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# Failure mode, impact and criticality analysis and fault tree analysis

Posted by: Chris Na 2021-05-26 Comments Off on Failure mode, impact and criticality analysis and fault tree analysis

Reliability analysis technology

In the reliability design of cnc machine tools, analysis is an important technical means. Reliability analysis is used to understand the defects in product design schemes, and to eliminate potential failure causes in the design stage through design improvements to achieve the purpose of design prevention. In the process of designing and manufacturing cnc machine tools, commonly used reliability analysis techniques include: failure mode, impact and criticality analysis (FMECA analysis), fault tree analysis (FTA analysis), thermal analysis, stress balance analysis, complete machine matching analysis, etc. content.

1  Failure mode, impact and hazard analysis

Failure mode, effects and criticality analysis (failure mode, effects and criticality analysis, FMECA) is to analyze all possible failure modes in the product and all possible effects on the product, and according to the severity of each failure mode and its A bottom-up analysis technique for classifying the probability of occurrence. It consists of two parts: failure mode and effect analysis (FMEA) and criticality analysis (CA).

1.1 FMECA use

The main uses of 　　FMECA are as follows:
(1) Find out all possible failure modes of the product and their effects, and conduct qualitative and quantitative analysis, and then take corresponding corrective measures, and confirm that the risk is below the acceptable level;
(2) Provide a qualitative and quantitative analysis basis for the list of failure modes and the list of single-point failure modes to determine the severity of the failure modes;
(3) As the input of maintainability (M), safety (S), testability (T), and supportability (S) design and analysis;
(4) Provide a basis for determining the list of product items for reliability test and life test;
(5) Provide qualitative and quantitative information for determining the list of key and important parts.

1.2 FMECA classification

Different FMECA analysis methods should be selected at different stages of the product life cycle. The FMECA classification method is shown in Figure 1. The application of the FMECA method at each stage of the product life cycle is shown in Table 1.

FMECA method is an effective reliability analysis method. It has been widely used in automobiles, CNC machine tools, robots, rail transit, aerospace products, electronic products and other industries, and has achieved fruitful results. Figure 4 is a partial view of the FMECA analysis report of a CNC turntable in a machining center.

1.4 Notes in FMECA analysis

1.4.1 Attach importance to FMECA planning work

Before the implementation of FMECA, a comprehensive and systematic plan should be carried out. During the implementation process, the principles of design, analysis, improvement, and “who design and analysis” should be implemented to ensure that FMECA analysis work is carried out concurrently with product design and development work. Improve the effectiveness of analytical work.

1.4.2 Strengthen the standardization work

For the same product, one FMECA table, unified initial agreement level, same severity level and definition, unified technical guidance, etc. should be used to ensure the correctness and comparability of analysis results.

1.4.3  Deeply understand and master the basic concepts in analysis

Severity is the severity of the final impact of a failure mode on the initial agreed level product; severity and criticality are two different concepts. The former is a measure of the severity of the failure mode’s impact, and the latter is the severity of the failure mode’s impact. It is a comprehensive measurement of its probability of occurrence; the fault detection method is a method of discovering faults during product operation or maintenance, rather than a method of exposing faults in development tests and reliability tests.

1.4.4 Accumulate experience and focus on information

The failure mode is the basis of FMECA. For this reason, the research, production, and user units should pay attention to collecting, analyzing, and sorting out the failure modes of products and similar products, and establish a corresponding failure database to provide support for follow-up work.

1.4.5 Pay attention to combining with other failure analysis methods

FMECA is an effective failure analysis method, but it is not a panacea. Designing FMECA is a static, single-factor analysis method, and it is not perfect in dynamic multi-factor analysis. In order to achieve a comprehensive analysis of the product, it needs to be combined with other methods.

2 Fault Tree Analysis (FTA)

fault tree analysis (fault tree analysis,
FTA) is an analysis technology that draws a fault tree by analyzing the hardware, software, environment, human factors, etc. that may cause product failures, thereby determining various possible combinations of product failure causes and/or their probability of occurrence . Since it was proposed by HA Watson and DF Haasl of Bell Laboratories in the United States in 1961, after years of development, system failures in automobiles, diesel engines, CNC machine tools, medical equipment, power transformers, hydraulic systems, etc. Played an important role in diagnosis, safety analysis and risk assessment.

Fault tree is a special inverted tree-like logic causality diagram, which uses event symbols, logic gate symbols and transition symbols to describe the causal relationship between various events in the system. The input event of the logic gate is the “cause” of the output event, and the output event of the logic gate is the “effect” of the input event. Among them, “event” is used to describe the failure state of the system, element and component, and “logic gate” connects the events and represents the logical relationship between the events. Common events and symbols in the fault tree are shown in Table 2; common logic gates and symbols are shown in Table 3.

2.1 Fault Tree Analysis (FTA) process

As a reliability analysis method, in order to ensure the effectiveness of its implementation, a set of reasonable analysis process is required as a guide. The FTA analysis process is shown in Figure 5.

The most important step after selecting the top event is the establishment of the fault tree. Only when the fault tree is correct and reasonable can the subsequent qualitative and quantitative analysis be meaningful and can truly play the role of fault tree analysis.

