However, there are bearings. And not any, but quite certain. They are durable, but not eternal, and when they fail, it is impossible to do without understanding the essence of the matter. Well, for professional repairmen, this is the most common matter.
How a bearing works
In modern automobile engines, plain bearings serve as supports for crankshafts and camshafts in almost all cases. Rolling bearings (ball, roller, needle) used for such purposes only in small motorcycle engines.
The necessary performance of plain bearings is achieved by using the effect of the so-called oil wedge. When the smooth shaft rotates, oil is supplied to the gap between the shaft and the hole. Since the load acting on the shaft causes its eccentric displacement, the oil is, as it were, drawn into the narrowing part of the gap and forms an oil wedge that prevents 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 (until the surfaces touch) withstands the 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.
Low-friction operation under certain conditions (also called liquid) may be broken. This happens when the viscosity of the oil decreases, for example, due to overheating due to insufficient supply, and a decrease in the speed of the crankshaft with increasing load.
Often, especially after engine repair, the non-optimal geometry of the assembly also affects. With a slight deviation of the shape of the surfaces from the cylindrical, with a misalignment of the axes and other defects in the parts, a local increase in the specific load is possible (those. load per surface area) above the allowable limit. Then the oil film in these places becomes thin, and the surfaces of the shaft and bearing begin to touch along the microroughnesses. A semi-fluid lubrication regime occurs, characterized by an increase in friction and a gradual heating of the bearing. Further, this can lead to the so-called boundary friction with full contact of the rubbing surfaces, which will result in overheating, seizure (badass), seizing, melting and destruction of the bearing.
It is clear that the boundary friction regime is unacceptable in operation. Nevertheless, it occurs when there is a violation of the oil supply, and this most often occurs due to its lack in the crankcase: either due to an oversight by the driver, or if the oil pan is damaged as a result of hitting an obstacle.
The semi-fluid lubrication regime is permissible only for a short time, when it does not have time to affect bearing wear. An example is starting a cold engine. True, there is another danger here: at very low temperatures, the oil can become too viscous and its normal supply is restored for too long (20–30 s or more). In this case, semi-fluid lubrication can also significantly affect the wear of parts.
Improving the design of automobile engines is associated with a constant increase in the frequency of rotation of the crankshaft and an increase in engine power. At the same time, there is a tendency to increase the compactness of structures, including a decrease 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 changes cyclically in magnitude and direction, the probability of the so-called fatigue failure of parts increases. Special designs, materials and technologies are required to ensure bearing performance under these conditions.
How is a plain bearing
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 place a circular groove in them to supply oil to the connecting rod bearings. The general trend is a decrease in the thickness of the liners, which now averages 1.8–2.0 mm for main bearings and 1.4–1.5 mm for connecting rod bearings. The thinner the shells, the better they adhere to the surface of the bearing housing (bed), the better the heat is removed from the bearing, the geometry is more precise, the permissible clearance and noise during operation are smaller, and the resource of the assembly is longer.
In order for the liner to accurately take its shape when installed in the bed, in the free state it must have an interference fit along the diameter of the bed (so-called straightening) and a non-cylindrical shape of variable radius. In addition, for a good fit to the surface and keeping it from turning, an interference is also necessary along the length of the liner - it is called protrusion. All these parameters depend on the thickness, width and diameter of the liners, the straightening is on average 0.5–1.0 mm, and the protrusion is 0.04–0.08 mm. However, this is still not enough for reliable operation of the bearing. Near the parting line, the thickness of the liners is reduced by 0.010–0.015 mm to avoid scuffing in these places. Seizure may occur due to the deformation of the bore in the bearing housing in the cylinder block under the action of the operating load, when the operating clearance in the bearing is small.
Insert materials may vary. Their choice depends on the choice of the crankshaft material and its heat treatment, the degree of engine forcing and the given resource. To a certain extent, the traditions of the automobile company also affect here.
Liners are always made multi-layered. The basis of the liner is a steel tape, which provides strength and secure fit in the bearing housing. A layer is applied to the base in various ways (or multiple layers) special anti-friction material, the thickness of which is 0.3–0.5 mm. The main requirements for an anti-friction material are low shaft friction, high strength and thermal conductivity (those. ability to conduct heat well from the shaft surface to the bearing housing). The first requirement is best met by soft metals, such as alloys with a high content of tin and lead (in particular, the well-known babbits).
