Study of the quality of plain bearings, used in mining machinery

. Plain bearings are an important part of many large and critical units and assemblies used in the mining industry. They are widely used in power equipment, powerful pumps, compressors, electric motors and internal combustion engines (ICE). Plain bearings usually include an anti-friction bushing, part of the shaft surface (bearing journal) and an oil layer between them. These are complex and critical parts in which dangerous defects may occur and which directly affect the durability, accuracy and reliability of the entire assembly. To ensure high reliability of equipment with plain bearings used in complex equipment, it is necessary to ensure their quality control, as well as a sufficient level of monitoring the technical condition and diagnosing emerging defects. This is one of the main operational tasks that the personnel of the maintenance and diagnostic services of enterprises have to constantly solve. This paper presents a comparative analysis of the quality of plain bearings for internal combustion engines of various manufacturers. The performance properties of bearings are given depending on the composition. The results of chemical analysis of the base, cover and intermediate layers of ICE liners are presented. Comparative characteristics of plain bearings are given in terms of the chemical composition of the layers and their thickness for various manufacturers: Cummins POL (Poland), Cummins CCEC (China), WETZ (Russia). Recommendations are formulated to improve the performance of plain bearings.


Introduction
Plain bearings are critical parts of internal combustion engines, their failure or malfunction can lead to engine failure and subsequent expensive repairs. Structurally, plain bearings (inserts) are high-tech products made of composite materials with a complex structure and precise dimensions.
Internal combustion engines are characterized by cyclic loading of bearings due to variable pressure in the cylinders, as well as inertial forces caused by moving parts.
Long-term and reliable operation of a plain bearing is achieved by providing a number of indicators, including: fatigue strength, wear resistance, cavitation resistance, run-in resistance, seizing resistance, absorption capacity.
The rotating components of internal combustion engines are equipped with plain bearings that perform different functions [1]: • The main bearings support the crankshaft and make it rotate. They are installed in the cylinder block, and each liner consists of an upper and lower half ring. On the inner surface of the upper half ring, as a rule, there is a groove and a hole to ensure the supply of lubricant; • connecting rod bearings provide rotation of the neck of the connecting rod, which rotates the crankshaft. Connecting rod bearings are installed in the lower head of the connecting rod; • Thrust rings prevent axial movement of the shaft. In many cases thrust rings are part of one of the main bearings. Such combined bearings are referred to as collar bearings; • bushings of the upper head of the connecting rod provide rotation of the piston pin connecting the piston to the connecting rod; • Camshaft bushings support and ensure rotation of the camshaft. They are installed at the top of the cylinder head.
Plain bearings are lubricated with engine oil, which is constantly supplied in sufficient quantities to their working surfaces and provides a hydrodynamic friction mode. Fluid (hydrodynamic) friction is characterized by the presence of an oil film under pressure and separating the surfaces of the bearing and shaft.
Under certain conditions, the hydrodynamic regime of friction can change to mixed. Mixed mode friction can lead to scuffing, increased bearing wear, and even seizing on the shaft. Cyclic loads on a bearing can lead to failure due to material fatigue. All of these shortcomings are compensated by the design of the inserts.
There is no direct contact between the surfaces rubbing in the hydrodynamic mode. This is provided by the force of hydrodynamic pressure arising in the oil, which is forced through the converging gap (oil wedge) formed by the surfaces of the bearing and shaft.
The oil film contributes to the distribution of the force applied to the bearing, thus preventing the local load concentration [1]. However, under certain conditions, the hydrodynamic friction mode is replaced by a mixed one. Reasons for switching to this mode the following [2]: • insufficient oil flow; • high loads; • low oil viscosity; • oil overheating, further reducing its viscosity; • high roughness of bearing and shaft surfaces; • oil contamination; • deformation and geometrical defects of the bearing, its seat or shaft. Mixed friction mode can lead to scoring, increased bearing wear, and even seizing with the shaft [3]. Cyclic loads on a bearing can lead to failure due to material fatigue.
The operating conditions of plain bearings are classified as complex stressed. They operate in a mixed friction mode, when during operation there is direct contact, alternating with hydrodynamic friction [4].
Based on the operating conditions, the following requirements are imposed on materials for plain bearings [2]: • fatigue strength (maximum load) -the maximum cyclic load that the bearing can withstand for an unlimited number of cycles. Exceeding this load leads to the formation of fatigue cracks in the material; • adhesion resistance (compatibility) -the ability of the bearing material to resist welding to the shaft material during direct physical contact between them; • wear resistance -the ability of the bearing material to maintain its dimensions, despite the presence of abrasive particles in the oil and also in conditions of metal contact with the shaft; • runnability -the ability of the bearing material to compensate for small geometrical defects of the shaft and seat due to slight local wear or plastic deformation; • absorption capacity -the ability of the bearing material to capture small foreign particles circulating with the oil; • corrosion resistance -the ability of the bearing material to resist the chemical attack of oxidized or contaminated oils; • cavitation resistance -the ability of a bearing material to withstand shock loads produced by collapsing cavitation bubbles (bubbles are formed as a result of a sharp drop in pressure in the flowing oil).
Long-term and reliable operation of the plain bearing is ensured, for example, by combining high wear resistance and running-in during operation. This is achieved (among other things) due to the complex design of the plain bearing (liner), consisting of several layers that perform different functions.
Currently, the most widely used bimetallic and trimetallic plain bearings (see Figure 1).

