Lecture No. 3
Under reliability is understood as the property of an object to maintain over time, within established limits, the values of parameters characterizing the ability to perform required functions in given modes and conditions of use Maintenance, repairs, storage and transportation. Reliability is a complex property, which, depending on the purpose of the object and the conditions of its use, consists of a combination of safety, maintainability and storability (Figure 1).
Figure 1 – Equipment reliability
For the vast majority of year-round technical devices, when assessing their reliability, the most important are three properties: reliability, durability and maintainability.
Reliability- the property of an object to continuously maintain an operational state for some time.
Durability- the ability to maintain operability until a limit state occurs with an established maintenance and repair system.
Maintainability- a property of a product that consists in its adaptability to maintaining and restoring its working condition through maintenance and repair.
At the same time, equipment for seasonal use (agricultural harvesting machines, some municipal vehicles, river vessels of frozen rivers, etc.), as well as machines and equipment for eliminating critical situations (fire-fighting and rescue equipment), which have a long service life in standby mode, must be assessed taking into account persistence, i.e. indicators of all four properties.
Storability- the property of a product to maintain within specified limits the values of parameters characterizing the ability of the product to perform the required functions during and after storage or transportation.
Resource(technical) - operating time of a product until it reaches the limit state specified in the technical documentation. The resource can be expressed in years, hours, kilometers, hectares, or number of inclusions. The resource is distinguished: full - for the entire service life until the end of operation; pre-repair - from the start of operation to overhaul restored product; used - from the start of operation or from the previous major overhaul of the product to the considered point in time; residual - from the moment in time under consideration until the failure of a non-repairable product or its overhaul, between-repairs.
Operating time- the duration of operation of a product or the amount of work it performs over a certain period of time. It is measured in cycles, units of time, volume, run length, etc. There are daily operating time, monthly operating time, and time to first failure.
MTBF- reliability criterion, which is a static value, the average operating time of the repaired product between failures. If operating time is measured in units of time, then MTBF refers to the average time between failures.
Finally, there is a whole range of products (for example, rubber products) that are assessed mainly by their storage and durability.
The listed reliability properties (reliability, durability, maintainability and storage) have their own quantitative indicators.
Thus, reliability is characterized by six indicators, including such important ones as probability of failure-free operation. This indicator is widely used in the national economy to assess the most various types technical means: electronic equipment, aircraft, parts, components and assemblies, Vehicle, heating elements. These indicators are calculated on the basis of state standards.
Refusal- one of the basic concepts of reliability, which consists in a malfunction of the product (one or more parameters of the product go beyond the permissible limits).
Failure rate- the conditional probability density of the occurrence of a failure of a non-repairable object is determined provided that the failure did not occur before the considered point in time.
Probability of failure-free operation- the possibility that within a given operating time, an object failure does not occur.
Durability is also characterized by six indicators representing different types of resource and service life. From a security point of view, the greatest interest is gamma percentage resource- operating time during which the object will not reach the limit state with probability g, expressed as a percentage. Thus, for metallurgical equipment objects (machines for lifting and moving liquid metals, pumps and devices for pumping harmful liquids and gases) g = 95% are assigned.
Maintainability is characterized by two indicators: probability and average time to restore a working condition.
A number of authors divide reliability into ideal, basic and operational. Ideal reliability is the maximum possible reliability, achieved by creating a perfect design of an object with absolute consideration of all manufacturing and operating conditions. Basic reliability is the reliability actually achieved during the design, manufacture and installation of an object. Operational reliability is the actual reliability of an object during its operation, determined both by the quality of the design, construction, manufacturing and installation of the object, and by the conditions of its operation, maintenance and repair.
The basic principles of reliability will be unclear without defining such an important concept as redundancy. Reservation- this is the use of additional means or capabilities in order to maintain the operational state of an object in the event of failure of one or more of its elements.
