Explanation of the basics of mechanical structure design, methods, requirements and design guidelines
First, Structural elements and design methods of mechanical structural parts
1.Geometric elements of structural parts
The function of the mechanical structure mainly relies on the geometry of mechanical parts and the relative position relationship between the parts to achieve. The geometry of the component is composed of its surface, a part usually has more than one surface, some of these surfaces are in direct contact with the surface of other parts, the part of the surface is called the functional surface. In between the functional surface of the linkage part is called the joint surface.
The functional surface of the part is an important factor in determining the mechanical function, the design of the functional surface is the core issue of the structural design of the part. The main geometric parameters describing the functional surface are the geometry of the surface, size, number of surfaces, location, order, etc. Through the variant design of functional surfaces, multiple structural solutions can be obtained to achieve the same technical function.
2.Coupling between structural parts
In a machine or machinery, no part exists in isolation. Therefore, in addition to studying the functions and other characteristics of the parts themselves, the interrelationships between the parts must also be studied in structural design.
The correlation of parts is divided into two categories: direct correlation and indirect correlation. Where the two parts have a direct assembly relationship, become directly related. There is no direct assembly relationship related to become indirectly related. Indirect correlation is further divided into two categories: position correlation and motion correlation. Position-related refers to two parts in mutual position requirements, such as two adjacent drive shaft in the reducer, its center distance must ensure a certain degree of accuracy, the two axes must be parallel to ensure the normal meshing of gears. Motion-related means that the motion trajectory of one part is related to another part, such as the motion trajectory of lathe tool holder must be parallel to the center line of spindle, which is guaranteed by the bed guide and spindle axis being parallel to each other, so the position between spindle and guide is related; and the motion between tool holder and spindle is motion-related.
Most parts have two or more directly related parts, so each part mostly has two or more parts related to other parts in the structure. In the structural design, the two parts directly related parts must be considered at the same time, in order to reasonably select the material heat treatment, shape, size, accuracy and surface quality, etc.. Also must be considered to meet the indirect correlation conditions, such as the dimensional chain and accuracy calculations. Generally speaking, if a part is directly related to the more parts, the more complex its structure; the more indirectly related parts of the part, the higher its accuracy requirements.
3.Structural design according to the material of structural parts
Mechanical design can be selected from a large number of materials, different materials have different properties, different materials correspond to different processing processes, structural design should be based on both the functional requirements of a reasonable selection of appropriate materials, but also according to the type of material to determine the appropriate processing process, and according to the requirements of the processing process to determine the appropriate structure, only through the appropriate structural design to make the most of the materials selected give full play to the advantages.
The designer must fully understand the mechanical properties, processing properties, cost of use and other information of the selected materials to achieve the correct choice of materials. The structural design should follow different design principles according to the characteristics of the selected material and its corresponding processing technology.
Second, the basic requirements of mechanical structure design
Mechanical products are used in all walks of life, and the content and requirements of structural design are very different, but all have the same common parts. The following three different levels of mechanical structure design to explain the requirements of structural design.
1. Functional design to meet the main mechanical functional requirements, in the technical specifics. Such as the realization of the working principle, the reliability of the work, process, materials and assembly.
2. Quality design takes into account the various requirements and constraints to improve the quality and performance-price ratio of the product, which is a characteristic of modern engineering design. Specifically for operation, aesthetics, cost, safety, environmental protection and many other requirements and constraints. In modern design, quality design is quite important and often determines the competitiveness of the product. That only to meet the main technical functional requirements of mechanical design era has passed, the integration of various requirements to improve the quality of the product, is the key to modern mechanical design.
3. Optimization design and innovative design with structural design variables and other methods to systematically construct the optimal design space, creative design thinking methods and other scientific methods of selection and innovation.
Third, the basic design criteria of mechanical structure
The final result of mechanical design is expressed in the form of a certain structure, which is processed and assembled according to the designed structure and manufactured into the final product. Therefore, the mechanical structure design should meet the requirements of many aspects as a product, the basic requirements of function, reliability, technology, economy and appearance of the shape and other requirements. In addition, it should also improve the force of the parts, improve the strength, stiffness, accuracy and life. Therefore, the design of mechanical structure is a comprehensive technical work. Due to the error or unreasonable design of the structure, it may cause undue failure of parts, so that the machine does not meet the requirements of the design accuracy, bringing great inconvenience to the assembly and maintenance. The following structural design guidelines should be considered in the process of mechanical structure design.
