1. The essence of the phenomenon of wear
Life time industrial equipment is determined by the wear of its parts - a change in the size, shape, mass or state of their surfaces due to wear, i.e., residual deformation from permanent loads or due to destruction of the surface layer during friction.
The amount of wear is characterized by established units of length, volume, mass, etc. Wear is determined by changing the gaps between the mating surfaces of parts, the appearance of leaks in seals, reducing the accuracy of processing the product, etc. Wear can be normal and emergency. Normal, or natural, is the wear that occurs during the correct, but long-term operation of the machine, that is, as a result of using a given resource of its work.
Emergency (or progressive) wear is called, which occurs within a short time and reaches such dimensions that further operation of the machine becomes impossible.
2. Types and nature of parts wear.
Types of wear are distinguished according to existing species wear:
Mechanical;
Abrasive;
fatigue;
Corrosive, etc.
Mechanical wear is the result of the action of friction forces when sliding one part over another. With this type of wear, abrasion (cutting) of the surface layer of the metal and distortion of the geometric dimensions of the jointly working parts occur. Wear of this type most often occurs during the operation of such common interfaces of parts as a shaft - a bearing, a bed - a table, a piston - a cylinder, etc.
The degree and nature of mechanical wear of parts depend on many factors:
Physical and mechanical properties of the upper layers of the metal;
Working conditions and the nature of the interaction of mating surfaces;
Pressure;
Relative speed of movement;
Lubrication conditions; roughness, etc.
The most destructive effect on parts is abrasive wear, which occurs when the rubbing surfaces are contaminated with small abrasive and metal particles. Typically, such particles fall on the rubbing surfaces during the processing of cast billets on a machine.
Mechanical wear can also be caused by poor maintenance of the equipment, such as irregularities in the supply of lubrication, poor quality repairs and non-compliance with its deadlines, power overload, etc.
fatigue wear is the result of variable loads acting on the part, causing fatigue of the material of the part and its destruction. Shafts, springs and other parts are destroyed due to fatigue of the material in the cross section. To prevent fatigue failure, it is important to choose the right cross-sectional shape of a newly manufactured or repaired part: it should not have sharp transitions from one size to another. The working surface eliminates the presence of scratches and scratches, which are stress concentrates.
Corrosive wear is the result of wear of parts of machines and installations that are under the direct influence of water, air, chemicals, temperature fluctuations.
Under the influence of corrosion, deep corrosions are formed in the parts, the surface becomes spongy, and loses mechanical strength.
Typically, corrosion wear is accompanied by mechanical wear due to the mating of one part with another. In this case, the so-called mechanical corrosion occurs, i.e. complex wear.
Seizure wear occurs as a result of sticking ("seizing") of one surface to another. This phenomenon is observed with insufficient lubrication, as well as significant pressure, at which two mating surfaces approach each other so tightly that molecular forces begin to act between them, leading to their seizure.
The nature of the mechanical wear of parts. Mechanical wear of equipment parts can be complete if the entire
the surface of the part, or local, if any part of it is damaged (Fig. 1).
As a result of the wear of the guides of the machine tools, their flatness, straightness and parallelism are violated due to the action of unequal loads on the sliding surface. For example, rectilinear guides 2 of the machine (Fig. 1, a) under the influence of large local loads become concave in the middle part (local wear), and the short guides 1 of the table mated with them become convex.
In rolling bearings due to various reasons (Fig. 2, a-d)
working surfaces are subject to wear - pockmarks appear on them, peeling of the surfaces of treadmills and balls is observed. Under the action of dynamic loads, their fatigue failure occurs; under the influence of excessively tight fits of bearings on the shaft and in the housing, the balls and rollers are pinched between the rings, as a result of which distortions of the rings during installation and other undesirable consequences are possible.
Various surfaces slides are also subject to characteristic wear patterns (Fig. 3).
During the operation of gears, due to contact fatigue of the material of the working surfaces of the teeth and under the action of tangential stresses, chipping of the working surfaces occurs, leading to the formation of pits on the friction surface (Fig. 3, a).
The destruction of the working surfaces of the teeth due to intense chipping (Fig. 3, b) is often called flaking (there is a separation from the friction surface of the material in the form of scales).
On fig. 3c shows a surface damaged by corrosion. The surface of the cast iron powder ring (Fig. 3, d) is damaged due to erosion wear, which occurs when the piston moves in the cylinder relative to the liquid; gas bubbles in the liquid burst close to the piston surface, which creates a local increase in pressure or temperature and causes wear on parts.
3. Signs of wear.
The wear of machine or machine parts can be judged by the nature of their work. In machines with crankshafts with connecting rods (internal combustion engines and steam engines, compressors, eccentric presses, pumps, etc.), the appearance of wear is determined by a dull knock at the points of mate parts (it is stronger, the greater the wear).
Noise in gears is a sign of tooth profile wear. Deaf and sharp shocks are felt every time when the direction of rotation or rectilinear movement is changed in cases of wear of parts of keyed and splined joints.
Traces of crushing on the turning roller installed in the conical bore of the spindle indicate an increase in the gap between the spindle necks and its bearings due to their wear. If processed on lathe the workpiece turns out to be conical, which means that the spindle bearings (mainly the front) and the bed guides are worn out. An increase in the backlash of the handles fixed on the screws in excess of the allowable is evidence of the wear of the threads of the screws and nuts.
The wear of machine parts is often judged by scratches, grooves and nicks that appear on them, as well as by a change in their shape. In some cases, the check is carried out with a hammer: a rattling sound when tapping a part with a hammer indicates the presence of significant cracks in it.
The operation of assembly units with rolling bearings can be judged by the nature of the noise they emit. It is best to perform such a check with a special device - stethoscope.
The operation of the bearing can also be checked by heating, which is determined by touch with the outer side of the hand, which painlessly withstands temperatures up to 60 ° C.
A tight turning of the shaft indicates a lack of alignment between it and the bearing, or an excessively tight fit of the bearing on the shaft or in the housing, etc.
4. Methods for detecting defects and restoring parts.
Most large and medium mechanical defects are detected during external examination. To detect small cracks, you can use various methods defectoscopy. The most simple capillary methods. If, for example, a part is immersed in kerosene for 15-30 minutes, then if there are cracks, the liquid penetrates into them. After thorough rubbing, the surfaces of the part are covered with a thin layer of chalk; the chalk absorbs the kerosene from the cracks, causing dark streaks to appear on the surface, indicating the location of the defect.
For more accurate detection of cracks, liquids are used that glow when irradiated with ultraviolet rays (capillary luminescent method). Such a liquid is, for example, a mixture of 5 parts of kerosene, 2.5 parts of transformer oil and 2.5 parts of gasoline. The item is immersed for 10-15 minutes in a liquid, then washed and dried, after which it is irradiated with ultraviolet rays (mercury-quartz lamp). In places of cracks, a light green glow appears.
Cracks are also detected by magnetic flaw detection methods. The part is magnetized and wetted with a magnetic suspension (iron oxide powder mixed in oil, kerosene or a water-soap solution). In places of cracks, accumulations of powder are formed (Fig. 4, a).
Longitudinal cracks are detected when magnetic lines pass along the circumference of the part (Fig. 4, b), and transverse cracks - during longitudinal magnetization (Fig. 4, c).
Defects located inside the material are detected by fluoroscopic method. X-rays, passing through the part being checked, fall on a sensitive film, on which voids appear as darker spots, and dense foreign inclusions as lighter spots.
Currently distributed ultrasonic method detection of cracks and other hidden defects. An ultrasonic probe is applied to the part under study, the main part of which is a crystal generator of high-frequency mechanical oscillations (0.5-10 MHz). These vibrations, passing through the material of the part, are reflected from internal boundaries (internal cracks, fracture surfaces, cavities, etc.) and fall back into the probe. The device registers the delay time of the reflected waves relative to the emitted ones. The longer this time, the greater the depth at which the defect is located.
Restoration of parts and mechanisms of machine tools is carried out by the following methods. Machining - repair size method- used to restore the accuracy of machine tool guides, worn holes or necks of various parts, threads of lead screws, etc.
Repair is called the size, up to which the worn surface is processed when restoring the part. There are free and regulated sizes.
Welding fix parts with kinks, cracks, chips.
Surfacing is a type of welding and consists in the fact that a filler material is deposited on the worn area, which is more wear-resistant than the material of the main part.
A method of restoring parts made of cast iron by welding - soldering with brass wire and copper-zinc rods has become widespread. tin alloys. This method does not require heating the edges to be welded to melting, but only to the melting temperature of the solder.
