Mechanical products are used in all walks of life, and the content and requirements of structural design vary widely. However, the design of mechanical structures in various industries still has the same common parts.
The final result of mechanical design is expressed in a certain structural form, which is processed and assembled according to the designed structure to manufacture the final product.
Therefore, the mechanical structure design should meet the multi-faceted requirements of the product. The basic requirements include function, reliability, craftsmanship, economy and appearance.
In addition, the stress of parts should be improved to increase strength, stiffness, accuracy and life.
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Therefore, mechanical structure design is a comprehensive technical work.
If the structural design is wrong or unreasonable, it may cause undue failure of parts and components, causing the machine to fail to meet the design accuracy requirements, causing great inconvenience to assembly and maintenance.
The following eight structural design criteria should be considered during the mechanical structure design process.
01Design guidelines for achieving expected functions
The main purpose of product design is to achieve predetermined functional requirements, so the design criteria to achieve expected functions is the first consideration in structural design.
To meet the functional requirements, the following must be done:
(1) Clarify the function: Structural design is to determine the parameter size and structural shape based on its function in the machine and its connection relationship with other parts.
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The main functions of parts are to bear loads, transmit motion and power, and ensure or maintain the relative position or movement 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 allocation of functions: When designing products, according to specific circumstances, it is usually necessary to reasonably allocate tasks, that is, to decompose a function into multiple sub-functions.
Each sub-function must have a certain structural responsibility, and there should be a reasonable and coordinated connection between the various parts of the structure to achieve the realization of the overall function.
Multiple structural parts carrying the same function can reduce the burden on the parts and extend their service life.
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The structure of the V-belt section is an example of a rational distribution of tasks.
The fiber rope is used to withstand the tensile force; 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 you only rely on the friction generated by bolt pretension to bear the lateral load, 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) Function 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.
Concentration of functions will make the shape of the parts more complex, but it must be done in a moderate manner, otherwise it will affect the processing technology and increase processing costs. The design should be based on the specific situation.
02Design criteria to meet strength requirements
(1) Equal strength criterion: Changes in the cross-sectional dimensions of parts should be adapted to changes in internal stress so that the strength of each cross-section is equal.
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With a structure designed according to the principle of equal strength, materials can be fully utilized, thereby reducing weight and cost. Such as the design of cantilever brackets and stepped shafts, etc.
(2) Reasonable force flow structure: In order to intuitively express the state of how force is transmitted in mechanical components, the force is regarded as like water flowing in the component, and these force lines converge into force flow.
Representing the flow of this force plays an important role in structural design investigations.
The force flow will not be interrupted in the component, and any force line will not suddenly disappear. 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 zone.
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The force flow in other parts is sparse or even non-existent. From a stress perspective, the material is not fully utilized.
Therefore, in order to improve the stiffness of the component, the shape of the part should be designed according to the shortest path of force flow as much as possible to reduce the load-bearing area, so that the cumulative deformation is smaller, the stiffness of the entire component is improved, and the material is fully utilized.
(3) Reduce stress concentration structure: When the direction of force flow turns sharply, the force flow will be too dense at the turning point, causing stress concentration. In the design, structural measures should be taken to make the force flow turn smoothly.
Stress concentration is an important factor affecting the fatigue strength of parts.
During structural design, stress concentration should be avoided or reduced as much as possible.
The methods will be introduced in the corresponding chapters, such as increasing excessive fillets, adopting unloading structures, etc.
(4) Balance the load 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 on parts such as shafts and bushings, but also reduce their accuracy and lifespan. , and also reduces the transmission efficiency of the machine.
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The so-called load balancing refers to taking structural measures to partially or completely balance the load without force to reduce or eliminate its adverse effects.
These structural measures mainly adopt balanced elements, symmetrical arrangements, etc.
03Meet design criteria for structural stiffness
In order to ensure that the parts can perform their functions properly during their service life, they must have sufficient stiffness.
04Consider design criteria for 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 that cannot be underestimated on the production cost and quality of product parts.
Therefore, in the structural design, we should strive to ensure that the product has good processing technology.
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The so-called good processing technology means that the structure of the parts is easy to process and manufacture. Any processing method may not be able to manufacture parts with certain structures, or the production cost will be high, or the quality will be 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 weaknesses when designing the structure.
