The task of mechanical design is to draw a specific structure drawings based on the design task on the basis of the overall conception, and to draw a specific structure drawings to achieve the functions requirement. The design process is to concretize abstract working principles into certain types of components or parts, including determining the materials, shapes, dimensions, tolerances, heat treatment methods, and surface treatments of structural parts. The processing technology, strength, rigidity, and problems such as accuracy and the relationship with other parts. Therefore, although the direct product of structural design is technical drawings, the work is not simple mechanical drawing. Drawings are only the engineering language to express the design plan. The use of various techniques of mechanism design to concretize design ideas is the basic content of structural design.
A must-see for mechanical design, what does structural design look like in the eyes of the god?
1 Structural elements and design methods of mechanical structural parts
1.1 Geometrical elements of structural parts
The function of mechanical structure is mainly realized by the geometric shape of mechanical parts and the relative positional relationship between each part. The geometry of a part is composed of its surface. A part usually has multiple surfaces, and some of these surfaces are in direct contact with the surfaces of other parts. This part of the surface is called a functional surface. The connecting part between the functional surfaces is called the connecting surface.
The functional surface of a part is an important factor that determines the mechanical function, and the design of the functional surface is the core of the structural design of the part. The main geometric parameters describing the functional surface include the geometric shape, size, number of surfaces, position, and order of the surface. Through different designs of functional surfaces, multiple structural solutions for achieving the same technical function can be obtained.
1.2 The relationship between structural parts
In a machine or machinery, no part exists in isolation. Therefore, in addition to studying the functions and related features of the parts themselves, the relationship between the parts must also be studied in the structural design.
The interrelationships between parts are divided into two types: direct correlation and indirect correlation. Two parts that have a direct assembly relationship become directly related. There is no direct assembly relationship becomes indirect correlation. Indirect correlation is divided into two types: position correlation and motion correlation. Position correlation means that two parts have requirements on the mutual position. 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. Motion correlation means that the motion trajectory of one part is related to another part. For example, the motion trajectory of the lathe tool post must be parallel to the centerline of the spindle. This is ensured by the parallelism of the bed guide rail and the spindle axis, so the spindle It is related to the position of the guide rail, while the tool post is related to the movement of the spindle.
Most parts have two or more directly related parts, so each part has two or more parts that are structurally related to other parts. In the structural design, the structure of 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, precision and surface of the material. At the same time, it must also consider satisfying indirect related conditions, such as dimensional chain and accuracy calculations. Generally speaking, if there are more directly related parts of a part, its structure is more complicated; the more indirectly related parts of a part, the higher the precision requirement.
1.3 Materials of structural parts in structural design There are many materials
that can be selected in the design of parts. Different materials have different properties, and different materials correspond to different processing techniques. In the design, appropriate materials must be selected reasonably according to functional requirements. Determine the appropriate processing technology according to the type of material, and determine the appropriate structure according to the requirements of the processing technology. Only through the appropriate structure design can the selected material give full play to its advantages.
The designer must fully understand the mechanical properties, processing performance, use cost and other information of the relevant materials in order to correctly select the materials of the parts. In the structural design, different design principles should be followed according to the characteristics of the selected material and the corresponding processing technology.
2 Basic requirements of mechanical structure design
Mechanical products are used in various industries, and the specific content and requirements of structural design are also very different, but the basic requirements of structural design are common. The following three different levels of mechanical structure design illustrate the requirements for structure design.
A must-see for mechanical design, what does structural design look like in the eyes of the gods?
2.1 Functional design
Meet the requirements of the main mechanical functions, technically specific. Such as the realization of the working principle, the reliability of the work, the process, the material and the assembly.
2.2 Quality design
Taking into account various requirements and restrictions, improving product quality and cost-effectiveness are the characteristics of modern engineering design. Specifically, factors such as operability, aesthetics, safety, cost, and environmental protection. In modern design, quality design is very important and often determines the competitiveness of products. The design idea that only focuses on meeting the main technical function requirements has become a thing of the past. It is the core of modern mechanical design that takes account of various requirements and makes appropriate balances and trade-offs to other requirements on the premise of meeting the main functions. .
2.3 Optimal design and innovative design
Use structural design variables and other methods to systematically construct an optimal design space, and use creative design thinking methods and other scientific methods for optimization and innovation.
