Mechanical equipment failure causes, some from the impact of the equipment itself defects; there are design problems, such as the original design structure, size, fit, material selection is unreasonable, etc.; there are parts material defects, such as uneven material, internal residual stress is too large, etc.; there are manufacturing problems, such as the manufacturing process of machining, casting, forging, heat treatment, assembly, standard parts, such as the existence of process problems; there are Assembly problems, such as parts of the optional, unreasonable adjustment, improper installation, etc.; and inspection, commissioning and other aspects of the problem.
1. Failure of mechanical parts
(1) the classification of mechanical parts failure
Mechanical parts to lose the specified function is called failure. A part in one of the following two states is considered a failure: one is unable to complete the specified function; two is not reliable and safe to continue to use.
Failure of parts is the main cause of mechanical equipment failure. Therefore, the study of the failure law of the parts, to find the causes of failure and take improvement measures to reduce the occurrence of mechanical failure and extend the service life of machinery has important significance.
The main manifestation of mechanical parts failure is the wear of the working mating surface of the parts, which accounts for the largest proportion of parts damage. Material corrosion, aging, etc. is another form of failure inevitable in the process of parts work, but its proportion is generally much smaller. These two forms of failure, basically summarizes the main forms of failure of mechanical parts under normal conditions of use. Other forms of failure, such as parts fatigue fracture, deformation and so on, although in practice also often occur, and belong to the most dangerous forms of failure, but more belong to the manufacturing, design defects, or the machine maintenance, improper use caused by.
Failure analysis refers to the analysis and study of the mechanism or process of wear, fracture, deformation, corrosion and other phenomena of the characteristics and laws, from which to find the main causes of failure, in order to use appropriate control methods.
The purpose of failure analysis is to provide a reliable basis for the development of maintenance technology solutions and to control certain factors that cause failure in order to reduce equipment failure rate and extend equipment life. In addition, failure analysis can also provide feedback for the design and manufacture of equipment, and provide an objective basis for the identification of equipment accidents.
(2) Wear of parts
(1) The wear law of parts
As we all know, a machine such as cars, tractors, its composition of the basic unit is the machine parts, many parts constitute the frictional vice, such as bearings, gears, pistons – cylinders, etc., they are under the action of external forces and thermal, chemical and other environmental factors, under the influence of a certain friction, wear until the final failure, which wear this failure mode, in a variety of mechanical failure occupies a considerable proportion. Therefore, it is very necessary to understand the wear pattern of the parts and their mating pairs.
① parts of the typical wear curve wear this failure mode belongs to the progressive failure. For example, the cylinder due to wear and tear failure and fan belt breakage, capacitors are different from the failure, the latter is a sudden failure, while the wear and tear failure is wear failure.
② Allowable wear and extreme wear
2) Abrasive wear
Abrasive wear is also called abrasive wear, which is caused by the presence of hard particles between the contact surfaces of the friction pair, or when the hardness of one side of the friction pair material is much greater than that of the other side, a similar metal cutting process occurs wear phenomena. It is a type of mechanical wear characterized by clear cutting marks on the contact surfaces. Among all kinds of wear, abrasive wear accounts for about 50%, which is a very common and most harmful kind of wear. Its wear rate and wear intensity are very high, which greatly reduces the service life of mechanical equipment and consumes a lot of energy and materials. .
According to the different stresses and impacts on the friction surface, abrasive wear can be divided into three types: chiseling, high stress crushing and low stress abrasion.
① The mechanism of abrasive wear belongs to the mechanical action of abrasive particles. One is the process of micro-cutting by abrasive particles along the friction surface; Intensively indented, eventually eroded due to surface fatigue. The sources of abrasive particles include external sand and dust, chip intrusion, fluid entrainment, surface wear products, surface hard spots and inclusions of material structure, etc.
The salient features of abrasive wear are: the wear surface has small grooves parallel to the direction of relative motion, and there are spiral, ring or curved fine chips and some powder.
②Measures to reduce abrasive wear Abrasive wear is caused by the mechanical action between abrasive particles and the surface of the friction pair, so the countermeasures to reduce or eliminate abrasive wear can start from the following two aspects.
a. Reduce the entry of abrasives. The friction pairs in mechanical equipment should prevent the entry of external abrasives and remove the abrasive debris generated during the running-in process of friction pairs in time. The specific measures are to equip air filters and fuel and oil filters; to increase the sealing device for dust prevention, etc.; to install magnets, debris collection rooms and oil pollution level indicators in the lubrication system; Oil filter unit.
b. To enhance the wear resistance of the friction surface of parts, one is to choose materials with good wear resistance; second, for parts that require wear resistance and impact load, heat treatment and surface treatment can be used to improve the surface properties of parts materials, Improve the surface hardness, and make the surface hardness exceed the hardness of the abrasive as much as possible; third, for parts that do not require high precision, wear-resistant alloys can be surfacing on the working surface to improve its wear resistance.
