O’zbekistоn respublikasi оliy va o’rta maxsus ta`lim vazirligi

Lecture №22
Tribology in the future.
The reliability and efficiency of mechanical systems largely depend on the reliability and efficiency of their friction units. In turn, the reliability and efficiency of friction units are determined by the interaction of dynamic processes occurring in friction and mechanical subsystems. However, these issues are practically not considered in the scientific literature, there is no description of the methods and techniques of the guest methods for assessing the mutual influence of dynamic processes occurring in mobile friction systems, as well as their dynamic monitoring. In contrast to the known theoretical and experimental data published in the scientific, technical and reference literature, the article substantiates the need to take into account the relationship between the dynamic processes of the mechanical subsystem and the subsystem of friction contacts when conducting laboratory and bench tests of mechanical systems with friction units. To implement this approach, it is necessary to ensure identical: the conditions for the functioning of the tribocontact of a full-scale object and its physical model, the parameters of macro- and microroughness of the contacting surfaces, the frequencies and forms of natural vibrations, the physico-mechanical properties of the friction contact by recording the amplitude-phase-frequency characteristics and a number of indirect integral indicators reflecting the dissipative nature of the processes friction in given octave (sub-octave) frequency bands. The problems arising in the calculation of the values of the complex transmission coefficient in the evaluation of the stability of the tribosystem based on the results of the analysis of oscillations in the normal and tangential directions of frictional interaction are considered. One of the most effective ways to study nonlinear friction systems is the method of their physical and mathematical modeling. In this case, the quasilinear subsystem is described by a system of differentiated equations, according to which an equivalent model of the mechanical subsystem is constructed. Friction processes are described by criterion equations. According to the proposed criterion equations, the conditions of a physical experiment are formed, which provide correct results corresponding to natural conditions. The proposed methods, methods and principles increase the reliability of studies of nonlinear systems, are the theoretical basis for dynamic monitoring and optimization of mechanical systems with friction units. Key words: friction systems, amplitude-phase-frequency characteristics, forecasting, physical and mathematical modeling.


