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Understand: The main failure modes of mechanical parts

Sources: | Release date: 2018-05-15 | Views:
Keywords: Understand: The main failure modes of mechanical parts

Failure: Mechanical parts cannot perform normal functions during the scheduled period of design and under specified conditions.

Forms of failure of mechanical parts: overall fracture, plastic deformation, corrosion, wear, gluing and contact fatigue.

Judgment basis for general design mechanical parts: static strength, fatigue strength, friction and wear.

1. Static strength failure
When mechanical parts are subjected to external loads such as tension, compression, bending, and twisting, the static stress on a dangerous section exceeds the strength limit of the part and breaks or breaks. For example, the bolt is pulled off after being pulled and the shear or crush of the key or pin is such a failure.

In addition, the part will be plastically deformed as the stress applied to the part exceeds the yield limit of the material. Plastic deformation will lead to a decrease in accuracy or inaccurate positioning, which will seriously affect the normal operation of the part, and therefore it is also a failure.

2. Fatigue strength failure
Most mechanical parts work under variable stress conditions, and the effect of variable stress can cause fatigue failure of parts and cause failure.

In addition, the long-term action of the surface of the part subjected to contact deformation stress may also cause cracks or particle peeling. Fatigue damage is a form of failure that occurs gradually with the continuation of working hours and is an important cause of mechanical component failure. For example, fractures due to fatigue crack propagation after shaft loading, fatigue fracture and pitting of roots, and fatigue fracture of chains are typical fatigue failures. The static strength failure of mechanical parts is due to the static force exceeding the yield limit, and often occurs before the fracture occurs, so the static strength failure is often found and predictable.

Fatigue strength failure is gradually formed but difficult to predict in advance, so it is more harmful.

3. Tribological failure
Tribological failures are mainly corrosion, wear, slip, gluing and contact fatigue.

Corrosion is an electrochemical or chemical attack that occurs on the surface of a metal. As a result, the surface of the part is rusted and the fatigue resistance of the part is reduced.

Wear is a phenomenon in which the surface material is lost or transferred during the relative movement of the two contact surfaces.

Gluing is caused by the destruction of the oil film between the two opposite moving surfaces. Under high-speed, heavy-duty working conditions, local sticking occurs. When the two surfaces slide relative to each other, the bonded parts are torn on the surface. Groove marks are formed along the relative movement direction, which is called gluing.

Contact fatigue is a phenomenon in which cracks or particle peeling occur on a surface that is subjected to long-term contact with stress.

Some parts only work properly if certain working conditions are met. For example, a liquid-friction sliding bearing can only work normally if there is a complete lubricating oil film. Otherwise, the sliding bearing will fail in the form of overheating, gluing, wear, etc., which is a tribological failure.

4. Other failures
In addition to the main failure modes noted above, mechanical parts have other forms of failure, such as excessive stiffness failure, unstable failure, and the like.

In addition, the specific failure mode of the mechanical parts depends on various factors such as the working conditions, materials, loaded state and stress properties of the parts. Even with the same type of parts, multiple forms of failure may occur due to different working conditions and mechanical requirements. For example, gear transmission may have failure modes such as gear fracture, wear, tooth surface fatigue pitting, gluing or plastic deformation.