1 Main Failure Modes and Mechanisms of Mechanical Products
The fundamental task of mechanical product reliability design is to prevent potential failures and correct them at the design stage. gjb 3554 “vehicle system quality and reliability information classification and coding requirements” divides the failure modes of mechanical products into six categories.
(1) Functional failure type, such as manipulation failure, non-starting, non-working, jamming, etc.
(2) functional malfunction type, such as too high or too low pressure, not in place, abnormal speed, insufficient power, etc.
(3) Damage damage type, such as fracture, broken, cracked, twisted deformation, pitting, peeling, etc.
(4) Loose and leaky plugging type, such as loose, fall off, oil leakage, water leakage, gas leakage, plugging, etc.
(5) Degradation and deterioration type, such as aging, deterioration, corrosion, rust, carbon accumulation, etc.
(6) Other types.
Although the failure modes and failure mechanisms of different mechanical products vary, but mostly due to the failure of the mechanical structure. The main causes of failure of mechanical structures include.
(1) insufficient strength, such as the occurrence of yielding, brittle fracture, fatigue, etc.
(2) insufficient stiffness, resulting in excessive structural deformation, instability, etc.
(3) Wear and corrosion problems.
(4) Loss of vibration stability, leading to damage by resonance
(5) Temperature effects, leading to thermal expansion and contraction of the structure, deterioration of performance, melting, etc.
Failure due to fatigue, wear, corrosion and failure of the entire mechanical products accounted for more than 80% of the failure, so the quantitative design of mechanical reliability is also often analyzed and calculated for these three failure mechanisms, where the fracture generated by insufficient strength often leads to major safety accidents, so in the design of mechanical products, strength is the most basic requirement of mechanical reliability.
2 The main parameters of mechanical product reliability
When designing mechanical products for reliability, mechanical products are often divided into whole machines (or systems) and parts. There are more parameters to measure reliability, the most commonly used are reliability, life or reliable life.
(1) Reliability (or probability of failure)
Reliability refers to the probability that a product will not fail under specified conditions and within a specified time. It should be noted that reliability here generally refers to the probability that a certain critical failure mode of a mechanical product does not occur, which reflects the characteristics of mechanical product reliability focusing on micro-mechanisms.
In the literature, it is recommended to use reliability as a metric parameter in accordance with the importance of failure effects of mechanical products and divide it into 5 levels, see Table 1. In the specific design calculation, the reliability level is determined according to the importance of failure effects as the basis of reliability analysis and evaluation.
(2) Life time and reliable life time
Any mechanical product has the problem of service life, for example, the life of a car is generally 500,000 km, and the life of an aircraft is 60,000 flight hours, so in addition to reliability as a reliability measure parameter, life is generally used to measure the working ability of the product.
The reliability of a product is related to its usage time, that is, reliability is a function of working time t. Reliability generally decreases gradually with the extension of working time. In the design life, not all mechanical products can work reliably, so the reliability life is often used to measure the reliability of the product, that is, the reliability is the working life of a given value of R. For example, bearings often use the life of the reliability of 0.9 as the reliability measurement parameter. At present, the B10 reliable life, which is widely used in the engine and automobile industries, also refers to the life when the reliability is 0.9.
There are two ways to determine the actual reliability parameter of mechanical products:
(1) Using tests or using data samples, obtained using statistical methods.
(2) Using expected methods, such as quantitative methods of mechanical reliability, expected methods based on failure rates, etc.