Metallization consists in melting the metal and spraying it with a jet of compressed air into small particles, which are embedded in surface irregularities, adhering to them. A layer from 0.03 to 10 mm and higher can be increased by metallization.
Metallizing installations can be gas (the metal melts in the flame of a gas burner) and arc (the diagram of which is shown in Fig. 5).
Chrome plating is a process of restoring the worn surface of a part by electrolytic chromium deposition (Fig. 6), chromium plating thickness up to 0.1 mm.
The whole variety of repair methods is clearly presented in Fig.7.
5. Modernization of machines.
At overhaul it is desirable to carry out the modernization of machine tools, taking into account the operating conditions and the latest achievements of science and technology.
Under the modernization of machine tools understand the introduction of partial changes and improvements in the design in order to increase their technical level to the level modern models for a similar purpose (general technical modernization) or for solving specific technological problems of production by adapting equipment to better perform a certain type of work (technological modernization). As a result of modernization, the productivity of equipment increases, operating costs decrease, rejects decrease, and in some cases the duration of the overhaul period increases.
An idea of the main directions of modernization of metal-cutting machines is given by the diagram shown in Figure 8.
LEKITSIA No. 6.
1.Technical diagnostics of equipment.
Technical diagnostics (TD)- an element of the PPR System that allows you to study and establish signs of a malfunction (operability) of equipment, establish methods and means by which a conclusion (diagnosis) is given on the presence (absence) of malfunctions (defects). Acting on the basis of studying the dynamics of changes in indicators technical condition equipment, TD solves the issues of forecasting (foresight) of the residual resource and trouble-free operation of equipment for a certain period of time.
Technical diagnostics proceeds from the position that any equipment or its component can be in two states - serviceable and faulty. Serviceable equipment is always operational, it meets all the requirements of the technical specifications established by the manufacturer. Faulty (defective) equipment can be both operational and inoperable, i.e., in a state of failure. Failures are the result of wear or misalignment of nodes.
Technical diagnostics is mainly aimed at finding and analyzing the internal causes of failure. External causes are determined visually, using a measuring tool, simple devices.
The peculiarity of TD is that it measures and determines the technical condition of the equipment and its components during operation, directs its efforts to search for defects. Knowing the technical condition of individual parts of the equipment at the time of diagnosis and the magnitude of the defect, in which its performance is impaired, it is possible to predict the period of failure-free operation of the equipment until the next scheduled repair, provided for by the standards for the frequency of the PPR System.
The standards of periodicity laid down in the basis of the PPR are experimentally averaged values. But Any average values have their own significant drawback: even if there are a number of clarifying coefficients, they do not provide a complete objective assessment of the technical condition of the equipment and the need for scheduled repairs. There are almost always two extra options: the residual resource of the equipment is far from exhausted, the residual resource does not provide trouble-free operation until the next scheduled repair. Both options do not provide the requirement federal law No. 57-FZ on establishing the useful life of fixed assets by objectively assessing the need to put it into repair or decommission from further operation.
An objective method for assessing the need for equipment for repair is constant or periodic monitoring of the technical condition of the facility with repairs only in the case when the wear of parts and assemblies has reached limit value, which does not guarantee the safe, trouble-free and economical operation of the equipment. Such control can be achieved by means of TD, and the method itself becomes an integral part of the PPR (control) System.
Another task of TD is to predict the residual life of the equipment and establish the period of its failure-free operation without repair (especially capital), i.e., adjusting the structure of the repair cycle.
Technical diagnostics successfully solves these problems with any repair strategy, especially a strategy based on the technical condition of the equipment.
The main principle of diagnosis is the comparison of the regulated value performance parameter or parameter of the technical condition of the equipment with the actual using diagnostic tools. Hereinafter, according to GOST 19919-74, a parameter is understood as a characteristic of equipment that reflects the physical value of its functioning or technical condition.
The objectives of the TD are:
Control of functioning parameters, i.e. the course of the technological process, in order to optimize it;
Monitoring the parameters of the technical condition of the equipment that change during operation, comparing their actual values with the limit values and determining the need for maintenance and repair;
Forecasting the resource (service life) of equipment, assemblies and assemblies in order to replace them or bring them out for repair.
2. Requirements for equipment transferred for technical diagnostics.
In accordance with GOST 26656-85 and GOST 2.103-68, when transferring equipment to a repair strategy based on technical condition, the issue of its suitability for installing TD means on it is first resolved.
The adaptability of the equipment in operation to the TD is judged by compliance with the reliability indicators and the availability of places for installing diagnostic equipment (sensors, instruments, wiring diagrams).
Next, a list of equipment subject to TD is determined by the degree of its influence on the capacity (production) indicators of production for the production of products, as well as on the basis of the results of identifying "bottlenecks" in terms of reliability in technological processes. As a rule, increased reliability requirements are imposed on this equipment.
In accordance with GOST 27518-87, the design of equipment must be adapted for TD.
To ensure the suitability of equipment for TD, its design should provide for:
Possibility of access to control points by opening technological covers and hatches;
Availability of installation bases (platforms) for installation of vibrometers;
Possibility of connection and placement in closed liquid systems of TD means (pressure gauges, flow meters, hydrotesters in liquid systems) and their connection to control points;
Possibility of multiple connection and disconnection of TD means without damage to the interface devices and the equipment itself as a result of leakage, contamination, ingress of foreign objects into internal cavities, etc.
The list of works to ensure the adaptability of the equipment to the TD is given in the terms of reference for the modernization of the equipment transferred to the TD.
After determining the list of equipment transferred for repair according to its technical condition, executive technical documentation is prepared for the development and implementation of TD tools and the necessary equipment upgrades. The list and sequence of development of as-built documentation are given in Table. one.
3. Choice of diagnostic parameters and methods of technical diagnostics.
First of all, parameters are determined that are subject to constant or periodic monitoring to check the functioning algorithm and ensure optimal operating modes (technical condition) of the equipment.
For all units and units of equipment, a list of possible failures is compiled. Preliminarily, data is collected on failures of equipment equipped with TD facilities, or its analogues. The mechanism of occurrence and development of each failure is analyzed and diagnostic parameters are outlined, the control of which, scheduled maintenance and current repairs can prevent failure. Failure analysis is recommended to be carried out in the form presented in Table. 2.
For all failures, diagnostic parameters are outlined, the control of which will help to quickly find the cause of the failure, and the TD method (see Table 3).
The range of parts whose wear leads to failure is determined.
In practice, diagnostic signs (parameters) have become widespread, which can be divided into three groups:
1) Workflow options
(dynamics of changes in pressure, effort, energy), directly characterizing the technical condition of the equipment;
2) Parameters of accompanying processes or phenomena
(thermal field, noise, vibration, etc.), which indirectly characterize the technical condition;
3) Structural parameters
(clearances in interfaces, wear of parts, etc.), which directly characterize the state of the structural elements of the equipment.
The possibility of reducing the number of controlled parameters through the use of generalized (complex) parameters is being studied.
For convenience and clarity of methods and means of TD, functional schemes for monitoring parameters are developed. technological processes and technical condition of the equipment.
When choosing TD methods, the following main criteria for assessing its quality are taken into account:
Economic efficiency of the TD process;
Reliability of TD;
Availability of manufactured sensors and devices;
Universality of methods and means of TD.
Based on the results of the analysis of equipment failures, measures are developed to improve the reliability of equipment, including the development of TD tools.
4. Means of technical diagnostics.
By execution, the funds are divided into:
- external- not being an integral part of the object of diagnosis;
- built-in- with a system of measuring transducers (sensors) of input signals, made in a common design with diagnostic equipment as its integral part.
External means of TD are divided into: stationary, mobile and portable.
If a decision is made to diagnose equipment by external means, then it should provide for control points, and in the operating manual for TD tools, it is necessary to indicate their location and describe the control technology.
Built-in TD tools control parameters whose values go beyond the standard (limit) values entail an emergency situation and often cannot be predicted in advance during periods Maintenance.
According to the degree of automation of the control process, TD tools are divided into automatic, manual (non-automatic) and automated-manual control.
The possibilities of automating diagnostics are greatly expanded with the use of modern computer technology.
When creating AP tools for technological equipment various converters (sensors) of non-electric quantities into electrical signals, analog-to-digital converters of analog signals into equivalent values of a digital code, sensory subsystems of technical vision can be used.
