Lecture #3
Under reliability is understood as the property of an object to keep in time within the established limits the values of parameters that characterize the ability to perform the required functions in specified 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 persistence (Figure 1).
Picture 1 - Equipment reliability
For the vast majority of year-round technical devices, when assessing their reliability, the most important are three properties: non-failure operation, durability and maintainability.
Reliability- the property of an object to continuously maintain a healthy state for some time.
Durability- the ability to remain operational until the limit state occurs with the established system of maintenance and repair.
maintainability- property of the product, which consists in adaptability to maintaining and restoring a working state through maintenance and repair.
At the same time, equipment for seasonal use (harvesting agricultural machines, some utility vehicles, river boats of freezing rivers, etc.), as well as machines and equipment for eliminating critical situations (fire fighting and rescue equipment), which, according to their purpose, have a long period of stay in standby mode, should be evaluated taking into account persistence, i.e. indicators of all four properties.
Persistence- the property of the product to maintain within the specified limits the values of the parameters characterizing the ability of the product to perform the required functions during and after storage or transportation.
Resource(technical) - the operating time of the product until it reaches the limit state specified in the technical documentation. The resource can be expressed in years, hours, kilometers, hectares, number of inclusions. There are a resource: 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 beginning of operation or from the previous overhaul of the product until the point in time under consideration; residual - from the point in time under consideration to the failure of a non-repairable product or its overhaul, overhaul.
Operating time- the duration of the operation of the product or the amount of work performed by it for 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, operating time to the first failure.
MTBF- reliability criterion, which is a static value, the average value of the operating time of the repaired product between failures. If the operating time is measured in units of time, then the mean time between failures is understood as the mean time of failure-free operation.
Finally, there is a whole range of products (for example, rubber products), which are evaluated mainly by their storability and durability.
The listed properties of reliability (non-failure operation, durability, maintainability and persistence) have their own quantitative indicators.
So reliability is characterized by six indicators, including such important ones as probability of failure. This indicator is widely used in the national economy to assess the most various kinds technical means: electronic equipment, aircraft, parts, components and assemblies, Vehicle, heating elements. The calculation of these indicators is carried out on the basis of state standards.
Refusal- one of the basic concepts of reliability, which consists in a violation of the product's performance (one or more parameters of the product go beyond the permissible limits).
Failure rate- the conditional probability density of the failure of a non-recoverable object, is determined under the condition that before the considered point in time the failure did not occur.
Probability of uptime- the possibility that, within the specified operating time, the failure of the object 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 most interesting gamma percentage resource- operating time during which the object does not reach the limit state with a probability g, expressed as a percentage. So for objects of metallurgical equipment (machines for lifting and moving liquid metals, pumps and devices for pumping harmful liquids and gases), g = 95% is prescribed.
Maintainability is characterized by two indicators: probability and average recovery time.
A number of authors subdivide reliability into ideal, basic and operational. Ideal reliability is the highest possible reliability achieved by creating a perfect object design 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 - the actual reliability of the object during its operation, due to both the quality of design, construction, manufacture and installation of the object, and the conditions of its operation, maintenance and repair.
The basic provisions of reliability will be unclear without defining such an important concept as redundancy. Reservation- this is the use of additional tools or capabilities in order to maintain the operable state of an object in case of failure of one or more of its elements.
One of the most common types of redundancy is duplication - redundancy with a one-to-one reserve ratio. 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 people's safety or entails severe economic consequences. So passenger and passenger-and-freight elevators are suspended on several ropes, aircraft are equipped with several engines, have duplicated electrical wiring, double and even triple brake systems are used in cars. Strength redundancy based on the concept of a safety factor has also become widespread. It is believed that the concept of strength is most directly related not only to reliability, but also to safety. Moreover, it is believed that engineering calculations of structures for safety 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, from 8 to 25.
When considering the production process in the relationship 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 totality of the reliability of its elements due to the influence of various connections.
In the theory of reliability, 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 values 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 the "reliability of the system".
At the same time, security and reliability are related but not identical concepts. They complement one another. So, from the point of view of the consumer, equipment can be reliable or not reliable, and in terms of safety, it can be safe or dangerous. At the same time, the equipment can be safe and reliable (acceptable in all respects), dangerous and not reliable (unconditionally rejected), safe and not reliable (most often rejected by the consumer), dangerous and reliable (rejected for safety reasons, but may be acceptable to the consumer, if the danger level is not too great).
