What are the general steps of mechanical design?

Mechanical design is a creative working process, and it is also a job to make the best use of the existing successful experience. Only by combining inheritance and innovation well can we design high-quality products. As a product design, it is required to determine the working principle, function, structure, part design and even manufacturing and assembly methods of the product. So different designers may have different design methods and steps. However, according to long-term design experience, mechanical design is divided into five steps: trend prediction, scheme design, technical design, construction design and trial production.

(1) trend forecast.

After putting forward the new product to be designed according to the actual needs, the trend prediction is only a planning preparation stage, and the product to be designed at this time is only a vague concept. At this stage, we should conduct a comprehensive investigation and analysis of the designed products.

The development process of a mechanical product is the same as that of any living individual. If you want your products to be continuously promoted and updated, you need to consider what kind of products to develop and when to put into production in the process of product development, and after careful consideration, careful planning and strict implementation, you can put them into the market without fail, and you can get feedback from buyers after listing as a reference for improving products in the future. That is, before the production of products, it is necessary to systematically investigate and analyze the functions, specifications, uses, sales markets and the characteristics of competitors' products. From the market point of view, a product must have a higher exchange value than its materials and processing costs, otherwise it will not be able to stand on the market. Therefore, before the design, it is necessary to conduct intelligence investigation and trend prediction, as shown in Figure 4- 16. In the design of incomplete new products, research and prediction generally choose similar products of some well-known brands as the research objects, and the research items include product functions, market sales, customer purchase motives and so on. Through product research, the advantages and disadvantages of this design product are made clear, and then the purchased parts and raw materials are counted, the man-hour quota and material consumption quota of each component are collected, the current cost of each component is calculated, and the actual cost of the product is preliminarily found out. Predict the competitiveness of improved products after they are put into the market, make decisions and write technical suggestions. According to the analysis of technical suggestions, reasonably determine the technical demonstration and technical and economic demonstration of product documents, and finally issue the design task book. In the design task book, the purpose and characteristics of the design object should be explained, so as to specify the indicators such as productivity, reliability and life, weight, overall size, driving energy and cost.

Figure 4- 16 Design Survey (II) Scheme Design.

The stage of scheme design plays a key role in the success or failure of design. This stage fully embodies the characteristics of multiple schemes in design work. According to the working requirements of the designed machine put forward in the design task book, firstly, various design principle schemes that can meet the working requirements are analyzed and compared. Since any working principle must be realized through a certain form of motion, this step is also to determine the motion scheme required for the design. When determining the design principle scheme, the design object can be regarded as a complete system, and it can be divided into subsystems and elements with various sub-functions. After analyzing the system, the optimal scheme is finally selected, as shown in Figure 4- 17.

At this stage, it is necessary to formulate the overall product and main component scheme according to the technical and economic demonstration needed to select the optimal scheme, and at the same time, it is necessary to determine the scope of problems related to working principle, reliability and strength. These problems must be studied, and some of them must be verified by experiments. Structural elements that have little influence on design objectives can be approximately calculated. If there is an actual prototype close to the design object, we can draw up various structural elements without calculation. However, in the scheme design, in order to carry out the technical and economic calculation related to the selection of the optimal design scheme, it is necessary to comprehensively and accurately estimate the parameters that have an impact on the final result.

Figure 4- 17 When determining the design principle scheme, designers must also reflect the technical development policy of the machinery industry. According to the actual working conditions of machinery, we should try our best to adopt microelectronic technology and new materials to design electromechanical and hydraulic integrated products.

(3) Technical design.

In technical design, it is necessary to draw up the overall design objects and components, and specifically determine the structure of the parts. The requirements for the designed mechanical products are: economical manufacture and maintenance, convenient and safe operation, high reliability and long service life. In order to meet these requirements, parts should meet some standards, the most important of which are: strength, stiffness, impact resistance, wear resistance, heat resistance, manufacturability and so on. Standardization is of great significance to the manufacturing cost and operation economy of designed products. Standardization can reduce the cost of mechanical products, shorten the design cycle and improve the reliability.

The designer simply calculates or estimates the stress, strength, shape, size and weight of the main parts of the machine according to the preliminary design general drawing drawn by himself, as shown in Figure 4- 18. If it is found that the initially selected structure is not feasible or practical, it is necessary to adjust or modify the structure. In addition, it is also necessary to consider whether there are dangerous components damaged by overheating, excessive wear and premature fatigue, and take measures to solve them.

In the technical design stage, the envisaged product is initially formed. Designers will find many contradictions in the shape, size and proportion of each part through the preliminary design of the general plan. When it is necessary to strengthen or improve one aspect, it may weaken or worsen another. At this time, it is necessary to weigh the trade-offs and maintain the balance in all aspects in order to achieve the best comprehensive effect. At this time, the designer's experience plays an important role.