It is recommended that the following requirements be imposed on the designs and types of converters used for TD facilities:
Small size and simplicity of designs;
Adaptability for placement in places with a limited amount of equipment placement;
Possibility of repeated installation and removal of sensors with minimal labor intensity and without equipment installation;
Compliance of metrological characteristics of sensors with information characteristics of diagnostic parameters;
High reliability and noise immunity, including the ability to operate in conditions of electromagnetic interference, voltage fluctuations and power frequency;
Resistance to mechanical influences (shocks, vibrations) and to changes in parameters environment(temperature, pressure, humidity);
Ease of regulation and maintenance.
The final stage in the creation and implementation of TD tools is the development of documentation.
Operational design documentation;
Technological documentation;
Documentation for the organization of diagnostics.
In addition to operational, technological and organizational documentation, programs for forecasting the residual and predicted resource are developed for each transferred object.
LECTURE №7.
1. Principles of modern service.
There are a number of generally accepted norms, the observance of which warns against errors:
· Mandatory offer. Globally, companies producing high quality goods, but poorly providing them with related services, put themselves in a very disadvantageous position.
· Optional use. The firm should not impose service to the client.
service elasticity. The package of service activities of the company can be quite wide: from the minimum required to the most appropriate.
Service convenience. The service must be presented in a place, at a time and in a form that suits the buyer.
Technical adequacy of the service.
Modern enterprises are increasingly equipped with the latest technology, which greatly complicates the actual manufacturing technology of products. And if the technical level of equipment and service technology is not adequate to the production level, then it is difficult to count on the necessary quality of service.
· Information return of the service. The management of the company should listen to the information that the service department can give out regarding the operation of goods, about the assessments and opinions of customers, the behavior and methods of service of competitors, etc.
Reasonable price policy. The service should be not so much a source of additional profit, but an incentive to purchase the company's products and a tool to strengthen customer confidence.
· Guaranteed conformity of production to service. A manufacturer conscientiously treating the consumer will strictly and strictly measure his production capacity with the capabilities of the service and will never put the client in the conditions of "serve yourself."
2. The main tasks of the service system.
In general, the main tasks in the service are:
Advising potential buyers before purchasing the company's products, allowing them to make an informed choice.
Training of the buyer's personnel or himself for the most efficient and safe operation of the purchased equipment.
Transfer of the necessary technical documentation.
Pre-sale preparation of the product in order to avoid the slightest possibility of failure in its operation during the demonstration to a potential buyer.
Delivery of the product to its place of use in such a way as to minimize the possibility of damage in transit.
Bringing equipment into working condition at the place of operation (installation, installation) and demonstrating it to the buyer in action.
Ensuring the complete readiness of the product for operation during the entire period of its stay with the consumer.
Prompt supply of spare parts and maintenance of the necessary network of warehouses for this, close contact with the manufacturer of spare parts.
Collection and systematization of information about how the equipment is operated by the consumer (conditions, duration, qualifications of personnel, etc.) and what complaints, comments, suggestions are made.
Participation in the improvement and modernization of consumable products based on the analysis of the information received.
Collection and systematization of information about how competitors conduct service work, what innovations they offer to customers.
Formation of a permanent clientele of the market according to the principle: "You buy our product and use it, we do the rest"
Assistance to the marketing department of the enterprise in the analysis and evaluation of markets, customers and goods.
3. Types of service by the time of its implementation.
By time parameters, the service is divided into pre-sales and after-sales, and after-sales, in turn, into warranty and post-warranty.
1. Pre-sales service
It is always free and provides for the preparation of the product for presentation to a potential or real buyer. Pre-sales service, in principle, includes 6 main elements:
Examination;
Conservation;
Completion of the necessary technical documentation, instructions for start-up, operation, maintenance, elementary repairs and other information (in the appropriate language);
Reopening and testing before sale;
Demonstration;
Conservation and transfer to the consumer.
2. After-sales service
After-sales service is divided into warranty and post-warranty on a purely formal basis: “free of charge” (in the first case) or for a fee (in the second) the work provided for in the service list is performed. The formality here is that the cost of work, spare parts and materials during the warranty period is included in the sale price or other (post-warranty) services.
Service during the warranty period covers the types of liability accepted for the warranty period, depending on the product, the concluded contract and the policies of competitors. Basically, it includes:
1) opening at the consumer;
2) installation and start-up;
3) checking and setting;
4) training of employees in proper operation;
5) training of customer's specialists in supporting service;
6) observation of the product (system) operation;
7) carrying out prescribed maintenance;
8) implementation (if necessary) of repair;
9) supply of spare parts.
The proposed list of services mainly relates to complex expensive equipment for industrial purposes.
Service in the post-warranty period includes similar services, the most common of which are:
Monitoring the product in operation;
Retraining of clients;
Various technical assistance;
Provision of spare parts;
Repair (if necessary);
Modernization of the product (as agreed with the customer).
The essential difference between the post-warranty service is that it is carried out for a fee, and its volume and prices are determined by the terms of the contract for this type of service, price lists and other similar documents.
Thus, the service policy covers a system of actions and decisions related to the formation of the consumer's conviction that with the purchase of a particular product or complex, he guarantees himself reliable rears and can concentrate on his main duties.
However, it should be emphasized that in order to form a competitive marketing service policy at the stage of product development, it is necessary to carry out the following actions:
a) study of consumer demand in the markets in that part that is associated with the forms, methods and conditions of service adopted by competitors for similar products;
b) systematization, analysis and evaluation of the collected information to select a solution for the organization of the service; development of solutions taking into account the characteristics of the product, market and goals of the organization;
in) comparative analysis options;
d) participation of service specialists in design and development activities to improve the product, taking into account subsequent maintenance.
In the case of the most complete implementation, a branded service includes a number of elements that reflect life cycle products from the moment of its manufacture to disposal (Fig. 1).
4. Types of service according to the content of work.
In stating recent trends, it should be noted that not purely engineering works, but various (including indirect) intellectual services. And it does not matter in what form these services are served: a special set of recipes for microwave ovens or a set of individual consultations for a given farmer on the processing of his particular plot.
For this reason, the service is divided according to the content of the work:
- hard service includes all services related to maintaining the operability, reliability and specified parameters of the product;
- soft service includes the whole complex intellectual services associated with individualization, i.e., with a more efficient operation of the product in specific working conditions for a given consumer, as well as simply with expanding the sphere of usefulness of the product for him.
A competent manufacturer strives to do the maximum possible for the buyer in any situation. When a manufacturer provides a farmer with a qualified assessment of the most effective tillage modes on a purchased tractor, this is a direct service. And if, in order to maintain a good relationship with the client, the dealer invites the farmer's wife to free courses"Home accountant", organized specifically for the wives of the firm's clients, here we can talk about an indirect service. This, of course, has nothing to do with buying a tractor, but it is useful and pleasant for the client. Thus, indirect service, although in complex ways, contributes to the success of the firm.
5. Basic approaches to the implementation of the service.
Based on the practice that has developed in developed countries, a number of Western authors have proposed the following classification of approaches to the implementation of the service:
1) Negative approach.
With this approach, the manufacturer considers the manifested product defects as random errors. The service is not seen as an activity that adds value to the product, but rather as an extra cost that needs to be kept as low as possible.
2) Research approach.
Organizationally, it is largely similar to the previous one. But in contrast to it, the emphasis is on the careful collection and processing of information about defects, which is used later to improve product quality. This approach relies more on finding out the cause of the defect rather than repairing the product itself.
3) Service as an economic activity.
Service can be a significant source of profit for an organization, especially if a large number of products and systems are sold that are already out of warranty. Any improvement in the product in the direction of increasing reliability limits the revenue from the service; but, on the other hand, creates the prerequisites for success in the competitive struggle.
4) Service is the responsibility of the supplier.
Practical work No. 1
"Independent study and note-taking of the topic: "Wear of parts of industrial equipment""
The essence of the phenomenon of wear
The service life of industrial equipment is determined by the wear of its parts.- a change in the size, shape, mass or state of their surfaces due to wear, i.e. residual deformation from permanent loads or due to destruction of the surface layer during friction.
The wear rate of equipment parts depends on many factors:
Ø conditions and mode of their work;
Ø material from which they are made;
Ø the nature of the lubrication of rubbing surfaces;
Ø specific force and sliding speed;
Ø temperature in the interface zone;
Ø state of the environment (dustiness, etc.).
Wear amount characterized by established units of length, volume, mass, etc.
Depreciation is determined:
Ø by changing the gaps between the mating surfaces of parts, \
Ø leakage in seals,
Ø decrease in the accuracy of processing the product, etc.
Wear is:
ü normal and
ü emergency.