Security requirements often act as constraints on the resource and lifetime of a piece of equipment or device. This happens when the required level of safety is violated before reaching the limit state due to physical or obsolescence. Limitations due to safety requirements play a particularly important role in assessing the individual residual life, which is understood as the duration of operation from a given point in time to reaching the limit state. As a measure of the resource, any parameter characterized by the duration of the operation of the object can be chosen. For aircraft, the measure of resource is 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 the general methodology and the theory of reliability 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 materials may change. Secondly, the use of economic and mathematical models to justify the assigned resource is possible only with the use of the assigned service life (the service life is defined as the calendar duration from the start of operation of an object or its renewal after a certain type of repair to the transition to the limit state and is measured in units of calendar time) . Thirdly, the calculation of the resource in units of time allows us to set forecasting problems in the most general form.
The initial impetus to the creation of numerical methods for assessing reliability was given in connection with the development aviation industry and a low level of flight safety in the initial stages. A significant number of aviation accidents with an ever-increasing intensity of air resources necessitated the development of reliability criteria for aircraft and requirements for the level of safety. In particular, there was comparative analysis one of the many aircraft in terms of successful completion of flights.
Indicative from the point of view of safety is the chronology of the development of the theory and technology of reliability. In the 40s, the main efforts to improve reliability were concentrated on a comprehensive improvement in quality, with the economic factor prevailing. To increase the durability of components and assemblies of various types of equipment, improved designs, durable materials, and perfect measuring instruments were developed. In particular, the electrical 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 temperature. Progress was made in the development of maintainable structures and in providing enterprises with equipment, tools and documentation to carry out preventive work and maintenance operations.
At the same time, the compilation and approval of standard schedules for periodic inspections and control charts for high-performance machine tools became widespread.
In the 1950s, great importance began to be attached to security issues, especially in such promising sectors as cosmonautics and nuclear energy. This period is the beginning of the use of many currently widespread concepts of 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 apparent. 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 first years of the era of spaceflight, considerable effort was expended on testing systems and individual elements. In order 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 flowcharts, a need arose for a different approach, the principle of analyzing systems using a fault tree was proposed and then widely used. It was first used as a program to evaluate the reliability of the MINITMAN missile launch control system.
Subsequently, the technique for constructing a fault tree was improved and extended to a wide range of different technical systems. Following catastrophic accidents at underground ICBM launch facilities, the United States officially introduced systems safety studies as a separate, independent activity. The US Department of Defense has introduced a requirement to conduct a reliability analysis at all stages of the development of all types of weapons. In parallel, requirements were developed for the reliability, performance and maintainability of industrial products.
In the 1970s, the most notable was the work on risk assessment associated with the operation nuclear power plants which was based on the analysis of a wide range of accidents. Its main focus was to assess the potential consequences of such accidents for the population in search of ways to ensure safety.
IN Lately The problem of risk has acquired a very serious significance and has attracted ever-increasing attention of specialists in various fields of knowledge. This concept is so inherent in both safety and reliability that the terms "reliability", "danger" and "risk" are often confused.
Among the technical causes of industrial accidents, the causes associated with the insufficient reliability of production equipment, structures, devices or their elements occupy a special place, since most often they appear suddenly and, therefore, are characterized by high rates of injury severity.
A large number of types used in industry, construction and transport of metal-intensive equipment and structures is a source of hazardous production factors due to the existing possibility of emergency failure of individual parts and assemblies.
The main purpose of the analysis of reliability and related safety of production equipment and devices is to reduce failures (primarily traumatic) and associated human casualties, economic losses and environmental disturbances.
Currently, there are quite a few methods for analyzing reliability and safety. So the most simple and traditional for reliability is the method of block diagrams. In this case, the object is represented as a system of individual elements, for which it is possible and expedient to determine the reliability indicators. Block diagrams are used to calculate the probability of failures, provided that only one failure is possible at a time in each element. Similar limitations have led to the emergence of other methods of analysis.
The preliminary hazard analysis method identifies hazards to the system and identifies elements for determining failure modes in the consequences analysis, as well as for building a fault tree. It is the first and necessary step in any research.
Failure Mode Consequence Analysis focuses primarily on hardware and considers all failure modes for each element. The disadvantages are that it is 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 take into account failures with a common cause of systems interaction.
Event tree analysis is used to identify basic sequences and alternative failure outcomes, but is not suitable for parallel sequences of events and for detailed study.
Hazard and operability analysis is an extended type of failure mode consequences analysis that includes the causes and consequences of changes in key production variables.
