Application of the hottest reliability theory and

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The application of reliability theory and methods in mechanical design

reliability theory is a new discipline developed in recent decades. Since the 1960s, it has gradually entered the field of mechanical and electrical product design, which has made unprecedented changes in mechanical design and mechanical and electrical product design. Especially in the past one or two decades, some representative works on mechanical and electrical product design at home and abroad have added the content of reliability technology. Under the guidance of these scientific theories, the mechanical products designed and manufactured can make the readings accurate, and the trouble free working time is increased by three times, the product defects are greatly reduced, and the user and social benefits are well maintained. The reliability design method has gradually become an important part of the modern mechanical product design method, which can better reflect the essence of things than the conventional design method

1 reliability theory

1.1 in the conventional mechanical design, the safety factor method or the stress method is usually used. It requires the operator to carefully remove dust and clean the equipment, so that the working stress s acting on the dangerous section is less than or equal to its allowable stress [s], and [s] is obtained by dividing the limit stress s by the safety factor n greater than 1; It can also make the calculated safety factor of mechanical parts, which is greater than the expected allowable safety factor for exhibitors and visitors [n]. Namely:

s ≤ [s]=s/n n = s/N ≥ [n]

this conventional design method has been used for many years. As long as the safety factor is selected appropriately, it is a feasible design method. However, with the increasing complexity of products and the higher requirements for their reliability, the conventional method is not perfect enough. First of all, a large number of experiments show that real design variables such as cut-off load, ultimate stress, material hardness, size and so on are random variables, which are discrete, large or small, and should be taken according to probability. Without considering this, the design results will inevitably be divorced from reality. Secondly, the key of the conventional design method is to select the safety factor, which is too large, causing waste, too small, and affecting normal use. However, there is often no exact selection scale when selecting the safety factor, and the result is that the design is very vulnerable to local experience. Therefore, in order to make the design more realistic, probability design should be carried out on the basis of conventional methods. The main characteristics of probabilistic design are: first, the relationship between probabilistic design and conventional design is not antagonistic, but inherited and developed. Various practical mechanical models, coefficients and empirical formulas are also used in probabilistic design. However, the data used in probabilistic design is based on statistical data, and all design variables should be observed on the basis of statistical distribution. For example, when selecting materials, only those with high mean and controlled standard deviation are good materials. Second, probability design uses the average safety margin (average safety factor) and reliability as the design goals, especially the latter is more important. Because the reliability comprehensively considers the statistical distribution characteristics of each design variable and quantitatively expresses the reliability of the designed product with probability, it can better reflect the actual situation. Third, probability design attaches importance to collecting and accumulating various reliability data, especially paying attention to information feedback, which objectively forms a virtuous circle and can organically combine design and management. Finally, probability design is gradually on the road of practicality

1.2 stress strength interference model

the model based on which the probability design is mainly based is stress strength. At this time, it is necessary to select the connected COM port to directly carry out the online interference model. In conventional design, the strength γ And stress s are regarded as constants. However, the inherent strength of the part itself is affected by many accidental factors. For example, the material and metallography of parts are uneven, the surface finish of parts is discrete, and the size processing of parts has random errors. Therefore, in practice, strength is a random variable. Of course, the working stress is also a random variable in practice due to the influence of accidental factors such as temperature, load, humidity and vibration. In this way, the strength and working stress of mechanical parts obey a certain probability distribution in practice, and their PDF curves are usually partially overlapped or interference. As shown in Figure 1. The overlapping degree or interference area directly reflects the reliability. A probabilistic design theory proposed by using the interference model of stress and strength is the basic basis for probabilistic design

Figure 1 stress strength interference model

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