FAQs
My gearbox supplier states a bearing life of 20 years - what does this mean?
A typical gearbox may contain many rolling bearings of different types and sizes. Since failure of a single bearing results in the need to overhaul or replace the gearbox, series reliability calculations are needed based upon the estimated life of each bearing. The supplier implies an L10 life of 20 years, that is, 10% of gearboxes will have a bearing failure within 20 years. This means that each bearing must have L10 life much longer than 20 years in order that the overall L10 life is 20 years due to series reliability.
What does L10 life mean?
There is a statistical distribution in rolling bearing life due to variability in microstructure and inclusion content in steels subjected to rolling contact fatigue. L10 life is that estimated to be achieved by 90% of bearings in a population of identical bearings (in a specific application) that are identically mounted, loaded, lubricated and maintained. L50 is the mean time to failure and is about 5 times the L10 life. Some bearings can be expected to last many times the L10 life. In the context of a single bearing, it represents the probability of failure and not the actual failure time.
How can higher reliability be specified?
For higher reliability, L2 or L1 life may be estimated, that is 98 or 99% survival. This may be necessary to achieve an overall L10 life in multi-bearing systems.
What is the definition of failure?
In ISO 281 and other procedures, failure is defined as the point at which the bearing raceway produces the first spall (that may be only a few millimetres across). This will be some time after the first crack is initiated at or below the surface, though in general the crack initiation time is longer than the time for the initial crack to grow to a gross spall.
Is it possible to detect crack initiation before a spall occurs?
Despite much research into techniques such as acoustic emission, it is not possible to detect crack initiation through sensor and signal processing technology. Whilst cracks are known to emit acoustic bursts as they grow, background noise in industrial machinery presents considerable challenges in discrimination and avoidance of false calls.