Mechanical parts refer to the inseparable basic units that make up the machine, such as bolts, screws, keys, belts, gears, shafts, springs, pins, etc. Mechanical parts are divided into general parts and special parts. Common parts refer to parts that can be widely used in various machines, and special parts refer to parts that can only be used in a specific type of machine.
1. Common materials for mechanical parts
The materials of mechanical parts are metal materials, non-metal materials, and composite materials.
Metal materials are divided into ferrous metal materials and non-ferrous metal materials. Ferrous metal materials include all kinds of steel, cast steel, and cast iron, which have good mechanical properties (such as strength, plasticity, toughness, etc.), are relatively cheap and easy to obtain, and can meet the requirements of various properties and uses. Among all kinds of ferrous metals, alloy steels are often used to manufacture important parts due to their excellent properties. Non-ferrous metal materials include copper alloys, aluminum alloys, bearing alloys, etc., which have the advantages of low density, good thermal conductivity, electrical conductivity, etc., and can usually be used in occasions where friction reduction, wear resistance, and corrosion resistance are required.
Non-metallic materials refer to polymer materials such as plastics, rubbers, synthetic fibers, and ceramics. Polymer materials have many advantages, such as abundant raw materials, low density, good elasticity in an appropriate temperature range, and good corrosion resistance. Its main disadvantage is that it is easy to age, and many of the materials have poor flame retardancy and, in general, poor heat resistance. The main characteristics of ceramic materials are extremely high hardness, wear resistance, corrosion resistance, high melting point, high stiffness, and lower density than steel. At present, ceramic materials have been used in structures such as seals, rolling bearings, and cutting tools. Its main disadvantages are brittleness, low fracture toughness, high price, and poor processability.
A composite material refers to a new type of material that is treated with two or more materials with significantly different physical and mechanical properties through a composite process to obtain the desired properties. For example, glass, graphite (carbon), boron, plastic, and other non-metallic materials can be composited into various fiber-reinforced composite materials. By attaching plastic to the surface of an ordinary carbon steel plate, a plastic composite steel plate with high strength and corrosion resistance can be obtained. The disadvantage of poor electrical conductivity. In addition, composite materials are more expensive. Therefore, at present, composite materials are mainly used in high-tech fields such as aviation and aerospace. In civilian products, composite materials also have some applications.
2. the principle of material selection of mechanical parts
Selecting suitable materials from a variety of materials is a work restricted by many factors. Commonly used materials for mechanical parts are steel, cast iron, non-ferrous metals, and non-metals, etc. The grades, properties, and heat treatment knowledge of commonly used materials can be found in the Mechanical Design Manual. Material selection is an important part of mechanical design. With the continuous development of material science, the requirements for parts in the machinery manufacturing industry are increasing. Therefore, when selecting materials, designers should fully understand the properties and applicable conditions of materials, and consider the requirements of the use, process, and economy of parts. The following is a brief introduction to the general selection principles of metal materials (mainly steel).
The principle of selecting materials for mechanical parts is: that the required materials should meet the requirements of the parts, and have good manufacturability and economy.
1. Requirements for use
The requirements for the use of mechanical parts are as follows:
(1) The working conditions and load conditions of the parts, as well as the requirements, to avoid the corresponding failure modes. The working conditions refer to the environmental characteristics, working temperature, friction, and wear degree of the parts. For parts working in a hot and humid environment or corrosive medium, the material should have good corrosion inhibition and corrosion resistance, such as stainless steel, copper alloy, etc.
The influence of working temperature on material selection should be considered. On the one hand, the coefficient of linear expansion of the materials of the two parts should not be too different, so as to avoid excessive thermal stress or loosening of the fit when the temperature changes; on the other hand, it should also be considered The change in the mechanical properties of a material with temperature. To improve the surface hardness of parts working under sliding friction to enhance wear resistance, quenched steel, carburized steel, nitrided steel and other varieties suitable for surface treatment should be selected, or those with good anti-friction and wear resistance should be selected. Material.