Normal or natural is called wear that occurs during the correct, but long-term operation of the machine, i.e. as a result of using a given resource of its operation.
emergency or progressive, called wear, which occurs within a short time and reaches such proportions that further operation of the machine becomes impossible.
At certain values of changes resulting from wear, wear limit, causing a sharp deterioration in the performance of individual parts, mechanisms and the machine as a whole, which causes the need for its repair.
Wear rate - this is the ratio of the values of the characterizing quantities to the time interval during which they arose.
The essence of the phenomenon of friction
The primary cause of wear of parts (especially mating and rubbing against each other) is friction.
Friction - the process of resistance to relative movement that occurs between two bodies in the areas of contact of their surfaces along the tangents to them, accompanied by the dissipation of energy, i.e., its transformation into heat.
In everyday life, friction is both beneficial and harmful.
Benefit lies in the fact that due to the roughness of all objects without exception, as a result of friction between them, slip does not occur. This explains, for example, that we can move freely on the ground without falling, objects do not slip out of our hands, a nail is firmly held in a wall, a train moves along rails, etc. The same phenomenon of friction is observed in the mechanisms of machines, whose work is accompanied by the movement of interacting parts. In this case, friction gives negative result - wear of mating surfaces of parts. Therefore, friction in mechanisms (with the exception of the friction of brakes, drive belts, friction gears) is an undesirable phenomenon.
Types and nature of wear parts
Types of wear are distinguished in accordance with the existing types of wear -
Types of wear:
Ø mechanical(abrasive, fatigue ),
Ø corrosive and etc.
Mechanical wear is the result of the action of friction forces when sliding one part over another.
With this type of wear, abrasion (cutting) of the surface layer of the metal and distortion of the geometric dimensions of the jointly working parts occur. Wear of this type most often occurs during the operation of such common interfaces of parts as a shaft - bearing, frame - table, piston - cylinder, etc. It also appears during rolling friction of surfaces, since sliding friction inevitably accompanies this type of friction, however, in such cases, wear is very small.
The degree and nature of mechanical wear of parts depend on many factors:
Ø physical and mechanical properties of the upper layers of the metal;
Ø working conditions and the nature of the interaction of mating surfaces; pressure; relative speed of movement;
Ø conditions for lubrication of rubbing surfaces;
Ø degree of roughness of the latter, etc.
The most destructive effect on the details has abrasive wear, which is observed in cases where the rubbing surfaces are contaminated with small abrasive and metal particles.
Usually, such particles get on the rubbing surfaces during the processing of cast billets on the machine, as a result of wear of the surfaces themselves, dust ingress, etc.
They retain their cutting properties for a long time, form scratches, scuffs on the surfaces of parts, and, when mixed with dirt, act as an abrasive paste, as a result of which intensive rubbing and wear of mating surfaces occurs. The interaction of the surfaces of parts without relative movement causes metal crushing, which is typical for keyed, slotted, threaded and other connections.
Mechanical wear can also be caused by poor maintenance of the equipment, such as irregularities in the supply of lubrication, poor quality repairs and non-compliance with its deadlines, power overload, etc.
During operation, many machine parts (shafts, gear teeth, connecting rods, springs, bearings) are subjected to long-term action of variable dynamic loads, which have a more negative effect on the strength properties of the part than static loads.
fatigue wear is the result of variable loads acting on the part, causing fatigue of the material of the part and its destruction. Shafts, springs and other parts are destroyed due to fatigue of the material in the cross section. In this case, a characteristic type of fracture is obtained with two zones - the zone of developing cracks and the zone along which the fracture occurred. The surface of the first zone is smooth, while the second zone is shelled and sometimes granular.
Fatigue failure of the material of a part does not necessarily lead to its failure immediately. It is also possible that fatigue cracks, peeling and other defects may occur, which, however, are dangerous, as they cause accelerated wear of the part and mechanism.
To prevent fatigue failure, it is important to choose the right cross-sectional shape of a newly manufactured or repaired part: it should not have sharp transitions from one size to another. It should also be remembered that a rough surface, the presence of scratches and scratches can cause fatigue cracks.
Seizure wearoccurs as a result of sticking (“seizing”) of one surface to another.
This phenomenon is observed with insufficient lubrication, as well as significant pressure, at which two mating surfaces approach each other so tightly that molecular forces begin to act between them, leading to their seizure.
Corrosive wear is the result of wear of parts of machines and installations that are under the direct influence of water, air, chemicals, temperature fluctuations. For example, if the air temperature in industrial premises is unstable, then every time it rises, the contained
Rice. one. The nature of mechanical wear of parts:
a- bed and table guides, b- internal surfaces of the cylinder,
in- piston, d, d- shaft, e, w- wheel teeth h- screw and nut threads,
and- disc friction clutch;
1 - table, 2 - bed, 3 - skirt, 4 - jumper, 5 - bottom, 6 - hole,
7 - bearing, 8 - shaft neck 9 - gap, 10 - screw, 11 - screw;
And- places of wear, R- active efforts
In the air, water vapor, in contact with colder metal parts, is deposited on them in the form of condensate, which causes corrosion, i.e., the destruction of the metal due to chemical and electrochemical processes developing on its surface. Under the influence of corrosion, deep corrosions are formed in the parts, the surface becomes spongy, and loses mechanical strength. These phenomena are observed, in particular, in parts of hydraulic presses and steam hammers operating in steam or water.
Typically, corrosion wear is accompanied by mechanical wear due to the mating of one part with another. In this case, the so-called corrosion-mechanical occurs, i.e. complex, wear.
Table 7.1 - Main types of mechanical wear| Conditions of occurrence | Destruction mechanism | Manifestation |
|---|---|---|
|
It is characterized by the appearance of adhesive bonds between parts with their subsequent destruction. | On the contact surface of a part made of a less durable material, randomly located tears are formed, and sticking is formed on a part made of a more durable material. |
|
The first stage (temperature up to 600 °C, the mechanical properties of materials decrease slightly). | Tears of particles on parts made of less durable material, alternating at approximately the same intervals. |
| The second stage (temperature 600-1400 °C, softening of the metal, a noticeable decrease in the mechanical properties of materials). | On the contact surface of a more durable part, sticking and smearing of the metal are visible, and on the surface of a less durable part, tears are visible. | |
| The third stage (temperature above 1400 °C, molten metal layers are carried away with lubricant). | Melted furrows.
|
|
|
It is determined by the interaction of the material of the parts with the oxygen of the environment with the formation of solid solutions and oxide films that protect the original materials from intense wear. Surface wear consists in the periodic appearance and chipping of hard and brittle oxide films. Minimum wear rate. | Matte stripes consisting of films of oxides, solid solutions and chemical compounds of metal with oxygen.
|
|
Repeated loading causes metal fatigue. Cracks appear on the planes of maximum stresses inside the part. Their development leads to rupture of the contact surface. The movement of the rolling elements through the surface rupture is accompanied by dynamic phenomena, as a result of which wear progresses. | Smallpox-like depressions appear at the points of chipping on the contact surfaces. The most characteristic type of wear of rolling bearing parts.
|
|
Abrasive particles deform microvolumes of surface layers and cause microcutting processes. | Unambiguously oriented in relation to the direction of movement, risks of various depths and lengths.
|
Erosive types of wear include:
- erosion wear- solid particles moving in a gas or liquid flow exert multiple local pulsed impacts on the metal surface, causing loosening and washing out of the surface layer of parts (erosion);
- electroerosive wear– erosion wear of the surface as a result of impact electric current, while there is a partial transfer of metal from one contact to another and metal spraying;
- cavitation wear– hydroerosive wear during movement solid body relative to the liquid and vice versa, in which gas bubbles collapse near the surface, creating a local increase in pressure.
Additional types of wear include ().
Table 7.2 - Additional types of wear
| Conditions of occurrence | Manifestation | A photo |
|---|---|---|
|
Spots at the points of contact of parts. |
|
|
Starts from the surface. It can be continuous (covers with an even layer and changes the surface roughness of parts without forming separate foci) and local (observed in the form of spots, the depth of which varies from a slight point depression to pits). |
|
7.2. Types of destruction and fractures
kink- the destruction of the part caused by poor quality of the material, manufacturing defects, violation of the rules of operation, accidental mechanical damage and other factors.
The type of fracture allows you to determine the causes of its occurrence ().
