The objective of mechanical structure design is, based on the overall design concept, to solidify the initial design principle into a detailed scheme that fulfills the required functions.
The design process transforms abstract working principles into specific components or parts, which involves determining the material, shape, size, tolerance, heat treatment method, and surface treatment of structural members.
Additionally, it is crucial to consider its manufacturing process, strength, rigidity, precision, and interrelationships with other components. Although the direct result of structural design is technical drawings, the task is not as simple as mechanical design.
Blueprints only express the design scheme in engineering language; Applying various techniques in the design of mechanisms to materialize the design concept is the fundamental content of structural design.
1. Structural Elements and Mechanical Component Design Methods
1.1 Geometric Elements of Components
The function of a mechanical structure is achieved mainly through the geometric shape of its components and the relative positional relationship between them. The geometric shape of a component is made up of its surfaces.
Typically, a component comprises multiple surfaces, some of which come into direct contact with the surfaces of other components. These contact surfaces are called functional surfaces. The areas that connect these functional surfaces are called connecting surfaces.
The functional surfaces of a component are crucial in determining its mechanical function. The design of these functional surfaces is at the heart of the structural design of the components.
The main geometric parameters used to describe functional surfaces include their geometric shape, size, number of surfaces, position, sequence, etc. Various structural solutions can be achieved to realize the same technical function through different designs of the functional surfaces.
1.2 Interrelationship between Components
In any machine or mechanical system, no component exists in isolation.
Therefore, in addition to studying the function and related characteristics of each component during structural design, it is also necessary to explore the interrelationships between components.
The interrelationships between components can be classified into two categories: direct and indirect relationships.
Two parts with a direct assembly relationship are considered directly related, while those without a relationship are considered indirectly related. Indirect relationships can be divided into positional and motion-related classes.
Positional relationships refer to the need for two components to maintain certain spatial arrangements.
For example, in a speed reducer, the center distance between two adjacent transmission shafts must maintain a specific accuracy, and the two shaft axes must be parallel to ensure normal gear engagement.
Motion-related relationships refer to the trajectory of movement of one component associated with another. For example, the movement path of a lathe tool holder must be parallel to the spindle centerline, which is guaranteed by the parallelism between the base rail and the spindle axis.
Therefore, the spindle and rail are related to position, while the tool holder and spindle are related to movement.
Most components have two or more directly related components. Thus, each part generally has two or more locations that are structurally related to other components.
During structural design, the structures of directly related parts must be considered simultaneously to reasonably select heat treatment methods, shapes, sizes, precision and surfaces of materials.
Additionally, requirements for indirect relationships such as dimension chain and precision calculations must also be considered.
Generally, the more directly related the parts of a component are, the more complex its structure becomes. On the other hand, the greater the number of indirectly related parts, the greater the precision required.
1.3 Selection of Materials in Structural Design
Various materials can be chosen in the design of the piece, each with unique properties. Different materials correspond to different manufacturing processes.
The design process requires not only the selection of appropriate materials based on functional requirements, but also the determination of the appropriate manufacturing process based on the type of material.
Furthermore, the structure must be determined according to the requirements of the manufacturing process.
Only through an adequate structural design can the chosen material be used to its maximum advantage.
In order for designers to correctly select materials for parts, they must fully understand the mechanical properties, machinability and cost-effectiveness of the relevant materials.
In structural design, different design principles must be followed based on the characteristics of the chosen materials and the corresponding manufacturing processes.
2. Fundamental Requirements for Mechanical Structure Design
Mechanical products are used in a variety of industries, with the specifics and requirements of structural design varying significantly.
However, the fundamental requirements for structural design are universal. Below we describe the requirements for mechanical structure design at three distinct levels.
2.1 Functional Design
Effort is put into materializing the technical aspects to meet the primary mechanical requirements.
Elements such as implementation of working principles, reliability of operation, processes, materials and assembly are covered.
2.2 Quality Project
Balancing diverse demands and constraints to improve product quality and cost-effectiveness exemplifies modern engineering design.
Specific areas include operability, aesthetics, safety, cost and environmental conservation. In contemporary designs, quality design has significant importance and often dictates competitive strength.
The design approach focused exclusively on satisfying primary technical functions has become obsolete.
The core of modern mechanical design lies in harmonizing diverse demands, establishing a balance and making appropriate trade-offs under the premise of fulfilling primary functions to improve product quality.
2.3 Optimized Design and Innovative Design
Structural design variables are systematically used to construct an optimized design space. Creative methods of design thinking and other scientific methods are employed for selection and innovation.
3. Fundamental Design Principles for Mechanical Structures
The end result of mechanical design is to express a given structural form in drawings. The final product is manufactured according to these designs through machining and assembly processes.