3) Adhesive wear
The two friction surfaces that constitute the friction pair, the wear caused by the transfer of the material on the contact surface from one surface to the other surface during relative motion is called adhesive wear. According to the damage degree of the surface of the friction pair of parts, adhesive wear can be divided into five categories: slight wear, smear, scratch, tear off and bite.
① Adhesive wear mechanism The friction pair works under heavy load conditions. The heat generated by poor lubrication, high relative motion speed, friction, etc. has no time to dissipate. The surface of the friction pair generates extremely high temperature. In severe cases, the surface metal partially softens or melts. The surface strength is reduced, so that the convex parts of the surface under high pressure adhere to each other, and then are torn off during relative movement, so that the material is transferred from the surface with low strength to the surface with high material strength, causing catastrophic damage to the friction pair. Such as bitten or scratched.
②Measures to reduce adhesive wear
a. Control the surface state of the friction pair. The cleaner and smoother the friction surface, the smaller the surface roughness, the more prone to adhesive wear. There is often an adsorption film on the metal surface. When there is plastic deformation, the metal slips and the adsorption film is destroyed, or the adsorption film will be destroyed when the temperature rises to 100-200 °C, which will easily lead to adhesive wear. In order to reduce adhesive wear, an appropriate lubricant should be selected according to its load, temperature, speed and other working conditions, or additives should be added to the lubricant to establish the necessary lubrication conditions. Oxygen in the atmosphere usually forms a protective oxide film on the metal surface, which can also prevent direct contact and adhesion of the metal, which is beneficial to reduce friction and wear.
b. Control the material composition and metallographic structure of the surface of the friction pair. Adhesive wear is most likely to occur between two metal materials with similar material composition and metallographic structure, because the materials on the surfaces of the two friction pairs tend to form solid solutions or intermetallic compounds. strong. Therefore, the materials used as the friction pair should be the two materials with the least tendency to form a solid solution, that is, materials with different material compositions and crystal structures should be selected. Covering one surface of the friction pair with lead, tin, silver, copper and other metals or soft alloys can improve the ability to resist adhesive wear. The ability to resist adhesive wear, the anti-adhesion performance of steel and cast iron is also good.
c. Improve heat transfer conditions By selecting materials with good thermal conductivity, cooling the friction pair or taking appropriate heat dissipation measures to reduce the temperature of the friction pair during relative motion and maintain the surface strength of the friction pair.
4) Fatigue wear
Fatigue wear is a phenomenon that fatigue cracks occur in local areas on the surface of friction pair materials under the periodic action of cyclic contact stress, and material particles fall off. According to the contact and relative motion between friction pairs, fatigue wear can be divided into rolling contact fatigue wear and sliding contact fatigue wear.
①Mechanism of fatigue wear The process of fatigue wear is the destruction process of crack generation and expansion, particle formation and shedding. Abrasive wear and adhesive wear are both related to the direct contact of the surfaces of the friction pair. If there is a lubricant to separate the two friction surfaces, these two types of wear mechanisms will not work. For fatigue wear, even if there is a lubricant between the friction surfaces, it may occur without direct contact, because the friction surfaces are subjected to large stresses transmitted through the lubricating oil film. Fatigue wear is different from abrasive wear and adhesive wear. It does not occur at the beginning, but the particles fall off after a certain number of stress cycles, so that the friction pair loses its working ability. According to the location of crack generation, the mechanism of fatigue wear has the following two cases.
a. Rolling contact fatigue wear rolling bearings, transmission gears, etc. have different depths of needle-shaped, pox-like pits (depth below 0.1-0.2mm) or large-area particles falling off on the surfaces of relative rolling friction pairs, all of which are caused by rolling Caused by contact fatigue wear, also known as pitting or pockmark wear.
b. Sliding contact fatigue wear The shear stress of the two sliding contact objects is 0.786b below the surface (b is the half-width of the plane contact area), where the plastic deformation is the most severe, and the repeated deformation under the action of periodic load will make the material A localized strength weakening occurs at the surface, where cracks first appear. Under the superposition of the shear stress caused by sliding friction and the shear stress caused by normal load, the maximum shear stress moves from 0.786b to the depth of the surface, forming sliding fatigue wear, and the depth of the peeling layer is generally 0.2-0.4mm.