Fig. 1. Typical designs of plain bearings
Bimetallic bearings. These bearings have a steel backing to provide rigidity and tightness under severe conditions of high temperature and cyclic loading. The second layer of the material consists of an antifriction alloy, which is able to run in and adapt to relatively large geometric defects, has good absorption capacity (see Figure 1, a) [5].
The main indicators of bearings are the thickness and chemical composition of the antifriction coating. The thickness of the coating is large enough -it is about 0.3 mm.
Typically, the working layer consists of an aluminum alloy containing 6-20% tin, which provides anti-friction properties, as well as 2-4% silicon in the form of small inclusions, which strengthens the alloy and has the ability to polish the shaft surface. Aluminum alloy can be additionally hardened with small additions of copper, nickel, manganese, vanadium and other elements.
The main area of application for bimetallic bearings is low-speed or light-loaded mechanisms such as piston pumps.
Trimetal bearings have a good combination of all the required characteristics for a bearing material. The best of these are tri-metal bearings with an intermediate layer of lead bronze. Such a bearing design includes a steel base, an intermediate layer of lead bronze and an anti-friction coating of babbit, separated from the intermediate layer by a nickel barrier (1-2 microns) (see Figure 1, b).
The intermediate layer serves as a substrate for the anti-friction coating, is made of lead bronze and must have anti-friction properties necessary to prevent scuffing in places of local wear of the anti-friction coating.
Anti-friction coating provides: low coefficient of friction, scuff resistance, run-in and the ability to absorb solids in the oil. As a rule, they are made from a lead alloy alloyed with tin and copper. The tin protects the lead alloy from corrosion in oxidized oil. Copper increases the strength and wear resistance of the coating.
The balance of properties, composition and thicknesses of the layers of the trimetallic insert guarantees a high level of its operational properties.
Depending on the operating conditions, the appropriate design of plain bearings is selected. For plain bearings used in heavily loaded conditions, trimetallic structures are mainly used [6,7,8]. It should be noted that the anti-friction coating of these bearings can be either from a lead alloy with a high tin content (for products operating under moderate loads) or from a lead alloy with a reduced tin content (for products operating in particularly difficult conditions).
Thus, the main field of application of trimetallic bearings are high-speed and heavily loaded mechanisms, such as internal combustion engines.
Comparative properties of the most popular bearing materials depending on the composition are presented in Table 1 [9].
As can be seen from Table 1 properties that characterize strength and softness are combined in different proportions for different materials. The soft properties of a bimetal are somewhat lower than those of a trimetal, but they are not limited by the thickness of the coating, so bimetallic bearings are able to run in to various geometric defects [7]. On the other hand, the fatigue strength (maximum load) of bimetallic bearings is lower (40-50 MPa) than that of trimetallic bearings (60-70 MPa).
Sufficient soft antifriction properties of the trimetal are limited by the thickness of the coating, which must be at least 12 µm. If a geometric defect or foreign particles exceed the thickness of the coating, its antifriction properties drop sharply [10,11].
In this work, comparative studies of trimetallic plain bearings of three manufacturing companies were carried out: Cummins POL (Poland), Cummins CCEC (China), WETZ (Russia).

