Formation of mechanical failures and analysis of their characteristics

Thread Content

Changes in mechanical usage and technical condition: Machines are subjected to various forms of energy during use, and these energies mainly include: (1) the energy from the surrounding environment, including the effects of operators who carry out tasks, maintenance personnel, and environmental conditions； (2) Internal energies related to mechanical operation and the functioning of various mechanisms, such as various loads, vibrations, temperature, etc ； (3) The latent energy accumulated in mechanical materials and components during manufacturing and assembly (internal stresses in castings and assembly stresses). These energies exist primarily in the forms of mechanical energy, thermal energy, and chemical energy. When these energies reach a certain level, they can lead to harmful processes that cause changes in the initial properties and conditions of mechanical components. For example, when mating parts move with certain forces and speeds, harmful friction occurs between them; as a result of this friction, the mating parts wear out, which in turn alters their movement. It can be seen that as harmful processes progress, damage will first occur to the mechanical components, manifesting as wear, deformation, cracks, fatigue, corrosion, etc. The occurrence of such damage leads to changes in the structural parameters of these components, such as variations in dimensional tolerances, form and position tolerances, and fit gaps. Changes in the structural parameters lead to changes in the mechanical performance parameters, such as variations in the machine’s output power and speed. As the degree of damage increases further, the structural parameters of the mechanical components gradually exceed the allowable values. If the structural parameters of mechanical components exceed the allowable values, while the functional output parameters remain within the permissible limits, it is considered that there is a potential fault in the machinery; such a condition represents an abnormal state, and it should be resolved through maintenance ； If, after the structural parameters exceed their limits, the functional output parameters also go beyond the allowable values, it is considered that the machine has experienced a functional failure; the corresponding state is then a fault state, and the relevant fault should be resolved through repair. If, after long-term use, the structural parameters of the main components of a machine reach their limit values, and the system’s functional output parameters exceed those limits significantly, the economic efficiency of using that machine drops markedly; at this point the machine is in a critical technical condition and requires major repairs or replacement. The characteristics of the mechanical technical condition are shown in Table 1, while the change process of the technical condition is illustrated in Figure 1. Table 1: Characteristics of technical condition
Mechanical technical condition, Structural parameters, Functional parameters, Technical measures, Remarks
Perfect condition: Within normal range, Within normal range, Operating normally
Abnormal condition: Beyond allowable values, Within normal range, Requires maintenance
Potential failure: Possible faults present
Faulty condition: Beyond allowable values, Beyond allowable values, Requires repair
Functional failure: Performance issues occur
Extreme condition: Beyond limit values, Beyond allowable values, Requires major repair or replacement
Poor economic efficiency

2. General process of mechanical failure
As mentioned above, when a machine is exposed to various harmful influences during use, it first leads to damage to its components; this damage in turn affects the machine’s performance parameters, causing changes in them. If the pattern of change in mechanical output parameters over time is represented by X(t), and the change in the degree of damage over time is represented by U(t), then the changes between X(t) and U(t) can either be consistent or quite different. Because there exists a transitional relationship of X = f(U(t)) that reflects the structure, purpose, and working principle of mechanical products. Furthermore, the damage is related to the physical phenomena occurring within the material of the product, whereas changes in the output parameters merely reflect the macroscopic changes in the product itself. After a random working time interval, the parameters of the machine reach the maximum allowable value Xmax, resulting in a failure; Figure 2 shows the main process by which the distribution function f(t) is formed. Initially, the output parameter f(a) exhibits a degree of dispersion relative to its mathematical expectation value a0; this degree of dispersion is related to the dispersion of the mechanical initial parameters as well as the extent of changes in operating conditions. Then, during use, as time passes, the degradation of the mechanical output parameters occurs gradually, through processes such as wear; this is precisely the typical process by which part wear leads to changes in mechanical properties. Under normal circumstances, changes in the parameters may begin after a certain time interval T; this time interval T is a random quantity related to the accumulation of damage (such as fatigue) or external influences, and it also exhibits a certain degree of discreteness. The variation of parameter X is also random; it is related to the wear and damage of various mechanical components. The degradation rate Vx of the machine’s output parameters is a function of the wear rates V1, V2, …, Vk, that is, Vx = dx/dt = f(V1, V2, …, Vk). 3 Analysis of mechanical failure characteristics Mechanical failures are closely associated with mechanisms such as wear, corrosion, fatigue, and aging. Based on the general process by which mechanical failures occur, these failures have the following characteristics: (l) Latency. Mechanisms suffer various types of damage during use, and such damage causes changes in the structural parameters of their components. When the damage progresses to the point where these structural parameters exceed acceptable values, the mechanism develops potential faults. Due to the presence of a certain margin (safety factor) in mechanical design, even if the structural parameters of some components exceed the allowable values, the functional output parameters of the machinery remain within acceptable limits; in other words, the machinery does not experience any functional failures. It generally takes a considerable amount of time for a potential fault to develop into a functional fault, as measures such as lubrication, cleaning, tightening, and adjustment can eliminate or slow down the progression of damage, thereby allowing potential faults to be controlled to a certain extent or even eliminated. The potential for mechanical failures can be reduced through maintenance, which helps to minimize the occurrence of functional failures and thus **extends the service life of the machinery. (2) Progressive. Since processes such as wear, corrosion, fatigue, and aging are closely related to time, mechanical failures mostly occur over time. In use, mechanical damage occurs gradually, the structural parameters of the components change slowly, and the mechanical performance also deteriorates over time. The vast majority of failures can be tested and monitored in advance using instruments. The probability of a failure occurring is related to the amount of time the machine has been in operation; the longer the machine is used, the greater the likelihood of a failure. The gradual nature of failures allows most mechanical failures to be prevented; fault diagnosis and condition-based maintenance are based on this principle. (3) Wear and tear. Processes such as mechanical wear, corrosion, fatigue, and aging are accompanied by changes in energy and mass, and these processes are irreversible. It is manifested as a gradual increase in mechanical aging and an increasing number of failures. As usage time increases, although the elimination of local failures can restore the machine’s performance, its failure rate continues to rise, and new failures keep emerging. At the same time, the elimination of damage is also incomplete; repairs cannot restore the machine’s performance to its condition before use. The wear nature of mechanical failures determines the differences in the level and depth of mechanical maintenance, and the distribution model of mechanical failures cannot be simply described by an exponential distribution. (4) Ambiguity. During operation, mechanical devices are affected by various operational and environmental conditions, which results in randomness and variability in both their damage and output parameters. Additionally, due to factors such as materials and manufacturing processes, the various limit values and initial values of these machines also follow certain distributions. For the same machine, under different operating conditions, the output parameters exhibit different distributions over time; this leads to variability in parameter changes as well as in the criteria used for detecting failures. As a result, both the occurrence of mechanical failures and the criteria for identifying them become somewhat ambiguous. The ambiguity of mechanical failures adds to the difficulty of diagnosing and identifying them, which also requires that research on mechanical failures combine macroscopic and microscopic approaches. (5) Diversity. During mechanical operation, due to the combined effects of wear, corrosion, fatigue, and aging, the same component often exhibits multiple failure mechanisms, resulting in various failure modes such as bending deformation, wear, and fatigue fracture of the shaft. These faults not only differ in their failure mechanisms and manifestations, but also vary in their distribution patterns and the extent of their impact at different levels, resulting in diversity among the faults. The diversity of mechanical failures requires that they be studied separately based on their different mechanisms and patterns.