Therefore, the design of mechanical structures must meet various requirements as a product, including functionality, reliability, processability, economic efficiency and aesthetic form.
Furthermore, it must improve the force-bearing capacity of the parts, increasing their strength, rigidity, precision and useful life.
Mechanical structure design, therefore, is a comprehensive technical task. Irrational or erroneous structural designs can lead to unexpected component failures, prevent machines from achieving the required precision, and cause considerable inconvenience during assembly and maintenance.
The following structural design principles should be considered in the mechanical structure design process.
3.1 Design Principles to Achieve Expected Functionality
The main objective of product design is to meet predetermined functional requirements.
Therefore, the design principle to achieve expected functionality is the first consideration in structural design. To meet these functional requirements, the following points must be respected:
(1) Explicit functionality:
The structural design must determine the parameters, dimensions and shape of the structure based on its function within the machine and its interconnection with other components.
The main functions of components include supporting loads, transmitting movement and power, and ensuring or maintaining the relative position or path of movement between related parts or components. The designed structure must meet its functional requirements considered from the perspective of the machine as a whole.
(2) Functional Allocation:
During product design, it is often necessary to delegate tasks reasonably based on specific circumstances, that is, decompose a function into several subfunctions.
Each subfunction must be supported by a defined structure and there must be a reasonable and coordinated link between different structural parts to achieve the overall function.
Several structural components that share a function can alleviate the load on individual parts, thus extending their service life.
For example, the structure of a V-belt cross section is an example of task distribution.
A fiber strand is used to carry the tension; a layer of rubber padding absorbs stretching and compression during belt bending; a layer of fabric interacts with the pulley groove to generate the friction necessary for transmission.
Another example is when the friction generated only by pre-tightening the screws is used to support lateral loads, which can result in oversized screws. This problem can be solved by adding shear-resistant components such as pins, sleeves and keys to share the lateral load.
(3) Functional Concentration: To simplify the structure of mechanical products, reduce manufacturing costs and facilitate installation, a single part or component can be assigned multiple functions in some circumstances.
Although functional concentration can make part shapes more complex, it must be moderated to avoid increasing machining difficulty and inadvertently increasing manufacturing costs. The design must be determined based on the specific situation.
3.2 Design criteria to meet strength requirements
(1) Equal strength criterion:
Changes in the cross-sectional dimensions of the parts must be adapted to changes in internal stresses so that the strength of each section is equal.
The structure designed according to the principle of equal strength can make the most of materials, thereby reducing weight and cost. Design of cantilever supports, stepped shafts, etc.
(2) Reasonable strength flow structure:
In order to visually demonstrate the state of how force is transmitted in mechanical components, force is considered to flow like water in the component, and these lines of force converge into the flow of force.
The flow of this force plays an important role in structural design considerations. The flow of power in the component will not be interrupted and no line of force will suddenly disappear. It must be transmitted from one place to another.
Another characteristic of the force flow is that it tends to propagate along the shortest route, resulting in a dense force flow near the shortest route and forming a high voltage zone.
The power flow in other parts is sparse and there is not even any power flow passing through. From a stress point of view, the material is not fully utilized.
Therefore, to improve the stiffness of the component, the shape of the component is designed according to the shortest possible force flow path, reducing the load-bearing area and thus reducing the accumulated deformation, increasing the stiffness of the whole component and making the most of the material.
(3) Minimize stress concentration in structures:
When the direction of force flow changes abruptly, the force becomes excessively concentrated at the curve, leading to stress concentration.
Measures must be implemented in the design to ensure a gradual change in force direction. Stress concentration is a significant factor that affects the fatigue strength of components.
In structural design, efforts should be made to avoid or minimize stress concentration, such as increasing transition radii, adopting stress relief structures, and so on.
(4) Establish load-balanced structures:
During machine operation, some unnecessary forces such as inertial forces and axial forces of helical gears are often generated.
These forces not only increase the load on parts such as shafts and bearings, reducing their precision and useful life, but also reduce the machine's transmission efficiency. Load balancing refers to structural measures that partially or fully balance these unnecessary forces to mitigate or eliminate their adverse effects.
These structural measures mainly involve the use of components of balance and symmetrical arrangement.
3.3 Design Guidelines for Achieving Structural Rigidity
To ensure that components function normally throughout their entire life cycle, it is essential to provide them with sufficient rigidity.
3.4 Design Guidelines Considering the Manufacturing Process
The main objective of the structural design of mechanical components is to ensure functionality, allowing the product to meet the required performance. However, the rationality of the project directly affects the production cost and quality of the components.
Therefore, it is crucial in structural design to seek good manufacturability of the component mechanisms. Good manufacturability means that the component structure is easy to manufacture.
Each manufacturing method has its limitations, which can result in high production costs or compromised quality.
