(1) The working condition and load of the parts and the requirements that put forward to avoid the corresponding failure forms refer to the environmental characteristics, working temperature, friction and wear degree of the parts. The materials of parts working in hot and humid environment or corrosion medium should have good corrosion inhibition and corrosion resistance, such as stainless steel, copper alloy and so on.

On the one hand, we should consider the influence of working temperature on the material selection, on the one hand, we should consider that the linear expansion coefficient of the two parts should not be too different, so as not to produce too large thermal stress or loosen the fit when the temperature changes. On the other hand, we should also consider the change of the mechanical properties of the material with temperature. In order to improve the surface hardness and wear resistance of the parts working under sliding friction, quenched steel, Carburized steel, nitride steel and other materials suitable for surface treatment should be selected or materials with good friction reduction and wear resistance should be selected.

Generally, the properties of metal materials can be improved and improved by heat treatment. Therefore, it is necessary to make full use of the means of heat treatment to give full play to the potential of materials. For the most commonly used quenched and tempered steel, the blank with different mechanical properties can be obtained because of its different tempering temperature. The higher the tempering temperature, the lower the hardness and strength of the material, and the better the plasticity. At the same time, its heat treatment specification should be stipulated and marked on the pattern.

(2) The limitation of part size and quality is related to the variety of materials and the preparation method of blank. When making blank with casting material, it is generally not limited by size and quality, but when making blank with forging material, attention should be paid to the production capacity of forging machinery and equipment. In addition, the size and quality of the parts are also related to the strength to weight ratio of the material. The material with large strength to weight ratio should be selected as much as possible in order to reduce the size and quality of the parts.

(3) The importance of the parts in the whole machine or parts.