The load condition refers to the magnitude and nature of the load and stress. In principle, brittle materials are only suitable for manufacturing parts that work under static load; in the case of more or less impact, plastic materials should be used as the main material; for parts with large contact stress on the surface, surface treatment should be selected. For parts subject to variable stress, fatigue-resistant materials should be selected; for parts subject to shock loads, materials with higher impact toughness should be selected; for size depends on strength, and size and quality are For restricted parts, materials with higher strength should be selected; for parts whose size depends on stiffness, materials with larger elastic modulus should be selected.
The properties of metal materials can generally be improved and improved by heat treatment. Therefore, it is necessary to make full use of the means of heat treatment to develop the potential of the material. For commonly used quenched and tempered steels, blanks with different mechanical properties can be obtained due to their different tempering temperatures. The higher the tempering temperature, the lower the hardness and strength of the material, and the better the plasticity. So when choosing the variety of materials. The heat treatment specification should be specified at the same time and marked on the drawing.
(2) Restrictions on the size and quality of parts The size and quality of parts are related to the variety of materials and the method of making blanks. When making blanks from casting materials, it is generally not limited by size and quality; when making blanks from forging materials, you need to pay attention to the production capacity of forging machinery and equipment. In addition, the size and mass of the parts are also related to the strength-to-weight ratio of the material. Materials with a large strength-to-weight ratio should be selected as much as possible to reduce the size and quality of the parts.
(3) The importance of the parts in the whole machine or component.
2. Process requirements
Consider that the materials used can be easily manufactured from blank to finished product. For example, parts with complex structures and large dimensions are difficult to forge, and can be cast or welded, and their materials need to have good casting properties or welding properties.
Depending on the selected process, the processing possibilities of the material for the process are considered. For casting, the fluidity of the material, the possibility of shrinkage cavities and segregation, etc. should be considered; for welding, the weldability of the material and the tendency to generate cracks should be considered; for forging, the elongation, hot brittleness and Deformation capacity, etc.; for parts that need heat treatment, the hardenability of the material, the tendency of quenching deformation, etc. should be considered; for parts that need to be machined, the hardness, machinability, cold work hardening degree and post-cutting properties of the material should be considered. Achievable surface roughness, etc.
3. Economic requirements
(1) On the premise that the relative price of the material itself meets the requirements of use, the material with a low price should be selected as much as possible. This is especially important for high-volume manufactured parts.
(2) Processing costs of materials When the quality of the parts is not large and the processing volume is large, the processing costs account for a large proportion of the total cost of the parts. Although cast iron is cheaper than steel plate, for some box-type parts produced in single or small batches, the cost of using cast iron is higher than using steel plate welding, because the latter can save the cost of mold manufacturing.
(3) The utilization rate of materials is processed without chips or with less chips, such as die forging, precision casting, stamping, etc., which can improve the utilization rate of materials.
(4) Local quality principle In many cases, parts have different requirements for materials in different parts. It is practically impossible to choose one material to meet different requirements, and even if it is possible, the price is very expensive. At this time, according to the principle of local quality, different materials or different heat treatment processes can be used in different parts to meet the requirements of each part. For example, the gear teeth of the worm gear need to have excellent wear resistance and high anti-glue ability, and other parts only need to have general strength. Therefore, a bronze ring gear is used in the outer casing of the cast iron wheel core to meet these requirements. Another example is that the sliding bearing only requires friction reduction at the surface in contact with the journal, so it is only necessary to use the friction-reducing material to make the bearing pad instead of the entire bearing. Local quality can also be obtained by carburizing, surface quenching, surface spraying, surface rolling and other methods.
(5) Substitute materials to save valuable and rare materials. Due to supply reasons or economic requirements, other materials can be substituted for the selected materials. For example, when strength is the main requirement, materials with higher strength and higher price can be used, or materials with lower strength and lower price can be used instead, and the structure size is appropriately increased; when wear resistance or corrosion resistance is the main requirement When it is necessary to use materials with good wear resistance or anti-corrosion, you can choose poor materials for various surface hardening treatment or anti-corrosion treatment; for rare materials, ordinary materials can also be used instead, such as aluminum bronze instead of tin bronze to make bearing bushes.
(6) Material supply From the perspective of simplifying the supply and storage of material varieties, for parts produced in small batches, the varieties, and specifications of materials used on the same machine should be reduced as much as possible.