Mechanic doing maintenance work
(1) Failure of mechanical parts and mechanical failure
The loss of working ability of machinery is called failure, and the loss of specified working ability of machine parts is called failure. Mechanical failure and component failure are inseparable. The failure caused by the failure of the parts due to the deformation, fracture, corrosion, etc. of the parts caused by normal wear or physical and chemical changes, such failures are also called natural failures.
1. Wear of parts
Wear is the most predominant and common form of component failure.
2. Deformation of parts
During the working process of the machine, the phenomenon that the size and shape of the parts are changed due to the force is called deformation. The deformation of metal includes elastic deformation and plastic deformation.
3. Breakage of parts
Under the action of external force, the parts first undergo elastic deformation. When the stress caused by the load exceeds the elastic limit and continues to increase, the material may undergo plastic deformation, and finally fracture occurs when the stress exceeds the strength limit.
4. Corrosion
The point-like spalling on the surface of the part under the action of cyclic contact stress is called fatigue pitting corrosion; the damage of the surface metal caused by the chemical and electrochemical action of the surrounding medium is called corrosion; the surface of the part produces needles under the action of temperature change and medium. Hole-like holes that expand continuously are called cavitations. Fatigue pitting, corrosion and cavitation are collectively referred to as erosion.
(2) Elimination (repair) method of mechanical failure
The elimination of man-made accidental failures is mainly achieved by improving the quality of use, management and maintenance personnel and strengthening the sense of responsibility. For natural failures, it can only be achieved by adjusting and repairing methods. Usually, there are the following methods:
1. The main repair method to restore the cooperative nature
(1) Adjustment method
Generally, adjusting the bolt tightness or adjusting the thickness of the gasket is used to restore the original matching relationship of the matching parts. When repairing, the matching parts are not processed (or only scraped), but only the method of increasing the gasket or adjusting the thickness of the gasket is used. Return it to the original mate clearance.
(2) Repair dimension method
During repairs, the more expensive part of the fitting is machined to restore its geometry and at the same time a new size is obtained, and then another worn part of the fitting is discarded and replaced with a new one that matches the machined part. Parts, so that the fitting clearance of the fittings is restored to the initial clearance, such as repairing the shaft and replacing the bearing bush, repairing the cylinder liner and replacing the piston, etc. This repair method should take into account the possibility of structural processing of the parts and the allowable mechanical strength of the parts after repair. On this premise, the number of repairs should be increased as much as possible; on the other hand, in order to facilitate the supply of spare parts, the repair size should be standardized.
(3) Supplementary parts method (additional parts method)
In this method, each part of the fitting is processed and shaped, and a reasonable diameter reduction or reaming is given to one of the parts, and then a bushing of the same material or higher quality is added to it, and it is pressed into it with an interference. Or screwed or welded to the original part, and then processed to the matching size, so that the matching properties meet the requirements.
2. Repair method to restore both the nature of the fit and the shape and size of the parts
(1) Welding repair method
Metal welding is to use the diffusion and connection between atoms to firmly combine the separated metal weldments into a whole. According to different welding equipment, welding includes gas welding and electric welding, etc. Many fractured and worn parts are mostly repaired and surfacing. For repairs, some parts are turned and ground after welding to restore the original geometry and size.
(2) Re-casting method
After the babbitt alloy of the sliding bearing is worn to the limit, the residual alloy is melted away, and the process of re-casting the new babbitt alloy is called the replacement casting method. In this way, the performance standard of the old plain bearing can be completely restored.
(3) Electroplating (brush plating, electrocoating) method
Electroplating is a process in which an electrochemical reaction occurs when a direct current is passed through an electrolyte to realize the deposition of metal on the surface of the plated part.
(4) Spraying and spray welding
Spraying is to spray the molten material particles on the surface of the prepared rough parts with a high-speed air flow to form a relatively firm mechanical bonding layer.
The spray welding process is developed on the basis of the spraying process. It re-melts the sprayed layer to obtain a coating with similar surfacing properties on the surface of the part.
(5) Bonding and bonding method
Bonding is a repair process that uses the combined chemical, physical and mechanical forces between the adhesive and the parts to bond the parts or repair the cracks, holes, wear and other defects of the parts.
(6) Features and applications of non-stop plugging technology
a. Plugging of straight pipes and containers: monolithic bonding, sticking method; fixture method; pressure-assisted method
b. Flange plugging method
(3) Disassembly, assembly, cleaning and inspection of machinery
1. Mechanical disassembly
(1) Preparations before disassembly
a. The workplace should be spacious, bright, flat and clean.
b. The dismantling tools are prepared and the specifications are appropriate.
c. Prepare benches, separate basins, oil drums, etc. for placing parts according to different purposes
(2) Basic principles of mechanical disassembly
a. According to the model and related information, the structural characteristics and assembly relationship can be clearly understood, and then the methods and steps of disassembly and disassembly can be determined.
