Mechanical assembly process foundation

1. The concept of assembly
Mechanical products are generally composed of many parts and components. In accordance with the prescribed technical requirements, the process of combining several parts groups into components or parts or combining several components and parts into a product is called assembly.

Mechanical assembly is the last stage in the entire mechanical manufacturing process and occupies a very important position in the manufacturing process. The quality of mechanical products is ultimately guaranteed by assembly work. The quality of parts is the basis of the quality of mechanical products, but the assembly process is not a process of simply combining qualified parts. Even with high-quality parts, low-quality assembly may produce low-quality products; high-quality assembly can assemble high-quality products on the basis of economic precision parts and components.

In recent years, due to the rapid increase in mechanization and automation in blank manufacturing and mechanical processing, the proportion of assembly workload in the manufacturing process has been increasing. Therefore, the technical level and labor productivity of assembly work must be improved to adapt to the development trend of the machinery industry.

For products with more complex structures, in order to ensure assembly quality and assembly efficiency, it is necessary to decompose the product into separate assembly units from the perspective of the assembly process according to the structural characteristics of the product.

Parts are the most basic units that make up a mechanical product. The parts are generally assembled into a composite, component or part before being assembled on the machine.

Composite parts are also called kits, which are made up of a number of parts that are permanently connected or connected and then processed. A component is a combination of several parts and assemblies. The components can perform a certain and complete function in the machine.

Assembly accuracy of mechanical products

The assembly accuracy of mechanical products is the technical requirements that should be met during product assembly, which can be divided into two categories: geometric parameters and physical parameters.

1. Geometrical accuracy requirements

The geometric accuracy requirements include clearance, fit properties, mutual position accuracy and relative motion accuracy, contact quality and so on.

Clearance and fit properties can be unified as dimensional accuracy requirements, which refer to the dimensional distance accuracy between related parts and components.

Mutual position accuracy in assembly includes related parts, parallelism between components, perpendicularity, coaxiality and various runouts, etc. Relative motion accuracy refers to the accuracy of the relative motion direction and relative speed direction between the relative motion of the parts and components in the product. The accuracy of the movement direction is mostly expressed as the parallelism and perpendicularity of the relative movement between zero and components; the relative speed accuracy is also called transmission accuracy, that is, a certain degree of precision must be maintained when the relative movement between components and components is required. The linear motion accuracy or circular motion accuracy of parts and components is the basis of relative motion accuracy.

Contact accuracy refers to the size and distribution of the actual contact area between the contact surfaces.

2. Physically required accuracy

The accuracy of the physical direction requires a lot of content, such as speed, weight, tightening force, static balance, dynamic balance, tightness, friction, vibration, noise, temperature rise, etc., depending on the type and purpose of the specific machine. The content varies.

Basic content of assembly work

Assembly fairness can be done by a series of assembly processes in an ideal order of work. Common basic assembly operations include the following:

1. Cleaning

The purpose of cleaning is to remove oil and mechanical impurities on the surface or inside of parts and components. Common basic cleaning methods include scrubbing, dipping, spraying, ultrasonic cleaning and so on. The elements of the cleaning process are the type of cleaning fluid (commonly used are kerosene, gasoline, lye and various chemical cleaning fluids), process parameters (such as temperature, pressure, time) and cleaning methods. The choice of cleaning process method should be determined according to factors such as the cleaning requirements of the workpiece, the material of the workpiece, the batch size, the nature of the oil and mechanical impurities, and the adhesion. In addition, the workpiece should have a certain intermediate anti-rust ability after cleaning.

The cleaning work is of great significance to ensure and improve the assembly quality of the machine and prolong the service life of the product, especially for the key components of the machine such as bearings, seals, precision parts, and lubrication systems.

2. Connect

There is a lot of connection work during the assembly process. Connection methods can generally be divided into two types: detachable connection and non-detachable connection.

can be disassembled to connect the interconnected parts and components without damaging any parts, and can be reconnected after disassembly. Common detachable connections include threaded connections, key connections and pin connections. Among them, threaded connection is the most widely used. The quality of threaded connection has a great relationship with the assembly process. The tightening force of each bolt, the tightening sequence and tightening of multiple bolts should be reasonably determined according to the shape of the connected parts and components, the distribution and force of the bolts Power balance and other requirements.

Non-detachable connection is not disassembled during the use of connected parts and components. If it is to be disassembled, some parts will often be damaged. Common non-detachable connections include welding, riveting and interference connection, etc., among which interference connection is mostly used for shaft and hole fitting. To realize the interference connection, methods such as press fit, thermal expansion fit and cold shrink fit are commonly used. General machines can use press-fitting method, and important or precise machines are willing to use thermal expansion and cold shrinkage fitting methods.

3. Correction, adjustment and configuration

Calibration refers to the alignment and leveling of the mutual positions of related parts and components. It is generally used in the assembly and general assembly of the base parts of large-scale machinery. The commonly used calibration methods include flat-square calibration, square-square calibration, level calibration, and wire-drawing calibration. , Optical correction and laser correction, etc.

Adjustment refers to the adjustment of the mutual position of related parts. The adjustment can cooperate with the correction to ensure the relative position accuracy of the parts and components. It can also adjust the gap in the motion pair to ensure the motion accuracy.

is used for assigning drilling, reaming, scraping and grinding, etc., which are some additional fitter and machining operations in the assembly process. Scratching is a pliers work on the surface of parts and components, and is mostly used for finishing of the matching surface of the moving pair. The drill and hinge are mostly used for fixed connection. Only after careful correction and adjustment to ensure the accurate geometric relationship of the relevant parts and components can it be matched.

4. Balance

The balance of the rotating body is an important requirement in assembly accuracy, especially for machines with higher rotation elements and higher requirements for stable operation. The requirements for the balance of rotating parts and components are more stringent. Some machines need to be balanced at the working speed after the final assembly of the product.

balance method can be divided into static balance method and dynamic balance method. Static balance method can eliminate static imbalance; dynamic balance method can eliminate force imbalance in addition to static imbalance. The general rotating body can be used as a rigid body for balance, and the one with larger diameter and smaller width can only be used for static balance. The parts and components with relatively large length and diameter need to be dynamically balanced. Among them, the rotating body whose working speed is more than 75% of the first-order critical speed should be dynamically balanced as a flexible rotating body.

For the unbalanced quality of the rotating body, the quality can be added by repair welding, riveting, cementing or threaded connection; the quality can be removed by means of drilling, milling, grinding, filing, scraping, etc.; the balance can also be changed in the prefabricated balance groove The location and number of blocks.

5. Acceptance test

In the assembly, component and final assembly process, intermediate inspections are often required before and after important processes. After the final assembly is completed, a comprehensive inspection and experiment of the product shall be carried out in accordance with the required technical standards and regulations.

The content and methods of inspection and experiment of various mechanical products are different. The acceptance work of metal cutting machine tools usually includes machine tool geometric accuracy inspection, dry running test, load test, work accuracy inspection, noise and temperature rise inspection, etc. The inspection content of automobile engine generally includes important fitting clearance, inspection of position accuracy and bonding condition between parts and so on. The final assembly of large power machinery is generally carried out on a special test bench, with detailed test procedures.