Cause-and-effect analysis demonstrates successive chains of events well, is flexible and rich enough, but too cumbersome and time-consuming.
The most widely used method in various industries, is a fault tree analysis. This analysis clearly focused on finding failures and in doing so identifies those aspects of the system that are important for the considered failures. At the same time, graphic, visual material is provided. Visibility gives the specialist the opportunity to penetrate deeply into the process of the system and at the same time allows you to focus on individual specific failures.
The main advantage of a fault tree over other methods is that the analysis is limited to identifying only those elements of the system and events that lead to this 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 would make two identical fault trees.
In order to find and visualize a causal relationship using a fault tree, it is necessary to use elementary blocks that subdivide and link a large number of events.
Thus, the currently used methods for analyzing the reliability and safety of equipment and devices, although they have certain drawbacks, still allow quite effectively to determine the causes of various types of failures even in relatively complex systems. The latter is especially relevant in connection with the great importance of the problem of the occurrence of hazards due to the insufficient reliability of technical objects.
Need to install windows, but do not know what to give preference to? On the one hand, the well-known wooden ones, and on the other, the now popular plastic ones. In both cases, environmental friendliness, safety and reliability of the design corresponds to the price and honesty of the manufacturer. And yet, when new windows need to be installed, a significant difference can be found 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 believe the companies that promise to bring the structure the day after tomorrow. In principle, this is impossible, because the minimum period for manufacturing a wooden structure is 30 days. The wood needs to be dried, painted or tinted, varnished if you need to install wooden windows. But when you need to install plastic window, then his company will be able to produce in a day. Especially if the manufacturer has its own production. 
When to Install Windows, then wooden structures are inferior to the palm for two reasons. This is a painstaking installation and a high price. To really install wooden Euro-structures, you will have to pay about 3-4 times than for a structure with a PVC profile. 
When need to install windows but, it should be remembered that even the most expensive plastic structures are made of polyvinyl chloride. And this means that at high temperatures, in extreme heat or during a fire, the greatest amount of harmful substances will be released 
Service life also needs to be considered when windows need to be installed. After all, plastic structures will last an average of about 40 years. They have already proven themselves well in the difficult Russian climate. Wooden structures will last for 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 it. When you need to install windows, you can mount plastic structures with your own hands, with minimal experience. Such a 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 to install plastic windows is easy maintenance. It will only be necessary to wipe the profile with a cloth, adjust and lubricate the fittings, change the seal. A wooden profile that cracks or absorbs moisture requires more attention. But on the other hand, the tree is subject to restoration, and the plastic will have to be completely changed. 
A double-glazed window in a plastic construction is easier to replace. You can do this in a few days. But in a wooden structure, this is much more difficult to do. In it, the double-glazed window is firmly glued into the sash with silicone sealant, and the glazing 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 windows need to be installed. 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 assembly or part to perform the specified functions, while maintaining its performance indicators (productivity, power, energy consumption, accuracy, etc.) within the specified limits for the required period of time or the 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. Subsystems can be considered Assembly units, parts, components or elements. If necessary, the concept of "object" may include information and its carriers, as well as human factor(for example, when considering the reliability of the "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, consisting in the general case of reliability, durability, maintainability and persistence. For example, for non-repairable objects, the main property may be failure-free operation. For repairable objects, one of the most important properties that make up the concept of reliability can be maintainability.
Reliability- the property of an object to continuously maintain a working state for some time or operating time.
Durability- the property of the object to maintain a working state until the limit state occurs with the established system of maintenance and repair.
maintainability- property of the object, which consists in adaptability to maintaining and restoring a working state through maintenance and repair.
Persistence- the property of an object to keep within the specified limits the values of the parameters that characterize the ability of the object to perform the required functions, during and after storage and (or) transportation.
An object- a technical product of a specific purpose, considered during the periods of design, production, testing and operation.
Element- the simplest component products, in reliability problems can consist of many parts.
System- a set of jointly acting elements, designed to independently perform the specified functions.
12 .Reliability indicators: probability of failure-free operation, mean time to failure, failure rate, failure rate parameter, time between failures. Weibull's law for characterizing the distribution of failures, a typical curve of change in the probability density of failures during the operation of objects.
Probability of uptime is the probability that within a given operating time the failure of the object does not occur. In practice, this indicator is determined by a statistical evaluation
where N0 is the initial number of healthy objects, n(t) is the number of failed objects during time t.
MTBF Mathematical expectation of the operating time of the object to the first failure.
Time to failure- equivalent parameter for a non-repairable device. Since the device is non-repairable, this is simply the average time that the device will work before it breaks.