One of the most common types of redundancy is duplication - redundancy with a reserve ratio of one to one. Due to the fact that redundancy requires significant material costs, it is used only for the most critical elements, components or assemblies, the failure of which threatens the safety of people or entails severe economic consequences. Thus, passenger and cargo-passenger elevators are suspended by several ropes, airplanes are equipped with several engines, have duplicate electrical wiring, and cars use a double and even triple brake system. Strength reservation, based on the concept of a safety factor, has also become widespread. It is believed that the concept of strength is directly related not only to reliability, but also to safety. Moreover, it is believed that engineering safety calculations of structures are almost exclusively based on the use of a safety factor. The values of this coefficient depend on specific conditions. For pressure vessels it ranges from 1.5 to 3.25, and for elevator ropes it ranges from 8 to 25.
By revising production process in the interrelation of its main elements, it is necessary to use the concept of reliability in a broader sense. In this case, the reliability of the system as a whole will differ from the total reliability of its elements due to the influence of various connections.
In reliability theory, it has been proven that the reliability of a device consisting of individual elements connected (in the reliability sense) in series is equal to the product of the probabilities of failure-free operation of each element.
The connection between reliability and safety is quite obvious: the more reliable the system, the safer it is. Moreover, the probability of an accident can be interpreted as “system reliability”.
At the same time, safety and reliability are related, but not identical concepts. They complement one another. So, from the consumer’s point of view, equipment can be reliable or unreliable, and in terms of safety precautions, it can be safe or dangerous. In this case, equipment can be safe and reliable (acceptable in all respects), dangerous and unreliable (unconditionally rejected), safe and unreliable (most often rejected by the consumer), dangerous and reliable (rejected due to safety regulations, but may be acceptable to the consumer, if the degree of danger is not too great).
Safety requirements often act as restrictions on the resource and service life of equipment or devices. This occurs when the required level of safety is compromised before a limit state is reached due to physical or mental aging. Limitations due to safety requirements play a particularly important role when assessing the individual residual life, which is understood as the duration of operation from a given point in time until the limit state is reached. Any parameter characterized by the duration of operation of the object can be selected as a resource measure. For aircraft, the measure of resource is the flight time in hours, for vehicles - mileage in kilometers, for rolling mills - the mass of rolled metal in tons, etc.
The most universal unit from the point of view of general methodology and reliability theory is the unit of time. This is due to the following circumstances. Firstly, the operating time of a technical object also includes breaks, during which the total operating time does not increase, and the properties of the materials may change. Secondly, the use of economic and mathematical models to justify the assigned resource is possible only using the assigned service life (service life is defined as the calendar duration from the start of operation of the object or its renewal after a certain type of repair until the transition to the limit state and is measured in calendar time units) . Thirdly, calculating the resource in time units allows us to pose forecasting problems in the most general form.
The initial impetus for the creation of numerical methods for assessing reliability was given in connection with the development aviation industry and low level of flight safety at the initial stages. A significant number of aviation accidents with an ever-increasing intensity of air resources has necessitated the development of reliability criteria for aircraft and requirements for the level of safety. In particular, it was carried out comparative analysis one of the many aircraft in terms of successful completion of flights.
Indicative from a safety point of view is the chronology of the development of reliability theory and technology. In the 1940s, the main efforts to improve reliability were concentrated on comprehensive quality improvements, with the economic factor being predominant. To increase the durability of components and assemblies of various types of equipment, improved designs, durable materials, and advanced measuring instruments were developed. In particular, the electrical engineering department of General Motors (USA) increased the active life of locomotive drive motors from 400 thousand to 1.6 million km through the use of improved insulation and the use of improved tapered and spherical roller bearings, as well as testing at high temperatures. temperature. Progress has been made in developing repairable structures and in providing businesses with the equipment, tools and documentation to carry out preventative work and maintenance operations.
At the same time, the preparation and approval of standard schedules for periodic inspections and control cards for high-performance machine tools has become widespread.
In the 50s, great importance began to be attached to security issues, especially in such promising industries as astronautics and nuclear energy. This period marks the beginning of the use of many currently widespread concepts on the reliability of elements of technical devices, such as expected durability, compliance of the design with specified requirements, and prediction of reliability indicators.