b. Correctly select tools and equipment. When the decomposition encounters difficulties, first find out the reasons and take appropriate methods to solve them. It is not allowed to smash and knock to prevent damage to parts and tools. .
c. When disassembling parts or assemblies with specified directions and marks, the directions and marks should be clearly marked, and if the marks are lost, they should be marked again.
d. In order to avoid damage or loss of the dismantled parts, they should be stored separately according to the size and accuracy of the parts, placed in the order of disassembly, and the important precision parts should be specially stored for safekeeping.
e. The removed bolts, nuts, etc. should be put back in place without affecting the repair, so as to avoid loss and facilitate assembly.
f. Disassemble as needed, and it can be judged that the condition is good without disassembly. On the one hand, it can save time and labor, and on the other hand, it can avoid damage during disassembly and assembly and reduce the accuracy of parts assembly. However, the parts that need to be disassembled must be disassembled. It is not easy to save trouble and sloppy, so that the repair quality cannot be guaranteed.
2. Mechanical assembly
The mechanical assembly process is an important link in determining the quality of mechanical repairs, so it must be done:
(1) The assembled parts themselves must meet the specified technical requirements, and any unqualified parts cannot be assembled. This part must be strictly inspected before assembly.
(2) The correct matching method must be selected to meet the requirements of matching accuracy. A large amount of work in mechanical repair is to restore the matching accuracy of mutual matching parts, and methods such as matching, repairing, and adjustment can be used to meet this requirement. The effect of thermal expansion should be considered for the fitting clearance. For fittings composed of materials with different expansion coefficients, when the ambient temperature during assembly is significantly different from the temperature during operation, the resulting clearance change should be compensated.
(3) Analyze and check the accuracy of the assembly dimension chain, and meet the accuracy requirements through matching and adjustment.
(4) The principle of handling the assembly sequence of the parts is: first inside and then outside, first difficult and then easy, first precision and then general.
(5) Select the appropriate assembly method and assembly equipment and tools.
(6) Pay attention to the cleaning and lubrication of parts. The assembled parts must be thoroughly cleaned first. For the moving and fitting parts, a clean lubricant that meets the working requirements should be applied to the relative motion surface.
(7) Pay attention to the sealing during assembly to prevent "three leaks". To use the specified sealing structure and sealing material, can not use arbitrary substitutes. Pay attention to the quality and cleanliness of the sealing surface. Pay attention to the assembly method and assembly tightness of the seal, and the static seal can be sealed with an appropriate sealant.
(8) Pay attention to the assembly requirements of the locking device and comply with safety regulations.
(9) Pay attention to the quality inspection of the intermediate links of assembly.
3. Mechanical cleaning and inspection
(1) Mechanical cleaning
1. Remove oil stains
Oil stains are adhesions of grease and dust, rust, etc., which do not dissolve into water, but into organic agents. In addition to mechanical decontamination, it can also be removed by chemical or electrochemical methods.
(1) Chemical degreasing method:
1. Organic solvent degreasing: Commonly used organic solvents are gasoline, kerosene, diesel oil, acetone, etc.
2. Alkaline solution for oil removal: such as caustic soda, sodium carbonate, sodium silicate, sodium phosphate, etc. Raising the solution temperature and stirring during cleaning can speed up the degreasing effect.
Generally, it can be heated to about 80 degree . After washing, it should be rinsed with hot water and dried with compressed air.
(2) Electrochemical degreasing method: The method of using the mechanical stirring and peeling action of the bubbles generated by the two electrodes during electrolysis to remove the oil from the surface of the parts is called the electrochemical degreasing method. The method has the advantages of high speed, high efficiency, and thorough degreasing.
(2) Mechanical inspection
The contents of the inspection are as follows:
1 Part inspection
Including the geometric accuracy inspection of parts, such as the size and shape of the parts; the inspection of the surface quality of the parts: such as surface roughness, surface damage and other defects; the mechanical properties inspection of the parts: such as the strength, hardness, and balance of the parts, The stiffness of the spring, etc.; the inspection of hidden defects of parts: such as voids, slag, microscopic cracks, etc.
2 Assembly inspection
Such as the relative position of the parts and the parts, the clearance or interference of the fittings; the balance between the parallel shafts, the coaxiality between the front and rear shafts, etc.
3 Whole machine inspection
The whole machine inspection is the inspection of the technical condition of the whole machine. Including the working ability of the machine, the power and economic performance, etc., the inspection methods are as follows:
A Inspection method: This method is simple, feasible, and widely used to test and judge only by seeing, touching, and hearing. It can be divided into:
(1) Visual inspection method: The surface damage of parts such as roughness, grooves, cracks, scratches, peeling (peeling), fractures, as well as large and obvious deformation of parts, severe wear, surface annealing and ablation, etc. are all visually or with the aid of a magnifying glass. Observation confirmed. There are also cracks in the paint film of rigid couplings, dislocation of elastic couplings, cracks in the paint film of threaded connections and riveted seals, etc., which can also be judged by visual inspection.
