals is equal to the number of cylinders in the engine. Although in V-shaped motors, two connecting rods often rest on one main journal.

To compensate for the centrifugal forces generated by the rotation of the crankpins, they in most cases, although not always, have counterweights (4 and 9). They can be located on both sides of the neck or only on one. The presence of counterweights avoids deformation of the shaft, which can cause incorrect operation of the engine. There are frequent cases when the bending of the crankshaft even leads to its jamming.

The so-called cheeks (5) connect the main and connecting rod journals. They also act as additional counterweights. The greater the height of the cheeks, the farther from the main axis are the connecting rod journals, and therefore, the higher the torque, but the lower the maximum speed that the engine is capable of developing.

There is a flange (7) on the crankshaft shank to which the flywheel is attached.

At the opposite end there is a seat (2) for the camshaft drive gear (timing belt).

In some cases, at one end of the crankshaft there is a ready-made gear for driving auxiliary units.

The crankshaft is mounted in the engine crankcase on the seating surfaces using main bearings, which are fixed from above with covers. Thrust rings near the main journals do not allow the shaft to move along its axis. From the side of the toe and shank of the shaft in the crankcase there are oil seals. 

To supply lubricant to the main and connecting rod journals, they have special oil holes. Through these channels, the so-called liners (sliding bearings) are lubricated, which are placed on the necks.

Winemaking

For the manufacture of crankshafts, high-strength steel grades and special types of cast iron with the addition of magnesium are used. Steel shafts are usually produced by stamping (forging) followed by heat and mechanical treatment. To ensure the supply of lubricant, special oil channels are drilled. At the final stage of production, the part is dynamically balanced to compensate for centrifugal moments that occur during rotation. The shaft is balanced and thus vibrations and beats are excluded during rotation.

Cast iron products are made by high-precision casting. Cast iron shafts are cheaper, and this method of production makes it easier to create holes and internal cavities.

In some cases, the crankshaft may have a collapsible design and consist of several parts, but such parts are practically not used in the automotive industry, except for motorcycles. 

What problems can arise with the crankshaft

The crankshaft is one of the most stressed parts of a car. Loads are mainly mechanical and thermal in nature. In addition, aggressive substances, such as exhaust gases, have a negative effect. Therefore, even despite the high strength of the metal from which the crankshafts are made, they are subject to natural wear. 

Increased wear is facilitated by the abuse of high engine speeds, the use of inappropriate lubricants and, in general, neglect of the rules of technical operation.

Liners (especially main bearings), connecting rod and main journals wear out. It is possible to bend the shaft with a deviation from the axis. And since the tolerances here are very small, even a slight deformation can disrupt the normal operation of the power unit up to the crankshaft jamming. 

The problems associated with the liners (“sticking” to the neck and scuffing of the necks) make up the lion's share of all crankshaft malfunctions. Most often they occur due to a lack of oil. First of all, in such cases, you need to check the lubrication system - oil pump, filter - and change the oil.

Crankshaft vibration is usually caused by poor balance. Another possible cause may be uneven combustion of the mixture in the cylinders.

Sometimes cracks may appear, which will inevitably end in the destruction of the shaft. This can be caused by a factory defect, which is very rare, as well as the accumulated stress of the metal or imbalance. It is highly likely that the cause of cracks is the impact of mating parts. A cracked shaft cannot be repaired.

All this must be considered before replacing or repairing the crankshaft. If you do not find and eliminate the causes of problems, in the near future, everything will have to be repeated again.

Selection, replacement, repair

To get the crankshaft, you have to dismantle the motor. Then the main bearing caps and connecting rods are removed, as well as the flywheel and thrust rings. After that, the crankshaft is removed and its troubleshooting is carried out. If the part has been previously repaired and all repair dimensions have already been selected, then it will have to be replaced. If the degree of wear allows, the shaft is cleaned, paying special attention to oil holes, and then proceed to repair.

Wear and tear on the surface of the necks are eliminated by grinding to a suitable repair size. This process is far from being as simple as it might seem at first glance, and requires special equipment and appropriate qualifications of the master.

Although, after such processing, the part is subject to mandatory re-dynamic balancing, crankshaft repair is often limited to grinding only. As a result, an unbalanced shaft after such a repair can vibrate, while the seats are broken, the seals are loosened. Other problems are possible, which ultimately lead to excessive fuel consumption, a drop in power, and unstable operation of the unit in certain modes. 

It is not uncommon for a bent shaft to be straightened, but experts are reluctant to undertake this work. Straightening and balancing is a very laborious and expensive process. In addition, editing the crankshaft is associated with the risk of fracture. Therefore, in most cases, a deformed crankshaft is easier and cheaper to replace with a new one.

When replacing, you need to install exactly the same part or an acceptable analogue, otherwise new problems cannot be avoided.

Purchasing a used crankshaft on the cheap is a kind of pig in a poke, which no one knows what will turn out in the end. At best, it is somewhat worn out, at worst, it has defects that are not noticeable to the eye.

By buying a new one from a trusted seller, you can be sure of its quality. The Chinese online store can offer various other components of your car at reasonable prices.

Do not forget also that when installing a new crankshaft, be sure to replace the connecting rod and main bearings, as well as oil seals.

After replacing the crankshaft, the engine must be run in from two to two and a half thousand kilometers in a gentle mode and without sudden changes in speed.