In the 60s, the urgent need for new methods of ensuring reliability and their wider application became obvious. The focus has shifted from analyzing the behavior of individual elements of various types (mechanical, electrical or hydraulic) to the consequences caused by the failure of these elements in the corresponding system. During the early years of the spaceflight era, significant effort was expended on testing systems and individual components. To achieve a high degree of reliability, block diagram analysis has been developed as the main models. However, with the increase in the complexity of block diagrams, the need for a different approach arose, and the principle of analyzing systems using a fault tree was proposed and then became widespread. It was first used as a program to evaluate the reliability of the MINITEMAN missile launch control system.
Subsequently, the methodology for constructing a fault tree was improved and extended to a wide range of different technical systems. After catastrophic accidents at underground intercontinental ballistic missile launch facilities in the United States, the study of system safety as a separate independent activity was officially introduced into practice. The US Department of Defense has introduced a requirement for reliability analysis at all stages of development of all types of weapons. At the same time, requirements for the reliability, performance and maintainability of industrial products were developed.
In the 1970s, the most notable work was to assess the risk associated with the operation of nuclear power plants, which was carried out based on the analysis of a wide range of accidents. Its main focus was to evaluate potential consequences similar accidents for the population in search of ways to ensure safety.
IN Lately The problem of risk has acquired very serious significance and to this day attracts increasing attention from specialists in various fields of knowledge. This concept is so inherent in both safety and reliability that the terms “reliability,” “hazard,” and “risk” are often confused.
Among the technical causes of industrial accidents, causes associated with insufficient reliability of production equipment, structures, devices or their elements occupy a special place, since most often they appear suddenly and are therefore characterized by high rates of injury severity.
A large number of types of metal-intensive equipment and structures used in industry, construction and transport are a source of hazardous production factors due to the existing possibility of emergency failure of individual parts and assemblies.
The main goal of analyzing the reliability and associated safety of production equipment and devices is to reduce failures (primarily traumatic ones) and associated human casualties, economic losses and environmental disturbances.
Currently, there are quite a few methods for analyzing reliability and safety. So, the simplest and most traditional method for reliability is the method of block diagrams. In this case, the object is presented as a system of individual elements for which it is possible and appropriate to determine reliability indicators. Structural diagrams are used to calculate the probability of failures, provided that only one failure is possible in each element at a time. Such limitations have led to the emergence of other methods of analysis.
The preliminary hazard analysis method identifies hazards to the system and identifies elements to determine failure modes in consequence analysis and to construct a fault tree. It is the first and necessary step in any research.
The analysis of consequences by failure mode is focused mainly on the equipment and considers all failure modes for each element. The disadvantages are that they are time consuming and that the combination of failures and human factors is often not taken into account.
Criticality analysis identifies and categorizes elements for system improvement, but often does not consider common cause failures between systems.
Event tree analysis is useful for identifying major sequences and alternative failure outcomes, but is not suitable for parallel sequences of events or for detailed study.
Hazard and performance analysis is an expanded form of consequence analysis by failure mode that includes the causes and consequences of changes in key production variables.
Cause-effect analysis demonstrates sequential chains of events well, is quite flexible and rich, but too cumbersome and time-consuming.
The most common method, widely used in various industries, is analysis using a fault tree. This analysis is clearly focused on finding failures and, in doing so, identifies those aspects of the system that are important for the failures in question. At the same time, graphic, visual material is provided. Visibility gives the specialist the opportunity to deeply penetrate the system’s operation process and at the same time allows him to focus on individual specific failures.
The main advantage of fault tree compared to other methods is that the analysis is limited to identifying only those system elements and events that lead to that particular system failure. At the same time, building a fault tree is a certain kind of art in science, since there are no analysts who could create two identical fault trees.
To find and visually represent a causal relationship using a fault tree, it is necessary to use elementary blocks that subdivide and connect a large number of events.
Thus, the currently used methods for analyzing the reliability and safety of equipment and devices, although they have certain disadvantages, still make it possible to quite effectively determine the causes of various types of failures, even in relatively complex systems. The latter is especially relevant due to the great significance of the problem of the emergence of hazards caused by insufficient reliability of technical objects.
Need to install windows, but don't know what to choose? On the one hand, the well-known wooden ones, and on the other, the now popular plastic ones. In both cases, the environmental friendliness, safety and reliability of the design corresponds to the price and honesty of the manufacturer. And yet, when it comes time to install new windows, you can find a significant difference between these two types. 