1. The design criteria of the present expected function
The main purpose of the product design is to achieve the predetermined functional requirements, so the design guidelines to achieve the expected function is the first consideration of the structural design. To meet the functional requirements, the following points must be achieved.
(1) Clarify the function: The structural design is to determine the parameter size and structural shape according to its function in the machine and the interconnection relationship with other parts. The main functions of the parts are to bear the load, transfer the movement and power, as well as to ensure or maintain the relative position or trajectory of movement between the parts or components concerned. The designed structure should be able to meet the functional requirements for it from the overall consideration of the machine.
(2) Reasonable distribution of functions: When designing a product, according to the specific situation, it is usually necessary to make a reasonable distribution of tasks, that is, to decompose a function into multiple sub-functions. Each sub-function should have a defined structure to undertake, and the structure of each part should have a reasonable and coordinated connection between them to achieve the total function. The structure of V-belt cross-section is an example of reasonable distribution 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; and the cladding layer acts with the pulley groove to generate the friction required for transmission. For example, if only rely on the friction generated by the bolt preload to bear the transverse load, it will make the size of the bolt too large, can increase the shear elements, such as pins, sleeves and keys, to share the transverse 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, a part or component can assume multiple functions. Function concentration will make the shape of the parts more complex, but to have the degree, otherwise it will affect the processing process, increase processing costs, the design should be based on the specific circumstances.
2. To meet the strength requirements of the design guidelines
(1) equal strength criterion: the change in the size of the cross-section of the part should be adapted to the change in its internal stress, so that the strength of each section is equal. The structure designed according to the principle of equal strength, the material can be fully utilized, thus reducing the weight and cost. Such as the design of cantilever bracket and stepped shaft.
(2) Rational force flow structure: In order to visually represent the state of how the force is transmitted in the mechanical member, the force is considered as if water flows in the member, and these force lines converge into a force flow. Representation of this force flow plays an important role in the structural design investigation. The flow of force in a member is not interrupted, and no line of force suddenly disappears, but must pass in from one place and out from another. Another characteristic of force flow is that it tends to pass along the shortest route, so that the force flow is dense near the shortest route, forming a high stress zone. In other parts, the force flow is sparse or even non-existent, and the material is not fully utilized from the stress point of view. Therefore, in order to improve the stiffness of the component, the shape of the part should be designed according to the shortest route of force flow as much as possible to reduce the load-bearing area, so that the cumulative deformation is smaller and the stiffness of the whole component is improved and the material is fully utilized.
(3) reduce the stress concentration structure: when the direction of the force flow turns sharply, the force flow at the turn will be too dense, thus causing stress concentration, the design should take measures in the structure to make the force flow turn gently. Stress concentration is an important factor affecting the fatigue strength of the part. The structure should be designed to avoid or reduce stress concentrations as much as possible. The methods will be introduced in the corresponding chapter, such as increasing the excessive rounding, the use of unloading structure, etc.
(4) make load balance structure: in the machine work, often produce some useless force, such as inertia force, helical gear axial force, etc., these forces not only increase the load of the shaft and bushings and other parts, reduce its accuracy and life, but also reduce the efficiency of the machine transmission. The so-called load balance means to take structural measures to partially or fully balance the no force to reduce or eliminate its adverse effects. These structural measures are mainly used to balance the elements, symmetrical arrangement, etc.
3. Meet the design guidelines for structural stiffness
In order to ensure that the parts in the service life of the normal realization of its function, must make it have sufficient stiffness.
4. Consider the design criteria of the processing process
The main purpose of the structural design of mechanical parts is: to ensure the realization of the function, so that the product to achieve the required performance. However, the results of the structural design of the product parts production costs and quality has a non-negligible impact. Therefore, in the structural design should strive to make the product has good processing technology. The so-called good processing technology refers to the structure of the parts is easy to process manufacturing, any kind of processing methods may not be able to manufacture certain structures of parts, or high production costs, or quality is affected. Therefore, it is very important for designers to recognize the characteristics of a processing method in order to design the structure as far as possible to avoid the shortcomings of the strengths. In practice, the structure of the parts and components process is subject to many factors, such as the size of the production batch will affect the billet generation method; production equipment conditions may limit the size of the workpiece; in addition, the shape, accuracy, heat treatment, cost and other aspects may have a restrictive effect on the structure of the parts and components process. Therefore, the structural design should fully consider the impact of the above factors on the processability.