3 Basic design criteria
The final result of mechanical design is to express the determined structure form in drawings, and process and assemble according to the designed drawings to produce the final product. Therefore, the mechanical structure design should meet various requirements as a product. The basic requirements include functionality, reliability, manufacturability, economy, and appearance. In addition, the force of the parts should be improved, and the strength, rigidity, precision and life should be improved. Therefore, mechanical structure design is a comprehensive technical work. Unreasonable or wrong structural design may cause unnecessary failure of parts and components, making the machine unable to meet the design accuracy requirements, and bringing great inconvenience to assembly and maintenance. The following structural design criteria should be considered in the process of mechanical structure design.
3.1 Design criteria for achieving expected functions
The main purpose of product design is to achieve predetermined functional requirements, so the design criteria for achieving expected functions are the first consideration in structural design. To meet the functional requirements, the following must be done.
(1) Clear function: The structural design is to determine the parameter size and structural shape according to its function in the machine and its connection with other parts. The main functions of components are to bear load, transmit motion and power, and to ensure or maintain the relative position or trajectory of related parts or components. The designed structure should be able to meet the functional requirements of the machine as a whole.
(2) Function allocation: During product design, it is usually necessary to reasonably allocate tasks according to specific conditions, that is, to decompose a function into multiple sub-functions. Each sub-function must have a definite structural commitment, and each part of the structure should have a reasonable and coordinated relationship to achieve the realization of the overall function. Multiple structural parts bearing the same function can reduce the burden on the parts and extend the service life. For example, a structure with a V-shaped cross-section is an example of a reasonable assignment of tasks. 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 interacts with the pulley groove to generate the friction required for transmission. For example, if the friction force generated by the bolt pre-tightening is used to bear the lateral load, the size of the bolt will be too large. Shear elements such as pins, sleeves and keys can be added to share the lateral load to solve this problem.
(3) Concentration of functions: 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. Concentration of functions will make the shape of the part more complicated, so it should be moderate, otherwise it will increase the processing difficulty and increase the processing cost. The design should be determined according to the specific situation.
3.2 Design criteria to meet strength requirements
(1) Equal strength criteria: the change in the size of the part’s section should be adapted to the change in the internal stress, so that the strength of each section is equal. With the structure designed according to the principle of equal strength, materials can be fully utilized, thereby reducing weight and cost. Such as cantilever support, stepped shaft design, etc.
(2) Reasonable force flow structure: In order to intuitively show how the force is transmitted in the mechanical component, the force is regarded as water flowing in the component, and these force lines converge into force flow. It means that the flow of this force plays an important role in structural design investigation. The flow of force will not be interrupted in the component, and any line of force will not suddenly disappear. It must be introduced from one place and transmitted from another place. Another characteristic of the 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 through. From the stress point of view, the material is not fully utilized. Therefore, in order to improve the rigidity of the component, the shape of the part should be designed according to the shortest path of the force flow as much as possible to reduce the load-bearing area, so that the smaller the cumulative deformation, the rigidity of the entire component is improved, and the material is fully utilized.
(3) Structure to reduce stress concentration: When the direction of force flow turns sharply, the force flow will be too dense at the turning point, which will cause stress concentration. Measures should be taken in the design to make the force flow turn smoothly. Stress concentration is an important factor affecting the fatigue strength of parts. In structural design, stress concentration should be avoided or reduced as much as possible, such as increasing excessive fillet, adopting unloading structure, etc.
(4) Load balancing structure: When the machine is working, some useless forces are often generated, such as inertial force, helical gear axial force, etc. These forces not only increase the load of shafts and bushings and other parts, reduce their accuracy and life. , It also reduces the transmission efficiency of the machine. The so-called load balance refers to the use of structural measures to partially or completely balance the useless force to reduce or eliminate its adverse effects. These structural measures mainly use balanced components, symmetrical arrangements, etc.
3.3 Satisfying the design criteria
In order to ensure that the parts can perform their functions normally within the service life, they must have sufficient rigidity.
3.4 Design criteria considering processing technology
The main purpose of the structural design of mechanical parts is to ensure the realization of functions and make the product reach the required performance. However, the rationality of structural design directly affects the production cost and quality of parts. Therefore, in the structural design, we should strive to have good processing technology for the parts mechanism. The so-called good processing technology refers to the easy processing and manufacturing of the structure of the parts. Any processing method has its limitations, or the production cost is high, or the quality is affected. Therefore, it is very important for designers to understand the characteristics of various processing methods in order to maximize the strengths and avoid weaknesses when designing the structure. In actual production, the processability of the part structure is restricted by many factors, such as the size of the production batch will affect the generation method of the blank; the conditions of the production equipment may limit the size of the workpiece; in addition, the shape, precision, heat treatment, cost, etc. It is possible to restrict the processability of the part structure. Therefore, the influence of the above factors on the processability should be fully considered in the structural design.
