Explanation of the basic knowledge of mechanical structure design, methods, requirements and design criteria

1. Structural elements and design methods of mechanical structural parts 1. Geometric elements of structural parts The function of mechanical structure is mainly realized by the geometric shape of mechanical parts and the relative position relationship between each part. The geometric shape of a part is composed of its surface. A part usually has multiple surfaces. Some of these surfaces are in direct contact with the surfaces of other parts. This part of the surface is called the functional surface. The connecting part between functional surfaces is called the connection surface. The functional surface of a part is an important factor in determining the function of a machine, and the design of the functional surface is the core issue of the structural design of a part. The main geometric parameters describing the functional surface include the geometric shape, size, number, position, and sequence of the surface. Through the variation design of the functional surface, a variety of structural schemes can be obtained to achieve the same technical function. 2. Connection between structural parts In a machine or machinery, no part exists in isolation. Therefore, in addition to studying the function and other characteristics of the part itself, it is also necessary to study the relationship between the parts in structural design. The correlation of parts is divided into two categories: direct correlation and indirect correlation. If two parts have a direct assembly relationship, they are directly related. Correlation without a direct assembly relationship is indirect correlation. Indirect correlation is further divided into position correlation and motion correlation. Positional correlation refers to the requirement of the relative position of two parts. For example, the center distance of two adjacent transmission shafts in the reducer must ensure a certain accuracy, and the two axes must be parallel to ensure the normal meshing of the gears. Kinematic correlation refers to the motion trajectory of one part being related to another part. For example, the motion trajectory of the lathe tool rest must be parallel to the center line of the spindle. This is ensured by the parallelism of the bed guide rail and the spindle axis. Therefore, the spindle and the guide rail are positionally related; while the tool rest and the spindle are kinematically related. Most parts have two or more directly related parts, so most parts have two or more parts that are structurally related to other parts. When designing a structure, the directly related parts of the two parts must be considered at the same time in order to reasonably select the heat treatment method, shape, size, accuracy and surface quality of the material. At the same time, it is also necessary to consider meeting the indirect related conditions, such as dimension chain and accuracy calculation. Generally speaking, the more directly related parts a part has, the more complex its structure is; the more indirectly related parts a part has, the higher its accuracy requirements are. 3 Structural design According to the material of the structural parts, there are many materials that can be selected in mechanical design. Different materials have different properties, and different materials correspond to different processing technologies. In structural design, it is necessary to reasonably select appropriate materials according to functional requirements, determine appropriate processing technologies according to the types of materials, and determine appropriate structures according to the requirements of processing technologies. Only through appropriate structural design can the selected materials give full play to their advantages. Designers must fully understand the mechanical properties, processing properties, and use costs of the selected materials in order to correctly select materials. Different design principles should be followed in structural design according to the characteristics of the selected materials and the corresponding processing technologies. 2. Basic requirements for mechanical structural design Mechanical products are used in all walks of life, and the content and requirements of structural design are also very different, but they all have the same common parts. The following three different levels of mechanical structural design will explain the requirements for structural design. 1. Functional design meets the main mechanical functional requirements and is technically specific. Such as the realization of working principles, reliability of work, process, materials and assembly. 2. Quality design takes into account various requirements and restrictions, improves the quality and performance-price ratio of products, and is a feature of modern engineering design. Specifically, there are many other requirements and restrictions such as operation, aesthetics, cost, safety, and environmental protection. In modern design, quality design is very important and often determines the competitiveness of products. The era of mechanical design that only meets the main technical functional requirements has passed. Taking into account various requirements and improving the quality of products is the key to modern mechanical design. 3. Optimization design and innovative design Use structural design variables and other methods to systematically construct the optimization design space, and use creative design thinking methods and other scientific methods for optimization and innovation. Non-standard mechanical design learning and sharing, professional focus 3. Basic design principles of mechanical structure The final result of mechanical design is expressed in a certain structural form, and it is processed and assembled according to the designed structure to manufacture the final product. Therefore, mechanical structure design should meet the various requirements of the product. The basic requirements include function, reliability, processability, economy and appearance. In addition, the force of parts should be improved to improve strength, rigidity, precision and life. Therefore, mechanical structure design is a comprehensive technical work. Due to errors or unreasonable structural design, it may cause unnecessary failure of parts and components, so that the machine cannot meet the requirements of design accuracy, which brings great inconvenience to assembly and maintenance. The following structural design principles should be considered in the process of mechanical structure design. 