In practice, the structural manufacturability of parts 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, aspects such as shape, accuracy, heat treatment, and cost are all affected. It may have a restrictive effect on the craftsmanship of the component structure.
Therefore, the impact of the above factors on processability should be fully considered in structural design.
05Consider design guidelines for assembly
Assembly is an important process in the product manufacturing process. The structure of parts has a direct impact on the quality and cost of assembly.
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The structural design guidelines for assembly are briefly summarized below:
(1) Reasonable division of assembly units: The entire machine should be able to be decomposed into several units (components or assemblies) that can be assembled individually to achieve parallel and specialized assembly operations, shorten the assembly cycle, and facilitate level-by-level technical inspection and maintenance.
(2) Ensure that parts are installed correctly: ensure accurate positioning of parts, avoid double coordination, and prevent assembly errors.
(3) Make parts easy to assemble and disassemble: In structural design, sufficient assembly space should be ensured, such as wrench space; avoid excessively long fits to avoid increasing assembly difficulty and causing scratches on the mating surfaces, such as the design of some stepped shafts; for To facilitate the disassembly of parts, the location for disassembly tools should be given, such as the disassembly of bearings.
06Consider design criteria for maintenance and repairs
(1) The configuration of the product should be arranged according to its failure rate, ease of maintenance, size and quality, and installation characteristics. All parts and components that need to be repaired should have good accessibility; the failure rate Highest parts that require frequent maintenance and emergency switches should provide optimal accessibility.
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(2) The disassembly and assembly of the product, especially wearing parts, frequently disassembled parts and additional equipment, should be simple. The entry and exit route of the parts during disassembly and assembly should be a straight line or a gentle curve.
(3) Product inspection points, test points and other system maintenance points should be arranged in easily accessible locations.
(4) Products that require maintenance and disassembly must have sufficient operating space around them.
(5) Internal operations should generally be visible during maintenance. In addition to being able to accommodate the maintenance personnel's hands or arms, the passage should also leave an appropriate gap for observation.
07Consider the guidelines for styling design
The design of the product must not only meet the functional requirements, but also consider the aesthetic value of the product shape to make it attractive to people.
From a psychological perspective, 60% of people's decisions depend on first impressions.
The social attributes of technical products are commodities. In the era of the buyer's market, designing an appearance for the product that can attract customers is an important design requirement; at the same time, beautiful-looking products can reduce operators' misoperations caused by fatigue.
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Appearance design includes three aspects: shape, color and surface treatment.
When considering styling, attention should be paid to coordinated size proportions, simple and unified shapes, and support and embellishment of colors and patterns.
Solid colors are only used for small widgets.
Large, especially moving components will appear monotonous and layerless if only one color is used. A small additional color block will liven up the entire tone.
When multiple colors coexist, there should be a dominant background color, and the color corresponding to the background color is called a contrasting color.
But on a product, the number of different tones should not be too many. Too many colors will give people a flashy feeling.
Comfortable colors range from light yellow, greenish yellow to brown.
This trend is getting warmer, yellow and green often look uncomfortable; strong gray tones look depressing.
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For cold environments use warm colors such as yellow, orange and red.
For hot environments use cool colors such as light blue.
All colors should be toned down.
In addition, certain color configurations can make the product appear safe and stable. Planes with small shape changes and large areas should be arranged in light colors, while components with moving and active contours should be arranged in dark colors; dark colors should be placed in the lower part of the machine, and light colors in the upper part.
08Design guidelines that consider cost
Product and maintenance operations should be simplified during design:
(1) When designing, analyze and weigh product functions, merge identical or similar functions, and eliminate unnecessary functions to simplify product and maintenance operations.
(2) When designing, the structure should be simple, the number of product levels and component units should be reduced as much as possible, and the shape of the parts should be simplified while meeting the specified functional requirements.
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(3) Products should be designed with simple and reliable adjustment mechanisms as much as possible 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 involve each other.
(4) Reasonably arrange the positions of each component, reduce the number of connecting parts and fixing parts, and make maintenance operations such as inspection and replacement of parts simple and convenient. When repairing any part, try to ensure that no disassembly, no movement, or less disassembly, Move other parts less to reduce the skill level requirements and workload of maintenance personnel.
