Mechanical reliability can generally be divided into structural reliability and institutional reliability. The structural reliability mainly considers the strength of the mechanical structure and the failure caused by fatigue, wear, and fracture due to the influence of the load; the reliability of the mechanism mainly considers the failure caused by the strength problem, but considers the mechanism in motion due to kinematics. Fault caused by the problem. Mechanical reliability design can be divided into qualitative reliability design and quantitative reliability design. The so-called qualitative reliability design is based on the impact of the failure mode and the analysis of hazards, targeted application of successful design experience so that the design of the product to achieve a reliable purpose. The so-called quantitative reliability design is to fully grasp the random distribution of stress distribution and design parameters of the designed parts, and to meet the specified reliability requirements by establishing the relationship between the implicit limit state function or the explicit limit state function. The product. The mechanical reliability design method is a commonly used method and is currently the most direct and effective method for carrying out mechanical reliability design. Both the structural reliability design and the mechanism reliability design are commonly used methods. Reliability quantitative design can be designed to meet the requirements of the appropriate parts in accordance with the reliability index, but due to the lack of specific data on the intensity distribution and load distribution of the material, coupled with many factors to be considered, thereby limiting its promotion and application, It is generally used when designing critical or important parts. Mechanical reliability design Due to differences in product and composition, the reliability design methods that can be adopted include: 1. Preventing faults. Designing mechanical products generally belongs to series systems. To improve overall machine reliability, the strict selection and control of components should be the first step. Start. For example, the use of standard parts and common parts is preferred; reliable parts and components that have been verified through the use of analysis are selected; strict selection according to standards and control of purchased parts; full use of the results of fault analysis, adoption of proven experience or verification through analysis tests The program. 2. Simplified design In the case of meeting predetermined functions, the mechanical design should be simple and the number of parts should be reduced as much as possible. The simpler and more reliable is a basic principle of reliability design, and is the most effective method to reduce the failure and improve the reliability. . However, other parts cannot be used to perform extraordinary functions or work under high stress because of the reduced parts. Otherwise, simplifying the design will not achieve the goal of improving reliability. 3. Derating Design and Safety Margin Design Derating design is a design method to make the stress of a component less than its rated stress. Derating can be achieved by reducing the stress on the part or increasing the strength of the part. Engineering experience has proved that most of the mechanical parts work under the conditions of rated bearing stress, the failure rate is lower, and the reliability is higher. In order to find the best derating value, a lot of experimental research needs to be done. When the load stress of the mechanical components and the strength of the specific components that bear these stresses are distributed in an undefined range, the average strength can be increased (such as by increasing the safety factor), reducing the average stress, and reducing stress changes. (such as through the use of restrictions on the conditions to achieve) and reduce the intensity changes (such as a reasonable choice of process methods, strictly control the entire process, or through the test or test reject defective parts) and other methods to improve reliability. For critical parts involving safety, limit design methods can also be used to ensure that they will not fail under the worst extreme conditions. 4. Redundancy Design Redundancy design refers to the design of repeating structures and spare parts that are required to complete the specified functions. In case of local failure, the entire machine or system will not be designed to lose the specified functions. When a certain part of the high reliability requirements, but the current level of technology is difficult to meet, such as the use of derated design, simplified design and other reliability of design sand, can not meet the reliability requirements, or improve the reliability of parts and components of the improvement of cost ratio When the repeat configuration is still high, the redundancy technology may be a better design method, such as a dual-pump or dual-engine mechanical system, but it should be noted that the redundancy design tends to make the overall volume, weight, The cost will increase accordingly. The redundancy design improves the task reliability of the mechanical system, but the basic reliability is reduced accordingly. Therefore, care must be taken when using redundancy design. 5. Environment-resistant design Environment-resistant design is designed to consider various environmental impacts that the product may encounter throughout its life cycle, such as assembly, transportation shocks, vibration effects, storage temperature, humidity, mold, etc. , When used, weather, dust, vibration, etc. Therefore, we must carefully select design plans and adopt necessary protective measures to reduce or eliminate the harmful environmental impact. Specifically, it can be considered from three aspects: understanding the environment, controlling the environment, and adapting to the environment. Understanding the environment means that you should not only pay attention to the product's working environment and maintenance environment, but also understand the environment in which the product is installed, stored, and transported. In the design and testing process, both environmental conditions of a single environment and a combined environment must be taken into consideration; the environment where the product is exposed should not be taken into account only in the natural environment where the product is located. The control environment means that, when conditions allow, a good working environment condition should be created for the designed parts within a small range, or artificially changing environmental factors that are unfavorable to product reliability. Adapting to the environment means that when it is impossible to artificially control all environmental conditions, measures are taken in terms of design options, material selection, surface treatment, and coating protection to improve the ability of mechanical components to withstand the environment. 6. Ergonomics Design The purpose of ergonomics design is to reduce errors in the use of human and machine to take advantage of their respective characteristics in order to improve the reliability of mechanical products. Of course, in addition to man's own reasons, man-made errors also have a close relationship with man's misoperations such as manipulating the console, controlling and manipulating the environment. Therefore, ergonomics design is to ensure the reliability of the system conveyed to people. For example, the indicator system not only monitors, but also the display mode, display configuration, etc. all make it easy for people to accept without mistakes. Second, the control and control system is reliable, not only the instruments and machinery have satisfactory accuracy, but also suitable for people's use habits. Easy to identify the operation, not easy to error, and safety-related, should have anti-misoperation design; Third, the design of the operating environment as much as possible for human needs, reduce the factors that cause fatigue, interfere with the operation, such as temperature, humidity, pressure, Light, color, noise, vibration, dust, space, etc. 7. Robustness Design The most representative method for robustness design is the Taguchi method created by Dr. Taguchi of Japan, that is, the design of a so-called product should be completed by three designs of system design, parameter design, and tolerance design. It is an optimization design that takes into account the internal and external interferences that affect the reliability of the system during the design process. This method has been adopted by the US Air Force in RM2000 as an anti-mutation design and an effective method to improve reliability. 8. Probabilistic design method The probabilistic design method is based on stress-intensity interference theory. Stress-intensity interference theory treats stress and intensity as random variables subject to a certain distribution. Chapters 3 and 4 of this book will introduce this aspect. 9. Trade-off design Trade-off design refers to the comprehensive balance between reliability, maintainability, safety, functional weight, volume, cost, etc., in order to achieve the best results. 10. Simulation Method Design With the development of computer technology, the simulation method is becoming increasingly sophisticated. It can be used not only for the quantitative design of reliability of mechanical parts, but also for system-level reliability quantitative design. Of course, the mechanical reliability design method must not leave the traditional mechanical design and other optimization design methods, such as mechanical computer aided design, finite element analysis, and so on.
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