Therefore, it is important for designers to be familiar with the characteristics of various manufacturing methods in order to maximize their strengths and minimize their weaknesses during design.
In actual production, the manufacturing capacity of component structures is limited by several factors. For example, the size of the production batch can affect the method of creating the blanks; Production equipment conditions may limit part sizes.
Additionally, factors such as molding, precision, heat treatment, cost, etc., could potentially restrict the manufacturability of the component structure.
Therefore, these factors must be carefully considered in structural design for their impact on manufacturability.
3.5 Design Guidelines for Assembly
Assembly is a crucial step in the product manufacturing process, and the structure of the components directly influences the quality and cost of assembly. The structural design guidelines for assembly are briefly described below:
(1) Rational division of assembly units:
The entire machine must be dissected into multiple independently assembled units (parts or components) to perform parallel, specialized assembly operations, shorten assembly cycles, and facilitate step-by-step technical inspections and repairs.
(2) Ensure correct installation of components:
This includes accurate positioning of parts, avoiding double coupling and avoiding assembly errors.
(3) Facilitate the assembly and disassembly of components:
The structural design must ensure sufficient mounting space, such as space for keys; avoid excessively long couplings to avoid increased assembly difficulty and possible damage to mating surfaces, as seen in some stepped shaft designs; To facilitate the disassembly of parts, places must be provided for placing disassembly tools, as in the case of removing bearings.
3.6 Design Guidelines for Maintenance and Repair
(1) A product's configuration should be organized based on factors such as failure rate, repair complexity, size, weight, and installation characteristics.
Any parts requiring maintenance must be easily accessible. Components with a high failure rate and emergency switches that require frequent maintenance must be provided with optimal accessibility.
(2) Products, especially consumable parts, frequently disassembled components and additional equipment, should be easy to assemble and disassemble.
The path for parts to enter and exit during disassembly and assembly should ideally be a straight line or a gentle curve.
(3) Product maintenance points such as inspection points and testing points should be located in easily accessible places.
(4) Products requiring maintenance and disassembly must have adequate operating space around them.
(5) During maintenance, operators must generally be able to view internal operations. In addition to accommodating the hand or arm of maintenance personnel, the passage must also leave adequate space for observation.
3.7 Guidelines for Aesthetic Design
The design of a product must not only satisfy its functional needs, but also consider its aesthetic value, making it appealing to users. Simply put, a product must be useful and attractive. Psychologically, 60% of human decisions are based on first impressions.
Given that technical products are commodities in the buyer's market, designing an attractive exterior is a crucial design requirement. Additionally, aesthetically pleasing products can help operators reduce errors caused by fatigue.
Design aesthetics encompasses three aspects: shape, color and surface treatment.
When considering shape, attention should be paid to the harmonious proportions of sizes, simple and unified shapes, and the enhancement and beautification provided by colors and patterns.
Monochrome is only suitable for small components. Large pieces, especially furniture, will look drab and flat if only one color is used. A small addition of a contrasting color can liven up the overall color scheme.
In multicolor situations, there must be a dominant base color, with the corresponding color known as the contrast color.
However, the number of different colors in a single product should not be excessive, as too many colors can give an impression of superficiality.
Comfortable colors usually range from light yellow and yellow-green to brown. This trend is towards warmer colors, with bright yellow and green often looking uncomfortable; Strong shades of gray can feel oppressive.
Warm colors like yellow, orange-yellow and red should be used for cold environments, while cool colors like light blue should be used for warm environments.
All colors must be muted. Additionally, a specific color configuration can make the product appear safe and sturdy.
Areas with minimal shape changes and larger surfaces should be set to light colors, while components with active and moving contours should be set to dark colors. Dark colors should be placed at the bottom of the machinery and light colors at the top.
3.8 Design Guidelines Considering Cost
The design must simplify both the product and maintenance tasks:
(1) During design, a cost-benefit analysis of the product's functionalities must be carried out.
Combine similar or identical features, remove unnecessary ones to simplify both the product and maintenance tasks.
(2) The design must strive for simplicity in structure and, at the same time, meet the specified functional requirements.
The number of hierarchical layers and components should be minimized and the shape of the parts simplified as much as possible.
(3) Products must be designed with easy-to-use yet reliable adjustment mechanisms to solve common problems caused by wear or runout.
For expensive parts prone to localized wear, design them as adjustable or removable assemblies for easy partial replacement or repair. Avoid or minimize the need for iterative adjustments due to interconnected parts.
(4) Components should be arranged logically to reduce the number of connectors and accessories, making inspection, parts replacement and other maintenance tasks simpler and more convenient.
As much as possible, the design should permit repair of any component without the need to disassemble, move, or minimally disassemble or move other parts. This approach reduces the skill level and workload required of maintenance personnel.