You need to install windows - the pros and cons of wooden and plastic structures
If you need to install a wooden window, then you should not trust companies that promise to deliver the structure the day after tomorrow. This is basically impossible, because the minimum production time for a wooden structure is 30 days. The wood needs to be dried, painted or tinted, and varnished if you need to install wooden windows. But when you need to install plastic window, then the company can produce it within 24 hours. Especially if the manufacturer has its own production. 
When to install windows, then wooden structures give way to the palm for two reasons. This is a painstaking installation and a high price. To actually install wooden European structures, you will need to pay about 3-4 times than for a structure with a PVC profile. 
When need to install windows However, you should remember that even the most expensive plastic structures are made of polyvinyl chloride. And this means that when high temperatures, in extreme heat or during a fire the greatest amount of harmful substances will be released 
Service life must also be taken into account when installing windows. After all, plastic structures will last on average about 40 years. They have already proven themselves well in the difficult Russian climate. Wooden structures will last about 10 years, and then the sun, wind and moisture will do their dirty work and gradually destroy the structure. 
If you need to install a plastic window, then at least because it is easier and faster to do. When you need to install windows, you can install plastic structures yourself with minimal experience. This trick with a wooden structure will no longer work. Installing a wooden structure is quite a process that requires both experience and special tools. 
Another reason why you need to install plastic windows is easy maintenance. You will only need to wipe the profile with a cloth, adjust and lubricate the fittings, and change the seal. A wooden profile that dries out or absorbs moisture requires more attention. But on the other hand, the wood must be restored, and the plastic will have to be completely changed. 
A glass unit in a plastic structure is easier to replace. This can be done in a few days. But in a wooden structure this is much more difficult to do. In it, the glass unit is firmly glued into the sash with silicone sealant, and the bead is securely attached. Therefore, it is very difficult to remove a double-glazed window without damaging the glazing bead. This is also taken into account when installing windows. If it is a domestic design, then it will take a week or two to replace the double-glazed window. And if the manufacturer turns out to be foreign, then you will have to wait at least a month for a replacement. 
Reliability.- this is the property of a machine, its component or part to perform specified functions, maintaining its performance indicators (productivity, power, energy consumption, accuracy, etc.) within specified limits for the required period of time or required operating time (in kilometers, hectares, cubic meters, cycles or others)
Reliability terminology in engineering applies to any technical objects - products, structures and systems, as well as their subsystems, considered from the point of view of reliability at the stages of design, production, testing, operation and repair. Can be considered as subsystems Assembly units, parts, components or elements. If necessary, the concept of “object” may include information and its media, as well as human factor(for example, when considering the reliability of a machine-operator system).
At the development stage, the term “object” is applied to a randomly selected representative from the general population of objects.
Reliability is a complex property, generally consisting of reliability, durability, maintainability and storability. For example, for non-repairable objects, the main property may be failure-free operation. For objects being repaired, one of the most important properties that make up the concept of reliability may be maintainability.
Reliability- the property of an object to continuously maintain an operational state for some time or operating time.
Durability- the property of an object to maintain an operational state until a limit state occurs with an established maintenance and repair system.
Maintainability- property of an object, which consists in its adaptability to maintaining and restoring an operational state through maintenance and repair.
Storability- the property of an object to maintain, within specified limits, the values of parameters characterizing the ability of the object to perform the required functions during and after storage and (or) transportation.
An object- a technical product for a specific purpose, considered during the periods of design, production, testing and operation.
Element- the simplest component products, in reliability problems may consist of many parts.
System- a set of jointly acting elements designed to independently perform specified functions.
12 .Reliability indicators: probability of failure-free operation, mean time to failure, failure rate, failure flow parameter, time between failures. Weibull's law to characterize the distribution of failures, a typical curve of changes in the probability density of failures during the operation of objects.
Probability of failure-free operation is the probability that, within a given operating time, an object failure does not occur. In practice, this indicator is determined by a statistical assessment
where N0 is the initial number of operational objects, n(t) is the number of failed objects during time t.