Table 7.3 - Main types of fractures
| Appearance | The nature of development | Cause |
|---|---|---|
| Ductile fracture | ||
| It has a fibrous structure, without a crystalline sheen (uneven areas scatter light - the fracture surface appears dull). A characteristic feature is the presence of side bevels along the edge of the fracture. | Accompanied by severe plastic deformation of the material of the part. Primary fractures are rarely viscous. A relatively slowly developing viscous crack is either detected in advance, or due to excessive plastic deformation, the part ceases to perform its functions even before failure. | The impact of significant short-term forces arising from the jamming of the mechanism or violation of the technological regime. May occur during prolonged action of forces that cause stresses that exceed the yield strength of the material of the part. |
| brittle destruction | ||
| It has a pronounced crystalline structure in non-deformable materials and smooth from shear in soft materials. Fracture edges are smooth, even, without bevels or with small bevels. A bevel on a brittle fracture indicates the location of the break (the end of the fracture). | In most cases, they begin to develop in areas of stress concentration (in places where stiffeners are welded, where welds, at holes and fillets, in areas of sharp changes in thickness). The centers are often welding defects (hot and cold cracks, lack of penetration, undercuts, slag inclusions, pores, metal delamination). | Occurs suddenly with a single application of force or under the action of repeated impact forces with a small degree of local plastic deformation. |
| fatigue failure | ||
| The following are clearly distinguished: the fatigue fracture zone, which has a fine-grained structure, with a porcelain-like or polished surface; the zone of static destruction - with a fibrous structure for ductile metals and coarse-grained for brittle ones. | They arise in the process of gradual accumulation of damage in the material of parts under the action of alternating stresses, which lead to the formation of microcracks, their development and the final destruction of the part. | It is one of the main types of damage from the action of cyclic loads. |
Rules for cleaning and inspecting a fracture:
- do not remove loose fragments from the fracture surface;
- do not try to put together the parts of the destroyed part;
- do not wipe the fracture with rags and brushes;
- the fracture is cleaned by blowing with compressed air, followed by immersion in kerosene.
Peculiarities hardening defects shown in .
Table 7.4 - Hardening defects
| Manifestation | Cause |
|---|---|
| The hardened layer is fine-grained, uniform. | The temperature regime is maintained. |
| The fracture surface is fibrous, the file leaves a noticeable mark on the part. | The product was not heated to the required temperature. |
| The fracture surface is uneven in grain size. | The product has been heated to a higher temperature than required. |
| The fracture is coarse-grained, with a strong white sheen. | The product has been heated to an excessively high temperature and has been at this temperature for a long time. |
| The fracture is inhomogeneous, in some places non-hardened and well-hardened grains, burnt grains are observed on the ribs and thin parts. | The product was heated too quickly and unevenly. |
7.3. Damage to rolling bearings
Traces radial force applied at one point, constant in direction, with a rotating inner and stationary outer ring, appear as a continuous mark on the inner ring and local wear of the outer ring ().
 |
 |
Figure 7.1 - Traces of a radial force, constant in direction: |
|
a) continuous wear on the inner ring; |
b) local wear of the outer ring |
If the inner ring is fixed and the outer ring is movable, then the effect of a constant radial force will appear as a continuous wear mark on the outer ring and local wear of the inner ring.
At deformation of the outer ring of the bearing as a result of deviations in the shape of the seat, pox-like chipping will appear at two points ().
Figure 7.2 - Smallpox chipping in two places on the tread of the outer ring of a double-row spherical roller bearing with a deviation in the shape of the bearing cap seat
Radial force applied at one point, performing periodic oscillatory motion in a limited sector leads to local wear of the outer and inner rings of the bearing (). This type of wear is typical for hinged mechanisms in which the shaft oscillates.
Figure 7.3 - Local wear of the treadmill of the outer ring of a double-row radial roller bearing during oscillatory motion
Radial force rotating with the shaft, will result in a permanent wear mark on the stationary outer ring and localized chipping on the inner ring ().
Figure 7.4 - Local spalling of the inner ring of a ball bearing with a rotating radial force, a stationary outer ring and simultaneous action of an axial force
Axial force acting in the longitudinal direction, causes displacement of wear marks on the bearing rings (). Additionally, the effect of axial force can be judged by the presence of lightening at the ends of the rollers ().
Figure 7.5 - Highlights on the ends of the rollers of one of the running tracks of a double-row radial roller bearing when exposed to axial force
The bearing unit has both fixed and movable contact surfaces of the parts. Inspection of the rolling bearing is carried out sequentially from the seating surface of the bearing in the mechanism housing to the seating surface of the inner ring on the shaft.
If the surfaces of the inner ring and the shaft are stationary, then the inner ring of the bearing has a matte surface ().
Figure 7.6 - Matte surface of the inner ring of the bearing with a fixed fit on the shaft
Loose bearing seat as a result of mounting errors, operation often leads to rotation of the bearing on the shaft and in the housing (). Bearing rotation is accompanied by an increase in the temperature of the assembly, a change in the nature of noise and vibration, and leads to unacceptable wear of body parts.
Figure 7.7 - Traces of turning the bearing rings
Fretting corrosion occurs when contacting surfaces move under the influence of variable forces or vibrations. Manifested in the form of intense oxidation of surfaces, dark spots on the seating surfaces of the bearing rings (). Leads to knocking, shock during operation of the bearing. With further development, it can cause the initiation of fatigue cracks.
 |
 |
Figure 7.8 - Traces of fretting corrosion on the seating surface of ball bearing rings: |
|
a) internal; |
b) outdoor |
If the load is unevenly distributed along the length of the roller or between the rows of rolling elements of a double-row bearing (), then the life of the bearing is significantly reduced. Reason - misalignment of the bearing housing.
 |
 |
Figure 7.9 - Uneven chipping when the shaft is bent: |
|
a) along the length of the rollers of a radial roller bearing; |
b) along the treadmills of a double-row radial spherical ball bearing |
Inspection of the outer end surfaces of the bearing rings allows you to confirm turning rings or define the presence of contact of the bearing with a nearby part ().
Figure 7.10 - Ring marks on the end surface of the inner ring - the result of contact of the bearing ring with a fixed part
Inspection of the running tracks of the outer and inner rings allows you to establish the nature of the contact between the rolling elements and the running track. Shaft misalignment relative to the bearing housing, it can be fixed along a triangular trace with the oscillatory nature of the bearing loading ().
Figure 7.11 - Triangular shape of the contact of the ring with the roller when the shaft is skewed relative to the housing of a double-row roller radial bearing
Cracks across treadmills are the result of impact dynamic loads, impacts or mounting errors(). Chips of the sides of the rings are the result of the dynamic effects of the axial force ().
 |
 |
Figure 7.12 - Results of impact loading: |
|
a) transverse crack on the bearing ring; |
b) chipped sides of the ring |
Cracks located along the bearing ring are the result of lack of thermal gaps when the machine heats up. The axial force arising from thermal expansion leads to the disappearance of the radial clearance and the appearance of significant radial forces that can lead to the destruction of the outer ring ().
Figure 7.13 - Destruction of the outer ring of a ball bearing in the absence of a thermal gap
Increased axial play a pair of angular contact ball bearings leads, when a longitudinal force occurs, to the appearance of facets or to pox-like chipping on the non-working part of the treadmill ().
 |
 |
Figure 7.14 - Non-working part of the treadmill of an angular contact ball bearing with increased axial play and longitudinal loading: |
|
a) facet; |
b) smallpox chipping |
Brinelling is manifested in the appearance of dents on the treadmills with a step equal to the step of the rolling elements. It is a consequence impact during installation ().
Figure 7.15 - Brinelling on the treadmills of the thrust ball bearing - dents with a pitch equal to the pitch of the rolling elements
False brinelling occurs when lubricant outflow from rolling surfaces of bearings idle machine as a result of mechanical vibrations transmitted from working mechanisms. It manifests itself in the form of damage to the working surface of the bearing, located with a step equal to the step of the rolling elements ().
Figure 7.16 - Traces of false brinelling on the working surface of the outer ring of a roller angular contact tapered single-row bearing
Separator damage is the most serious type of damage. If the separator is damaged, other parts may be damaged due to vibration, wear, jamming and distortion (). The most common cause of separator failure is lubrication problems and deformation of the outer rings. This leads to the appearance of uneven forces on the rolling elements and the impact of destructive forces on the separator.
 |
 |
Figure 7.17 - Destruction of the separator |
|
Rolling bearings must be replaced with one of the following damages:
- fatigue or corrosion shells on the tracks and rolling elements;
- cracks, chips of boards, rings, rolling elements;
- cracks, fracture of the separator;
- wear, breakage of separator rivets;
- nicks on the separator;
- scuffing, corrugation, wear or dents on the working surfaces of the rings and rolling elements;
- surface corrosion or discoloration on work surfaces;
- increase in radial clearance.
