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Recently, an article titled “The fatigue life of key components of my country’s aero-engines exceeds 50,000 hours” has been widely disseminated on the Internet. At that time, I was quite shocked when I saw this number, NB! Because it is usually very good for aviation bearings to reach tens of thousands of hours. ISO9001 Approved Manufacturer of stainless steel CNC machining services
Today we will take the opportunity to talk to you about the following two topics about bearings.
Factors Affecting Bearing Fatigue Life
Accelerated Equivalent Life and Bearing Reliability
First look at the factors that affect the fatigue life of the bearing. Here we will not talk about the influence of the load, assuming that the design and selection are correct. Then there are two main points to consider, one is the application environment (affecting lubrication conditions), and the other is the quality of the bearing itself (mainly the purity of the bearing steel).
Usually the bearing has a very smooth super-finishing surface, which can form better lubrication conditions (with a sufficient thickness of the oil film). ), the basic dynamic load rating listed on the bearing catalog that you see is based on the data when the bearing is damaged due to this internal fatigue. ISO9001 Approved Manufacturer of stainless steel CNC machining services
However, in practice, due to insufficient lubrication conditions, such as insufficient oil film formation due to low lubricating viscosity, or impurities or moisture mixed into the lubricant, etc. Bearings often fail before internal fatigue develops, and these failures usually originate at the contact surfaces. This is an impact of the application environment we mentioned.
When the application environment remains normal, the quality of the bearing itself, the most important of which is purity, can better distinguish the quality of the bearing. The internal structure of the bearing, the uniformity of chemical composition, the non-metallic inclusions contained, the size and distribution of carbide particles, the degree of decarburization, etc. are very strict, otherwise, these defects will significantly shorten the service life of the bearing and the applicability of the material . The research on the purification of bearing steel has always been the focus of research in various countries. Like most high-strength materials, bearing steels generally lack damage tolerance in the form of a particularly high susceptibility to pre-existing material defects such as non-metallic inclusions that are by-products of the steelmaking process. This is because when the rolling bearing rotates, the contact surface of the bearing parts is under the action of pulse load, and the non-metallic inclusions cause local damage to the continuity of the matrix structure of the steel, causing internal stress concentration, plastic deformation around the non-metallic inclusions, and the formation of microcracks . At present, the mainstream process routes at home and abroad have adopted technologies such as raw material purification, out-of-furnace refining, and double vacuum smelting to ensure the purity of bearing steel, which greatly improves the fatigue life of bearings. ISO9001 Approved Manufacturer of stainless steel CNC machining services
Next, let’s talk about accelerated equivalent life and reliability.
At present, the life determination of aero-engine main bearings is basically carried out in accordance with GJB 7268-2011 “Aero-engine Bearing Test Life Determination Procedures and Requirements”. The load spectrum is usually integrated and determined according to the actual operating conditions of the engine. Apparently, this test method is too costly and inefficient for bearings whose service life needs to be measured for tens of thousands of hours, and the development progress cannot meet the requirements of the engine. This is why accelerated equivalent life testing is used.
The so-called accelerated equivalent test of bearing life is to accelerate the failure process by increasing the stress level of the test bearing without changing the failure mechanism of the bearing and without adding new failure modes, and then use the mathematical statistics theory to estimate the Bearing life under normal stress. ISO9001 Approved Manufacturer of stainless steel CNC machining services
The failure of bearings under different stress loads basically follows the Weibull distribution. According to public information, taking deep groove ball bearings as an example, the life of foreign advanced aviation bearing products is generally more than 8 times the calculated life (up to more than 30 times), and the reliability is more than 98%, while the life of my country’s aviation bearings is generally For 3 to 5 times the calculated life, the reliability is about 96%. In order to let everyone have an intuitive understanding, let’s do a calculation example:
Requirements: The target life is 50,000 hours, the reliability is 98%, and the confidence is 90%.
Suppose we tested 5 samples, all of which exceeded 50,000 hours, and the measured values were 52,000h, 58,000h, 63,000h, 85,000h and 100,000h, are the data very good? Do you have enough confidence to say that our bearings will be absolutely fine after 50,000 hours?
According to the reliability analysis, in the case of 90% confidence level, the reliability of reaching the 50,000-hour target is only 59.93%, which is far from the 98% target. Did you not think of it? then what should we do? Either increase the number of test samples (costing money); or reduce the confidence level (unsure); or reduce the target value, which needs to be reduced from 50000h to 12250h to achieve a reliability of 98%. You may be surprised to see here, 50000h compared to 12250h, the difference is so big!
After this simple small example, I actually want to tell you that once we consider reliability and confidence, things are not as simple as everyone imagined. As a major part of aviation, the reliability requirements for the main bearing are actually quite high, after all, it is related to the safety of human life. In our example above, there is also a data that reaches 100,000 hours, which does not explain anything, so next time you see a lifespan of 100,000 or 200,000 hours in the news, there is no need to be surprised. ISO9001 Approved Manufacturer of stainless steel CNC machining services