Mean time to failure Mathematical expectation of the operating time of an object until the first failure.
Run-to-failure- equivalent parameter for a non-repairable device. Since the device is not repairable, this is simply the average time that the device will work before it breaks.
Operating time- duration or volume of operation of an object, measured in hours, engine hours, hectares, kilometers, switching cycles, etc.
It is measured statistically, by testing many instruments, or calculated by methods of reliability theory.
T = 1/m * Σti where ti is the operating time of the i-th object between failures; m is the number of failures.
Failure rate. Conditional probability density of the occurrence of an object failure, determined under the condition that the failure did not occur before the considered point in time . The failure rate is the ratio of the number of failed equipment samples per unit of time to the average number of samples that work properly in a given period of time, provided that the failed samples are not restored or replaced with serviceable ones.
Failure flow parameter. The ratio of the mathematical expectation of the number of failures of a restored object over a sufficiently short operating time to the value of this operating time.
If the child begins to get up and move more actively, then it’s time to limit access to some cabinets and drawers for his safety.
In principle, we were not going to choose, because... IKEA locks inspired the greatest confidence. But the presence of 2 large chests of drawers (and that’s already 11 drawers) and besides them 12 other important and dangerous doors forced us to take a closer look and evaluate other cheaper analogues. We sampled different manufacturers and almost all of them had to be replaced with IKEA ones.
About the advantages (and no disadvantages other than cost were found)
They have been serving for a year without any complaints. Sticks to any surface. The main thing is to degrease it before gluing it.
There is an adjustment for different doors in terms of the width of the lock - we installed it on a cabinet in the bathroom,
where the distance is small, and on the drawer under the crib, where the maximum length of the lock was required. Adjustable by cutting the tape. The truth is already irrevocable))
The lock is quite difficult to open. With long nails, I think it’s more difficult; with small nails, opening and closing takes seconds. The main thing is to get used to it. Well, of course, a child can’t do it at all. Unlike other locks we've tried.
The color is only white. We were more than satisfied with this, because... in the room everything is mostly bright, but where things don’t match is not scary for us - safety comes first.
Reliability – this is the property of an object to perform specified functions, maintaining over time the values of established operational indicators within specified limits, corresponding to specified modes and conditions of use, maintenance, repair, storage and transportation. This is a quality that extends over time. Therefore, the concept of reliability is close to the concept of quality, and therefore the problems of quality management are directly reflected in the concept of reliability.
Reliability is an objective property of a product; reliability can be measured. To measure reliability, the concepts of “failure”, “probability of failure-free operation”, “failure rate”, etc. were introduced. The concepts of failure and reliability are among the basic ones in reliability theory. Usually under reliability understand the ability of products to remain operational for a long time. Refusal– this is a complete or partial loss of the product’s functionality.
American authors D. Lloyd and M. Lipov in the book “Reliability” write: “Reliability affects cost, time costs, psychologically - in the form of inconvenience, and in certain cases also threatens the safety of people and the nation. Typically, losses due to unreliability are not only the cost of the unit that fails, but also the cost of the associated equipment that deteriorates or is destroyed as a result of the failure... A classic example of the psychological effect of unreliability is the Avangard satellites of sad memory. The United States, acutely experiencing the successes of Russia, which launched " Sputnik-1" tried to enter the competition using for this purpose an almost untested rocket, which had to work almost to the limit of its capabilities. The failures and the ensuing despondency and loss of prestige were very serious."
American writer, poet and scientist of the 19th century. There is a poem by Oliver Holmes called "The Priest's Masterpiece, or the Wonderful One-Horse Carriage." It talks about a priest who built a carriage, remarkable in that all its parts had exactly the same strength. This stroller lasted exactly 100 years and fell apart right along the road. All parts broke at the same time.
A product that would be destroyed in this way is the dream of any engineer and quality management specialist. But real mechanisms are destroyed randomly and at random times. Therefore, statistical methods and the probabilistic apparatus of mathematics are used to assess reliability. The probability of failure-free operation is the probability that a product failure will not occur in a given time interval or within a given operating time.
