Inspection method and classification of machinery parts of Zhejiang Huajian

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1. Inspection and classification of mechanical parts

in the process of mechanical equipment maintenance, the inspection of parts is an important process, which not only affects the maintenance quality, but also affects the maintenance cost. after the parts are removed from the equipment, the technical state can be determined by testing, and it can be divided into three categories: available, to be repaired and to be scrapped.

 

Usable parts refer to their technical state can still meet the specified requirements, without any repair can be directly assembled. If the technical state of the part has exceeded the specified requirements, it is a part in need of repair. however, some parts, although the technical requirements can be met by repair, but the cost is high and uneconomic, at this time, usually do not repair and replace with new parts. When the parts in the technical state (such as material deterioration, insufficient strength, etc.) has been beyond repair, should be given scrap treatment.

 

When inspecting and classifying mechanical parts, the following technical conditions must be considered comprehensively.

 

(1) Working conditions and performance requirements of parts, such as mechanical properties, heat treatment and surface characteristics of parts materials.

 

(2) Parts may produce defects (such as cracking, cracks, etc.) on the impact of its performance, master its detection methods and standards.

 

Limit wear and allowable wear standard of vulnerable parts.

 

(4) Limit fit clearance and allowable fit clearance standard of matching parts.

 

(5) Other special scrap conditions of parts, such as coating performance, bearing alloy and matrix bonding strength, balance and seal damage.

 

the working surface of the parts is abnormal, such as scratches and corrosion on the working surface of the precision parts.

 

 

 

2. Inspection content of mechanical parts

 

Different parts of mechanical equipment, their respective requirements and importance are not the same, so the detection items are also different, generally can be tested from the following aspects of mechanical parts.

 

(1) geometric shape accuracy of parts. the main inspection items are roundness, cylindricity, flatness, straightness, line profile, feather surface profile. In inspection, general measuring tools are used, such as vernier measuring tools, spiral micrometer measuring tools, gauges, etc.

 

(2) the precision of the surface position of the parts. the test items are coaxiality, symmetry, position, parallelism, perpendicularity, skew and runout. during inspection, the mandrel, gauge and dial gauge are generally used to measure with each other.

 

the surface quality of the parts. Mainly check fatigue spalling, corrosion pitting, cracks and scratches. the cracks can be examined by magnetic particle inspection, eddy current inspection and ultrasonic inspection.

 

(4) Internal defects of parts. Internal defects are cracks, pores, loose, inclusions, etc. Can be used for radiation and ultrasonic inspection of near surface defects, some can also be used for magnetic particle inspection and eddy current inspection.

 

Mechanical and physical properties of parts. Mainly check hardness, hardening layer depth, permeability, etc., and use electromagnetic induction method to do most of the nondestructive testing. Hardness can also be checked by ultrasonic and other methods. The surface stress state of parts can be detected by X - ray, magnetic and ultrasonic methods.

 

Weight and balance of parts. the weight difference between piston and piston connecting rod group should be checked. Some high speed rotating parts, such as crankshaft flywheel set, car drive shaft and car wheels need to check the balance of action. this is because the piston parts and high-speed rotating parts, parts of the imbalance will cause the vibration of the machine, and to the parts themselves and bearings caused additional load, so as to accelerate the wear and other damage parts. Dynamic balancing needs to be carried out in a special dynamic balancing machine, such as crankshaft dynamic balancing machine, car wheel dynamic balancing machine, etc. In the absence of dynamic balance inspection equipment, if disassembly and processing repair, pay attention to not destroy the original assembly state, in advance before disassembly mark.

 

3. Inspection method of mechanical parts

 

at present, the commonly used detection methods of mechanical parts include inspection method, measurement method and nondestructive detection method of hidden defects. In general, the corresponding detection method is selected according to the specific situation of production, so as to make a comprehensive and accurate identification of the technical state of the parts.

