How to inspect and examine components

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The inspection of spare parts is an important part of the mechanical maintenance process. Through inspection, the technical condition of the machinery and its components is determined, as well as the corrective actions that need to be taken; subsequently, inspection is also used to assess the technical quality of the machinery after repairs. 1. Main aspects of part inspection In mechanical maintenance, parts are generally inspected one by one, and the main aspects of this inspection can be divided into the following categories: 1) Inspection of the geometric accuracy of parts. Geometric accuracy includes dimensional accuracy as well as shape and position accuracy. However, in maintenance work, the focus is sometimes not on the geometric dimensions of individual parts, but rather on the relative fit between them, which is often an important aspect of the inspection process. Common precision requirements for shape and position during maintenance include roundness, cylindricity, concentricity, coaxiality, parallelism, and perpendicularity. 2) Inspection of surface quality: The inspection of the surface quality of parts during maintenance work is not limited to checking the surface finish; it also involves checking for defects such as scratches, burns, and roughness on the surfaces of used parts. 3) Inspection of mechanical properties: Given the characteristics of mechanical maintenance, aside from checking the hardness, which is one of the mechanical properties of the material used for parts, other parameters are generally not examined. However, certain properties that arise during part manufacturing and maintenance, such as balance and spring stiffness, cannot be ignored. 4) Inspection for hidden defects: During the manufacturing process, parts may have inherent defects such as inclusions and voids, and microscopic cracks may arise during use. These defects cannot be detected directly through ordinary observation and measurement, but they can have serious consequences for the machinery; therefore, during mechanical maintenance, certain components must be thoroughly inspected purposefully. 2. Methods of part inspection There are many methods for inspecting parts, and new inspection technologies are constantly evolving. However, based on the actual practices of mechanical maintenance, it can be summarized into the following aspects: 1) Visual inspection method. This is a method that does not require examining the equipment; instead, it relies on the inspector’s intuitive senses to assess the technical condition of the parts. This method is simple, and it also makes it easier to identify defects in the numerous components that are disassembled during mechanical maintenance; therefore, it remains useful. However, this method cannot be used to inspect parts with high precision requirements, and it demands that the inspector have extensive experience. 2) Instrument and tool inspection method: A large amount of inspection work is carried out using instruments and tools. Due to the varying principles of operation and types of instruments and tools, they can be classified into general-purpose measuring tools, specialized measuring tools, mechanical instruments and meters, optical instruments, electronic instruments, etc. 3) Physical inspection method: This is a method that utilizes physical quantities such as electricity, magnetism, light, sound, and heat, as well as the changes they cause in the workpiece, to determine the technical condition of the parts. The implementation of this method is also combined with instrument and tool inspection methods. This method is typically used to detect hidden defects inside parts without damaging the parts themselves, and it is commonly known as non-destructive testing. Non-destructive testing techniques have been developing rapidly in recent years, and the methods widely used in production today include magnetic particle testing, penetrant testing, ultrasonic testing, and radiographic testing. 3. Visual inspection of parts 1) Visual inspection: This is the main aspect of visual inspection in terms of part evaluation; issues such as fractures and macroscopic cracks in the parts, obvious bending and warping, burns and scratches on the surface of the parts, as well as severe wear, can usually be identified with the naked eye. To improve the accuracy of visual inspection, a magnifying glass can also be used in some cases. To compensate for the limitations of vision in inspecting the interior of certain walls, optical fibers can also be used as endoscopes for light transmission. 2) Auditory inspection: A method that relies on the auditory capabilities of the human ear to determine whether mechanical parts have defects is an established inspection technique. This method has been used for the routine inspection of railway vehicles for decades. During inspection, the part under examination is tapped; when there are no defects in the part, the sound is clear, while if there are internal shrinkages, the sound becomes dull. If cracks appear internally, the sound is hoarse. Thus, by listening to the different sounds, it is possible to determine whether the part has any defects. 3) Tactile inspection: By touching the surface of the part with the hand, its surface condition can be felt. By shaking the mating parts relative to each other, it is possible to sense their fitting condition. For machinery that is in operation, by touching its components, it is possible to sense its temperature level, thereby assessing the condition of its mechanisms. 4. Measures to ensure the quality of part inspection: 1) Strictly adhere to technical standards. Most mechanical parts and fitting components have specified technical standards, which serve as the main basis for inspection work. Unless there is reliable evidence suggesting that these standards need to be modified, it is essential to strictly comply with them; it is absolutely not permissible to lower these standards in order to use substandard parts. 2) Select inspection equipment according to the requirements of the items being inspected. In addition to choosing the equipment based on the nature and scope of the inspection tasks, special attention should be paid to the accuracy requirements. For example, when measuring length and a precision of within 1 mm is required, a steel ruler is sufficient ； When a precision of around 0.1 mm is required, a vernier caliper is generally used ； When a precision of 0.01 mm is required, a micrometer or dial indicator is needed ； When a precision of 0.001 mm is required, a comparator is needed. If the accuracy of the inspection equipment is lower than the accuracy required for the object being tested, it is impossible to meet the quality inspection requirements, and care must be taken to prevent this. 3) Improve the technical level of inspection operations. The technical level of inspection operations has a direct impact on the accuracy of inspections; whether it is self-inspection by maintenance personnel or inspections carried out by specialized inspectors, it is essential that the operators be proficient in using the inspection equipment and understand the requirements for inspecting the specific items. To this end, it is important to focus on improving inspection techniques; specialized training should be provided for the use of special and critical inspection equipment, and the personnel responsible for inspections should remain relatively stable. 4) Reducing inspection errors: Any inspection result is inevitably subject to errors, whose impacts are multifaceted. To prevent and eliminate such errors, the following measures should be taken: (1) Inspection equipment has its own precision level; it is necessary to calibrate it regularly and take good care of it to maintain its proper precision ； (2) Correct the errors caused by temperature; among the environmental factors that affect inspection accuracy, the impact of temperature is the greatest. Factors such as the effect of linear expansion in length measurements, and changes in properties caused by temperature variations in electronic devices, all require necessary corrections in more precise measurements ； (3) Errors caused by improper operation or inaccurate reading are mostly random errors; therefore, the average of multiple measurements can be taken to **reduce the degree of such errors. 5) Establish sound and reasonable inspection rules and regulations. A proper inspection system serves as an organizational guarantee for effective inspection work; it is necessary to set up post responsibility systems to clarify duties and ensure that everyone is accountable. Additionally, it is recommended to implement certain acceptance and handover procedures as well as measurement calibration systems.