There are many numerical characteristics to assess reliability. For example, availability factor is the probability that the product will be operational at specified or random moments, – the time during which the product is operational, referred to the time of its operation.
by the consumer means the time during which a product with a manufacturer's guarantee maintains its quality parameters expected by the consumer, and therefore this time is usually called the guaranteed service life of the product.
Product service life guaranteed by the manufacturer called the durability of the product. Durability depends on the possibility of repair, after which its quality parameters can be restored, i.e. on the maintainability of the product.
Based on the actual service life, the consumer judges mainly the quality of the product he purchased, which subsequently affects his attitude towards the corresponding manufacturer and, ultimately, the image of this manufacturer in the eyes of the consumer.
The most widely used indicator in reliability studies is failure rate (λ ):
Where n– number of failed products; N- total number
products; – average test time.
The average test time is determined by the formula
where is the number of products in the test group; – duration of the test for this group.
If the number of failed products exceeds 5-10%, then adjustments are made to the calculation:
(2.3)
where is the number of failed products in this group;
– number of failures during the same test time;
Duration of tests to disable the product.
To calculate the average failure rate, it is important to select the correct time interval, since failure density usually varies over time.
EXAMPLE 2.1
When testing a certain piece of electronic equipment, λ can be determined after 1000–2000 hours. Testing is carried out in 4 groups of 250 products for 2000 hours.
The test results are as follows:
Let's calculate:
In total, 20 products failed during the tests (7 + 5 + + 4 + 4).
Parts and assemblies may fail due to manufacturing defects and other reasons.
At a constant level of failure rate per unit time, the probability distribution of failure-free operation intervals is expressed by the exponential distribution law of operational durability.
The main quality parameters for products are:
- – functional characteristics – compliance of the product with its intended purpose;
- – reliability – the number of repairable failures over the service life;
- – durability (service life) – an indicator related to reliability;
- – defect-free – the number of defects detected by the consumer.
Reliability is a concept associated primarily with technology. It can be interpreted as failure-free-
agility, ability to perform a specific task or howprobability of execution specific function or functions for a certain time and in certain conditions .
As a technical concept, "reliability" is the probability (in a mathematical sense) of satisfactorily performing a specified function. Since reliability is a probability, statistical characteristics are used to evaluate it. The results of reliability measurements should include data on sample sizes, confidence limits, sampling procedures, etc.
In technology, the concept of “satisfactory performance” is also used. The precise definition of this concept is related to the definition of its opposite - “unsatisfactory performance” or “refusal”.
The general concept of “reliability” is opposed to the concept of “reliability itself” of a sample of equipment, which is the probability of failure-free operation in accordance with specified technical conditions under specified verification tests for the required period of time. Reliability testing measures the actual reliability. It essentially represents the “operational reliability” of the equipment and is a consequence of two factors: actual reliability and operational reliability. Operational reliability, in turn, is determined by the compliance of the equipment with its use, the procedure and method of operational use and maintenance, the qualifications of personnel, the ability to repair various parts, factors environment and etc.
For each characteristic to be measured, in technical conditions a tolerance is specified, the violation of which is considered as a “failure”. The tolerance defining a failure must be optimal with the necessary allowance for wear of parts, i.e. it must be wider than the normal factory tolerance. Therefore, factory tolerances are set taking into account the fact that parts wear out over time.
Let us characterize the basic concepts related to reliability.
- 1. Serviceability – the state of the product in which it is in this moment time meets all the requirements established both in relation to the main parameters characterizing the normal performance of specified functions, and in relation to secondary parameters characterizing ease of use, appearance and so on.
- 2. Malfunction – the state of a product in which it does not currently meet at least one of the requirements characterizing the normal performance of specified functions.
- 3. Performance – the state of the product in which it currently meets all the requirements established in relation to the basic parameters characterizing the normal performance of specified functions.
- 4. Refusal – an event consisting in the complete or partial loss of a product’s functionality.
- 5. Complete refusal – a failure, until the elimination of which the use of the product for its intended purpose becomes impossible.
- 6. Partial failure – failure, until elimination of which partial use of the product remains possible.
- 7. Reliability – the property of a product to continuously maintain performance over a certain period of time.
