Common failure forms and main causes of mechanical parts

Date：2022-03-28 14:38:08Views：2433

When mechanical parts lose their working capacity or fail to meet the design requirements for some reasons, it is called failure. The failure of mechanical parts does not simply mean damage, which can be summarized into three situations: completely unable to work; Although it can work, its performance is poor and exceeds the specified index; Serious injury and loss of ability to work safely. In order to prevent the failure of parts, it is necessary to analyze the failure of parts, that is, by judging the failure form of parts, determining the failure mechanism and causes of parts, selecting materials pertinently, determining reasonable processing routes, and putting forward failure prevention measures.

1. Failure form of mechanical parts

The common failure modes of mechanical parts can be divided into three types: excessive deformation failure, fracture failure and surface damage failure.

(1) Excessive deformation failure: failure of parts caused by excessive deformation exceeding the allowable range. It mainly includes excessive elastic deformation, plastic deformation and creep at high temperature.

(2) Fracture failure: the phenomenon that parts are separated into two or more parts that are not connected to each other due to excessive bearing or fatigue damage. Fracture is the most serious failure form, which includes ductile fracture failure, low-temperature brittle fracture failure, fatigue fracture failure, creep fracture failure and environmental fracture failure.

(3) Surface damage failure: failure caused by damage or size change on the surface of parts due to relative friction of the surface or corrosion by environmental media during working. It mainly includes surface wear failure, corrosion failure, surface fatigue failure and so on.

It should be pointed out that the same mechanical parts often work in more than one failure mode. However, one way always plays a leading role in the failure of parts. The core problem of failure analysis is to find out the main failure modes.

2. Causes of failure of mechanical parts

There are many and complex factors causing the failure of mechanical parts, including structural design, material selection, material processing and manufacturing, product assembly, use and maintenance and so on.

(1) Design: it mainly refers to the incorrect or unreasonable structure and shape of parts, such as notches, small arc corners, transition zones of different shapes, etc. On the other hand, it refers to the insufficient estimation of the working conditions and overload of the parts, resulting in the insufficient actual working capacity of the parts, resulting in the early failure of the parts.

(2) Material selection: wrong judgment on the failure form of parts in the design, and the selected material properties can not meet the needs of working conditions; The performance index based on material selection can not reflect the resistance of the material to the actual failure form, and the wrong material is selected; The quality of the selected materials is too poor, and the composition or performance is unqualified, resulting in failure to meet the design requirements. All these belong to unreasonable material selection.

(3) Processing technology: improper processing technology of parts may produce various defects, resulting in early failure of parts in the process of use. Such as overheating, overburning and banded structure during hot processing; Decarburization, deformation and cracking during heat treatment; Deep knife marks and grinding cracks appear during cold machining.

(4) Installation and use: the assembly and installation process does not meet the technical requirements, such as too tight and loose coordination during installation, inaccurate alignment, unstable fixation, etc., which may cause the parts to fail to work normally or fail prematurely; In addition, illegal operation, overload, overspeed, untimely repair and maintenance during use will also cause premature failure of parts.

Early failure period is the failure of mechanical parts in the early stage of use. The failure rate is high, but it decreases at a very fast speed. Early failure problems are mostly related to improper design, manufacturing, installation or use. The failure rate of accidental failure period is low and stable. It is the normal working period of mechanical parts. The failure of parts at this stage is generally caused by accidental factors, so the failure is random. If we want to reduce the failure rate in this period, we must take measures from the aspects of material selection, design, manufacturing process, correct use and maintenance.

After the accidental failure period, due to long-term use, the failure efficiency increases sharply due to wear and fatigue crack propagation of parts, indicating that the service life of mechanical parts has exceeded the service life. This stage is called wear failure period. At this stage, although important equipment or parts have not failed, they should be replaced or repaired according to corresponding criteria to prevent major accidents.

In conclusion, although there are many possible failure forms of mechanical parts, the main reasons are due to the influence of strength, stiffness, wear resistance, temperature on working capacity, vibration stability and reliability.