Types of wear. Types and causes of parts wear Types of parts wear
During operation, the equipment and its elements, being subjected to various influences, change in condition, size and properties. These changes can proceed smoothly (regular change) and abruptly (irregular change). The reasons for these changes are wear phenomena, estimated by changing the geometric dimensions of machine elements, their mass, or by some other indirect signs (wear due to shape changes without mass loss, etc.).
Wear- a process that leads to a change not only in the external, but also in the strength characteristics of machine elements, which gradually reduces their reliability and leads to failures in operation.
The most intensive wear process occurs in the conjugated elements of machines, especially when they are mutually moving. On fig. 7 shows the main factors that determine the wear processes in machines.
Rice. 7. Main factors determining wear processes in machines and equipment.
Wear- the result of wear, manifested in the form of separation or permanent deformation of the material of the part. The consequence of wear, as a rule, is a violation of interfaces, kinematic connections and the operation of parts of a given unit or mechanism as a whole.
Machine wear can be mechanical, molecular-mechanical, corrosion-mechanical, corrosion.
Mechanical wear occurs as a result of mechanical influences and includes the following types of wear: abrasive, hydroabrasive, gas-abrasive, erosive, fatigue, cavitation.
abrasive wear occurs as a result of the cutting and scratching action of solid particles. These particles, which have entered from the outside or separated (chipped, planed, etc.) from mutually contacting and rubbing parts, significantly increase their wear.
Waterjet wear occurs as a result of the action of solid particles trapped in the flow of an oily liquid that serves as a lubricant between parts.
Gas-abrasive wear arises as a result of the action of solid particles trapped between rubbing parts with gas flows.
erosion wear surfaces of parts occurs as a result of exposure to liquid or gas flows containing excessively small solid particles or inclusions.
Gas-abrasive wear is typical for internal combustion engines, and erosive wear is typical for its parts: valve system, injector nozzles, carburetor jets, etc.
fatigue wear occurs as a result of repeated deformation of the material of the parts. It arises and develops in the most stressed, mainly working, surface layers of parts due to the long-term action of loads, especially those that are variable in value and direction. In this type of wear, the cause of parts failures are fatigue cracks that begin to develop in that part of the surface where tensile stresses act, and, as a rule, from the place where various kinds of risks, nicks, and delaminations appeared.
cavitation wear manifests itself in the relative movement of solids in a liquid medium. Most often, it is observed in the liners of the cylinder block, cooling and lubrication systems of internal combustion engines, blades of oil and water pumps, etc.
Molecular mechanical wear occurs as a result of the simultaneous action of mechanical and molecular or atomic forces. Due to their unevenness and roughness, the mutually contacting and rubbing surfaces of the mating parts have contacts through which significant specific loads are transmitted, therefore, breaks in the lubricating film (oils, ointments) are possible, and at high relative speeds of movement of the surfaces of the parts, excessive heating occurs, leading to the evaporation of the lubricating oil film or ointments and to the setting of particles in contact parts. In the future, separation and destruction of the places of setting of parts occurs. In this case, a recess is formed on one of the surfaces, and a protrusion is formed on the other, i.e. transfer of metal from one surface to another.
The considered type of wear is observed in the process of running-in parts and elements of machines.
Corrosion-mechanical wear occurs during the friction of materials that have entered into chemical interaction with the environment (air oxygen and other gases). Under the action of an aggressive oxidizing environment, oxide films are formed on the mutually contacting and rubbing surfaces of the parts, which are removed as a result of mechanical friction, and the surfaces freed from these films are oxidized again, etc., i.e. wear process takes place. An example is the wear of parts of the cylinder-piston group of engines due to the presence in the environment of such corrosion agents as sulfuric, sulphurous and organic acids.
The most significant influence on the wear process is exerted by friction forces, causing mechanical and other types of wear of mutually contacting surfaces. Moreover, the wear resulting from friction is a whole series of simultaneously occurring processes: abrasion, crushing, oxidation, etc.