Almost any machine or mechanism is a friction system, i.e., it consists of quasi-linear mechanical subsystems and essentially non-linear subsystems (friction contacts). In a number of cases, the reliability and efficiency of friction units determines the technical, economic and environmental performance of the entire mechanism or machine, the reliability and safety of their operation, and, ultimately, their competitiveness. As a rule, the cause of instability, abnormal modes of operation of friction systems is the mutual influence of dynamic processes occurring in the friction contact and in the mechanical subsystem. The values of tribological parameters vary over a wide range and depend nonlinearly on dozens of external and internal factors [1–3]: the materials used for friction pairs, the roughness of their surfaces, the types of lubricants and lubricants, the rigidity of the links of the mechanical subsystem, the load-speed modes of friction pairs, etc. Solving the issues of research, optimization of friction systems, creation of express methods for non-destructive testing of them during their operation without disturbing technological processes and with the ability to control friction processes is associated with the creation of their mathematical models with the subsequent stage of searching for the optimum of the desired, or desired, parameters and their values , as well as with a number of difficulties and contradictions due to the need to take into account the elastic-dissipative nature of friction processes. It is difficult to solve tribotechnical problems using mathematical modeling methods. For example, using the method of mathematical planning of an experiment with 5 variable and 30 controlled factors in order to obtain a regression equation that connects any triboparameter with factors influencing it, it is possible to obtain about 142 thousand variants of mathematical models of a particular friction friction pair. This number must be multiplied by the number of friction pairs in the mechanical system of the machine or mechanism. In addition, the dynamic characteristics of specific mechanical systems of machines and mechanisms should be taken into account, since a slight change in their dynamic characteristics can lead to a significant change in the output tribological parameters of friction systems. To assess the complex influence of external and internal factors on friction processes, in practice, in each specific case, one chooses their own methods of model testing and methods for optimizing friction systems [1–11]. It is difficult to carry out comprehensive studies of a specific mobile friction system during operation. This is due, for example, to the need to ensure the safety of the operation of vehicles (for example, railway or road transport), their significant dimensions, speeds, cost of work, etc. Effective methods for studying and optimizing such mechanical systems are methods of full-scale experiment based on theoretical foundations. physical and mathematical modeling [2, 3, 5, 8]. The dynamic processes occurring in frictional contact are more complex and ambiguous compared to aerodynamic and hydrodynamic processes, they depend significantly non-linearly on a much larger number of interrelated factors. Linearization of these dependences leads to results that are directly opposite to real processes, and the use of superposition methods leads to significant errors due to the nonlinear dependence of friction processes on interrelated external and internal factors. Physical modeling of friction processes was considered in the works of well-known tribologists I.V. Kragelsky, A.V. Chichinadze, Yu.A. Evdokimova, Yu.N. Drozdova, E.D. Brown and other scientists. The main task of the methods and techniques of laboratory studies of friction systems is to ensure the identity of the dynamic characteristics of mechanical subsystems, tribocharacteristics of the surface layers of contacting bodies, output triboparameters (type of wear, wear intensity, friction coefficient, stability and stability, etc.) for full-scale and model objects. However, the works of these authors did not take into account the mutual influence of dynamic processes occurring in the friction and mechanical subsystems of full-scale and model objects, the conditions for the similarity of loads and velocities of relative sliding of the contacting friction surfaces were violated, and modeling of the dynamics of the interaction of micro- and macroroughnesses was not provided. The purpose of the work is the development and improvement of the theoretical foundations of tribology for solving tribotechnical problems of land transport. Let us consider the possibility of implementing dynamic monitoring methods using the example of the “traction rolling stock – track” system [7, 10]. To improve the reliability, resource and other reliability indicators of tribosystems, modern information technologies and dynamic monitoring should be used. Monitoring includes the following stages: 1) comprehensive studies of the model system; 2) diagnostics of the current state of the model and natural systems; 3) forecasting changes in the states of systems; 4) control of model and full-scale systems using automatic control systems. The first stage of dynamic monitoring is the most time-consuming and responsible. Errors made at this stage can lead to inadequate physical models and, accordingly, to false data on diagnostics and subsequent prediction of full-scale transport systems. At this stage, the issues of creating a physical model, determining the main parameters and possible states of the system are solved. The basis of this stage is the methods of tribospectral identification of friction processes and mathematical planning of the experiment, and the instrumental base of research is modern computer systems, including analog-to-digital, digital-to-analog converters and software. At this stage, a database of tribotechnical and tribospectral identification characteristics is collected, which have the highest possible level of correlation with the controlled parameters or states of the natural system. The number of identification characteristics depends on the number of controlled factors, their physical nature, the accepted level of diagnosis and prediction probability, and a number of other conditions. The second stage of dynamic monitoring consists in non-destructive testing of nodes, subassemblies and the transport system as a whole, in collecting a database of tribological and tribospectral characteristics, in the analysis and statistical processing of similar studies. Statistical processing of studies allows checking the obtained results for ergodicity, and analysis of tribospectral characteristics reveals identification signs of the evolution of the dynamic properties of the tribosystem. Analysis of the databases collected in the course of research and identification signs of the transition of the dynamic properties of the tribosystem from one stationary state to another makes it possible to implement the third stage of dynamic monitoring - short-term or long-term prediction of changes in dynamic characteristics, i.e. observation of the evolutionary change in their trajectories from the start of the system up to the current point in time, forecasting the trajectories of the future movement of the system at a given time interval with a given degree of accuracy and efficiency. In order to prevent critical modes of operation (for example, athermal or thermal seizure) or uncontrolled movement (slipping, skidding, loss of stability, etc.) of the transport system, automated control of the corresponding drives is carried out through the use of automatic control systems, devices for rapid response to changing conditions work, on-board computers or a single center for ensuring traffic safety using GLONASS satellite technologies. To solve the tasks set, we represent a full-scale mechanical system in the form of a traditional equivalent model with constant coefficients, consisting of n concentrated masses interconnected by elastic-dissipative bonds. The number of concentrated masses Mi of an equivalent quasi-linear model of a mechanical subsystem in the study of the general dynamics of mobile transport systems (cars, locomotives, cars) - bouncing, galloping, rolling, etc., is usually limited to three to five concentrated masses and the corresponding number of degrees of freedom. In the study of resonant oscillations of drives of mechanical systems, the number of concentrated masses of the equivalent model can increase significantly. Any mechanical system consists of one or more friction systems. The friction contact model is determined by the parameters of a specific full-scale system, its operating conditions, the type of tasks being solved, as well as the values of the actual contact area. The adequacy of the developed model of friction contact is based on the principles of correct solution of contact problems, consideration of the principles of breakage and formation of friction bonds, the influence of edge effects on friction processes, determination of accurate models of the actual area of contact, etc. As a result of the mutual displacement of contacting friction surfaces, depending on ratios of internal factors (physico-mechanical characteristics of the materials of the first and second friction surfaces, normal 1, 2 and tangential 1, 2 stresses and their gradients, physical and mechanical characteristics of the "third body" introduced into the friction contact and their gradients, corresponding bulk temperatures 1, 2 and their gradients, etc.) and external factors (speed of relative sliding of friction surfaces Vc, contact pressure Q, mutual overlap coefficient kvz, influence of mechanical system parameters, etc.), sign-alternating deformations occur and their corresponding normal and tangential nye stresses, accompanied by fluctuations of the so-called active microvolumes of materials of the friction pair. Under the influence of normal and tangential contact stresses during frictional interaction of friction surfaces, deformation and destruction of their active microvolumes occur. Equilibrium roughness is formed, characterized by relatively stable geometric characteristics of the friction surfaces. A forced change in the stiffness of frictional bonds (or inertial characteristics) contributes to the implementation of transient dynamic processes, after which a new equilibrium roughness is established. The flow of thermal energy generated from the frictional interaction passes through microroughnesses in contact with each other. Oscillations of contacting microvolumes in the normal and tangential directions of force interaction determine tribospectral characteristics, thermodynamics of contact interaction. When solving various engineering problems, taking into account the real tribological characteristics of a friction contact, the concepts of bulk temperature, surface temperature, and the so-called flash point are usually used. Under certain assumptions, one can proceed to a schematic representation of thermodynamic processes in the context, taking into account fluctuations in bulk temperature and its railway transport gradient.