3.5 Consider the design criteria
Assembly is an important process in the product manufacturing process, and the structure of components has a direct impact on the quality and cost of assembly. A brief description of the structural design criteria for assembly is as follows:
(1) Reasonably divide assembly units: The whole machine should be able to be decomposed into several units (components or components) that can be assembled separately to achieve parallel and specialized assembly operations and shorten assembly cycles. And it is convenient for step-by-step technical inspection and maintenance.
(2) The parts are correctly installed: to ensure the accurate positioning of the parts, avoid double coordination, and prevent assembly errors.
(3) Make the components easy to assemble and disassemble: In the structural design, ensure that there is enough assembly space, such as wrench space; avoid excessively long fitting to avoid increasing assembly difficulty and scratching the mating surface, such as the design of some stepped shafts; To facilitate the disassembly of parts, the location for disassembly tools should be given, such as the disassembly of bearings.
A must-see for mechanical design, what does structural design look like in the eyes of the gods?
3.6 Design criteria for considering maintenance and repair
(1) The configuration of the product should be arranged according to its failure rate, difficulty of maintenance, size and quality, and installation characteristics. All parts and components that need to be repaired should have good quality Accessibility: The best accessibility should be provided for parts and emergency switches that have a high failure rate and require frequent maintenance.
(2) The disassembly and assembly of the product, especially the vulnerable parts, frequently disassembled parts and additional equipment, should be simple, and the route of the parts in and out during disassembly and assembly is best to be a straight or flat curve.
(3) System maintenance points such as product inspection points and test points should be arranged in locations that are easy to access.
(4) For products that need to be repaired and disassembled, there must be sufficient operating space around them.
(5) In general, the internal operations should be visible during maintenance. In addition to the passageway that can accommodate the hands or arms of the maintenance personnel, there should also be a proper gap for observation.
3.7 Criteria for considering model design
The design of a product must not only meet the requirements of use, but also consider the aesthetic value of the product model to make it attractive to people. Simply put, it is both easy to use and beautiful. From a psychological point of view, 60% of a person’s decision depends 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 a product that can attract customers. At the same time, beautiful products can reduce the operator’s misoperation due to exhaustion.
The appearance design includes three aspects: shape, color and surface treatment.
When considering styling, attention should be paid to coordinated size and proportion, simple and unified shape, and support and embellishment of colors and patterns.
Monochrome is only used for small widgets. The big ones, especially the moving parts, will appear monotonous and without gradation if only one color is used, and a small additional color block will make the whole color alive. In the case of multiple colors coexisting, there should be a dominant background color, and the color corresponding to the background color is called a contrasting color. But in a product, the number of different shades should not be too many, too many colors will give people a flashy feeling.
Comfortable colors are approximately in the range from light yellow, green and yellow to brown. This trend is getting warmer, yellow and green often appear uncomfortable; strong gray tones appear depressive. For cold environments, apply warm colors such as yellow, orange, and red. For hot environments, use cool colors, such as light blue. All colors should be faded. In addition, a certain color configuration can make the product appear safe and stable. The plane with small changes in shape and large area is configured with light colors, and the components with moving and active contours are configured with dark colors; dark colors should be placed on the lower part of the machine, and light colors on the upper part.
A must-see for mechanical design, what does structural design look like in the eyes of the gods?
3.8 Design criteria for considering cost
Product and maintenance operations should be simplified during design :
(1) When designing, product functions should be analyzed and weighed, the same or similar functions should be combined, and unnecessary functions should be eliminated to simplify product and maintenance operations.
(2) When designing, under the premise of meeting the specified functional requirements, the structure should be simple, the product level and the number of constituent units should be reduced as much as possible, and the shape of the parts should be simplified.
(3) The product should be designed as simple and reliable as possible to adjust the mechanism to facilitate the elimination of common failures caused by wear or drift. Valuable parts that are prone to local wear should be designed as adjustable or detachable assemblies to facilitate local replacement or repair. Avoid or reduce the repeated adjustments that involve each other.
(4) Reasonably arrange the position of each component, reduce the connecting parts, fixing parts, make it easy and convenient for maintenance operations such as detection, replacement, etc., as far as possible, when repairing any part, no disassembly, no movement or less disassembly, Move other parts less to reduce the skill level requirements and workload of maintenance personnel.