Process abrasion occurs when one part of the machine or its element slides relative to another. This phenomenon is called friction of the first kind and occurs due to the fact that the contacting surfaces, as a rule, have irregularities (roughness) that prevent the free movement (sliding) of one part over another. The process of abrasion is the more intense, the more rough the contacting surfaces. Wear intensity increases if abrasive or other inclusions get between the contacting surfaces.
The process of abrasion also occurs during mutual rolling of the surfaces of machine parts under load and during impacts. This phenomenon is called friction of the second kind. It occurs due to the fact that, as a result of rolling or impacts, microcracks, and often macrocracks, appear on the surfaces of the contacting parts, with their subsequent development in depth and the formation of a thin metal film, which subsequently crumbles and peels off, resulting in the so-called wear at major destruction. The causes of such wear can be surface fatigue, as well as structural damage to the metal of the contacting surfaces due to heat and shock. The considered type of mechanical wear often appears on the working surfaces of gear and worm gears, rolling bearings, various support devices, etc.
Fig.8. Wear in mating parts: a - increase in wear; b - wear rate
Corrosion wear- destruction of metal parts of machines under the influence of the environment, especially moistened. Destruction in this type of wear begins, as a rule, from the outer surfaces, gradually penetrating deep into. The most common type of corrosion is rusting, i.e. combination of metal with atmospheric oxygen. As a result of corrosion, the unpainted surfaces of the metal parts of machines are first covered with a dark coating, and then with deep (if the necessary measures are not taken) corrosive flaws, while the metal parts acquire a spongy fragile structure. Parts of machines with a low carbon content are subjected to the greatest damage and wear as a result of corrosion. The intensity of corrosion increases in the presence of a number of gases and liquids containing acids and alkalis.
There are two types of corrosion wear processes: chemical And electrochemical.
Chemical corrosion manifests itself under the influence of atmospheric oxygen and various gases (carbon dioxide, sulfur dioxide), as well as liquids that do not conduct electric current (oils and ointments of oil refining, various resins). The intensity of chemical wear of parts depends on the quality of the materials from which they are made, the degree of oxidation at high temperatures and operating conditions (neutral or aggressive environment, etc.).
Electrochemical corrosion occurs in media that conduct electric current, i.e. in electrolytes - solutions of salts, acids, alkalis, as well as in a humid atmosphere and soil.
The pattern of increasing wear of equipment elements, especially in their joints, is expressed by a curve that has three clearly defined sections that characterize the periods of operation of the joints (Fig. 8):
I- the running-in period, when the joints wear out very intensively, but the wear rate gradually decreases;
II- the period of normal operation, when the conditions on the surface of the articulated parts become constant, and wear proceeds at a constant rate;
III- the period of emergency, most intensive wear, when wear (clearances) reach unacceptable values.
The period of normal operation of a piece of equipment (assembly unit, parts, etc.):
(31)
where - the duration of the running-in parts; - wear corresponding to the maximum allowable wear (clearance) in the interfaces of parts; - wear corresponding to the end of running-in parts; tg- coefficient characterizing the rate of wear of parts.
The following main factors influence the wear rate of a normal period of operation: operating conditions - pressure, nature of loads, relative speeds, temperatures, etc.; properties of materials, their variability in work; mating conditions, the nature of the contact of mating elements, the quality of processing of the material from which these elements are made; timeliness and quality of technical services; compliance of the used fuels and lubricants.
In addition to wear, the phenomena of plastic deformation of equipment elements are possible due to unacceptable loads on these elements.
Changes in machines and their elements are expressed by the following functional dependence:
where - operational factors (the nature and characteristics of the production of works, modes of use of machines, climatic conditions, etc.); - design factors (kinematic and dynamic features of machines, properties of materials from which their elements are made, etc.); - technological factors (type of materials from which machine elements are made, methods and quality of their processing, etc.); - subjective features and qualifications of the personnel servicing the machine (drivers, mechanics, tankers, etc.).
Wear and tear in machines and their elements are divided into moral and physical.
Obsolescence- decrease in the cost of equipment under the influence of technical progress.