Plain bearing research methodology
To control the quality of plain bearings (liners), a methodology has been developed for compliance with reference samples. It includes the following items [12]: • layer-by-layer control of the chemical composition of the investigated alloys; • carrying out metallographic studies to determine the number and depth of layers of applied coatings; • layer-by-layer control of the microhardness distribution of the applied coatings and the liner base.
Chemical analysis A Q4 TASMAN optical emission spectrometer was used to carry out layer-by-layer chemical analysis of metal coatings and the basis of liners.
Spectrometers of this type are the most common analytical instruments. They are intended for analysis (most often elemental analysis) of the composition of various substances in various aggregate states.
Metallographic studies They included the preparation of samples and directly the study of the microstructure. Preparation of samples for metallographic studies is one of the important stages in the conduct of metallographic studies of the structure and mechanical properties of metals and is of great importance in metallography. Metallographic sample preparation is the process of obtaining quality samples for research. The quality of sample preparation directly affects the quality of research results [9,10].
To conduct metallographic studies to determine the number and depth of layers of applied coatings on bearings, it is necessary to provide a number of conditions [14,15]. Among them: • carry out cutting of specimens with minimal residual deformations; • ensure the safety of surface layers during the preparation of thin sections; • form a high quality of the investigated surface of the samples. Studies of the structure of the liner in order to assess the number and thickness of the deposited layers of metal coatings on the base were carried out on a special inverted optical metallographic microscope EPIQUANT, which provides a magnification of up to 1500×. Both the depth of the deposited coatings and the size of the phases were measured using quantitative metallography methods [12].
Distribution of microhardness Microhardness was measured by indentation of a diamond pyramid according to the Vickers method using a PMT-3 model microhardness tester [14].
The microhardness test made it possible to make measurements in all the main layers of the plain bearing. To measure the microhardness of the base and the intermediate layer, a load on the indenter of 100 g was used, and to determine the microhardness of the thin upper layer, 50 g [16].

Research results
This paper presents the results of a study of trimetallic liners of various sizes and manufacturers: Cummins POL (Poland), Cummins CCEC (China), WETZ (Russia). Their list is given in Table 2.
The design of all bearings is of the same type [12,17]  5. Steel base with a thickness of (2-4) × 103 microns; 6. Protective tin layer 1-2 µm thick. The results of studies of the base, intermediate layer and cover layer of plain bearings are summarized in Table 3, Table 4 and Table 5 respectively. These tables present the results of studies of the chemical composition, microhardness (HV) and layer thickness (h). Based on the presented results, a comparative analysis of the quality of bearings from various manufacturers was carried out.    Studies have shown that various manufacturers use low-carbon, extra-high-quality steels for the manufacture of plain bearing bases. As shown by microanalysis, the method of manufacturing blanks is cold-rolled steel. The microstructure is a fine-grained ferrite with a microhardness of 160-210 HV (see Table 3) [17]. The increased microhardness of the steel base from low-carbon steel indicates that all the manufacturers presented used the cold rolling method (cladding) to manufacture the steel-bronze strip. This method provides the highest quality adhesion between the steel base and the bronze strip, which means high performance for heavily loaded plain bearings.
The conducted studies made it possible to establish some differences in the chemical composition, depth, and microhardness of the working layers (see Table 4, Table 5).
The intermediate layer in all the studied samples is cast lead-tin bronze with a reduced tin content. According to the chemical composition, bronze can be identified by GOST 613-79 as a grade close to the BrO5S25 alloy [18]. Thus, the conducted studies testify to the close chemical composition of the intermediate layer of the internal combustion engine liners of various manufacturing companies. The homogeneity of the alloys is confirmed by metallographic studies and similar values of microhardness (two-phase microstructure) in the range of 100-160 HV (Figure 2, Figure 3; Table 4) [12,17]. In the production of the cover layer, babbits based on lead and tin (samples No. 1, 2) or based on lead (samples No. 3, 4, 5) are used [19]. In all the studied samples, the depth of the cover layer is in the range of admissible values of 12-25 µm. The chemical composition of babbits does not significantly affect the microhardness (see Figure 2; Table 5).
The exact chemical composition of the cover layer could not be determined due to its shallow depth. The results of chemical analysis were influenced by the presence of a thin nickel layer between the coating layer and the bronze insert, which is based on copper. However, from the results of chemical analysis it is clear that the Russian manufacturer (WETZ) uses lead babbits for the top layer, which have proven themselves well under conditions of increased loads; Polish manufacturer (Cummins POL) -tin-lead babbits; Chinese manufacturer (Cummins CCEC) -both lead and tin-lead babbits. The choice of composition of babbitt is determined by the operating conditions.
It should be borne in mind that the service life of internal combustion engine liners is determined not only by their quality, but also to a large extent by the conditions of their operation. In this regard, it is advisable to formulate the conditions for trouble-free operation of plain bearings.