In the past, babbits were widely used on low-powered low-speed engines. With increasing loads, the strength of such liners with a thick layer of babbit turned out to be insufficient. The problem was solved by replacing this entire layer with a peculiar «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 with four layers (under the babbit there is still a very thin layer of nickel) or even five-layer, when the thinnest layer of tin is applied on top of the working surface to improve the running-in. This is what bearings look like on many foreign engines.
Along with this, steel-aluminum liners are also widely used. The antifriction material here is aluminum alloys with tin, lead, silicon, zinc or cadmium, both with and without coatings. Most often in world practice, an aluminum alloy with 20% tin without coating is used. It well withstands the high loads and speeds of modern engines, including
diesel engines, and at the same time has a satisfactory «softness». However, steel-aluminum liners are stiffer than babbitt liners (or babbitt coated), so are more prone to scuffing in under-lubricated conditions.
Auxiliary and camshafts of engines rotate, as a rule, at a lower frequency than crankshafts, and experience much lower loads, so their working conditions are easier. The bushings and bushings of these shafts are usually made from materials similar to those described. In addition, uncoated babbitt or 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 alloy of aluminum with silicon, which has good anti-friction properties.
The common thing for bearings of modern engines, especially when it comes to crankshaft bearings, is the conformity of the material and design of the liners with the material and operating conditions of the shaft (speed, loads, lubrication conditions, etc.). Therefore, an arbitrary replacement of parts, when, for example, liners from another engine are installed during repairs, cannot be recommended. Otherwise, the durability of the repaired unit may be very short. to take such a step, you need to have the appropriate information.
Plain bearing shells are very precise (precision) details. to guarantee small but well-defined (on average 0.03–0.06 mm) working clearances in bearings, during manufacture, the thickness of the liner is maintained with an accuracy of approximately 5–8 microns, and the length is 10–20 microns. Violation of these requirements can lead to a change in the working clearance in the bearing or the tightness of the liner in the housing, which is unacceptable, as it can lead to a decrease in the reliability and service life of the entire engine as a whole.
Who manufactures plain bearings
The complexity of the whole range of problems associated with the creation of high-quality automotive plain bearings has led to the fact that their production has gradually moved to specialized firms. Abroad, many of these firms simultaneously produce other parts for engines, and deliveries go both to the conveyors of automobile plants and to sale - in the form of spare parts. Some firms of this kind are part of well-known transnational manufacturing and commercial corporations. Of the global manufacturers of plain bearings for engines, Kolbenschmidt should be noted first of all (KS), Glyco, TRW, Sealed Power, Glacier, Clevite, Bimet. In recent years, bearings have begun to be made by such companies «luminaries», like Mahle and Goetze. Among «young» it is worth mentioning the specialized company King (Israel), which began producing bearings in the early 80s of the last century. Most of the listed manufacturers produce a huge range of bearings and supply their products for spare parts everywhere, including our market (through dealers or wholesalers). Basically, of course, these are bearings for engines of foreign cars - European, Japanese and American.
On sale you can find liners in both standard and various repair sizes (different from the standard ones, as a rule, by no more than 0.75 mm) for most popular models. For less common models, as well as if you need to buy larger repair liners, you usually have to place an order and wait an average of 5-10 days (different companies have different time frames).
The quality of such products is usually not in doubt either in terms of geometry or materials. Although, if there is a choice and doubts about which manufacturer to give preference to, the following should be borne in mind. Firms such as Kolbenschmidt, Glyco, Glacier are some of the main suppliers for mass production. When buying their products, you can even get the same liners that are put on engines «at birth». The difference lies only in the absence on the new parts of the emblem of the company - the manufacturer of the car. By the way, search «relatives» (or so-called original) oversized liners can be problematic. Not all automotive companies supply repair inserts for spare parts, and the price of inserts in «original» packaging is usually markedly higher than directly from their manufacturer.
Inserts made by other, less well-known companies are usually cheaper, although it will be difficult to detect differences in workmanship. Moreover, if there is a choice, then here you can try to take into account the operating conditions of the car. So, relatively cheap liners, oddly enough, resist low-quality oils and oil filters somewhat better, «walking» in our stores and markets than the more expensive steel-bronze babbitt ones. This, in particular, was shown by the practice of using King steel-aluminum liners instead of standard bronze-babbitt liners in the repair: such a replacement does not damage the reliability of engines, but it allows significant savings.