1. Design criteria for expected functions The main purpose of product design is to achieve the predetermined functional requirements. Therefore, the design criteria for achieving the expected functions are the first consideration in structural design. To meet the functional requirements, the following points must be achieved. (1) Clear function: Structural design is to determine the parameter size and structural shape based on its function in the machine and the connection relationship with other parts. The main functions of parts are to bear loads, transmit motion and power, and ensure or maintain the relative position or motion trajectory between related parts or components. The designed structure should be able to meet the functional requirements of the machine as a whole. (2) Reasonable distribution of functions: When designing a product, it is usually necessary to reasonably distribute the tasks according to the specific situation, that is, to decompose a function into multiple sub-functions. Each sub-function must be undertaken by a certain structure, and the various structures should have a reasonable and coordinated connection to achieve the overall function. Multiple structural parts that undertake the same function can reduce the burden on the parts and extend their service life. The structure of the V-belt section is an example of reasonable task distribution. The fiber rope is used to bear the tension; the rubber filling layer bears the tension and compression when the belt is bent; the cloth layer acts on the pulley groove to generate the friction required for transmission. For example, if the lateral load is only supported by the friction force generated by the bolt pre-tightening, the size of the bolt will be too large. Shear-resistant elements such as pins, sleeves and keys can be added to share the lateral load to solve this problem. (3) Functional concentration: In order to simplify the structure of mechanical products, reduce processing costs and facilitate installation, in some cases, one part or component can assume multiple functions. Functional concentration will make the shape of the part more complex, but it should be done in moderation, otherwise it will affect the processing technology and increase processing costs. The design should be based on specific circumstances. 2. Design criteria that meet strength requirements (1) Equal strength criterion: The change in the cross-sectional size of the part should be consistent with the change in its internal stress, so that the strength of each cross-section is equal. Structures designed according to the equal strength principle can make full use of materials, thereby reducing weight and cost. Such as the design of cantilever brackets and stepped shafts. (2) Reasonable force flow structure: In order to intuitively represent how force is transmitted in mechanical components, the force is regarded as water flowing in the component, and these force lines merge into force flow. Representing this force flow plays an important role in structural design. The force flow will not be interrupted in the component, and any force line will not disappear suddenly. It must be transmitted from one place to another. Another characteristic of force flow is that it tends to be transmitted along the shortest route, so that the force flow is dense near the shortest route, forming a high stress area. The force flow in other parts is sparse, or even no force flow passes through. From the perspective of stress, the material is not fully utilized. Therefore, in order to improve the stiffness of the component, the shape of the part should be designed as much as possible according to the shortest force flow route, reducing the load-bearing area, thereby reducing the cumulative deformation, improving the stiffness of the entire component, and making full use of the material. (3) Reduce stress concentration structure: When the force flow direction turns sharply, the force flow will be too dense at the turning point, causing stress concentration. In the design, measures should be taken in the structure to make the force flow turn smoothly. Stress concentration is an important factor affecting the fatigue strength of parts. When designing a structure, stress concentration should be avoided or reduced as much as possible. The methods will be introduced in the corresponding chapters, such as increasing the excessive fillet and using unloading structures. (4) Make the load balanced structure: When the machine is working, some useless forces are often generated, such as inertial force, axial force of helical gears, etc. These forces not only increase the load on parts such as shafts and bushings, reduce their accuracy and life, but also reduce the transmission efficiency of the machine. The so-called load balance refers to taking structural measures to partially or completely balance the useless forces to reduce or eliminate their adverse effects. These structural measures mainly adopt balancing elements, symmetrical layout, etc. 3. Design criteria to meet structural stiffness To ensure that the parts can normally perform their functions within their service life, they must have sufficient stiffness. 4. Design criteria considering processing technology The main purpose of the structural design of mechanical parts is to ensure the realization of functions and enable the product to achieve the required performance. However, the results of structural design have an impact on the production cost and quality of product parts that cannot be underestimated. Therefore, in the structural design, efforts should be made to make the product have good processing technology. The so-called good processing technology refers to the structure of the parts being easy to process and manufacture. Any processing method may not be able to manufacture certain structural parts, or the production cost is very high, or the quality is affected. Therefore, it is very important for designers to understand the characteristics of a processing method so that they can maximize their strengths and avoid their weaknesses when designing the structure. In practice, the processability