1- PRACTICE

MACHINES FOR THE STUDY OF WEAR AND FRICTION
Surface roughness parameters are determined by probe and optical instruments. To estimate the surface roughness ­by the parameter R _z use a double optical microscope MIS-11. In it, microroughnesses are illuminated by a light strip directed from the lighting tube at a certain angle to the surface. The line of intersection of the light band and microroughness ­is observed in an enlarged form in the visual tube. Microroughness (for values of R _g 80 to 2 µm) are measured with an ocular micrometer or photographed using a photo­ nozzles. Interchangeable lenses achieve magnification up to 517 times. The disadvantage of this method is the high labor intensity.
The most widely used methods for determining the parameters of roughness with the help of probe profilometers and profilographs. The action of the profilometer is based on the principle of feeling the surface under investigation with a diamond needle with a small radius of curvature and converting its oscillations into voltage oscillations by the inductive method. On the scale of the indicating device of the profilometer, the roughness assessment is given by R _a .
To record the microprofile of surfaces in the form of profilograms, profilographs are used. On fig. 1 shows a block diagram of a probe profiler - profilometer model 201 of the Kalibr plant ­. The electrical part of the device includes a sensor, an electronic unit 5 with an indicating device 6 and a recording device 7.
The magnetic system of the sensor consists of a core 9 with two coils 2. The sensor coil and two halves of the primary ­winding of transformer 4 form a balance bridge powered by generator 3. When the sensor moves along the surface under study, ­the diamond needle /, feeling the surface irregularities, oscillates, causing armature 10 to oscillate about axis 8. Armature oscillations change the ­air gaps between the armature and the core and thereby change the voltage at the output of transformer 4. The resulting voltage changes ­are amplified by an electronic unit, to the output of which recording or indicating devices are connected.



Figure 1. Block diagram of a probe profiler-profilometer.


profiler - profilometer "Abris-PM7" works on the basis of ECM ( Fig. 2), signals are processed on this device and the results are displayed on the screen.
It performs the following functions :
- measured surface : rectilinear, cylindrical , conical ;
- measured parameters : Ra, Rz, Rmax, Sm, t p ;
- measuring range:

Indicators : Ra, microns	0.04...12.5	0.04...12.5	0.02...65	0.02...65
Rz, Rmax, µm	0.16...50	0.16...50	0.1...250	0.1...250
Sm, µm	-	8...250	-	8...250
t p , %	-	0.1...99.9	-	0.1...99.9
measurement range , µm	-	0.04...50	-	0.02...250

Figure - 2. Profiler - profilometer "Abris-PM7" .
To study the state of friction surfaces and the structure of surface layers, various physical methods are currently used.
With the help of optical metallography , studies are carried out on optical microscopes (magnification 100-2000 times) in bright ­and dark fields in order to qualitatively determine the phase and structural composition of alloys, as well as the quantitative content of phases, size, shape and distribution of structural components. This method is also used to assess the state of friction surfaces (presence of damage, scratches, foci of corrosion, traces of fatigue wear, etc.).