Abstract: This article briefly analyzes the causes of common damages in shaft parts, introduces the inspection methods for defects in shaft parts, and emphasizes the precautions to be taken to improve the repair quality of shaft parts.
Keywords: agricultural machinery; Shaft parts; Repair quality
Shaft parts are common and important components in agricultural machinery. The shaft parts on agricultural machinery include crankshaft, camshaft, transmission shaft, transmission shaft, half shaft, etc. Shaft parts account for a large number of various parts sent for repair. Their repair is not only due to their good craftsmanship, but also to their high economic benefits. Therefore, they are one of the main objects for repairing old parts in agricultural machinery repair departments.
1. Common damages and causes of shaft type parts
Shaft parts are also rotating parts, with a structural feature that their length is several to several tens of times larger than their diameter, making them prone to bending; High speed requires some accessories on the shaft to rotate with it, and the journal is prone to wear. Some shafts are subjected to a lot of force during operation, such as the engine crankshaft, which is subjected to significant gas pressure and inertial force impact during operation, which can easily cause abnormal wear, burning, bending, twisting, fracture, and other faults. The common damage analysis of shaft parts is as follows (taking the crankshaft as an example):
(1) Journal wear. This is manifested as a reduction in the size of the main journal and connecting rod journal, as well as the generation of ovality and taper. The reason is that the abrasive in the shaft oil passage accumulates on the journal, mechanical impurities accumulate in the lubricating oil, the connecting rod is bent, the cylinder barrel is longitudinally skewed in the engine body, the force distribution of the connecting rod pad on the connecting rod journal is uneven, and the grinding of the crankshaft does not meet the technical requirements.
(2) Scratches. There are scratches and spots on the surface of the journal. The reason is that abrasive particles were brought in during assembly, and there were impurities, metal objects, etc. in the lubricating oil.
(3) Burns. The surface of the journal is not bright, and sometimes there is alloy sticking to the bearing pad. The reason is that the surface lubrication of the journal is poor, mainly due to a lack of lubricating oil, poor oil quality, incorrect lubricating oil grade, blocked oil channels, and small assembly clearance between the bearing bush and journal, which leads to metal melting on the surface of the journal, often referred to as burning the bush to hold the shaft.
(4) Bending and twisting. If crankshaft and other shaft parts are bent and twisted, it will lead to accelerated wear of related mating parts and deterioration of engine working conditions. The reason is that the crankshaft is subjected to a large impact, such as engine detonation, excessive heavy load starting, and sudden occurrence of a burning of the bearing and holding of the shaft accident.
(5) Cracks and fractures. Cracks and fractures in the journal are prone to occur at the edge of the oil hole and the shoulder of the shaft, as this area is prone to stress concentration. Reason: ① In manufacturing, the main bearing seat hole is deformed (exceeding the allowable value), and the flywheel and crankshaft connecting plate are not concentric. ② In terms of repair, when grinding the crankshaft, the radius of the shoulder fillet is too small and there is no transitional fillet left, resulting in stress concentration In use, a lack of engine oil can cause severe burning of the bearing and shaft, causing the crankshaft to twist and break In terms of material, the material used to manufacture the crankshaft does not meet the requirements.
2. Inspection of axis parts
(1) Bending inspection: Support both ends of the shaft horizontally with V-shaped iron, so that the differential meter contact pin is pressed against the middle of the shaft, and then slowly rotate it for one cycle. Observe and record the swing of the watch pin. The maximum value of the swing Z of the watch pin reflects the bending degree of the shaft (1/2 of the swing is the actual bending degree of the shaft). During inspection, it should be noted that if the journal is singular, the intermediate journal can be measured; If the journal is an even number, measure the two intermediate journals and take the larger value of Z. In order to reduce the impact of journal misalignment on inspection accuracy, the differential table Z should be positioned at one end of the journal during inspection. For some simple shaped shafts, they can be directly placed on a flat plate for rolling observation or inserted with a thickness gauge to check for bending.
(2) Wear inspection: Generally, an external micrometer is used for inspection. For shafts with uniform wear, only the outer diameter size needs to be checked. But for some shafts with uneven wear, it is also necessary to check their out of roundness and out of column degree. To measure the wear of the crankshaft journal, find the larger and smaller diameters of Z on the same cross-section, and the difference between the two is the out of roundness; Find the difference between the larger and smaller diameters of Z in the same vertical section, and the difference between the two is the non columnar degree.
(3) Inspection of fractures: Inspection methods such as inspection, immersion hammering, or magnetic testing can be used.
3. Measures to improve repair quality
(1) Pay attention to protecting the center hole of shaft parts during disassembly. The importance of the center hole is manifested in the following aspects: ① From the perspective of maintaining the accuracy of parts, shaft parts are generally composed of several or more surfaces. However, daily maintenance is often required on one or two of these surfaces. Therefore, when repairing parts, in addition to restoring the accuracy of the worn surface itself, it is also necessary to comply with the technical requirements on the part drawing, that is, a certain value of form and position error (shaft parts refer to coaxiality, parallelism, etc.). The center hole is the implementation of the centerline, which is the combined reference feature of various surfaces of shaft parts. Therefore, it is very important to maintain the mutual position of the surfaces of the parts in accordance with the requirements of the technical drawings and maintain the accuracy of the center hole From the perspective of the process of repairing parts, the current repair process for shaft parts by the repair department can be roughly divided into two parts: firstly, selecting appropriate methods to fill the damaged surface based on the wear status and degree of the parts; The second is to use reasonable process methods according to the technical requirements of the parts to repair surfaces that have lost accuracy. From the above two processes, whether it is the commonly used vibration surfacing, submerged arc welding, spray welding, and brush plating to fill the defect surface, or the commonly used processes such as turning and grinding to restore surface accuracy, center holes are generally required From the perspective of assembly, use, and inspection, firstly, the threaded central hole is used to fix other parts or for disassembly and assembly; The second is that the center hole is also used as a basis for inspection and assembly when available.
The central hole of shaft parts not only plays a different role in manufacturing, testing, assembly, and use, but also has greater significance in maintaining the accuracy of the parts and extending their service life. However, in practice, the damage to the center hole of some shaft parts for repair often leads to undue losses in the repair work of old parts. Therefore, we should maintain the accuracy of the center hole of shaft parts during maintenance and disassembly.
(2) Before repair, the crankshaft should be carefully cleaned, especially the residue in the oil channel and the oil stains on the surface should be removed. Then, necessary technical identification, inspection, and measurement should be conducted on the crankshaft for cracks, bending, twisting, and journal size; Tighten the tile cover according to the specified torque, otherwise the crankshaft will be bent; When installing the crankshaft, the centerline of each main bearing shell should be on the same axis, and the clearance of each bearing shell should comply with regulations.
(3) When assembling shaft parts, attention should be paid to adjusting the fit clearance to ensure smooth lubrication oil flow. When the fit clearance between the shaft and bearing shell exceeds the limit, repair or replacement must be carried out.
(4) Improper storage of shaft parts can lead to deformation of the parts. For example, if long shaft parts are laid flat for too long and there are heavy objects on the shaft, it is easy to cause bending deformation; The multi cylinder crankshaft should have the flywheel adapter plate down and the crankshaft standing on the ground or on a pad.
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2023-11-22