Reliability analysis technology

In the reliability design of CNC machine tools, analysis is an important technical means. Reliability analysis is used to understand the defects in product design schemes, and to eliminate potential failure causes in the design stage through design improvements to achieve the purpose of design prevention. In the process of designing and manufacturing CNC machine tools, commonly used reliability analysis techniques include: failure mode, impact and criticality analysis (FMECA analysis), fault tree analysis (FTA analysis), thermal analysis, stress balance analysis, complete machine matching analysis, etc. content.

1  Failure mode, impact and hazard analysis

Failure mode, effects and criticality analysis (failure mode, effects and criticality analysis, FMECA) is to analyze all possible failure modes in the product and all possible effects on the product, and according to the severity of each failure mode and its A bottom-up analysis technique for classifying the probability of occurrence. It consists of two parts: failure mode and effect analysis (FMEA) and criticality analysis (CA).

1.1 FMECA use

The main uses of 　　FMECA are as follows:
(1) Find out all possible failure modes of the product and their effects, and conduct qualitative and quantitative analysis, and then take corresponding corrective measures, and confirm that the risk is below the acceptable level;
(2) Provide a qualitative and quantitative analysis basis for the list of failure modes and the list of single-point failure modes to determine the severity of the failure modes;
(3) As the input of maintainability (M), safety (S), testability (T), and supportability (S) design and analysis;
(4) Provide a basis for determining the list of product items for reliability test and life test;
(5) Provide qualitative and quantitative information for determining the list of key and important parts.

1.2 FMECA classification

Different FMECA analysis methods should be selected at different stages of the product life cycle. The FMECA classification method is shown in Figure 1. The application of the FMECA method at each stage of the product life cycle is shown in Table 1.

FMECA method is an effective reliability analysis method. It has been widely used in automobiles, CNC machine tools, robots, rail transit, aerospace products, electronic products and other industries, and has achieved fruitful results. Figure 4 is a partial view of the FMECA analysis report of a CNC turntable in a machining center.

1.4 Notes in FMECA analysis

1.4.1 Attach importance to FMECA planning work

Before the implementation of FMECA, a comprehensive and systematic plan should be carried out. During the implementation process, the principles of design, analysis, improvement, and “who design and analysis” should be implemented to ensure that FMECA analysis work is carried out concurrently with product design and development work. Improve the effectiveness of analytical work.

1.4.2 Strengthen the standardization work

For the same product, one FMECA table, unified initial agreement level, same severity level and definition, unified technical guidance, etc. should be used to ensure the correctness and comparability of analysis results.

1.4.3  Deeply understand and master the basic concepts in analysis

Severity is the severity of the final impact of a failure mode on the initial agreed level product; severity and criticality are two different concepts. The former is a measure of the severity of the failure mode’s impact, and the latter is the severity of the failure mode’s impact. It is a comprehensive measurement of its probability of occurrence; the fault detection method is a method of discovering faults during product operation or maintenance, rather than a method of exposing faults in development tests and reliability tests.

1.4.4 Accumulate experience and focus on information

The failure mode is the basis of FMECA. For this reason, the research, production, and user units should pay attention to collecting, analyzing, and sorting out the failure modes of products and similar products, and establish a corresponding failure database to provide support for follow-up work.

1.4.5 Pay attention to combining with other failure analysis methods

FMECA is an effective failure analysis method, but it is not a panacea. Designing FMECA is a static, single-factor analysis method, and it is not perfect in dynamic multi-factor analysis. In order to achieve a comprehensive analysis of the product, it needs to be combined with other methods.

2 Fault Tree Analysis (FTA)

fault tree analysis (fault tree analysis,
FTA) is an analysis technology that draws a fault tree by analyzing the hardware, software, environment, human factors, etc. that may cause product failures, thereby determining various possible combinations of product failure causes and/or their probability of occurrence . Since it was proposed by HA Watson and DF Haasl of Bell Laboratories in the United States in 1961, after years of development, system failures in automobiles, diesel engines, CNC machine tools, medical equipment, power transformers, hydraulic systems, etc. Played an important role in diagnosis, safety analysis and risk assessment.

Fault tree is a special inverted tree-like logic causality diagram, which uses event symbols, logic gate symbols and transition symbols to describe the causal relationship between various events in the system. The input event of the logic gate is the “cause” of the output event, and the output event of the logic gate is the “effect” of the input event. Among them, “event” is used to describe the failure state of the system, element and component, and “logic gate” connects the events and represents the logical relationship between the events. Common events and symbols in the fault tree are shown in Table 2; common logic gates and symbols are shown in Table 3.

2.1 Fault Tree Analysis (FTA) process

As a reliability analysis method, in order to ensure the effectiveness of its implementation, a set of reasonable analysis process is required as a guide. The FTA analysis process is shown in Figure 5.

The most important step after selecting the top event is the establishment of the fault tree. Only when the fault tree is correct and reasonable can the subsequent qualitative and quantitative analysis be meaningful and can truly play the role of fault tree analysis.