This type of wear has two forms of manifestation. Obsolescence of the first form is the depreciation of machines due to the constant growth of labor productivity in the industries that produce these machines, as well as manufacturing products, materials, etc. for them. The area of distribution of this form of obsolescence is determined by the rate of technical progress of that sector of the national economy and related industries that produce these machines or components for them, materials, etc.
Loss of equipment value due to obsolescence of the first form:
(33)
where is the initial cost of the equipment, rub.; - the replacement cost of the machine or the cost of its complete reproduction at the time of physical wear and tear, taking into account the appearance of more advanced designs, rub.
Replacement cost of equipment after a certain time T:
(34)
where is the initial cost of the car, rub.; R- the average annual increase in labor productivity in the industry and related industries that produce the specified type of equipment.
Obsolescence of the second form is the depreciation of equipment due to the emergence of new technology, i.e. machines similar or close to them, but more advanced designs. An indicator of the obsolescence of this form is the coefficient of reduction in the cost of machines due to technical progress, expressed as a fraction of its original cost:
(35)
Physical deterioration arises as a result of mechanical molecular-mechanical and corrosion-mechanical wear and consists of the wear of structural and non-structural elements of machines. Physical wear appears both as a result of the direct action of machines and their elements (wear as a result of the direct action of machines), and as a result of the indirect action of equipment and its individual elements (wear as a result of equipment inactivity - during downtime, when they are affected by atmospheric and other adverse conditions ). Depreciation is determined as a percentage: new elements in the equipment (parts, assembly units, etc.) are taken as 100% serviceability, and worn ones, the use of which is impossible, are taken as 100% wear.
In value terms, the physical wear and tear of equipment is determined (% of the cost of reproduction):
(36)
where is the estimated cost of equipment repair, rub.; - replacement cost of equipment or the cost of complete reproduction of equipment at the time of its physical wear and tear, taking into account the appearance of more advanced designs, rubles; a is the relative value of residual wear, which is established from the experimental data of the repair of such equipment, %.
Repair of equipment is expedient if the cost of restoring equipment is less than the cost of acquiring a new one, i.e.< , где - стоимость нового оборудования. При этом нельзя не учитывать степени совершенства конструкции, соответственно, и технико -экономических показателей как старого, так и нового оборудования.
General depreciation of equipment due to physical and moral depreciation as a share of the initial cost:
Fig. 9 Graph of the total wear of a machine, consisting of elements that are replaceable or completely renewable at different service lives (according to aggregated indicators)
The total wear of equipment is determined by two methods - analytical And graphic. The most obvious is the graphical method.
On the horizontal axis (Fig. 9), the full service life of the machine T (accepted according to the standard service life tables) is plotted, and on the vertical axis - the maximum indicator of total wear. First, the wear of the main non-replaceable and non-renewable element of equipment (main frame, bed, etc.) is determined with the absolute wear value . A straight line drawn from the origin to the point represents the line of total wear of the element in question. By the end of the life of the equipment, this element will be completely worn out over time (partial wear
The service life of machine elements is taken according to the results of experimental verification, test data or according to regulatory reference books.
When determining the total wear, the following order is followed: make a list of all structural and non-structural elements of the machine; determine their service life; select constructive and non-constructive elements in groups so that each of them can be considered as one enlarged element; determine the service life and cost of all simultaneously replaceable or renewable elements of each of these groups; make a table and calculate the total wear for any interval of using the machine or build a wear graph based on aggregated indicators.
In determining the life or cost of enlarged elements and the frequency of their renewal, data on the frequency and average cost of the corresponding maintenance and repair, together with the average cost of the spare parts replaced, can be used.
A significant share of the costs of the enterprise - the costs associated with the use of machinery, equipment, production facilities. Their use has a characteristic feature: unlike material resources, they are not consumed in one production cycle. Capital resources last for years and wear out.
Depreciation of equipment is the loss of its value and performance. Wear and tear can occur due to many reasons: aging of equipment, loss of its competitiveness, etc. Today, the fight against wear and the extension of the service life of equipment is a very urgent task.