 

(1) examine method

 

It is mainly based on human organs (eyes, hands and ears, etc.) feeling or with the help of simple tools (magnifying glass, hammer, etc.), standard blocks, etc., to test, compare and judge the technical state of the parts of a method. this method is simple and easy to operate, and is not restricted by conditions, so it is widely used. but the accuracy of inspection mainly depends on the production practice experience of inspectors, and can only be qualitative analysis and judgment.

 

(2) measurement method

 

the dimensional accuracy, shape accuracy and position accuracy of the parts are tested by measuring tools and instruments. this method is the most commonly used and basic checking method.

 

(3) Nondestructive detection of hidden defects

 

Nondestructive testing is mainly to determine the nature, size, location and orientation of hidden defects of parts, so nondestructive testing is selected in detail

the risk measurement method must be combined with the working conditions of the parts, the stress state, production technology, testing requirements and economy, etc. At present, the non-destructive testing methods commonly used in production mainly include magnetic powder method, osmotic method, ultrasonic method and ray method.

 

(1) magnetic particle method

 

this method has the advantages of simple equipment, reliable detection and convenient operation, but it is only suitable for the detection of defects on the surface and near surface of ferromagnetic materials. Its principle is that the use of ferromagnetic materials under the action of electromagnetic fields can produce the phenomenon of magnetization, parts to be tested under the effect of electromagnetic field, due to its surface or near surface (within a few millimeters) flaws, the lines of magnetic force had to bypass defects, magnetic leakage or gathered to form local magnetic adsorption absorption magnetic powder, which shows the location of the defects, shape and orientation. Figure 1 shows the principle of magnetic particle detection.

 

When USING magnetic particle method, we must pay attention to the selection of magnetization method, so that the direction of magnetic force line is as vertical as possible or at a certain Angle through the orientation of the defect, in order to obtain the best detection effect, and at the same time, attention should be paid to demagnetization after detection.

 

 

(2) the osmosis

 

the microcracks of 1υm width on the surface of the parts made of any material and any shape can be detected by penetration method. this method is simple and convenient to detect. the principle and process is that the permeant is coated on the surface of the cleaned part, and the permeant enters the defect through the capillary action of the surface defect. In this case, the position and shape of the defect can be shown by the penetration agent in the defect with the characteristics of color or fluorescence under ultraviolet irradiation.

 

Ultrasonic method

 

the main characteristics of this method are strong penetration ability, high sensitivity, wide range of application, not limited by materials, light equipment, easy to use, can be detected on site, but only suitable for the detection of internal defects of parts. the principle is that when the ultrasonic wave generated by the piezoelectric effect of some substances (quartz, barium titanate, etc.) propagates in the medium, it encounters the interface of different media (internal cracks, slag inclusion, shrinkage hole and other defects), which will produce reflection, refraction and other characteristics. The reflection generated by the ultrasonic wave at the defect can be transferred by the detection instrument.

the refraction wave is displayed on the fluorescent screen to determine the location, size and nature of the defects inside the part.

 

(4) ray method

 

the most important feature of this method is that it is easier to determine the shape, size and nature of the defects on the photographic film, and the film can be stored for future reference for a long time. however, the investment and cost of testing equipment are relatively high, and corresponding safety measures against radiation are required. It is only used for the detection of important parts or when the defects cannot be determined by ultrasonic testing. Its principle is that the use of radiation (X - ray) irradiation, so that it through the parts, if encountered defects (cracks, pores, loose or slag, etc.), the ray is easier to pass through the characteristics. In this way, more energy is transmitted from the defect of the measured part than from other places. When these rays illuminate the film, after photosensitivity and development, the formation of different blackness (contrast), so as to analyze and judge the shape, size and location of parts defects.

 

It must be pointed out that when detecting and classifying parts, we must also pay attention to the corresponding special tests combined with the special requirements of parts, such as the balance test of high-speed movement, the elastic test of elastic parts and the sealing test of seals, etc., only in this way can we make a comprehensive, accurate identification and correct classification of the technical state of parts.