However, there are bearings there. And not just any, but very specific ones. They are durable, but not eternal, and when they fail, you cannot do without understanding the essence of the matter. Well, for professional repairmen this is the most common matter.
How does a bearing work?
In modern automobile engines, plain bearings serve as supports for crankshafts and camshafts in almost all cases. Rolling bearings (ball, roller, needle) they are used for such purposes only in small motorcycle engines.
The required performance of plain bearings is achieved by using the so-called oil wedge effect. When the smooth shaft rotates, oil is supplied into the gap between the shaft and the hole. Since the load acting on the shaft causes its eccentric displacement, the oil is drawn into the narrowing part of the gap and forms an oil wedge, preventing the shaft from coming into contact with the walls of the hole. The greater the pressure and viscosity of the oil in the gap, the greater the load (before the surfaces touch) can withstand plain bearing.
Attention! The actual oil pressure in the wedge zone reaches 50–80 MPa (500–800 kg/cm²), and in some designs even more. This is hundreds of times higher than in the supply system (engine lubrication system). However, one should not think that the supply pressure has little effect on the operation of the bearing. The higher it is, the more intensively the oil is pumped through the bearing and the better it is cooled.
Under certain conditions, low friction operation (it is also called liquid) may be violated. This occurs when the viscosity of the oil decreases, for example due to its overheating due to insufficient supply, and the crankshaft speed decreases with increasing load.
Often, especially after engine repair, the non-optimal geometry of the unit also plays a role. With a slight deviation of the surface shape from cylindrical, with axes misalignment and other defects of the parts, a local increase in the specific load is possible (i.e. the load referred to the surface area) above the permissible limit. Then the oil film in these places becomes thin, and the surfaces of the shaft and bearing begin to touch along micro-roughnesses. A semi-liquid lubrication regime arises, characterized by an increase in friction and gradual heating of the bearing. This can then lead to so-called boundary friction with full contact of the rubbing surfaces, which will result in overheating, seizure (scuffing), sticking, melting and destruction of the bearing.
It is clear that the boundary friction regime is unacceptable in operation. However, it does occur when the oil supply is disrupted, and this most often occurs due to a lack of oil in the crankcase: either due to driver oversight, or damage to the oil pan as a result of hitting an obstacle.
The semi-liquid lubrication mode is only permissible for a short period of time, when it does not have time to affect the wear of the bearing. Example: starting a cold engine. However, there's another danger here: at very low temperatures, the oil can become too viscous, and normal flow takes too long to restore (20–30 seconds or more). In this case, even a semi-fluid lubricant can significantly affect component wear.
Improvements in the design of automobile engines are associated with a constant increase in crankshaft speed and an increase in engine power. At the same time, there is a tendency towards increasing the compactness of designs, including a reduction in the width and diameter of bearings. This means that the specific stresses in the friction unit increase. And since the load on the bearing during engine operation cyclically changes in magnitude and direction, the likelihood of so-called fatigue failure of parts increases. To ensure the performance of bearings under such conditions, special designs, materials and technologies are required.
How a plain bearing works
Typically, crankshaft bearings in modern engines are made in the form of thin-walled liners or bushings with a thickness of 1.0 to 2.5 mm (rarely more). The crankshaft main bearing shells are made thicker due to the need to accommodate a circular groove in them to supply oil to the connecting rod bearings. The general trend is a reduction in the thickness of the bearing shells, which now averages 1.8–2.0 mm for main bearings and 1.4–1.5 mm for connecting rod bearings. The thinner the liners, the better they fit to the bearing housing surface (bed), the better heat is dissipated from the bearing, the more precise the geometry, the smaller the permissible clearance and noise during operation, and the longer the service life of the unit.
To ensure that the insert takes the exact shape of the bed when installed, it must be tensioned according to the bed's diameter when in a free state (so-called straightening) and a non-cylindrical shape of variable radius. In addition, to ensure a good fit to the surface and to prevent rotation, tension is also required along the length of the liner – this is called protrusion. All these parameters depend on the thickness, width, and diameter of the bearing shells. Straightening averages 0.5–1.0 mm, and protrusion is 0.04–0.08 mm. However, this is not sufficient for reliable bearing operation. Near the parting line, the thickness of the liners is reduced by 0.010–0.015 mm to avoid scoring in these areas. Scoring can occur due to deformation of the bearing housing bore in the cylinder block under operating load when the operating clearance in the bearing is small.
The materials for the liners may vary. Their choice depends on the choice of crankshaft material and its heat treatment, the degree of engine boost and the specified service life. To a certain extent, the traditions of the automobile company also play a role here.
The liners are always made multi-layered. The core of the bearing shell is a steel strip, which ensures a strong and secure fit in the bearing housing. A layer is applied to the base using various methods (or several layers) a special antifriction material with a thickness of 0.3–0.5 mm. The main requirements for the antifriction material are low friction on the shaft, high strength, and thermal conductivity (i.e. the ability to effectively dissipate heat from the shaft surface to the bearing housing). The first requirement is best met by soft metals, such as alloys with a high tin and lead content (in particular, the widely known babbitts).