7.4. Gear damage
external factors:
- The value of the applied force load determines the following nature of damage on the working surface:
- the nominal load does not lead to a change in the shape of the tooth and does not leave traces of deformation on the working surface of the gear ();
Figure 7.18 - Absence of deformations - a sign of the impact of the rated load:
a) the working surface of the teeth;
b) the end surface of the teeth
- variable or alternating forces, lead to the appearance of stresses on the contact areas that exceed the endurance limit of the material, leave smallpox-like depressions on the working surface caused by material fatigue ();
Figure 7.19 - Exceeding the endurance limit of the material leads to pox-like chipping of the working surface:
a) initial stage;
b) further development;
c) limit state
- plastic shifts on the working surface of the teeth occur when the stresses acting on the contact areas, the yield strength, the surface layer of the metal moves from the pitch diameter to the top of the tooth, forming a protrusion ();
Figure 7.20 - Plastic shears on the working surface of the gear - the stresses on the contact pads have exceeded the yield strength:
a) initial stage;
b) further development
Intermediate manifestations of the acting forces are: peeling of metal particles from the working surface of the teeth, hardening due to strong impacts in the presence of a gap in the engagement.
- the nominal load does not lead to a change in the shape of the tooth and does not leave traces of deformation on the working surface of the gear ();
- The nature of the applied power load associated with the constancy or inconstancy of the speed, change in the direction of rotation, the value of the dynamic component. Dynamic impacts often lead to tooth breaks (). With an increase in the rotational speed, the requirements for the accuracy of manufacturing and installation of gears increase, otherwise the wear of the teeth increases. In non-reversible gears, it is mandatory to inspect the reverse (non-working) surface of the tooth. It may show manufacturing or installation errors. For example, due to a small lateral clearance, contact marks () may appear on the back surface of the tooth.
Figure 7.21 - Fracture of the teeth due to the impact of dynamic impacts
Figure 7.22 - Contact patch on the non-working surface of the wheel tooth
- The presence of abrasive particles or substances that cause corrosion, leads to abrasive wear, corrosion of the tooth surface, contributes to the occurrence of gas or liquid erosion. The main cause of corrosion - the presence of water in the lubricant - appears as a uniform () or uneven layer () of rust on the surface of the teeth.
The initial manifestation of abrasive wear is the appearance of scratches or scratches on the working surface in the direction of movement of the abrasive material (). The development of abrasive wear is facilitated by the use of contaminated or grease, which is an accumulator of abrasive particles. Worn gears have increased engagement gaps; noise, vibration and dynamic overloads increase; the shape of the tooth is distorted; the cross-sectional dimensions and strength of the tooth () decrease.
Figure 7.24 - The initial stage of abrasive wear of the gear pump wheel - the appearance of scratches on the working surface of the teeth
Figure 7.25 - The limiting stage of abrasive wear of the rack gear
The performance of the gearing is affected by such internal factors:
- Immobility of the landing surfaces gear and shaft meets the requirements if the mating parts are stationary when the load is applied (). The appearance of small movements of the mating parts leads to fretting corrosion, which manifests itself in the form of dark spots on the seating surface ().
Subsequently, traces of mutual movement of the mating surfaces appear in the form of shiny polished surface areas. This increases the rate of development of wear processes, creating prerequisites for the occurrence of impacts at the last stage of damage development. When the joint of the mating parts is opened, the rigidity of the joint decreases, dynamic shocks occur, leading to hardening and destruction.
Lecture 2. Types of wear. Lubricants. Ways to deal with wear
Technological processes carried out in the chemical industry are characterized by a variety of parameters. The operating conditions of the equipment are mainly determined by the temperature, pressure and physico-chemical properties of the medium.
Under reliability equipment understand full compliance with its technological purpose within the specified operating parameters.
Durability– the duration of maintaining the minimum allowable reliability under the operating conditions of the equipment and the accepted maintenance system (maintenance and repair).
1.1. Main types of wear
The decrease in reliability and decrease in the durability of equipment are due to the deterioration of its condition as a result of physical or obsolescence.
Under wear and tear one should understand the change in the shape, dimensions, integrity and physical and mechanical properties of parts and assemblies, which is established visually or by measurements.
Obsolescence equipment is determined by the degree of lag of its technical and design purpose from the level of advanced technology (low productivity, product quality, efficiency, etc.).
1.1.1. Mechanical wear
Mechanical wear can be expressed in breakage, surface wear and a decrease in the mechanical properties of the part.
- Breaking
Complete failure of the part or the appearance of cracks on it is the result of exceeding the permissible loads. Sometimes the cause of the breakdown lies in non-compliance with the manufacturing technology of the equipment (poor-quality casting, welding, etc.).
- Surface wear
Under any operating and maintenance conditions, surface wear of parts in contact with other parts or media is inevitable. The nature and amount of wear depends on various factors:
physical and mechanical properties of rubbing parts and media;
specific loads;
relative speeds of movement, etc.
- Wear due to friction forces
Wear is a gradual destruction of the surface of the material, which may be accompanied by the separation of particles from the surface, the transfer of particles of one body to the surface of the conjugated body, a change in the geometric shape of the rubbing surfaces and the properties of the surface layers of the material.
- Abrasion
Abrasion is the relative movement of parts pressed against each other. Rubbing surfaces with any processing have a roughness, i.e. recesses and tubercles. With mutual movement, the tubercles are smoothed out. As a result of the gradual running-in of rubbing surfaces, the work of friction will decrease and wear will stop. Therefore, it is very important to observe the established break-in regime for new equipment.
Another cause of abrasion may be the molecular contact of the surfaces in separate areas, in which they merge by welding. With the relative movement of the surfaces, the welding points are destroyed: many particles come off the friction surfaces.
During friction, the surfaces of the parts heat up. As a result of this, the amorphous layers of the run-in surfaces soften under certain conditions, are transferred to certain distances, and, having fallen into the depressions, harden.
- Bullying
Scoring is the formation of rather deep grooves on the surface, which serves as a prerequisite for further intense abrasion. It has been established that the most frequent cases of scuffing are in rubbing pairs made of the same metal.
- Abrasive abrasion
In addition to solid particles formed during abrasion, a lot of small particles in the form of dust, sand, scale, soot fall on the rubbing surfaces. They are brought in with the lubricant or formed under certain operating conditions. The effect of these particles is small if their dimensions are less than the thickness of the lubricant layer.
- Collapse deformation and fatigue spalling
With a low quality of processing of rubbing surfaces, the actual contact area is much less than the theoretical one: the parts are in contact only with protruding ridges. When the limiting pressure is reached, the deformation of the crushing of the sections protruding beyond the average contact surface occurs.
A frequent change in the direction and magnitude of the load on the friction surfaces leads to metal fatigue, as a result of which individual particles peel off from the surfaces (fatigue chipping).
1.1.2. Erosive wear
Many media that parts come into contact with contain solid particles (salts, sand, coke in oil streams; catalyst, absorbent, etc.) that cause abrasion or wear. Similar wear is observed with strong and prolonged impacts on the surface of liquid and steam jets. The destruction of the surface of the part, which occurs under the action of friction and impact from the working environment, is called erosive wear .
1.1.3. fatigue wear
There are frequent cases when a part subjected to variable loads breaks at stresses much lower than the tensile strength of the part material. The complete or partial destruction of a part under the action of stresses, the value of which is less than the tensile strength, is called fatigue wear .
1.1.4. Corrosive wear
Corrosion is understood as the destruction of the metal surface, which is a consequence of the occurrence of chemical or electrochemical processes. Corrosion can be continuous, local, intergranular and selective.
At solid corrosion, the surface of the part wears out relatively evenly. According to the degree of uniformity of corrosion destruction of the surface layer, continuous uniform (see Fig. 2.1, a) and continuous uneven (see Fig. 2.1, b) are distinguished.
At local Corrosion destruction does not spread over the entire surface of contact with the medium, but covers only certain areas of the surface and is localized on them. In this case, craters and depressions are formed, the development of which can lead to the appearance of through holes. Varieties of local corrosion are: corrosion individual spots (see Fig. 2.1, c), ulcerative (see Fig. 2.1, d), point (see Fig. 2.1, e).
Intergranular (or intercrystalline) corrosion - the destruction of metals along the grain boundary (Fig. 2.1, e). This type of corrosion is typical for parts made of chromium-nickel steels, copper-aluminum, magnesium-aluminum and other alloys.