of component structures is restricted by many factors. For example, the size of the production batch will affect the method of generating blanks; the conditions of the production equipment may limit the size of the workpiece; in addition, modeling, precision, heat treatment, cost and other aspects may have a restrictive effect on the processability of component structures. Therefore, the influence of the above factors on processability should be fully considered in structural design. Public account: Non-standard mechanical design learning and sharing, professional focus 5. Consider the design criteria for assembly Assembly is an important process in the product manufacturing process. The structure of the parts has a direct impact on the quality and cost of assembly. The structural design criteria for assembly are briefly described as follows: (1) Reasonable division of assembly units: The whole machine should be able to be decomposed into several units (components or assemblies) that can be assembled separately to achieve parallel and specialized assembly operations, shorten the assembly cycle, and facilitate step-by-step technical inspection and maintenance. (2) Ensure that the parts are correctly installed: ensure accurate positioning of parts, avoid double matching, and prevent assembly errors. (3) Make parts easy to assemble and disassemble: In the structural design, sufficient assembly space should be ensured, such as wrench space; avoid too long fit to increase the difficulty of assembly and cause scratches on the fitting surface, such as the design of some stepped shafts; in order to facilitate the disassembly of parts, the location of the disassembly tools should be provided, such as the disassembly of bearings. 6. Design criteria for maintenance and repair (1) The configuration of the product should be arranged according to its failure rate, the difficulty of maintenance, the size and quality, and the installation characteristics. All parts and components that need to be repaired should have good accessibility; for parts with high failure rates and frequent maintenance and emergency switches, the best accessibility should be provided. (2) The disassembly and assembly of products, especially vulnerable parts, frequently disassembled parts and additional equipment, should be simple. The route for the entry and exit of parts during disassembly and assembly should preferably be a straight line or a gentle curve. (3) The inspection points, test points and other system maintenance points of the product should be arranged in a position that is easy to access. (4) There should be enough operating space around products that need to be repaired and disassembled. (5) During maintenance, the internal operation should generally be visible. In addition to being able to accommodate the hands or arms of maintenance personnel, the passage should also leave appropriate gaps for observation. Non-standard mechanical design learning and sharing, professional focus 7. Consider the criteria for styling design The design of a product should not only meet functional requirements, but also consider the aesthetic value of the product shape to make it attractive to people. From a psychological point of view, 60% of people's decisions depend on the first impression. The social attribute of technical products is commodities. In the era of buyer's market, it is an important design requirement to design an appearance that can attract customers for the product; at the same time, beautiful products can reduce the operator's misoperation caused by fatigue. Appearance design includes three aspects: shape, color and surface treatment. When considering shape, attention should be paid to the coordination of size proportions, simple and unified shapes, and the support and embellishment of colors and patterns. Monochrome is only used for small components. Large, especially moving components, will appear monotonous and without layers if only one color is used. A small additional color block will make the entire tone lively. When multiple colors coexist, there should be a dominant background color, and the color corresponding to the background color is called a contrasting color. However, the number of different tones on a product should not be too many. Too many colors will give people a gaudy feeling. Comfortable colors are roughly in the range from light yellow, green-yellow to brown. The trend is to gradually warm up. Pure yellow and pure green often appear uncomfortable; strong gray tones appear depressing. For cold environments, use warm colors, such as yellow, orange-yellow and red. For hot environments, use cool colors, such as light blue. All colors should be diluted. In addition, a certain color configuration can make the product look safe and stable. Light colors should be used for planes with small shape changes and large areas, while dark colors should be used for components with moving and active contours; dark colors should be placed at the bottom of the machine and light colors at the top. 8. Design principles considering cost The product and maintenance operations should be simplified during design: (1) During design, the product functions should be analyzed and weighed, the same or similar functions should be merged, and unnecessary functions should be eliminated to simplify the product and maintenance operations. (2) During design, the structure should be simple, the number of product levels and components should be reduced as much as possible, and the shape of the parts should be simplified while meeting the specified functional requirements. (3) The product should be designed with a simple and reliable adjustment mechanism to eliminate common faults caused by wear or drift. Valuable parts that are prone to local wear and tear should be designed as adjustable or detachable assemblies to facilitate local replacement or repair. Avoid or reduce repeated adjustments that are mutually involved. (4) The positions of various components should be arranged reasonably, and the number of connecting parts and fixing parts should be reduced to make the inspection, replacement and other maintenance operations simple and convenient. When repairing any part, do not disassemble or move other parts or disassemble and move other parts as little as possible to reduce the requirements on the skill level and workload of maintenance personnel.