Operating time- the duration or volume of work of the object, measured in hours, moto-hours, hectares, kilometers of run, switching cycles, etc.
It is measured statistically by testing a variety of instruments, or calculated by methods of reliability theory.
Т = 1/m * Σti where ti is the operating time of the i-th object between failures; m is the number of failures.
Bounce rate. The conditional density of the probability of 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 are working properly in a given period of time, provided that the failed samples are not restored and are not replaced by serviceable ones.
Bounce flow parameter. The ratio of the mathematical expectation of the number of failures of the restored object for a sufficiently small operating time to the value of this operating time.
If the child began to get up and move more actively, then it's time to restrict access to some cabinets and drawers for his safety.
In principle, we were not going to choose, because locks from IKEA inspired the greatest confidence. But the presence of 2 large chests of drawers (and this is already 11 drawers) and, in addition to them, 12 other important and dangerous doors, forced us to take a closer look and evaluate other cheaper analogues. They took samples from different manufacturers and almost all of them had to be replaced with IKEA ones.
About the advantages (and there were no disadvantages except for the cost)
They've been running for a year now with no problems. Stick to any surface. The main thing is to degrease it before sticking it.
There is an adjustment for different doors in terms of the width of the lock - we also installed it on a cabinet in the bathroom,
where the distance is short, and on the box under the crib, where the maximum length of the lock was required. Adjustable by cutting the tape. The truth is already irreversible))
The lock is hard enough to open. With long nails I think it's more difficult, with small opening-closing takes seconds. The main thing is to adapt. Well, the child certainly can not do it at all. Unlike other castles we have tried.
The color is only white. We were more than satisfied with this, because. everything in the room is mostly bright, but where it didn’t match, it’s not scary for us, safety comes first.
Reliability – this is the property of an object to perform the specified functions, keeping in time the values of the established performance indicators within the specified limits, corresponding to the specified modes and conditions of use, maintenance, repair, storage and transportation. It is a quality that extends through 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. are introduced. The concepts of failure and reliability are among the basic ones in the theory of reliability. Usually under reliability understand the property of products to maintain performance for a long time. Refusal- this is a complete or partial loss of performance of the product.
American authors D. Lloyd and M. Lipov in the book "Reliability" write: "Reliability affects cost, time, psychologically - in the form of inconvenience, and in certain cases also threatens the security of people and the nation. Usually, losses due to unreliability are not only the cost of a failing unit, but also the cost of associated equipment that deteriorates or is destroyed as a result of a failure ... A classic example of the psychological effect of unreliability is the Vanguard satellites of sad memory. The United States, keenly experiencing the success 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."
The American writer, poet and scientist of the XIX century. Oliver Holmes's poem "The Priest's Masterpiece, or the Remarkable One-Horse Carriage". It speaks of a priest who built a carriage, remarkable in that all its parts had exactly the same strength. This stroller served exactly 100 years and fell apart right on 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 in a given time interval or within a given operating time there will be no failure of the product.
There are many numerical characteristics for evaluating reliability. For example, readiness factor - is the probability that the product will be operable at given or random moments, - the time during which the product is operable, referred to the time of its operation.
by the consumer refers to the time during which a product with a manufacturer's warranty retains its quality parameters expected by the consumer, and therefore this time is usually called the guaranteed service life of the product.
Manufacturer's guaranteed product life called the durability of the product. Durability depends on the possibility of repair, after which its quality parameters can be restored, i.e. from the maintainability of the product.
By the actual service life, the consumer judges mainly the quality of the goods he has 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 in reliability studies is the indicator failure rate (λ ):
where n- the number of out-of-service products; N- total number
products; is the average test time.
The average test time is determined by the formula
where is the number of products in the test group; – the duration of the test for this group.
If the number of products that are out of order exceeds 5-10%, then adjustments are introduced into the calculation:
(2.3)
where is the number of failed products in this group;
- the number of failures for the same test time;
Duration of tests to disable the product.
To calculate the average failure rate, it is important to choose the correct time interval, since usually the density of failures varies with time.
EXAMPLE 2.1
When testing some piece of electronic equipment, λ can be determined after 1000–2000 hours. 4 groups of 250 items are tested 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 of time, the distribution of probabilities of failure-free operation intervals is expressed by an exponential law of distribution of operational durability.
The main quality parameters for products are:
- - functional characteristics - compliance of the product with the intended purpose;
- – reliability – the number of maintainable failures during the service life;
- - durability (service life) - an indicator associated with 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-
capacity, the ability to perform a specific task or howexecution probability certain function or functions for a certain time and under certain conditions .