Recommendations for improving the performance of plain bearings
To ensure high performance of plain bearings used in mining machinery, a number of conditions must be met during operation related to the quality of their parts in the state of delivery and during operation. Then internal combustion engines will be able to work for a longer time (up to 1.5 million kilometers). However, to ensure such a lifetime, the following conditions must be met: -correct installation of bearings; -high quality processing of the crankshaft with the contours of the necks; -ensuring the required performance of routine maintenance; -compliance with the recommended oil change intervals and the use of oils of the required SAE viscosity grades and the API quality standard; -preventing coolant and fuel from getting into the engine oil; -control of operating conditions of the engine to eliminate overloads, high speeds and overheating of the engine.
The long period of trouble-free operation of motor bearings depends on the implementation of a quality standard, which should regulate verification tests.

Conclusions
The conducted metallographic studies made it possible to establish that all the samples under study (Figure 2 and Figure 3) include three main layers: a steel base, an intermediate layer of a copper alloy, which is a cast lead-tin bronze with a reduced tin content, and a cover a layer of babbits based on lead and tin or based on lead. In the design of the investigated bearings, the presence of protective tin layers of small depth, a few micrometers, and the presence of a nickel gasket (barrier) between the cover layer and the bearing shell were also found. All bearings under study have layer thicknesses at the level of recommended values. [20,21].
1. A patent-literature analysis was carried out on typical designs of plain bearing shells, on chemical composition, on the production technology of crankshaft bearings for internal combustion engines.
2. Typical designs of crankshaft liners for internal combustion engines of various manufacturers were studied: materials for plain bearings, requirements for these materials, as well as the properties of bearing materials.

3.
A technique for quality control of internal combustion engine liners has been developed. It includes: chemical analysis of alloys, metallographic studies, microhardness measurement, layer depth measurement. 4. As a result of approbation of the developed method for quality control of ICE crankshaft bearings, it was revealed that all the investigated bearings of ICE crankshafts are trimetallic in structure. 5. A comparative analysis of ICE liners from various manufacturers (Cummins POL, Cummins CCEC, WETZ) showed that they differ slightly from each other in chemical composition, depth and microhardness of the working layers: 5.1. Cummins POL: tin lead babbitt: nickel interlayer -tin lead bronze -low carbon steel base. 5.2. Cummins CCEC: tin lead babbitt (or lead babbitt) -nickel interlayer -tin lead bronze -mild steel steel base. 5.3.WETZ: lead babbitt: nickel interlayer -tin lead bronze -mild steel steel base. 6. Based on a comparative analysis of the quality of internal combustion engine liners from different manufacturers, it was concluded that all of them are suitable for operation.
7. The choice of a plain bearing for large and critical components and assemblies used in the mining industry depends on its operating conditions and perceived loads.