5. Consider the design guidelines for assembly
Assembly is an important process in the manufacturing process of the product, the structure of the component has a direct impact on the quality and cost of the assembly. The structural design guidelines for assembly are briefly described as follows.
(1) reasonable division of the assembly unit: the machine should be broken down into a number of individually assembled units (parts or components), in order to achieve parallel and specialized assembly operations, shorten the assembly cycle, and to facilitate level-by-level technical inspection and maintenance.
(2) so that the parts are correctly installed: to ensure accurate positioning of parts, to avoid double fit, to prevent assembly errors.
(3) so that the parts are easy to assemble and disassemble: structural design, should ensure that there is sufficient assembly space, such as wrench space; avoid too long fit to avoid increasing the difficulty of assembly, so that the fit surface abrasion, such as the design of some step shaft; to facilitate the disassembly of parts, should be given to place the location of disassembly tools, such as bearing disassembly.
6. Consider the design guidelines for maintenance and repair
(1) The configuration of the product should be based on its failure rate, the ease of maintenance, size and quality of the size and installation characteristics of the overall arrangement, where the parts need to be repaired parts, should have good accessibility; high failure rate and the need for frequent maintenance of the parts and emergency switch, should provide the best accessibility.
(2) products, especially wearing parts, often disassembled parts and additional equipment disassembly and installation should be easy, disassembly parts in and out of the route is best straight or gentle curve.
(3) The inspection points of the product, test points and other maintenance points of the system, should be arranged in a location that is easily accessible.
(4) need to repair and disassembly of the product, its surrounding to have sufficient operating space.
(5) Maintenance should generally be able to see the internal operation, and its access should leave appropriate clearance for observation in addition to accommodating the hands or arms of maintenance personnel.
7. Consider the guidelines for modeling design
The design of the product should not only meet the functional requirements, but also should consider the aesthetic value of the product shape, so that it is attractive to people. From a psychological point of view, 60% of people’s decisions depend on the first impression. The social attributes of technical products are commodities, and in the era of buyer’s market, designing a product with an attractive appearance for customers is an important design requirement; at the same time, a product with beautiful shape can make the operator reduce the misuse due to energy fatigue.
Appearance design includes three aspects: shape, color and surface treatment.
When considering the shape, attention should be paid to the proportional coordination of size, simple and uniform shape as well as the support and embellishment of color and pattern.
Monochrome is only used for small components. Large and especially sports components will look monotonous and without layers if only one color is used, a small additional color block will make the whole tone active. In the case of multiple colors co-existing, there should be a dominant base color, and the color corresponding to the base color is called the contrast color. However, the number of different shades on a product should not be too large, too many colors will give a flashy feeling.
Comfortable colors are located approximately in the area from light yellow, greenish yellow to brown. This trend is gradually warm, positive yellow and green often look uncomfortable; strong gray tones look depressing. 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 muted. In addition, through a certain color configuration can make the product appear safe and solid. The shape of small changes, a large area of the plane configuration of light colors, while the movement, active contours of the components of the configuration of dark colors; dark colors should be placed in the lower part of the machinery, light colors placed in the upper part.
8. Consider the cost of design guidelines
Design should simplify the product and maintenance operations.
(1) design, the product function should be analyzed and weighed, merge the same or similar functions, eliminate unnecessary functions to simplify the product and maintenance operations.
(2) The design should be designed to meet the specified functional requirements under the condition that it is simple in construction, reduce the number of product levels and component units as much as possible, and simplify the shape of the parts.
(3) The product should be designed as simple and reliable adjustment mechanism to facilitate the exclusion of common failures caused by wear and tear or drift. For the valuable parts prone to local wear and tear, should be designed to adjust or disassemble the combination of parts to facilitate local replacement or repair. Avoid or reduce the interlocking repeated tuning.
(4) To rationalize the location of the components, reduce the number of connections, fixed parts, so that its detection, replacement and other maintenance operations simple and convenient, as far as possible, in the maintenance of any part, not to disassemble, not to move or less disassembly, less mobile other parts, in order to reduce the requirements of the level of skills and workload of maintenance personnel.