I. Definition of Failure
During the use of mechanical equipment, it is inevitable that wear, breakage, corrosion, fatigue, deformation, aging and other situations will occur, causing the equipment's performance to degrade and lose its specified functions or even production capabilities.
This phenomenon of degradation of equipment performance and loss of specific functions is “failure” or “malfunction”.
In general, “failure” and “malfunction” are synonymous. However, strictly speaking, according to GB 3187-1994, “malfunction refers to the loss of the specified function of the product, often referred to as failure for repairable products”.
II. Failure Judgment Criteria
The meaning of failure has been clarified above, however, failure cannot be determined by intuitive feeling alone and must be based on certain judgment criteria.
Firstly, it is necessary to clarify what “specified function” the product maintains, or to what extent the loss of the product function is considered a failure.
Some specified functions are very clear and will not cause divergent understandings, such as damage to the engine cylinder forcing a stop for repairs.
Sometimes it is difficult to determine the specified function, especially when the fault is formed due to the gradual decrease of the function, for example, if the engine wear exceeds a certain limit, it will aggravate the wear, cause power reduction, increase the consumption rate of fuel, and when this situation occurs, it can be considered a failure.
However, it is difficult to determine the limit of wear in use, as in the engine situation mentioned, if the load is reduced, the lubricating oil is increased, an engine with a certain amount of wear can still barely continue to be used, and may not be considered a failure. , which requires establishing standards in advance.
Secondly, when determining whether it is a failure, the consequences of the failure also need to be analyzed, mainly to see whether the failure affects the production of products and equipment and personal safety.
In addition to using any non-compliance with the permissible limits specified in the technical parameters as a judgment criterion for failure, we must also consider whether unacceptable failure consequences will occur if work continues in this state.
Therefore, when judging product failure, it not only depends on the “specified function” of the product, but it is also necessary to consider the consequences of failure.
Generally speaking, product failure refers to: under specific conditions, it cannot complete the specified functions; under specific conditions, one or several performance parameters cannot be maintained within the specified upper and lower limits; When the product is operating within the specified voltage range, it causes various cracks, leaks, wear, rust, damage and other conditions to mechanical parts or components.
Different products have different fault judgment standards and the starting point of research work is different, so the defined faults are also different and it is difficult to unify them. However, within the same user department, there must be unified standards.
In conclusion, when determining failure judgment criteria, the following principles must be followed: It cannot lose function under conditions of use; failure judgment criteria “determined according to acceptable performance”; Different products can be measured according to key product performance indicators.
III. Classification of mechanical equipment failure levels
1. Failure mode
According to GB 3187-82, failure mode refers to the “manifestation of product failure (malfunction)”.
Failure modes are obtained through human senses or measuring instruments.
When generally researching product failure, we often start from the phenomenon of product failure and then discover the cause of failure through the phenomenon, so it is necessary to clarify the failure modes of the product at various functional levels.
The failure modes of mechanical equipment and its components can be roughly divided into the following categories:
- Type of damage – Breakage, cracking, cracking, sintering breakdown, short circuit, bending, excessive deformation, pitting corrosion, melting.
- Type of degradation – Aging, deterioration, insulation deterioration, oil quality deterioration, peeling, corrosion, premature wear.
- Loose Type – Loosening, dropping, desoldering.
- Type of misadjustment – Inadequate clearance, inadequate flow, inadequate pressure, inadequate stroke, inadequate sound, inadequate lighting.
- Type of blockage and leakage – Blockage, adhesion, contamination, irregularity, oil leakage, oil infiltration, air leakage, water leakage.
- Whole machine and subsystem – Unstable performance, abnormal function, functional failure, difficult starting, insufficient fuel supply, unstable idling speed, total assembly noise, brake bypass.
2. Fault classification
In managing mechanical equipment maintenance and failure analysis, it is crucial to understand and master failure classifications. This will help clarify the physical concepts of various faults and resolve them systematically.
Failure classification methods vary according to the research objectives.
1) According to the nature of faults, they are divided into natural and human-induced faults.
Human-induced failures are caused by intentional or unintentional actions of machine users.
2) Based on the location of faults, they are categorized into global and localized faults.
Most failures occur in the weakest parts of the product, and these areas must be reinforced or structurally modified.
3) Based on the time of failures, they are classified into running-in period, normal use period and wear period.