②Countermeasures to reduce or eliminate fatigue wear The countermeasures to reduce or eliminate fatigue wear are to control the factors that affect the occurrence and expansion of cracks, mainly in the following two aspects.
a. Reasonable selection of material and heat treatment The existence of non-metallic inclusions in steel can easily cause stress concentration, and the edges of these inclusions are most likely to form cracks, thereby reducing the contact fatigue life of the material. The organizational state and internal defects of materials also have an important impact on wear. Usually, the grains are fine and uniform, and the carbides are spherical and evenly distributed, which are beneficial to improve the rolling contact fatigue life. Under the condition of the same undissolved carbide state, when the mass fraction of carbon in martensite is about 0.4% to 0.5%, the strength and toughness of the material are well matched, and the contact fatigue life is high. For the undissolved carbides, through proper heat treatment, it tends to be less, the grains are fine and uniform, and the appearance of coarse acicular carbides is avoided, which is beneficial to the elimination of fatigue cracks. As the hardness increases within a certain range, the contact fatigue resistance will also increase accordingly. For example, when the surface hardness of the bearing steel is about 62HRC, its anti-fatigue wear ability is the largest; for the tooth surface of the transmission gear, the hardness is best in the range of 58-62HRC. In addition, it is also very important to match the surface hardness of the two contacting rolling elements. For example, in rolling bearings, the hardness of the raceway and the rolling element are similar, or the hardness of the rolling element is 10% higher than that of the raceway.
b. Reasonable selection of surface roughness Practice has shown that appropriately reducing the value of surface roughness is an effective way to improve fatigue resistance. For example, when the surface roughness value of the rolling bearing is reduced from Ra 0.40μm to Ra 0.20μm, the life can be increased by 2 to 3 times; Ra 0.05μm or less will have little effect on the improvement of life. The level of surface roughness requirements is related to the contact stress on the surface. Usually, when the contact stress is large or the surface hardness is high, the surface roughness value is required to be small.
In addition, the surface stress state, the level of fit accuracy, the nature of lubricating oil, etc. will all have an impact on the speed of fatigue wear. Usually, excessive surface stress, too small or too large fit clearance, and corrosive substances produced by lubricating oil during use will aggravate fatigue wear.
5) Corrosive wear
①Mechanism of corrosion and wear During the friction process of the kinematic pair, the metal reacts chemically or electrochemically with the surrounding medium at the same time, causing corrosion and peeling off the metal surface. This phenomenon is called corrosion wear. It is a wear phenomenon formed by the combination of corrosion and mechanical wear. Therefore, the mechanism of corrosion wear is different from that of abrasive wear, adhesive wear and fatigue wear. It is an extremely complex wear process that often occurs under high temperature or humidity. In the environment, it is more likely to occur under the conditions of special media such as acids, alkalis, and salts. According to the different properties of corrosive media and materials, corrosive wear is usually divided into two categories: oxidative wear and corrosive wear in special media.
a. Oxidative wear During the friction process, the oxide film formed on the friction surface under the action of oxygen in the air or oxygen in the lubricant is quickly removed by mechanical friction, which is called oxidative wear. Most of the metals used in industry can be oxidized to form surface oxide films, and the properties of these oxide films have an important impact on wear. If a dense and complete oxide film is formed on the metal surface, which is firmly combined with the substrate, and the wear resistance of the film is good, the wear will be slight; if the wear resistance of the film is not good, the wear will be severe. For example, both aluminum and stainless steel are easy to form oxide film, but the wear resistance of the oxide film on the surface of aluminum is not good, and the wear resistance of the oxide film on the surface of stainless steel is good, so stainless steel has stronger oxidation resistance than aluminum.
b. Corrosive wear in special media During the friction process, the corrosion products formed by the action of electrolytes such as acid and alkali in the environment on the friction surface are quickly removed by mechanical friction. The wear form is called corrosive wear in special media. The mechanism of this wear is similar to oxidative wear, but the wear rate is much higher than oxidative wear. The nature of the medium, the ambient temperature, the strength and adhesion of corrosion products all have an important impact on the wear rate. The probability of this kind of corrosive wear is very high. For example, when a fluid delivery pump is conveying a corrosive fluid, especially a fluid containing solid particles, the parts in contact with the fluid will be subject to corrosive wear.
