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Basic design criteria for mechanical structures

Views: 6     Author: Site Editor     Publish Time: 2022-04-11      Origin: Site

The final result of mechanical design is to determine the structure of the form expressed in drawings, according to the design drawings for processing, assembly, manufacturing the final product. Therefore, the mechanical structure design should meet the requirements of many aspects as a product, the basic requirements of functionality, reliability, technology, economy and appearance modeling and other requirements. In addition, should also improve the parts of the force, improve the strength, stiffness, accuracy and life. Therefore, mechanical structure design is a comprehensive technical work. Due to unreasonable or wrong structural design, parts may not be due to failure, so that the machine cannot meet the requirements of design accuracy, to assembly and maintenance bring great inconvenience. The following structural design criteria should be considered in the process of mechanical structure design.

Design criteria for achieving desired functions

The main purpose of product design is to achieve the desired functional requirements, so the design criterion to achieve the desired function is the first consideration of structural design. To meet the functional requirements, the following must be done.

(1) Clear function: structural design is to determine the size and shape of the structure according to its function in the machine and the connectionhalf shaft with double universal joint - Xinsanming relationship with other parts. The main functions of parts are to bear loads, transfer motion and power, and to ensure or maintain the relative position or movement track of relevant parts or components. The design structure should be able to meet the functional requirements of the machine as a whole.

(2) Function allocation: In product design, according to the specific situation, it is usually necessary to reasonably allocate tasks, that is, to decompose a function into multiple sub-functions. Each sub-function should have a certain structure to undertake, each part of the structure should have a reasonable, coordinated connection, in order to achieve the realization of the total function. Multi-structure parts can reduce the burden of parts and prolong the service life. For example, the V-shaped structure with cross section is an example of reasonable task allocation. Fiber rope is used to withstand tension; The rubber filling layer withstands the stretching and compression when the belt is bent; The coating layer acts with the pulley groove to produce the friction required for transmission. For example, if only the friction generated by bolt pretightening to bear the transverse load, the size of the bolt will be too large. Shear elements such as pins, sleeves and keys can be added to share the transverse load to solve this problem.

(3) Functional concentration: in order to simplify the structure of mechanical products, reduce processing costs, easy installation, in some cases, can be a part or components bear multiple functions. Function concentration will make the shape of parts more complex, so it should be moderate, otherwise it will increase the difficulty of processing, but increase the cost of processing, design should be based on the specific situation.

Meet the strength requirements of the design criteria

(1)  Equal strength criterion: the change of section size of parts should be adapted to the change of internal stress, so that the strength of each section is equal. The structure designed according to the principle of equal strength can make full use of materials, thus reducing weight and cost. Such as cantilever bracket, ladder shaft design.

(2)  Reasonable force flow structure: in order to directly express the state of how force is transferred in the mechanical component, force is regarded as water flowing in the component, and these force lines converge into force flow. The flow of this force plays an important role in structural design investigations. Force flow in the component will not be interrupted, any force line will not suddenly disappear, must be introduced from one place, from another. Another characteristic of force flow is that it tends to travel along the shortest route, so that the force flow is dense near the shortest route, forming a high stress zone. Other parts of the force flow is sparse, even no strong flow, from the point of view of stress, the material is not fully utilized. Therefore, in order to improve the stiffness of the components, as far as possible according to the shortest route of force flow to design the shape of the parts, reduce the bearing area, thus the cumulative deformation is smaller, improve the stiffness of the entire component, make full use of the material.

(3)  Reduce stress concentration structure: when the direction of force flow turns sharply, the force flow will be too dense at the turning point, resulting in stress concentration. Structural measures should be taken in the design to smooth the force flow. Stress concentration is an important factor affecting fatigue strength of parts. In structural design, stress concentration should be avoided or reduced as far as possible, such as increasing excessive rounded corners and adopting unloading structure.

(4)  Load balance structure: when the machine is working, often produce some useless force, such as inertia force, helical gear axial force, etc., these forces not only increase the load of shaft and bushing parts, reduce the accuracy and life, but also reduce the transmission efficiency of the machine. The so-called load balance refers to the structural measures taken to balance part or all of the non-force, in order to reduce or eliminate its adverse effects. These structural measures mainly adopt balanced elements, symmetrical arrangement, etc.

Consider the design criteria of machining process

The main purpose of mechanical parts structural design is to ensure the realization of the function, so that the product to achieve the required performance. However, the rationality of structural design directly affects the production cost and quality of components. Therefore, in the structural design, we should strive to have good processing technology. The so-called good processing technology refers to the part structure is easy to process manufacturing, any kind of processing method has its limitations, or production cost is high, or quality is affected. Therefore, it is very important for designers to understand the characteristics of various machining methods in order to maximize their strengths and avoid their weaknesses when designing structures. In the actual production, the structural technology of spare parts is restricted by many factors, such as the size of production batch will affect the generation method of blank parts; Production equipment conditions may limit the size of the workpiece; In addition, modeling, precision, heat treatment, cost and other aspects may restrict the process of parts and components. Therefore, the influence of the above factors on the manufacturability should be fully considered in the structural design.

Consider the design criteria for assembly

Assembly is an important process in the manufacturing process of products. The structure of parts has a direct impact on the quality and cost of assembly. The structural design criteria for assembly are summarized as follows:

(1)  Reasonable division of assembly units: The whole machine should be able to be decomposed into several units (parts or components) that can be assembled separately, so as to realize parallel and professional assembly operations, shorten the assembly cycle, and facilitate step-by-step technical inspection and maintenance.

(2)  Ensure the correct installation of parts: ensure the accurate positioning of parts, avoid double coordination and prevent assembly errors.

(3)  Make the parts easy to assemble and disassemble: in the structural design, there should be enough assembly space, such as wrench space; Avoid too long matching so as not to increase the difficulty of assembly, so that the matching surface abrasion, such as some ladder shaft design; To facilitate the disassembly of parts, the location of disassembly tools, such as bearings, should be given.

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  : The Innovation Industrial Park, No.36, Jingwei Road ,Xi'an Economic and Technological Development Zone,Xi'an City,Shaanxi Province,China
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