(2) Percussion method: For the inconspicuous cracks of the casing parts, the combination of the bearing alloy and the bottom tile, etc., the quality can be judged by percussion listening to crisp or hoarse sound.
(3) Comparison method: compare the new standard parts with the inspected parts to identify the technical status of the inspected parts. Such as the free length of the spring, the length of the chain, the quality of the rolling bearing and so on.
B measurement method: After the parts are worn or deformed, the size and shape will change, or the technical performance (such as elasticity) will decrease due to fatigue. It can be measured by measuring tools and instruments and compared with the allowable standards to determine whether to continue to use, or to be repaired or scrapped. For example, the measurement of rolling bearing clearance, the measurement of temperature rise, the measurement of gear wear, the measurement of spring elasticity, etc.
C detection method: The detection of hidden defects of parts, especially minor defects of important parts, is of great significance to ensure the quality of repair and safety of use, and must be carried out carefully. There are mainly the following methods:
(1) Penetration display method: Immerse the cleaned parts in kerosene or diesel for a while, take out and wipe the surface dry, sprinkle a layer of talcum powder, and then tap the non-working surface of the parts with a small hammer, if the parts have cracks When the vibration caused the oil immersed in the crack to seep out, the talc at the crack showed yellow line marks.
(2) Fluorescent display method: first wash the surface of the inspected part, preheat it with an ultraviolet lamp for 10 minutes, so that the surface of the workpiece is dark purple when observed under the ultraviolet lamp, and then evenly apply the fluorescent display liquid on the working surface of the part, Can show yellow-green defect traces.
(3) Flaw detection method: magnetic particle inspection, ultrasonic inspection, radiographic inspection. It is mainly used to measure the internal defects of parts and the quality of welds.
(4) Balance of the rotor
1. Types of rotor unbalance
(1) Static unbalance: The unbalanced weight on the rotor can be integrated into a centrifugal force that generates only one centrifugal force when the rotor rotates, and can be determined in a static state, which is called static unbalance.
(2) Dynamic unbalance: If two unbalanced weights that are equal in size and opposite in direction, but not on the same diameter can be synthesized on a rotor, the rotor can be balanced in static state, but it will produce an unbalanced weight when rotating. A balanced force couple, which cannot be determined in a static state but can only be determined in a dynamic state, is called dynamic unbalance.
(3) Mixing unbalance: If there is both static and dynamic unbalance on a rotor, it is called mixing unbalance.
2. The balance of the rotor
In order to eliminate the unbalanced force or couple on the rotor, the position and size of the unbalanced mass must be measured, and then try to balance it. This operation process is called rotor balancing. Generally, it can be divided into static balance and dynamic balance.
(1) Static balance: The method in which the unbalanced mass and orientation of the rotor can be measured under static conditions, and the weight of the rotor can be eliminated by de-weighting or aggravating, so as to balance the rotor, is called static balance.
(2) Dynamic balance: The position of the unbalanced mass of the rotor can only be determined under dynamic conditions, and the position and size of the balance mass should be determined. This method of finding balance is called dynamic balance. Dynamic balance can not only eliminate the dynamic unbalanced force couple, but also eliminate the static unbalanced centrifugal force, so it can be used to find the balance of various cylindrical and conical rotors.
(5) Overall inspection of equipment
The overall inspection of the equipment is a comprehensive quality appraisal after the mechanical equipment is repaired, and it is an important link to ensure that the mechanical equipment has good performance, safety and reliability after delivery. The overall inspection includes steps such as no-load test run, load test run and post-test run inspection. Pressure tests and compactness tests are also required for important equipment.
1. No-load trial operation: firstly check the connection, tightening, lubrication, sealing, operation of each part, test the operation and function of the control system, adjustment control system, safety device, and make appropriate adjustments, and check the indications of various instruments at the same time Whether the situation meets the specified standards. If the performance test of the assembly has not been carried out, the test run shall be carried out in parts, and the load test shall not be carried out until the faults and abnormal noise, temperature rise, jumping, etc. found in the test run are eliminated.
2. Load test run: The load test run is carried out after the no-load test run is normal. Determine whether the dynamic performance, economic performance, operating conditions of the machinery and the functions of manipulation, adjustment, control and safety devices meet the operating requirements through the load test run.
3. Inspection after trial operation: After the load trial operation, it is necessary to actively check whether there is deformation, looseness, overheating, damage, etc. of each part, and at the same time, check the sealing of relevant parts and the contact of friction surfaces.
4. Pressure test and compactness test of equipment:
(1) Hydraulic test: It is usually used to check the compactness and strength of welds and joints. Generally, water is used as the medium, so it is also called hydraulic test.
(2) Air pressure test: For the container that is not allowed to have trace residual liquid due to institutional reasons or the container, the air pressure test is used to detect.
(3) Tightness test: For various pressure vessels storing gas or liquid, the tightness test of the weld should be carried out to ensure no leakage. Usually, methods such as air tightness test, kerosene leakage test and ammonia penetration test can be used.
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