As a technical concept, "reliability" is the probability (in the mathematical sense) of a satisfactory performance of a certain function. Since reliability is a probability, statistical characteristics are used to evaluate it. The reliability measurement results should include data on sample size, confidence limits, sampling procedures, etc.
In technology, the concept of "satisfactory performance" is also used. The exact definition of this concept is connected with the definition of its opposite - "unsatisfactory performance" or "refusal".
The general concept of "reliability" is opposed by the concept of "intrinsic reliability" of a piece of equipment, which is the probability of failure-free operation in accordance with specified specifications under specified verification tests for a required period of time. Reliability testing measures the reliability itself. It represents, in essence, the "operational reliability" of the equipment and is the result of two factors: reliability itself and operational reliability. Operational reliability, in turn, is determined by the conformity of the equipment to its use, the procedure and method of operational use and maintenance, the qualifications of personnel, the possibility of repairing various parts, factors environment and etc.
For each characteristic to be measured, in specifications a tolerance is set, the violation of which is considered as a "failure". The tolerance that determines the failure must be optimal with the necessary allowance for the 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 the specified functions, and in relation to the secondary parameters characterizing the ease of use, appearance etc.
- 2. Malfunction – the state of the product, in which it at a given time does not meet at least one of the requirements that characterize the normal performance of the specified functions.
- 3. performance – the state of the product, in which it at a given moment of time meets all the requirements established in relation to the main parameters characterizing the normal performance of the specified functions.
- 4. Refusal - an event consisting in the complete or partial loss of the product of its performance.
- 5. Complete failure - a failure, until the elimination of which the use of the product for its intended purpose becomes impossible.
- 6. Partial failure – failure, until the elimination of which it remains possible to partially use the product.
- 7. Reliability – the property of a product to continuously maintain operability for a certain period of time.
- 8. Durability – the property of a product to remain operational (with possible interruptions for maintenance and repair) until destruction or other limiting state. The limit state can be set according to parameter changes, according to safety conditions, etc.
- 9. maintainability – product property, expressed in its suitability for maintenance and repair operations, i.e. to the prevention, detection and elimination of faults and failures.
- 10. Reliability (in a broad sense) – property of the product, due to the reliability, durability and maintainability of the product itself and its parts and providing
which preserves the performance of the product under specified conditions.
- 11. Recoverability - the property of the product to restore the initial values of the parameters as a result of eliminating failures and malfunctions, as well as to restore the technical resource as a result of repairs.
- 12. Persistence - the property of the 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 is either on or off.
In practice, sometimes Special attention on the improvement of the main components of the product, losing sight of the fact that structural components that are of an auxiliary nature can be the cause of unreliability and the subsequent accident.
To measure (estimate) reliability, it is necessary to test an apparatus that would describe random events or random processes. It's about about the theory of probability and mathematical disciplines. The main quantitative indicator of reliability is the probability of non-failure operation of the product for a given period of time.
Probability of uptime- is the probability that in a given time interval or within a given operating time ns there will be a failure of products. With the introduction of this concept, it becomes possible to measure the reliability and compare the reliability of the product according to this indicator. The probability of failure-free operation of the same product is not the same at different points in its operation.
To assess the reliability, there are many characteristics, in particular: the probability of failure-free operation; readiness factor(the probability that the product will be operational at a given or random moment); time utilization factor(the time during which the product is operable, referred to the time of its operation).
Product uptime by the consumer implies the time during which a product with a manufacturer's warranty retains its quality parameters expected by the consumer, and therefore this time is usually called guaranteed service life of the product.
Guaranteed service life of the goods, 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 goods are restored, i.e. depends on maintainability. Durability characterizes the actual service life of the product. By the actual service life, the consumer judges the quality of the purchased goods, 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 the product is essential at the time of its purchase in comparison with a similar product of competitors, and the rigor of the subsequent fulfillment of all pre-agreed conditions, guarantees when purchasing a product determines the consumer's attitude to 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 expectations of the consumer, which are guaranteed by the manufacturer, then the responsibility for this lies with the manufacturer of the goods (supplier), who must repair at his own expense, and in case of impossibility of repair, replace defective goods for quality.
The manufacturer must guarantee the quality of the product both during its storage and during its operation.
In order to predict future failures, actual data on the frequency of failures over the time the equipment is used for its intended purpose is needed.
When processing information, the reciprocal of the failure rate is used "mean time between failures".
