Abstract: The assembly quality of agricultural machinery parts determines the repair quality and the service life of the machinery, with a focus on discussing the process requirements for the assembly of agricultural machinery parts.
Keywords: Agricultural machinery; Parts; assembling
The use of any machinery cannot be separated from maintenance and repair, and maintenance and repair cannot be separated from disassembly, replacement, and assembly. Here, we focus on discussing the assembly process requirements and technical points of agricultural machinery parts. The assembly process of parts refers to the process of installing mechanical parts, components, components, and assemblies on basic components in a certain assembly sequence according to certain assembly technical requirements. The assembly of mechanical components must meet technical requirements such as clearance, tightening torque, mutual position, balance, and sealing. Otherwise, the mechanical assembly will have poor performance, short service life, and high failure rate.
1. Assembly accuracy requirements
1.1 Matching accuracy
One of the main purposes of mechanical repair is to restore the normal coordination requirements of each part. During mechanical assembly, in addition to meeting technical requirements for each part and component, it is also necessary to meet the requirements of fitting accuracy to ensure the correct fitting clearance between parts. If the dynamic fitting part is a clearance fit, a small clearance will exacerbate wear and increase power consumption; The parts that are tightly fitted should maintain a certain amount of interference. If the interference is too large, it is easy for the parts to be damaged or even broken during the assembly process. The following methods are commonly used in assembly:
(1) Optional method: In agricultural machinery repair, it is found that even if each component is within the allowable error range, any combination assembly may not necessarily meet the requirements of fitting accuracy. Therefore, optional matching must be carried out during assembly. The selection method can not only meet the requirements of fitting clearance, but also meet technical requirements such as balance.
(2) Repair method: Leave appropriate repair allowance during part processing, and perform simple mechanical processing before assembly to achieve high-precision fit between the processed part and the matching part. This method is called repair method. The repair method is widely used in mechanical repair, such as cutting steel sleeves based on piston pin hinges; Scrape the bearing shells according to the contact marks; Research valves and valve seats; Repair the gap between the piston ring openings, etc.
(3) Adjustment method: The method of achieving the required fitting accuracy by adding or removing shims, changing the position of adjustment screws, and other measures is called adjustment method. The clearance of agricultural machinery cone bearings, meshing marks of bevel gears, meshing clearance, valve clearance, etc. must be adjusted during assembly to meet the technical requirements for fitting.
1.2 Dimensional chain accuracy
In mechanical assembly, sometimes although the fitting accuracy of each fitting part meets the requirements, the dimensional chain error caused by accumulated errors may exceed the required range. This can be seen from the dimensional chain during the assembly of the engine crank connecting rod mechanism shown in Figure 1. In the figure, A is the distance from the centerline of the main bearing seat hole to the upper plane of the cylinder block, B is the crankshaft turning radius, C is the distance between the centerline of the connecting rod's large and small ends, and D is the distance from the centerline of the piston pin hole to the piston top plane, δ The distance between the upper plane of the piston at the top dead center and the upper plane of the cylinder block is the closed ring of this dimension chain. δ It has a significant impact on the compression ratio of diesel engines and should be ensured during assembly. δ The size of the value is not only affected by the manufacturing and repair accuracy of sections A, B, C, and D, but also by the fit clearance at O, O1, and O2. When A is Z large, the dimensions of B, C, and D are Z small, and the gap between O, O1, and O2 is Z large, the compression is completed δ Large value; On the contrary, at the end of compression δ The value Z is small, and its value may exceed the specified range. Therefore, it is necessary to inspect after assembly. If it does not meet the requirements, it should be re selected or replaced with certain parts to ensure that the accumulated error of all components in the dimension chain meets the accuracy requirements of the dimension chain.
Figure 1 Assembly dimension chain of crank connecting rod mechanism
2. Assembly sealing requirements
In mechanical use, the phenomenon of "three leaks" (oil leakage, water leakage, and air leakage) often occurs due to seal failure. The main reason for the occurrence of "three leaks" is due to the assembly process of the sealing device not meeting the requirements or the wear, deformation, aging, and corrosion of the sealing components. The early damage of sealing components is related to assembly factors (including the selection of sealing material, hardness, assembly position, etc.). Therefore, sufficient attention must be paid during assembly.
(1) The selection of sealing materials should be appropriate. Sealing materials are generally selected based on pressure, temperature, and medium. Paper gaskets are only used for low pressure and low temperature conditions; Rubber has weak pressure and high temperature resistance, and its oil resistance, acid resistance, and alkali resistance should be considered; Plastic has high pressure resistance, but is not resistant to high temperatures; Asbestos has good strength and high temperature resistance; Metal has the ability to withstand both high temperature and high pressure.
(2) The assembly tightness should meet the requirements. The assembly tightness of the seal must meet the requirements and the compression force must be uniform. When the clamping force is insufficient, it can cause leakage, or after working for a period of time, the tightness is reduced due to vibration and the tightening screws are stretched, resulting in leakage; Excessive compression force can cause the static sealing gasket to lose its strength, leading to early failure of the gasket; Dynamic sealing components can cause heating, accelerate wear, increase friction power, and other adverse consequences.
(3) Using sealant. Choose the appropriate sealant according to the working conditions of the machinery. The temperature range for the use of sealant is generally between -60 to 250 ℃, and the pressure resistance is not greater than 6000 MPa.
3. Assembly cleanliness requirements
The cleanliness of the parts has a significant impact on the quality of repairs, and they must be thoroughly cleaned during assembly. If the dirt deposits in the lubricating oil passage are not completely removed, it will reduce the cross-sectional area of the oil passage? Increasing the flow resistance of lubricating oil, reducing the flow rate of lubricating oil, and deteriorating the lubrication conditions of the operating parts. The dust on the surface of the parts mixed into the lubricating oil will circulate with the lubricating oil, causing varying degrees of abrasive wear. During assembly, it is necessary to clean thoroughly and blow dry with compressed air. The scale in the engine water jacket must be cleaned thoroughly during repair. The scale accumulated in the engine water jacket not only reduces the capacity of the cooling system and affects the circulation of cooling water, but also has extremely poor thermal conductivity, which can easily cause the engine to overheat.
4. Assembly Tool Requirements
During assembly, special tools should be used as much as possible, and random knocking and striking are prohibited to prevent damage to parts and affect assembly quality. When installing tightly fitted parts, a press and specialized fixtures should be used. When installing the rolling bearing onto the journal, Z should heat the bearing to 100-120 ℃ in engine oil before proceeding.
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https://www.vafem.com/products/Agricultural-Wheel-Hub-Unit/898.html
2024-01-05