In the past, babbitts were widely used in low-power, low-speed engines. As loads increased, the strength of such liners with a thick layer of babbitt turned out to be insufficient. The problem was solved by replacing this entire layer with a kind of "sandwich" – lead-tin bronze covered with a thin (0.03–0.05 mm) layer of the same babbitt. The liner has become multi-layered.
In modern engines, steel-bronze-babbitt liners are usually made in four layers (there is also a very thin layer of nickel under the babbitt) or even five-layer, when, to improve the burnishing, a very thin layer of tin is applied to the working surface on top. This is exactly what bearings look like on many foreign engines.
Along with this, steel-aluminum liners have also become widespread. The antifriction material here is an alloy of aluminum with tin, lead, silicon, zinc or cadmium, both with and without coatings. The most commonly used alloy in world practice is aluminum with 20% tin without coating. It withstands well the high loads and rotation speeds of modern engines, including
diesel engines, and at the same time has a satisfactory "softness." However, steel-aluminum bearings are stiffer than babbitt ones (or with a babbitt coating), therefore more prone to scuffing under conditions of insufficient lubrication.
Auxiliary and camshaft engines generally rotate at a lower speed than crankshafts and experience much lower loads, so their operating conditions are easier. The liners and bushings of these shafts are usually made from materials similar to those described. In addition, babbitt or uncoated bronze is sometimes used here. Often these bearings do not have bushings or liners at all and are formed directly by boring holes in the cylinder head. In such designs, the head is made of an aluminum-silicon alloy, which has good anti-friction properties.
What is common to bearings in modern engines, especially when it comes to crankshaft bearings, is that the material and design of the bearings must match the material and operating conditions of the shaft (rotation speed, loads, lubrication conditions, etc.). Therefore, arbitrary replacement of parts, when, for example, during repairs, liners from another engine are installed, cannot be recommended. Otherwise, the durability of the repaired unit may be very short. to decide on such a step, you need to have the relevant information.
Plain bearing liners are very precise (precision) to ensure small but well-defined (on average 0.03–0.06 mm) bearing operating clearances, the bearing shell thickness is maintained during manufacturing with an accuracy of approximately 5–8 µm and the shell length is maintained with an accuracy of 10–20 µm. Failure to comply with these requirements may result in changes in the bearing operating clearance or the tightness of the shell fit in the housing, which is unacceptable, as it may reduce the reliability and service life of the entire engine.
Who produces plain bearings?
The complexity of the entire range of problems associated with the creation of high-quality automotive plain bearings has led to their production gradually being transferred to specialized firms. Abroad, many of these companies simultaneously produce other engine parts, with deliveries going both to automobile plant assembly lines and for sale in the form of spare parts. Some firms of this kind are part of well-known transnational manufacturing and trade-industrial corporations. Among the world's leading manufacturers of plain bearings for engines, the most notable are Kolbenschmidt (KS), Glyco, TRW, Sealed Power, Glacier, Clevite, and Bimet. In recent years, established companies such as Mahle and Goetze have also begun producing bearings. Among the newer companies, it's worth mentioning the specialized firm King (Israel), which began producing bearings in the early 1980s. Most of the listed manufacturers produce a wide range of bearings and supply their products as spare parts everywhere, including our market (through dealers or wholesale trading companies). Basically, of course, these are bearings for engines of foreign cars – European, Japanese and American.
You can find both standard and various repair sizes of liners on sale (differing from the standard ones, as a rule, by no more than 0.75 mm) for most common models. For less common models, as well as when it is necessary to purchase liners of a larger repair size, you usually have to place an order and wait an average of 5-10 days (these terms vary among different trading companies).
The quality of such products usually does not raise any doubts either in terms of geometry or materials. However, if there is a choice and doubts about which manufacturer to give preference to, you should keep the following in mind. Companies such as Kolbenschmidt, Glyco, Glacier are some of the main suppliers for mass production. When purchasing their products, you can even get the same bearings that are installed in engines "at birth." The only difference is the absence of the vehicle manufacturer's logo on the new parts. By the way, searching for "relatives" (or so-called original) repair size liners may prove problematic. Not all automobile companies supply repair bearings as spare parts, and the price of bearings in "original" packaging is usually significantly higher than that of bearings directly from the manufacturer.
Inserts from other, less reputable companies are usually cheaper, although it will be difficult to detect differences in manufacturing quality. Moreover, if there is a choice, then here you can try to take into account the operating conditions of the car. So, comparatively cheap liners, oddly enough, are somewhat better at resisting low-quality oils and oil filters found in our stores and markets than more expensive steel-bronze-babbitt ones. This has been demonstrated, in particular, by the use of King steel-aluminum bearings in repairs instead of the standard bronze-babbitt ones: such a replacement does not affect the reliability of engines, but allows for significant savings.