Deeply penetrating intergranular corrosion is called transcrystalline (Fig. 2.1, g).
selective(structural-selective) corrosion consists in the destruction of one or several structural components of the metal at the same time (Fig. 2.1, h).
Rice. 2.1. The nature and forms of the spread of corrosive wear:
a - continuous uniform; b - continuous uneven; c - local;
g - ulcerative; d - point; f – intergranular; g - transcrystalline;
h - structural-selective
According to the mechanism of action, chemical and electrochemical corrosion are distinguished.
Chemical corrosion - corrosion of metal by chemically active substances (acids, alkalis, salt solutions, etc.).
Widespread electrochemical corrosion occurring in aqueous solutions of electrolytes, in an environment of moist gases and alkalis under the action of an electric current. In this case, metal ions pass into the electrolyte solution.
Underground (soil ) corrosion is the result of the action of soil on the metal. In most cases, it occurs during aeration and is local in nature. Soil corrosion is biocorrosion (microbiological corrosion) caused by microorganisms. She most often appears in earthen ground, in ditches, in sea or river mud.
External surfaces of equipment, pipelines, metal structures are subject to atmospheric corrosion occurring in the presence of an excess amount of oxygen under the alternating action of moisture and dry air on the metal.
In chemical equipment, the so-called contact corrosion. It occurs at the site of contact between two different or identical metals in different states.
1.1.5. Thermal wear
A significant part of the equipment of chemical and petrochemical plants operates at high temperatures. Under these conditions, being in a stressed state, the steel structure undergoes creep and relaxation over time.
Phenomenon creep consists of slow plastic deformation structural element under a constant load. If the stresses are small, then the growth of deformation over time may stop. At high stresses, deformations can increase until the product fails.
Under relaxation is understood as a spontaneous decrease in stress in a part, with a constant value of its deformation, under the influence of high temperature. Relaxation can lead to equipment depressurization and accidents.
Violation of the stability of the structure at high temperatures is due to graphitization, spheroidization and intergranular corrosion.
Process graphitization is the destruction of carbide with the formation of free graphite, resulting in a decrease in the impact strength of the metal. Gray cast iron, carbon and molybdenum steels are susceptible to graphitization at temperatures above 500 °C.
Spheroidization does not significantly affect the strength of steels. It lies in the fact that lamellar perlite takes on a round granular shape over time.
1.2. Ways to control and measure wear
Qualitative and quantitative methods are used to assess corrosion damage.
The qualitative method consists in visual inspection of the sample and its examination under a microscope in order to check the state of the surface, detect corrosion products on these surfaces or in the medium, establish changes in color and physicochemical properties of the medium.
quantitative method consists in determining the corrosion rate and the actual mechanical characteristics of the metal.
An indicator of the magnitude of corrosion is the depth of damage to the metal at individual points, determined with the help of special instruments. The nature of corrosion and its rate are determined by systematic inspections and measurements made periodically during the entire service life of the equipment. However, such periodic examinations require a fairly frequent shutdown of the devices, their preparation and opening, which reduces the productive time.
Therefore, preference is given to the method of continuous monitoring using probes. The principle of operation of the probe is based on the control of changes in the electrical resistance of samples made of the same material as the equipment under study. A sample of certain sizes and shapes is placed inside the apparatus in those areas where the study of the nature of metal corrosion or the aggressive properties of the medium is of greatest interest. The readings of all probes are placed on one shield.
It is more difficult to control the corrosion damage of non-metallic materials. The mechanism of destruction of polymeric materials differs from the corrosion of metals and is not well understood. The difficulty lies in the fact that the polymer swells in the medium and quickly dissolves. These processes propagate deep into the polymer material due to diffusion.
The simplest and most common method for determining the amount of wear is micrometerage , i.e., measuring the actual dimensions of parts using a variety of tools (calipers, micrometers, gauges, templates, etc.).
For a more accurate determination of the total amount of wear, a method is used that consists in determining the mass loss of the sample as a result of wear. This method requires thorough cleaning and rinsing of the parts and a highly sensitive balance.
In some cases, when it is required to control the wear of equipment during its operation (on the go), they use integral method , which provides for determining the amount of steel or cast iron that has passed into lubricating oil as a result of wear of friction surfaces. To do this, take a sample of oil for chemical analysis.
In addition to normal wear, in practice there are frequent cases of so-called catastrophic wear, which occurs very quickly, and sometimes instantly (breakdown). The possibility of catastrophic wear should be established as soon as possible to prevent accidents. To do this, use all possible ways visual inspection and touch test.
During an external examination, they check the correct relative position of parts and components of the machine, the density and strength of the joints, fastening to the foundation, etc. The temperature of the rubbing parts and the vibration of the machine or its individual components are determined by touch. Increased temperature and unacceptable vibration may be the result of increased wear.
Breakage of moving parts is easy to establish by knocking or noise by ear or with the help of a special hearing aid.
Wear is a random process, because it depends on a large number factors. Therefore, the analytical description of wear is performed on the average values of wear indicators.
Wear rate- the absolute wear of the part in time, expressed in linear, mass or volume units, and is measured in microns / h, g / h, mm 3 / h, respectively.
Wear intensity is the ratio of absolute wear to the sliding distance (µm/km, m/m).
The intensity of linear wear is determined by the equation
I h = h/L,
where h is the height of the worn layer;
L is the length of the friction path.
The intensity of mass wear is determined by the equation
I m = M/FL
where M- mass of worn metal;
F is the nominal surface of the friction area.
Relationship between I h and I m is determined by the formula
I h = I mρ,
where ρ is the density of the metal.
As the temperature rises, the hardness of the material decreases, and the equation is used to describe the wear rate as a function of temperature:
I = A exp( BT),
where A, B- permanent.
To describe the dependence of wear rate on pressure P usually a power equation is used
I = CPn,
where C, n- permanent.
The surface finish determines the actual contact surface of the rubbing parts. The cleanliness of processing determines mainly the wear during the break-in period. On fig. 2.2 shows the change in surface roughness over time for different initial finishes. Time τ 1 characterizes the running-in period, i.e., when a noticeable change in roughness is observed. At τ >τ 1, a period of steady wear is observed.
The optimum roughness depends on the properties of the materials, the shape of the parts, the working conditions of the friction pairs and the presence of lubricant.
The nature of the wear of parts over time is shown in Fig. 2.3. The initial value of the gap in the connection is determined by the design of the connection. The wear curve can be divided into the following sections:
I is the running-in period, characterized by increased wear due to the rapid destruction of microroughnesses;
II - the period of normal wear, characterized by a constant wear rate;
III - the period of emergency wear, characterized by an increase in the wear rate.
The gap δ 2 corresponding to the transition from the period of normal wear to emergency wear is the maximum allowable. The numerical values of δ 2 are given in specifications for car repairs.
It follows from the wear curve that the wear rate (the tangent of the slope of the tangent to the wear curve) decreases during the running-in period, during the period normal operation remains constant, and increases with emergency wear. AT general view the wear equation will look like
The simplest linear dependence has the form
where A, B- coefficients.
RELIABILITY AND REPAIRABILITY OF EQUIPMENT
Any device after manufacture or repair must work for a certain time. The need and frequency of repairs are determined by its reliability.
Reliability- the property of the product to perform its functions, maintaining performance within the specified limits for the required period of time.
performance- the state of the object, in which it is able to perform the specified functions, while maintaining the values of the specified parameters within the limits established by the regulatory and technical documentation.
Inoperability- the state of the object, in which the value of at least one of the specified parameters does not meet the requirements of regulatory and technical documentation.
Reliability- the property of an object to continuously maintain operability for a certain period of time.
Refusal- an event consisting in a violation of the object's operability.
limit state- this is the state of the object, in which its further operation must be terminated due to an unrecoverable violation of safety requirements.
Operating time- the duration or scope of the object's work.
Technical resource– operating time of an object from the beginning of operation or its resumption after a major overhaul until the limit state occurs.
Durability- the property of the object to remain operational until the limit state occurs with the established system of maintenance and repair.
maintainability- the property of the object, which consists in adaptability to the prevention and detection of the causes of its failures and the elimination of their consequences by carrying out repairs.
Object under repair- this is an object whose serviceability and operability in the event of a failure or damage is subject to restoration.
Non-repairable object- this is an object, the serviceability and operability of which in the event of a failure or damage cannot be restored.