Depreciation in the economic sense means the loss of value of the equipment during its operation. In this case, two types of wear are distinguished: physical and moral. Physical wear and tear occurs due to the aging of equipment and the loss of its performance, and moral wear due to the loss of competitiveness.
Physical depreciation is the loss of fixed assets of their original consumer value, as a result of which they become unusable and require replacement with new funds. This is normal wear and tear. It is the result of past periods of operation, environmental influences and downtime. As a result of physical wear, the technical characteristics of the object worsen, the probability of breakdowns and accidents increases, the residual service life of the object as a whole or some of its components and parts decreases. This leads to an increase in waste, the risk of serious accidents, the inability of machines and equipment to meet the requirements for proper functioning. Production costs (materials, energy), maintenance and repair costs also increase.
The physical type of wear is divided into subspecies:
1. For the reason that caused the wear, wear of the first and second kind is distinguished. Depreciation of the first kind accumulates as a result of operation. Depreciation of the second kind occurs due to accidents, natural disasters, violations of operating standards, etc.
2. According to the flow time, wear is divided into continuous and emergency. Continuous is a gradual decrease in the technical and economic indicators of objects. Emergency - wear, rapidly flowing over time.
3. According to the degree and nature of the distribution, wear can be global and local. Global - wear, uniformly spreading over the entire object. Local - wear, affecting individual parts and components of the object.
4. According to the depth of flow, partial and complete wear are distinguished. Partial - depreciation, allowing repair and restoration of the object. Full involves the replacement of this object with another.
5. If it is possible to restore the lost consumer properties, wear can be removable and irreparable.
6. According to the form of manifestation, technical and structural wear are distinguished. Structural wear is manifested in the deterioration of the protective properties of external coatings and the increase in fatigue of the main parts and components of equipment, which increase the likelihood of accidents. Technical wear and tear is wear and tear, expressed in a decrease in the actual values of technical and economic parameters compared to standard or passport values.
The decrease in the value of capital goods may be associated not only with the loss of their consumer qualities. In such cases, we speak of obsolescence.
Obsolescence is understood as a decrease in the cost of equipment and other fixed assets until the end of their service life due to a decrease in the cost of their reproduction, as new types of fixed assets begin to be produced cheaper, have higher productivity and are technically more advanced. Therefore, the use of obsolete machines and equipment becomes economically unprofitable as a result of their low productivity and high cost.
The time of obsolescence and its degree are determined by the influence of many factors. First of all, these are the features and scale of production. Machinery and equipment, the use of which becomes unprofitable in some conditions of production, can be successfully used in others. In this case, we can talk about partial obsolescence of equipment. Losses from obsolescence can be eliminated by upgrading and refurbishing equipment, as well as using it to perform work where it remains cost-effective.
Losses from complete obsolescence are eliminated only by replacing obsolete machines and equipment with new, more advanced and cost-effective ones. Sometimes the improvement of existing equipment and machinery is more effective than its replacement. Therefore, a more rational way to reduce obsolescence is the modernization of machinery and equipment.
There are two forms of obsolescence.
Obsolescence of the first kind is due to the growth of the efficiency of production of capital goods. It is caused by the appearance of similar, but cheaper means of labor.
Obsolescence of the second kind - depreciation of fixed assets due to the creation of new, more productive and improved equipment.
To assess the degree of physical wear and tear, the following assessment methods are used:
Expert method based on the examination of the actual technical condition of the object;
Lifetime analysis method based on comparison of actual and standard life of equipment.
The types of wear are distinguished in accordance with the existing types of wear - mechanical (abrasive, fatigue), corrosion, etc.
Mechanical wear is the result of friction forces when one part slides 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 mates of parts as a shaft - a bearing, a frame - a table, a piston - a cylinder, etc. It also appears during rolling friction of surfaces, since this type of friction is inevitably accompanied - there is also sliding friction, but in such cases, wear is very small.