Quite complex analytical techniques are used to study reliability. For example, in the study of electronic systems, an engineer selects a number of key characteristics, selects the most important of them, selects options for action and one of these options, studies the operating conditions and evaluates them.
In connection with rapidly contemporary scientific and technological progress it is important to choose the optimal moment for the transition from scientific research And preparatory work to product manufacturing. In a competitive environment, a well-timed release into production is an important factor that works in two directions: "too early" release into production can lead to the same negative consequences as well as "too late".
The reasons for the manufacture of unreliable products can be:
- – lack of regular verification of compliance with standards;
- – errors in the use of materials and incorrect control of materials during production;
- – incorrect accounting and reporting on control, including information on technology improvement;
- – non-compliant sampling schemes;
- - lack of testing of materials for their compliance;
- – non-compliance with standards for acceptance tests;
- – lack of instructive materials and instructions for the control;
- – occasional use of control reports for improvement technological process.
Mathematical models used for quantitative assessments of reliability depend on the "type" of reliability. Modern theory identifies three types.
- 1. Instant Reliability(e.g. fuses).
- 2. Reliability with normal service life(for example, computer technology). Normal service reliability studies use "mean time between failures" as the unit of measure. The range recommended in practice is from 100 to 2000 hours.
- 3. Extremely long service life(for example, spaceships). If the service life requirements exceed 10 years, they are classified as extremely long service life.
Under normal operational reliability, the technical prediction of reliability can be theoretical, empirical, and experimental.
At theoretical test tools develop a scheme for this operation and check the compliance of the scheme using a mathematical model. If the schema does not match the operation, refinements are made until a match is achieved. This is the so-called scientific research.
Empirical approach is to perform the necessary measurements in relation to the actually manufactured products and conclusions about the 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. After that, the information is subjected to statistical analysis using modern computer technology, which ensures the reliability and reliability of the conclusions.
Any type of test is preceded by an experimental plan.
Because reliability is a probabilistic characteristic, scores are used to estimate the "average reliability" calculated from samples from the entire population, as well as to predict future reliability. Reliability is tested using statistical methods and can be corrected with their help.
It should be noted that the service life is not the only indicator of operational properties.
In some cases, other indicators are used (mileage, duration of active use, etc.); the service life of products depends on both the 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 manufacturing process at the manufacturer.
The manufacturer must:
apply statistical quality control;
- – check at regular intervals the state of process controllability;
- – strive to improve the quality and reliability of the manufactured equipment;
- – ensure that customer requirements are properly understood and met.
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 the study of reliability is that the possibilities of sampling are limited by the small number of units of equipment in the early stages of its development. As a rule, the customer chooses the number of units to be tested. However, the level of confidence in the test results varies depending on the number of units tested. The duration of the expected operational time and the degree of wear of the samples during testing have the same effect.
In practice, sampling for reliability testing is carried out according to a plan that initially (and then each time the sampled product has a reduced mean time between failures) assumes a 10% consumer risk at an acceptable quality level corresponding to 10% units, with below-standard reliability. Let us note some difference between statistical quality control and spot checks in connection with the technical support of reliability. In the latter case, in addition to questions of the representativeness of the sample, the question of the required test time arises.
Naturally, 100% testing of batches until the samples are completely worn out is impossible. Therefore, the sampling schemes used in the study of reliability provide for the ongoing random inspection of manufactured products with a relaxed inspection regime until a product with characteristics below the norm is found. In other words, the weakened control procedure continues until a defective specimen appears in the sample. When a unit of output with a characteristic reduced against the norm is found, the normal control mode is restored, which can switch to the enhanced control mode, depending on the number of defects found in the sample. As a rule, such sampling plans are developed taking into account the given average time between failures and monthly production sizes.
In the study of reliability, the method of sequential analysis is often used to decide whether to accept or reject a lot. First of all, it is revealed that the mean time of failure-free operation under given conditions is at the level of the established minimum or exceeds it. Such tests are planned after the specimens and test equipment to be tested have been properly inspected. Testing is terminated as soon as an acceptance decision is made. But they do not stop if a decision is made to reject the batch. In the latter case, they continue according to a well-defined plan of statistical control.
Failure is understood as the appearance of the first signs of malfunction or malfunction of the equipment. Each failure is characterized by a certain time of its occurrence.
The results of the reliability study are important in the certification of products and quality systems Mazur I. I., Shapiro V. D. Quality management: textbook. allowance. Moscow: Omega-L, 2011.
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