Abstract: This article briefly analyzes the causes of common damage to 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; Axis components; Repair quality

Shaft components are common and important parts in agricultural machinery. The shaft components on agricultural machinery include crankshafts, camshafts, transmission shafts, drive shafts, half shafts, etc. Shaft parts account for a large number of various parts sent for repair. Their repair is not only due to their good processability, but also because they can achieve high economic benefits. Therefore, they are one of the main objects of old parts repair in agricultural machinery repair departments.

1. Common damages and causes of shaft components

Shaft components are also rotating components, with a structural characteristic that their length is several to tens of times greater than their diameter, making them prone to bending; At high speeds, some accessories need to be attached to the shaft to rotate together, and the shaft neck is prone to wear. Some shafts also experience a lot of force during operation, such as engine crankshafts, which are subjected to high gas pressure and inertial impact, which can easily cause abnormal wear, tile burning, bending, twisting, fracture and other faults. The common damage analysis of shaft components is as follows (taking the crankshaft as an example):

(1) Axle neck wear. Manifested as a reduction in the size of the main journal and connecting rod journal, as well as the generation of ellipticity and taper. The reason is that the abrasive in the shaft oil passage accumulates on the shaft neck, mechanical impurities accumulate in the lubricating oil, the connecting rod bends, the cylinder barrel tilts longitudinally in the engine body, the force distribution of the connecting rod bearing on the connecting rod shaft neck is uneven, and the crankshaft grinding does not meet the technical requirements.

(2) Scratches. There are scratches and spots on the surface of the shaft neck. The reason is that abrasive particles were brought in during assembly, and there are slag, metal objects, etc. in the lubricating oil.

(3) Burns. The surface of the shaft neck is not shiny, and sometimes there is adhesion of bearing alloy. 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 passages, and too small assembly clearance between the bearing shell and the journal, resulting in metal melting on the surface of the journal, often referred to as burning the bearing and holding the shaft.

(4) Bending and twisting. If crankshaft and other shaft components are bent or twisted, it will accelerate the wear of related mating parts and deteriorate the working condition of the engine. The reason is that the crankshaft is subjected to significant impact, such as engine detonation, excessive starting under heavy load, and sudden occurrence of tile burning and shaft hugging accidents.

(5) Cracks and fractures. Neck cracks and fractures are prone to occur at the edges of oil holes and shaft shoulders, as these areas are prone to stress concentration. Reason: ① In manufacturing, the main bearing seat hole is deformed (exceeding the allowable value), and the flywheel and crankshaft joint are not concentric. ② In terms of repair, when grinding the crankshaft, the shoulder fillet radius is too small and there is no transition fillet left, resulting in stress concentration In use, the lack of engine oil resulted in severe burning of the bearing and shaft, causing the crankshaft to twist and break In terms of materials, the material used to manufacture the crankshaft does not meet the requirements.

2. Inspection of shaft components

(1) Bending test: Support the two ends of the shaft horizontally with V-shaped iron, so that the differential gauge contact pin is in the middle of the shaft, then slowly rotate it once, observe and record the swing of the gauge pin, and the Z-value of the swing difference of the gauge pin reflects the degree of bending of the shaft (1/2 of the swing difference is the actual bending degree of the shaft). When inspecting, it should be noted that if the shaft neck is odd, measuring the shaft neck is sufficient; If the journal is a double number, the two middle journals should be measured and the value of Z should be taken. In order to reduce the impact of journal misalignment on inspection accuracy, the differential table Z should be placed at one end of the journal during inspection. For some shafts with simple shapes, 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 inspected. But for some shafts with uneven wear, it is necessary to check the size of their out of roundness and out of column. To measure the wear of the crankshaft journal, the difference between the diameters of Z and Z in the same cross-section is called the out of roundness; Find the difference between the diameters of Z large and Z small on the same longitudinal section, and the difference between the two is the non cylindricity.

(3) Fracture inspection: Inspection methods such as inspection, immersion hammer striking, or magnetic flaw detection can be used for inspection.

3. Measures to improve repair quality

(1) Pay attention to protecting the center hole of shaft components during disassembly. The importance of the central hole is manifested in the following aspects: ① From the perspective of maintaining the accuracy of the parts, shaft parts are generally composed of several or more surfaces. However, daily maintenance often involves one or two of these surfaces. Therefore, when repairing the parts, in addition to restoring the accuracy of the worn surface itself, it is also necessary to follow 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 a composite reference element of the surfaces of shaft components. Therefore, it is very important to maintain the mutual position of each surface of the parts in accordance with the technical drawings and to maintain the accuracy of the central hole From the perspective of repairing parts, the current repair process for shaft parts in the repair department can be roughly divided into two parts: one is to select appropriate methods to fill the damaged surface according to the wear state and degree of the parts; The second is to select reasonable process methods according to the technical requirements of the parts to repair the surface that has lost accuracy. From the above two processes, whether it is commonly used vibration welding, submerged arc welding, spray welding, and brush plating to fill the missing surface, or processes such as turning and grinding to restore surface accuracy, a center hole is generally required From the perspective of assembly, use, and inspection, firstly, the threaded center hole is used to fix other parts or for disassembly and assembly; Secondly, the central hole is sometimes used as a basis for inspection and assembly.

The center hole of shaft parts not only plays different roles 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 sent for repair often causes undue losses to the repair work of old parts. Therefore, when conducting maintenance and disassembly, we should maintain the accuracy of the center hole of shaft components.

(2) Before repair, the crankshaft should be carefully cleaned, especially the debris in the oil passage and the oil dirt on the surface should be removed. Then, necessary technical identification, inspection, and measurement should be carried out on whether the crankshaft has cracks, bends, twists, and journal dimensions; Tighten the tile cover according to the prescribed torque, otherwise the crankshaft will be bent and bent; When installing the crankshaft, the centerlines of each main bearing tile should be on the same axis, and the clearance between each tile should comply with regulations.

(3) When assembling shaft components, attention should be paid to adjusting the fit clearance to ensure smooth lubrication of the oil circuit. If the fit clearance between the shaft and the bearing exceeds the limit, repair or replacement must be carried out.

(4) Improper storage of shaft components can lead to deformation, such as placing long shaft components flat for too long and having heavy objects on the shaft that can cause them to bend and deform; The multi cylinder crankshaft should have the flywheel attached to the lower part, and the crankshaft should be placed on the ground or a cushion plate.


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