The degree and nature of mechanical wear of parts depend on many factors: the 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; conditions for lubrication of rubbing surfaces; the degree of roughness of the latter, etc. The most destructive effect on the parts is abrasive wear, which is observed in cases where the rubbing surfaces are contaminated with small abrasive and metal particles. Usually, such particles fall on rubbing surfaces during the processing of cast billets on a machine, as a result of wear of the surfaces themselves, dust ingress, etc. They retain their cutting properties for a long time, form scratches, scuff marks on the surfaces of parts, and also, mixing with dirt, they 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, for example, irregularities in the supply of lubrication, poor repair and failure to meet its deadlines, power overload, etc.
In. 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 a part, causing fatigue of the part material 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 one is shelled and sometimes granular.
Fatigue failure of the material of the part does not necessarily lead to its failure immediately. It is also possible the occurrence of fatigue cracks, peeling and other defects, 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 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 closely 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, and temperature fluctuations. For example, if the air temperature in industrial premises is unstable, then every time it rises, the contained
Rice. one.
a - bed and table guides, b - internal surfaces of the cylinder, c - piston, d, d - shaft, f, g - 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 , // -- screw; And - places of wear, P "acting forces
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 erosions 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-mechanics occurs, i.e., complex and wear.
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, a-i).
As a result of wear of the guide machines, their flatness, straightness and parallelism are violated due to the action of unequal loads on the sliding surface. For example, the 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 mating with them become convex.
Cylinders and piston liners in engines, compressors, hammers and other machines also wear out unevenly (Fig. 1, b). Wear occurs in the area of movement of the piston rings and manifests itself in the form of wear of the inner walls of the cylinder or liner. The shape of the cylinder bore is distorted - deviations from cylindricity and roundness (barrel shape) are formed, scratches, scuffs * and other defects occur. In the cylinders of internal combustion engines, their upper part, which experiences the highest pressures and the highest temperatures, is subjected to the greatest wear. In forging and pressing equipment, on the contrary, the greatest wear appears in the lower part of the cylinder - where the piston is located during impacts. Piston wear (Fig. 1, c) manifests itself in abrasion and scoring on the skirt
The wear of the shafts (Fig. 1, d, e) is manifested by the appearance of various defects: the shafts become bent, twisted, and also broken due to material fatigue; bullies form on their necks; cylindrical necks become conical or barrel-shaped. Deviations from roundness are also acquired by the holes of plain bearings and bushings. The uneven wear of the necks of the shafts and the surfaces of the holes in the bushings during the rotation of the shaft is the result of the action of various loads in different directions. If only the force of its gravity acts on the shaft during rotation, then wear appears in the lower part of the bearing (see Fig. 1, d, left).
In gears, the teeth wear out most often: scuffs form, the teeth change their shape, size and break out. Breakage of teeth, the appearance of cracks in the spokes, rim and hub of gears, wear of mounting holes and keys occurs for three main reasons: 1) gear overload; 2) ingress of foreign bodies into it; 3) incorrect assembly (for example, mounting gears on a shaft with misaligned axes).
The lead screws have a trapezoidal or rectangular thread. The threads of the screw and its nut wear out, the turns become thinner (Fig. 1, Z.). Thread wear on screws is usually uneven
* Seizure - damage to the friction surface in the form of wide and deep grooves in the direction of sliding. dimensional, since the vast majority of parts processed on machines have a shorter length than the lead screw. The part of the thread that works more wears out more strongly. Lead screw nuts wear faster than screws. The reasons for this are as follows: the thread of the nuts is inconvenient to clean from contamination; nuts in some cases are unsatisfactorily lubricated; for a nut associated with a screw, all threads are involved in the work, while for a screw only a small part of its turns, equal to the number of turns of the nut, work at the same time.
In disc couplings, as a result of the action of friction forces, the ends of the discs are subjected to the greatest wear (Fig. 1, i); their surfaces are abraded, scratches, scuffs appear on them, flatness is disturbed.