The above definitions show that the reliability of equipment depends on the quality of maintenance and repairs. Reliability issues should be of the greatest importance in the development of new equipment. In the chemical industry, a major role in improving reliability is assigned to repair services.
The failure of parts most often occurs not due to insufficient strength, but due to wear of the working surfaces.
secondary resource, i.e., the resource acquired after the first overhaul is not always equal to the primary resource of the new machine. In the car, fatigue or aging accumulates, as it were, not eliminated during a major overhaul. However, the main reason for the low secondary resource is the lower quality of repair work compared to the quality of work carried out during the manufacture of the machine at a specialized machine-building plant.
Quantitative indicators of reliability are expressed in the form of any absolute or relative values. Reliability cannot be accurately measured or predicted; it can only be estimated approximately by specially organized tests or collection of operational data.
Reliability is also failure rate λ is the number of equipment failures per unit of time, related to the number of equipment of the same type in operation.
In accordance with the physical picture of wear, a component failure rate curve is constructed (Fig. 2.4). Section I characterizes the change in the failure rate during the running-in period, section II - the failure rate during the normal operation period, section III - the change in the failure rate during the period of increased wear.
Rice. 2.4. Part sudden failure rate curve λ
Possible failure modes:
1. Failures in early period machine operation. Burn-in failures are the result of imperfection in the manufacturing technology of parts or poor-quality assembly and control.
2. Sudden failures - take place with a sudden load concentration that exceeds the calculated one. They occur randomly, and it is impossible to predict their occurrence, but it is possible to determine the probability of random failures.
3. Failures caused by wear parts are the result of machine aging. Timely inspections, lubrication, repair and replacement of worn parts serve as a means of preventing them.
maintainability It is characterized by the adaptability of the machine to the detection of damage, maintainability and maintainability.
Adaptability to determine damage, to diagnose the technical condition without disassembling the machine depends on the design, the presence of safety, signaling, measuring devices and nodes open for viewing.
Maintainability is evaluated by the ease of access to units and individual parts for inspection and repair and depends on the availability of hatches and covers that can be opened.
Maintainability is determined by the ability of the machine to replace parts and the ability of parts to recover.
Quantitatively, maintainability is characterized by the proportion of the time of correct operation of the device:
where T b – duration of no-failure operation;
T p is the duration of downtime for repairs;
T o is the time spent on maintenance.
The main requirements for the maintainability of equipment can be divided into two groups.
The 1st group includes requirements that ensure the maintainability of equipment during inspection and repair on site:
a) free access to units and parts subject to inspection, adjustment or replacement;
b) quick replacement of wearing parts;
c) adjusting the interaction of units and parts, broken in the process of work;
d) checking the quality of the lubricant, its replacement or replenishment at the place of operation of the equipment;
e) rapid determination of the causes of accidents and equipment failures and their elimination.
The 2nd group includes requirements that ensure maintainability during repairs at the RMC of enterprises:
a) ease of disassembly and assembly of units, as well as complexes;
b) the use of simple means of mechanization in the operations of disassembly and assembly;
c) the maximum possibility of restoring the nominal dimensions of wearing elements;
d) ease of checking the condition of parts and assemblies after bench tests;
e) the possibility of checking the interaction of all parts of the equipment after repair.
The tendency to depreciation is inherent in many types of property accounted for in the company, including fixed assets. About what are the types of depreciation of fixed assets and how to determine it, will be discussed in the publication.
The concept and types of depreciation of fixed production assets (OPF)
OPF - assets designed for operation in production for a long time (more than 1 year) and wear out in the course of work.
Depreciation is considered to be the gradual loss of consumer qualities of an object and, accordingly, its value. It happens in different ways. Some objects wear out due to the obsolescence and dilapidation of constituent materials, mechanical wear, metal fatigue under the influence of production processes, natural phenomena and other factors, while others - due to the loss of expediency of use and reduction economic efficiency in application. And since production assets wear out for completely different reasons, they classify this phenomenon in accordance with them.
Based on the above criteria, the types of depreciation of fixed assets include physical and moral depreciation.
Obsolescence of fixed assets
The obsolescence of the fixed assets is found in the depreciation of the fixed assets, as a result of the appearance of technical innovations, sometimes long before the end of the JFS. Distinguish obsolescence of the 1st and 2nd orders.
The first category includes depreciation caused by an increase in labor productivity in the industries producing OF. This process leads to a reduction in the cost of manufactured objects that already have increased competitiveness due to lower prices.
The obsolescence of fixed assets of the 2nd order occurs as a result of the creation of the most cost-effective fixed assets, the emergence of new facilities that increase production productivity.
Obsolescence can be partial or complete. Partial depreciation is recognized, which is a shared loss of the consumer value of the object. Depending on the specifics of production, it is possible to prevent partial obsolescence of an object by using it in other operations where efficiency will be higher.
Complete obsolescence is the complete depreciation of the object. In such cases, its use in production becomes unprofitable.
Physical depreciation of fixed assets
The physical deterioration of the OS means the loss of use value. Distinguish between productive and unproductive wear. Productive is characterized by loss of value, which is the result of operation, unproductive wear and tear is an invariable attribute of objects that are under conservation for various reasons, such as the impossibility of use, natural aging, etc.
Physical deterioration can be complete or partial. In full, OS items are replaced by new assets as the lifespan has expired and the cost of the OS has fully passed into the price of the products being released. An example is capital construction, when an erected building replaces a worn one. Partial physical depreciation implies the possibility of further operation of the object, carrying out repair work, reconstruction, if appropriate, or the implementation of appraisal work to determine the percentage of depreciation of the object and establish the possibility of its operation or sale.
Wear Calculation Methods
The degree of physical wear and tear of fixed assets depends on such factors as the intensity and duration of operation, the characteristic features of the OS designs and the circumstances of work. We will consider methods for calculating the depreciation of buildings, since they most often require professional evaluation.
In the special literature on assessment, 5 methods for calculating the physical deterioration of buildings are described. These are the methods:
- cost compensation;
- chronological age;
- effective age;
- expert;
- breakdowns.
Consider the features of each of them.
- Cost compensation consists in equating the amount of depreciation to the cost of its elimination, which is an excellent justification for the amount of depreciation. The disadvantage of the method is its laboriousness of calculations, especially for large buildings.
- With the chronological calculation method, the formula is used:
And physical \u003d B x / B ss x 100, where B x is the age of the object in fact, B ss is the service life of the building according to the standard.
Let's calculate the physical deterioration of the building, example:
Let us determine the depreciation of a building that has served 750 months with a standard service life of 1200 months.
And physical \u003d 750 / 1200 x 100 \u003d 62.5%
The advantage of the method is the simplicity of calculation, but it does not take into account the repairs and replacements that took place during operation, which often happens in practice. Therefore, this method is considered effective for calculating depreciation in the first years of OS operation; if the building is more than 10 years old, you should not use it .;
- Calculation by the effective age method has 3 variations:
And physical \u003d V e / V ss x 100%, where where V e is the effective age of the object, i.e. the expert evaluates the structure by appearance.
And physical \u003d (V ss - V ost) / V ss x 100%
And physical \u003d (1 - B st / V ss) x 100%, where B st - the remaining life of the building.
Substituting the initial data of the previous example into the formulas and adding the expert's estimate of 720 months, we get the values:
And physical \u003d 720 / 1200 x 100 \u003d 60%
And physical \u003d (1200 - 450) / 1200 x 100 \u003d 62.5%
And physical \u003d (1 - 450 / 1200) x 100 \u003d 62.5%
The disadvantage of the method is the impossibility of a strong justification of the effective age of the structure. There is a large calculation error (this can be seen from the first formula).
- The expert method is based on the rating scale for depreciation, proposed in the "Rules for assessing the physical depreciation of residential buildings" VSN 53-86r. Its value is determined by external damage to the elements. This method is used by BTI employees when issuing registration certificates. Wear is determined by the formula:
And physical \u003d ∑ (I k x HC k) x 100%, where I k is the amount of wear of a certain element in the building, calculated according to the rules of VSN 53-86r, UV k - specific gravity this element in the building.
The specified NLA describes in detail the expert methodology, we introduce only the principle of calculation. expert method is the most commonly used.
- The breakdown method proposes the establishment of physical depreciation as a whole by summing the depreciation values for individual groups, expressed in:
- Correctable wear (deferred repair);
- Irreparable wear of short-lived (i.e., repeatedly replaced during operation) elements;
- Irreparable wear and tear of long-lived (recovery of which is possible only with the overhaul of the building) elements.
At different stages of determining wear, all of the above methods for calculating physical wear can be used.
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