10 methods for summarizing basic knowledge of reliability design

Date：2022-06-27 18:01:59Views：1014

In the process of product design, technical activities are taken to eliminate potential defects and weak links of products, prevent failures, and ensure that the specified inherent reliability requirements are met. Reliability design is an important part of reliability engineering and the most critical link to realize the inherent reliability requirements of products. It is realized by formulating and implementing reliability design criteria on the basis of reliability analysis.

Specify qualitative and quantitative reliability requirements

Specify qualitative and quantitative reliability requirements. With the reliability index, the reliability design can have the goal, the reliability of the developed products can be assessed, and the interests of developers and customers can be avoided due to frequent failures in the use of customers. The most commonly used reliability indexes are mean time between failures (MTBF) and service life.

Establish reliability model

The system level and subsystem level reliability models of products are established, which can be used to quantitatively allocate, estimate and evaluate the reliability of products. Reliability model includes reliability block diagram and reliability mathematical model. For one or more functional modes of a complex product, block diagrams are used to represent the faults of various components or their combinations. The block diagram is divided into series model and parallel model. The method is to predict or estimate the series model and parallel model block diagram of the designed product reliability model, quantitatively calculate the reliability and failure rate of the product by using the mathematical formula, and finally derive the reliability index.

Reliability allocation

Reliability allocation is to allocate the quantitative requirements of the overall reliability of the product to the specified product level. The overall and partial reliability quantitative requirements are coordinated through allocation. It is a decomposition process from whole to part and from top to bottom. There are many methods for reliable distribution, such as score distribution method, proportion distribution method, etc. Let's take the scoring distribution method as an example:

Score distribution method is a common distribution method. In case of lack of product reliability data, experts who are familiar with the product and have practical engineering experience can be invited to score each factor (1-10 points) according to the complexity, technical maturity, importance and environmental conditions of several factors that affect the product reliability.

Complexity: evaluated according to the number of components constituting the subsystem and the difficulty of their assembly and commissioning. 10 points for the most complicated and 1 point for the simplest.

Technical maturity: evaluated according to the technical level and maturity of the subsystem. 10 points for low technical maturity and 1 point for high technical maturity.

Importance: evaluated according to the necessity of the subsystem. 10 points for the lowest importance and 1 point for the highest importance.

Environmental conditions: evaluate according to the environmental conditions of the subsystem. 10 points for those who have experienced severe conditions and 1 point for those with the best environmental conditions.

The mean time between failures (MTBF) of reliability index is calculated quantitatively by mathematical formula, so the reliability index can be allocated to each component by score distribution method.

Reliability prediction

Reliability prediction. Reliability prediction is a quantitative estimation of system reliability at the design stage. It estimates the reliability of components and systems of the system according to similar product reliability data, system composition and structural characteristics, system working environment and other factors. The reliability prediction results can be compared with the required reliability to estimate whether the design meets the requirements. Through the reliability prediction, the units with high failure rate in each unit of the system can be found, and the weak links can be found for improvement. There are many methods for reliability prediction, such as component counting method, stress analysis method, upper and lower limit method, etc.

The component counting method is applicable to the early stage of product design and development. Its advantage is that the failure rate of the product can be quickly estimated without a detailed understanding of the application of each component and the logical relationship between them, but the predicted results are rough.

The stress analysis method is applicable to the detailed design stage of electronic products. It has a detailed list of documents, electrical stress ratio, ambient temperature and other information. This method is more accurate than the component counting method. The stress analysis method is divided into three steps. The first step is to calculate the working failure rate of various components; The second step is to calculate the working failure rate of the product; The third step is to calculate the reliability index mean time between failures (MTBF).

Note: the above failure rate and environmental coefficient can be found in the national military standard gjb299b

Reliability design criteria

Reliability design criteria. It is to summarize the existing engineering experience of similar products, make it organized, systematic and scientific, and become the principles and requirements that designers should follow in reliability design. Reliability design criteria are generally aimed at a certain product, but the common contents of reliability design criteria of various products can also be integrated into a certain type of reliability design criteria, such as helicopter reliability design criteria. Of course, these common reliability design criteria can be tailored and supplemented to become specific product specific reliability design criteria.

Reliability design criteria shall generally be formulated according to product type, importance, reliability requirements, use characteristics, reliability design experience of similar products and relevant standards and specifications.

Environment resistant design

Environment resistant design. The influence of product service environment on product reliability is very obvious. Therefore, anti vibration, anti impact, anti noise, moisture-proof, mould proof, anti-corrosion and thermal design should be carried out during product development.

Component selection and control

Component selection and control. Electronic components are the basic circuit units that complete the specified functions of the product and can not be further divided. They are the basis of the reliability of electronic products. To ensure the reliability of products, it is very important to strictly control the components used. The formulation and implementation of component outline is an effective way to control the selection and use of components.

EMC design

Electromagnetic compatibility design. EMC design is very important for electronic products. It includes electrostatic immunity, surge and lightning immunity, power fluctuation and instantaneous drop immunity, RF electromagnetic field radiation immunity, etc.

Derating and thermal design

Derating design and thermal design. The failure rate of components and parts is closely related to the stress they bear. Reducing the stress they bear can improve their reliability in use. Therefore, their working stress should be designed below the specified rated value and allowance should be made in the design. The high ambient temperature around products, especially electronic products, is an important reason for the increase of failure rate. Therefore, the principles of heat conduction, convection and heat radiation should be used in combination with necessary natural ventilation, forced ventilation, water cooling and heat pipe technologies for reasonable thermal design to reduce the ambient temperature.

finite element analysis

Finite element analysis is one of the most important and commonly used methods in reliability design and analysis. It is widely used in engineering machinery, rail transit, aerospace, bridge construction and other fields. Through finite element analysis and reliability simulation, the weak links of reliability can be determined, possible potential faults can be found as soon as possible, and they can be improved to improve the reliability design level of products.

During the design process, analyze and evaluate the mechanical strength and thermal characteristics of the product, and try to find the weak links of the bearing structure and materials and the overheated parts of the product, so as to take design improvement measures in time.

Key points of finite element work: finite element analysis shall be carried out when the product development progresses to the basic determination of design and materials; The key of finite element analysis is to correctly establish the model of the influence of product structure and materials on load or environment; Finite element analysis shall be carried out for safety and task critical mechanical structural parts and products as far as possible.