In the process of modifying the general plan of the preliminary design, it is necessary to make a technical and economic analysis of the preliminary design. The general principle is: firstly, analyze those parts with complex structure, large quality, large size, expensive materials, poor performance and low technical level, as well as those parts with large batch, complex technology, large consumption of raw materials and low yield, and modify the design accordingly, so as to obtain the preliminary design general drawing with high technical and economic indicators.

Figure 4- 18 General Plan of Preliminary Design of Product Engineering Drawing, which is drawn in proportion after repeated revisions. Special attention should be paid to possible interference and collision, and there should be enough views and sections in all directions to expose possible contradictions in all aspects. After the general layout of the preliminary design is completed, the manufacturing cost is preliminarily estimated (for review and quotation) and preliminarily reviewed.

From the preliminary design general drawing to the technical design assembly drawing, attention should be paid to:

① Try to use standard parts, common parts or parts designed and manufactured in the past to save production costs.

(2) Determine the blank material, whether the blank is supplied by other factories or produced by our factory.

(3) Improve the processing and installation technology, such as adopting group processing technology and parallel assembly operation, so as to reduce the manufacturing cost and make it possible to adopt advanced technology in the design.

④ Improve the structure according to the principle of modeling design.

⑤ Consider the safety design requirements.

⑥ Conduct technical and economic analysis. Finally, on the basis of the above work, adjust the size proportion of the parts and draw the technical design assembly drawing. For high-speed moving machinery, it is necessary to carry out systematic dynamic check calculation, including the natural frequency and vibration mode of the whole structure, determine the external load borne by the structure, calculate the dynamic stress under the dynamic load, and take measures to avoid * * * vibration and reduce the dynamic stress.

According to the preliminary evaluation opinions, the general assembly drawing of technical design is obtained, and the structural drawing of each part is drawn. Because the size of the part is known at this time, the load borne by the part can be accurately calculated, and then the stress distribution of the part after loading can be calculated by using the special program for part design, so as to find out its dangerous point, and make structural improvement to reduce the peak stress of the dangerous point or optimize the design of some main dimensions of the part. Then consider the requirements of material selection, machining and assembly, determine the size of the part, calculate the stress response under the working load spectrum for the dangerous points of the part, calculate the fatigue strength and life, and modify the part design according to the life requirements. Complete lubrication design and electrical design (drive and control), and finally draw the general assembly drawing of technical design for secondary review.

In the second review, experts and user representatives from all sides should still be invited to conduct the review. If the design is changed at this time, the cost will be high, but if it is necessary to change, it must be changed. The best way to avoid repainting is to discuss with users, manufacturing technicians and other related experts during the design process. Some important and mass-produced machines sometimes need to make a model. After the second review, the general assembly drawing and component drawing (sub-assembly drawing) of technical design are formally drawn.

(4) Architectural design.

Design parts according to the general assembly drawing of technical design. Draw a part drawing, make sure every dimension of the part is not missed, and determine the tolerance. Where there are standards, the drawing must conform to the national standards. Then draw the overall assembly drawing of construction design according to the actual part size. Then, start proofreading the pattern. First, check the dimensions of the part drawings, check whether there is any omission or contradiction in the dimensions of each drawing, whether there is any error in dimension marking, whether there is any contradiction between each drawing and its adjacent drawings, and whether there is any possibility of interference or collision with the general drawing. Then, the manufacturability of the drawings should be reviewed. Skilled technicians should read each drawing, check whether each part is easy to manufacture and install, and modify parts that are difficult or even impossible to process. The parts, dimensions and fit of the machine have standards. Although the designers are familiar with the standards, it is best to have a special person to review the standards. In addition, the drawings should be audited for lubrication, and the lubrication methods and lubricant varieties should be studied. Finally, prepare various technical documents such as parts list and instructions.

(5) Trial production.

According to the construction design drawings and various technical documents, the prototype is trial-produced, the functional test of the prototype is carried out, and the cost accounting of various expenses is carried out to feed back and improve the design. The prototype passed the audit, and then small batch trial production was carried out. After improvement, it was officially put into small batch production.

After the products produced in small batches are put on the market, if the "trial sale rate" and "resale rate" of the products are high, it means that the products are welcomed by users and can be approved for mass production; If the "trial sale rate" is low and the "resale rate" is high, it means that users don't understand the products, and advertising should be strengthened before mass production; If the "trial sale rate" is high, the "resale rate" is low, or both are low, it means that there is something wrong with the product quality, and the design should be modified to improve the quality and reduce the cost.

When products can be mass-produced, it is necessary to study the process suitable for mass production and carry out mass trial production according to this process. In batch trial production, it may be found that the thoughtlessness in process audit leads to difficulties in batch production, difficult to ensure stable quality, high consumption and high cost, and the problem cannot be solved only by improving processing technology. This requires some corresponding modifications to the design and improvement of the process of mechanical design, and then formal batch or batch production can be started. When a large number of products are put into use, it is necessary to collect information on use and maintenance from users in time, and improve and modify the design when necessary.