In threaded connections, the thread profile most often wears out, as a result, the gap increases in them. This is observed in
Rice. 2. Wear of rolling bearings:
a - due to misalignment, b - when turning the inner ring on the shaft, c - due to excessive tightness, d - due to a faulty stuffing box; I - wear points
interfaces not only running, but also clamping, for example, clamping screws of frequently unscrewed mounting bolts. Wear of threaded connections is the result of insufficient or, conversely, excessive tightening of screws and nuts; wear is especially intense if the working connection perceives large or alternating loads: bolts and screws are stretched, the thread pitch and its profile are distorted, the nut begins to “seize”. In these cases, emergency breakdowns of the connection parts are possible. The edges of the heads of bolts and nuts most often wear out because they are unscrewed with the wrong wrenches.
In keyed connections, both the keys and the keyways wear out. Possible reasons for this phenomenon are loosening of the fit of the part on the shaft, improper fitting of the key in the socket.
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.
The different sliding surfaces 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, i.e. separation of particles of material
Fig.3.
a - chipping, b - peeling, c - corrosion, d - erosion, e - scratches, e - scuffing, g - sticking, h - deep tearing of the material and its transfer from another friction surface rial, 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 near the piston surface, which creates a local increase in pressure or temperature and causes wear on parts. The surface of the brake drum (Fig. 3, e) shows the risks that appear when a solid body or solid particles act on a rotating drum. Seizures (Fig. 3, f) are formed as a result of seizing surfaces during friction due to the action of molecular forces between them. On fig. 3, g shows the working surface of the part with foreign particles adhering to it, and in fig. 3, h - the surface of the part with wear during jamming as a result of setting - deep tearing of the material and its transfer from another friction surface.
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 holds firmly in the wall, a train moves along rails, etc. The same friction phenomenon 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 and 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, for example, 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 closely 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:
but- 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.
Lecture number 3. Wear of equipment parts. Types of wear.
Wear - gradual surface destruction of the material with a change in the geometric shapes and properties of the surface layers of parts.
There is wear:
Normal;
- emergency.
Depending on the causes, wear is divided into 3 categories:
1. chemical;
2. physical;
3. thermal
Normal wear - dimensional change that occurs in a short time due to improper installation, operation and maintenance.
Chemical wear - consists in the formation of the thinnest layers of oxide on the surface of parts, followed by exfoliation of these layers. The ongoing destruction is accompanied by the appearance of rust, metal corrosion.
Physical deterioration - the reason may be:
Significant loads;
Surface friction;
Abrasive and mechanical impact.
And at the same time, the details appear:
microcracks;
cracks;
The metal surface becomes rough.
Physical wear is:
Smallpox;
- tired;
- abrasive;
Thermal wear - characterized by the formation and subsequent destruction of molecular bonds within the metal. Occurs due to high or low temperature.
Reasons for wear:
1. The quality of the material of the parts.
As a rule, for most parts, wear resistance is higher, the harder their surface, but not always the degree of hardness is directly proportional to wear resistance.
Materials with only high hardness have high wear resistance. However, this increases the likelihood of scratches and detachment of material particles. Therefore, such parts must have a high viscosity, which prevents the detachment of particles. If two parts made of homogeneous materials experience friction, then with an increase in the coefficient of friction they wear out quickly, therefore, more expensive and difficult-to-replace parts must be made from a harder, high-quality and expensive material, and cheaper simple parts should be made from a material with a low coefficient of friction.
2. The quality of the surface treatment of the part.
There are three parts wear periods:
The initial period of running-in is characterized by a rapid increase in the gap of movable joints;
- the period of steady wear - there is a slow, gradual wear;
A period of rapid, increasing wear - caused by a significant increase in clearances and a change in the geometric shapes of parts.
To increase the service life of parts, you must:
To shorten the first period as much as possible, by very precise and clean processing of parts;
Increase the maximum second period;
Prevent the third period.
3. Lubrication.
A layer of lubricant introduced between the rubbing parts fills all the roughness and irregularities and reduces friction and wear many times over.
4. The speed of movement of parts and specific pressure.
Based on experimental data, it has been established that under normal specific loads and speeds of movement from 0.05 to 0.7, the oil layer does not break and the part works for a long time. If you increase the load, then the wear of the part will increase many times.
5. Violation of rigidity in fixed parts.
6. Violation of landings.
7. Violation of the mutual arrangement of parts in mates.