Throughout the product life cycle, the probability of failures occurring mainly occurs during the wear period.
4) According to the failure occurrence rate, failures are divided into sudden and progressive failures.
Sudden failures are characterized by the absence of detectable signals prior to component failure. For example, parts may develop thermal deformation cracks due to interruption of lubrication, or component fractures may occur due to improper use or overloading of the machine. Sudden failures result from several adverse factors and unexpected external influences, and their occurrence is unpredictable and is not related to the time of use.
Progressive failures, on the other hand, result from the gradual deterioration of certain machine parts, causing their performance parameters to exceed the permitted range. Most mechanical equipment failures fall into this category. The causes of these failures are closely related to product material wear, corrosion, fatigue and creep. These failures occur in the later stages of a component's effective useful life, during the wear period, and can be avoided. The probability of occurrence of such failures is related to the operating time of the machinery.
There is a connection between sudden and progressive failures. It can be said that all failures are progressive, as changes in things follow a process that goes from quantitative change to qualitative change.
5) Faults are categorized into unrelated and related faults based on their correlation.
Unrelated failures refer to those that are not caused by the failure of other parts of the machine. On the other hand, related failures are those caused by the failure of other components.
For example, the sticking of a crankshaft bearing in an engine due to a failure in the oil supply is a related failure. However, if a failure in the engine's valve timing mechanism is not related to a failure in the transmission components, it will be classified as an unrelated failure.
6) Based on external characteristics, faults are divided into visible and hidden faults.
Visible faults are those observable to the naked eye, such as oil or water leaks. On the other hand, hidden faults are those that are not easily visible, such as a broken engine valve.
7) Failure severity is divided into complete and partial failures.
The severity of a failure is measured by the possibility of continued use of the product. A total failure implies that the product's performance has exceeded a certain threshold, causing the total loss of its designated function. A partial failure indicates that the product's performance has exceeded a certain threshold but has not completely lost its specified function.
8) Failures are categorized into those caused by design, production process and use.
Reasons for these failures include design or calculation errors that lead to an unreasonable product structure, inadequate strength calculations or testing methods, substandard material quality, inadequate machining methods, inadequate precision of machining equipment, assembly that does not meet technical requirements, non-compliance with operating procedures. during use or not carrying out maintenance, transportation or storage in accordance with technical requirements.
9) Based on the consequences, failures can be classified into fatal, serious, general and minor failures.
The severity of the consequences of a failure mainly refers to its impact on the assembly, the system, the machine and personal safety. Fatal failures put equipment and personal safety at risk, cause the disposal of important parts, result in significant economic losses or cause serious damage to the environment.
Serious faults can cause serious damage to key components, affect production safety, and cannot be eliminated in a short time even with replaceable parts.
General failures cause a decrease in equipment performance, but do not cause serious damage to the main components and can be eliminated by replacing consumable parts in a short period of time.
Minor failures generally do not cause a drop in equipment performance, do not require replacement of parts, and can be easily eliminated.
10) Based on the consequences, failures can also be classified into functional and parameter failures.
Functional failures are those that prevent the product from performing its function, such as a reducer that does not rotate and transmit power, an engine that does not start or an oil pump that does not supply oil.
Parameter failures are those that cause the parameters or characteristics of the product to exceed the permitted limit, such as a machine impairing its machining precision or not reaching its maximum speed.
3. Classification of failure levels
When performing qualitative or quantitative failure analysis, it is essential to pre-define failure levels. This is the only way to evaluate the impact and consequences of each failure mode on the system.
In fact, classifying failure levels is essentially applying the principle of the effect of consequences of failure on the system. Fatal failures are typically classified as Level I failures, major failures as Level II, general failures as Level III, and minor failures as Level IV.
The factors considered in classifying failure levels are the following:
- The extent of injury or death to workers or the public caused by a component failure.
- Damage to the product itself caused by the failure of a component.
- The inability of equipment to perform its primary function or perform tasks following a component failure, i.e., the extent of the impact on the completion of the specified function.
- The cost, labor and downtime required to restore its function after a component failure, i.e. the difficulty and duration of the repair.
- The economic loss caused by the loss of equipment functioning after a component failure, that is, the impact on the system.
In summary, the classification of failure levels must take into account factors such as performance, cost, cycle, safety, etc. These include the wide-ranging impact of component failure on personal safety, task completion, economic losses and other aspects.