②Measures to reduce corrosion and wear
a. Reasonable selection of materials and anti-oxidation treatment on the surface. Steel materials containing chromium, nickel, molybdenum, tungsten and other components can be selected to improve the oxidation resistance and wear resistance of the surface of the moving pair. Or carry out strengthening treatment such as shot blasting and rolling on the surface of the moving pair, or carry out anodizing treatment on the surface, etc., to form a dense structure or oxide film on the metal surface, and improve its resistance to oxidation and wear.
b. For the corrosive wear under the action of a specific medium, the corrosive wear rate can be reduced by controlling the formation conditions of the corrosive medium, selecting suitable wear-resistant materials and changing the action mode of the corrosive medium.
6) Fretting wear
The wear of two fixed contact surfaces due to relatively small amplitude vibration is called fretting wear, which mainly occurs on the joint surfaces of relatively static parts, such as key connection surfaces, interference or transition fit surfaces, bolted connections and Surfaces connected by rivets, etc., are therefore often overlooked.
The main harm of fretting wear is to reduce the fit accuracy, reduce or even loosen the interference of the interference fit parts, loosen or even separate the connecting parts, and even cause accidents in severe cases. Fretting wear is also easy to cause stress concentration, leading to fatigue fracture of connectors.
① Mechanism of fretting wear Fretting wear is a composite wear form combining abrasive wear, adhesive wear and oxidative wear. Fretting wear is usually concentrated in a local area, and the contact stress causes plastic deformation of the asperities on the bonding surface, and metal adhesion occurs; the adhesion point is sheared under the repeated action of external small-amplitude vibration, and the adhered metal falls off, shearing The surface at the joint is oxidized; the two joint surfaces will never be out of contact, and the wear products are not easy to be removed. The wear debris acts as an abrasive on the joint surface due to vibration, so fretting wear has the functions of adhesive wear, oxidative wear and abrasive wear.
②Countermeasures to reduce or eliminate fretting wear Practice shows that material performance, load, amplitude and temperature are the main factors affecting fretting wear. Therefore, the countermeasures to reduce or eliminate fretting wear mainly include the following aspects.
a. Improving material properties Selecting appropriate material pairing and increasing hardness can reduce fretting wear. Generally speaking, the pairing of materials with good anti-adhesion performance is also good at resisting fretting wear, while the pairing of materials with poor anti-adhesion ability such as aluminum to cast iron, aluminum to stainless steel, and tool steel to stainless steel has poor anti-fretting wear ability. When increasing the surface hardness of carbon steel from 180HV to 700HV, fretting wear can be reduced by 50%. The use of surface vulcanization treatment or phosphating treatment and the coating of polytetrafluoroethylene surface coating are also effective measures to reduce fretting wear.
b. Control the load and increase the prestress. Under certain conditions, the amount of fretting wear increases with the increase of the load, but the rate of increase will continue to decrease. When a certain critical load is exceeded, the amount of wear decreases. Therefore, the fretting wear can be effectively slowed down by controlling the prestress or interference amount of the interference fit.
c. Control the amplitude. Experiments have shown that when the amplitude is small, the wear rate is relatively small; when the amplitude is 50-150 μm, the wear rate will increase significantly. Therefore, the amplitude should be effectively controlled within 30 μm.
d. Reasonably control the temperature of low-carbon steel above 0°C, the wear amount will gradually decrease with the temperature rising; the wear amount will suddenly decrease at 150-200°C; continue to increase the temperature, the wear amount will increase, and the temperature will increase from 135°C At 400°C, the amount of wear increases 15 times. When the other conditions remain the same for medium carbon steel, the fretting wear turns at a temperature of 130°C. Above this temperature, the fretting wear is greatly reduced.
e. Selecting a suitable lubricant Experiments show that ordinary liquid lubricants are not effective in preventing fretting wear; lubricating grease with high viscosity, high drop point, and strong shear resistance has a certain effect on preventing fretting wear; the effect The best is a solid lubricant, such as MoS2, etc.
7) Wear control
①Control factors The factors affecting wear are very complicated, but generally there are four aspects, namely material properties, operating conditions, geometric factors and working environment, and each aspect contains many specific contents. It should be pointed out that these factors must not be fully considered in the control of any wear process. For a given wear condition, some factors are very important and must be considered, but some factors may not be important or even irrelevant. .
②General considerations for material selection of wear parts No matter what the wear conditions are, correct material selection is very important to control the wear of parts and ensure product quality. Correct selection of the first