- 8. Durability – the property of a product to maintain operability (with possible interruptions for maintenance and repair) until destruction or another limiting state. The limit state can be set based on changes in parameters, safety conditions, etc.
- 9. Maintainability – property of a product, expressed in its suitability for carrying out maintenance and repair operations, i.e. to the prevention, detection and elimination of malfunctions and failures.
- 10. Reliability (in a broad sense) – property of a product due to the reliability, durability and maintainability of the product itself and its parts and ensuring
ensuring the preservation of the performance characteristics of the product under specified conditions.
- 11. Recoverability – the ability of a product to restore the initial values of parameters as a result of eliminating failures and malfunctions, as well as restore the technical life as a result of repairs.
- 12. Storability – the property of a product to maintain serviceability and reliability under certain conditions and transportation.
For some products that are relatively simple in design, the concept of “failure” can be introduced quite clearly. For example, a light bulb either lights up or doesn’t light up.
In practice, they sometimes turn Special attention on improving the main components of the product, losing sight of the fact that the cause of unreliability and subsequent accidents may be structural components that are of an auxiliary nature.
To measure (estimate) reliability, it is necessary to test an apparatus that would describe random events or random processes. It's about about probability theory and mathematical disciplines. The main quantitative indicator of reliability is the probability of failure-free operation of a product for a given period of time.
Probability of failure-free operation is the probability that a product failure will not occur within a given time interval or within a given operating time. With the introduction of this concept, it becomes possible to measure reliability and compare the reliability of a product according to this indicator. The probability of failure-free operation of the same product is not the same at different moments of its operation.
To assess reliability, there are many characteristics, in particular: probability of failure-free operation; availability factor(the probability that the product will be operational at a given or random moment); time utilization ratio(the time during which the product is operational, referred to the time of its operation).
Time of trouble-free operation of the product consumer refers to the time during which a product with a manufacturer's guarantee maintains its quality parameters expected by the consumer, and therefore this time is usually called guaranteed product service life.
Guaranteed product service life, as a rule, less than its actual service life, which is characterized by the durability of the product.
Durability depends on the possibilities of repair, after which the quality parameters of the product are restored, i.e. depends on maintainability. Durability characterizes the actual service life of a product. Based on the actual service life, the consumer judges the quality of the purchased product, which subsequently affects his attitude towards the manufacturer and, ultimately, the image of this manufacturer in the eyes of the consumer.
At the same time, the guaranteed service life of a product is of significant importance at the time of its purchase in comparison with a similar product from competitors, and the strictness of the subsequent fulfillment of all pre-agreed conditions and guarantees when purchasing a product determines the consumer’s attitude towards the reliability of not only the supplier (seller), but also the manufacturer .
If during the guaranteed service life the value of the quality parameters does not meet the consumer’s expectations, which are guaranteed by the manufacturer, then the responsibility for this lies with the manufacturer of the product (supplier), who must carry out repairs at his own expense, and if repairs are impossible, replace defective goods for quality.
The manufacturer must guarantee the quality of the product both during its storage and during its operation.
To predict failures in the future, actual data on the frequency of failures during the period of use of the equipment for its intended purpose is necessary.
When processing information, the inverse of the failure rate is used "mean time between failures".
Quite complex analytical techniques are used to study reliability. For example, when studying electronic systems, an engineer selects a number of key characteristics, selects the most important one, selects options for action and one of these options, examines the operating conditions and evaluates them.
Due to at a fast pace modern scientific and technological progress it is important to choose the optimal moment to transition from scientific research And preparatory work to the production of products. In a competitive environment, the timing of production launch is an important factor, acting in two directions: launching production “too early” can lead to the same negative consequences as launching “too late”.
The reasons for manufacturing unreliable products may be:
- – lack of regular verification of compliance with standards;
- – errors in the use of materials and improper control of materials during production;
- – incorrect accounting and reporting of controls, including information on technology improvements;
- – sampling schemes that do not meet the standards;
- – lack of testing of materials for their compliance;
- – failure to comply with acceptance testing standards;
- – lack of instructional materials and instructions for conducting control;
- – irregular use of control reports for improvement technological process.
