1. Material selection for mechanical parts
Material selection primarily refers to the parts in processing in the material properties, including mechanical properties, physical properties and chemical properties. For most parts, the mechanical properties are the main necessary index, mechanical properties are mainly characterized by the strength limit σb, elastic limit σe, yield strength σs or σ0.2, elongation δ, reduction of area ψ, impact Toughness ak and hardness HRC or HBS and so on. The strength of these parameters is the main performance index of mechanical properties, and only in the case of strength to meet the requirements, to ensure the normal operation of the parts, and durable. In the study of mechanics of materials, it has been found that the allowable stresses used in the design of the dangerous cross-sectional dimensions of components and the degree of safety in checking are all based on the material strength data.
2. the processing machinery parts - machinery parts failure reasons:
Failure due to a variety of reasons, in actual production, parts failure is rarely due to a single factor, often in the result of several factors combined effect. To sum up can be divided into design, materials, processing and installation of four aspects. The possible reasons are as follows:
design reason: firstly due to the design structure and shape unreasonable lead to failure, such as parts of the high stress area there is a clear source of stress concentration (a variety of sharp corners, notches, too small transition fillet; Secondly, the parts of estimated working conditions failure, such as possible over-estimation of the work of inadequate, so that the design of the carrying capacity is not enough.
the improper material selection is the main reason for the failure of the material. The most common is the designer to make decisions based only on the general performance indicators of the material, and these indicators do not at all reflect the type of failure of the material by the force; the defects of the material itself (such as shrinkage, porosity, porosity, inclusions, Micro-cracks, etc.) also lead to parts failure.
processing reasons due to poor processing controlling will cause a variety of defects caused by failure. Such as improper heat treatment process control lead to overheating, decarburization, insufficient tempering, etc .; forging process with bad bands, overheating or over-burning phenomenon; cold processing caused by poor finishing, too much cutter marks, grinding cracks can lead to Parts failure.
Some parts caused by improper processing defects has a great relationship with part design, such as heat treatment of some of the shortcomings. Part shape and structure design unreasonable will promote the heat treatment defects (such as deformation, cracking). In order to avoid or reduce the occurrence of parts quenching or cracking, the design of the parts should pay attention to: cross-sectional thickness is not uniform to avoid easy cracking in thin shelter; symmetrical structure, as far as possible to use of closed structure to avoid the large deformation; uniform cross-section transition, Prevent stress concentration.