The mathematical models used to quantify reliability depend on the “type” of reliability. Modern theory distinguishes three types.
- 1. Instant Reliability(eg fuses).
- 2. Reliability with normal service life(for example, computer technology). In normal serviceability studies, the unit of measurement is "mean time between failures". The range recommended in practice is from 100 to 2000 hours.
- 3. Extremely long-term operational reliability(For example, spaceships). If service life requirements exceed 10 years, they are classified as extremely long service reliability.
Under normal operational reliability, technical prediction of reliability can be theoretical, empirical and experimental.
At theoretical testing means develop a scheme for this operation and check the compliance of the scheme using a mathematical model. If the diagram does not correspond to the operation, clarifications are made until compliance is achieved. This is the so-called scientific research.
Empirical approach consists of performing the necessary measurements on the actual products produced and drawing conclusions about reliability.
Experimental approach occupies an intermediate position between theoretical and empirical. The experimental approach uses both theory and measurements. At the same time, methods of mathematical modeling of processes are widely used, creating experimental data on this basis. The information is then subjected to statistical analysis using modern means computer technology, which ensures the reliability and validity of the conclusions.
Any type of test is preceded by an experimental plan.
Because reliability is a probabilistic characteristic, quantitative ratings are used to estimate the “average reliability” calculated from samples of the entire population, as well as to predict future reliability. Reliability is examined using statistical methods and can be clarified with their help.
It should be noted that service life is not the only indicator of performance properties.
In some cases, other indicators are used (mileage, duration of active use, etc.); The service life of products depends on both manufacturing conditions and operating conditions.
The reliability of many products can be revealed in the conditions of their consumption. A scientifically based system for monitoring the operation of products makes it possible to identify defects caused by violations of the manufacturer’s technological process.
The manufacturer must:
apply statistical quality control;
- – check the controllability state of processes at certain intervals;
- – strive to improve the quality and reliability of manufactured equipment;
- – ensure a correct understanding of customer requirements and their satisfaction.
An analysis of various definitions of reliability available in the literature leads to a generalized conclusion that reliability is understood as the failure-free operation of products under regulated operating conditions for a certain period of time.
Selective control. A characteristic feature of control in reliability research is that the possibilities of compiling samples are limited by the small number of pieces of equipment at the early stages of its development. As a rule, the number of units for testing is chosen by the customer. Moreover, the level of reliability of the test results varies depending on the number of units tested. The duration of the expected operating time and the degree of wear of the samples during testing have the same effect.
In practice, sampling for reliability testing is carried out in accordance with a plan that initially (and then each time a sampled product is characterized by a reduced mean time between failures) provides for a 10% consumer risk at an acceptable quality level corresponding to 10% of the units. with reliability below normal. Let us note some differences between statistical quality control and random checks in connection with technical reliability assurance. In the latter case, in addition to questions of sample representativeness, the question of the required test time arises.
Naturally, 100% testing of batches until the samples are completely worn out is impossible. Therefore, sampling schemes used in reliability studies provide for ongoing random testing of manufactured products with a weakened control regime until products with characteristics below the standard are detected. In other words, the weakened control procedure continues until a defective specimen appears in the sample. If a unit of manufactured products is detected with a characteristic lower than the norm, the normal control mode is restored, which can switch to an enhanced control mode depending on the number of defects identified in the sample. Typically, such sampling plans are developed taking into account a given mean time between failures and monthly production volumes.
When studying reliability, the method of sequential analysis is often used to decide whether to accept or reject a batch. First of all, it is determined that the mean time between failures under given conditions is at or above the established minimum. Such tests are planned after the specimens and test apparatus to be tested have been properly verified. Testing stops as soon as a decision on acceptance is made. But they do not stop if the decision is made to reject the batch. In the latter case, they continue in accordance with a precisely defined statistical control plan.
Failure is understood as the appearance of the first signs of malfunction or malfunction in the operation of the equipment. Each failure is characterized by a certain time of its occurrence.
The results of reliability research are important for the certification of products and quality systems Mazur I. I., Shapiro V. D. Quality management: textbook. allowance. M.: Omega-L, 2011.
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