What is the useful life of a bearing?
Firstly, it is essential to clarify that the question “How long can a bearing be used?” implies that the bearing is already in service and asks about its useful life.
This discussion does not include storage time for unused bearings.
The usage time of a bearing also depends on another standard, that is, under what conditions is a bearing considered unusable?
Generally, we consider a bearing to have failed when it can no longer provide its designed performance during operation. Bearing failure is categorized into two situations:
- First, the bearing completely loses its functionality and cannot rotate.
- Secondly, the bearing partially loses its performance, but can still function. However, anomalies have occurred in the operating accuracy, vibration, etc., of the shaft system.
Clear explanations for the above two types of bearing failures can be found in the corresponding bearing failure analysis standards. However, it is clear that the possibility of a “defective” bearing being considered “unusable” depends on the specific situation of the user.
Typically, there are several ways to define the point at which a bearing becomes “unusable”:
Based on the definition of bearing fatigue failure, it is when the first fatigue spalling appears on the rolling element or raceway surface. At this point, the bearing has suffered fatigue failure. In other words, once this state is reached, the bearing becomes “unusable”.
Determining when a bearing becomes “unusable” can be done in several ways:
Firstly, according to the definition of bearing fatigue failure, it is the moment when the first fatigue chip appears on the rolling element or the raceway surface. At this point, the bearing has suffered fatigue failure. In other words, the bearing becomes “unusable” in this state.
Secondly, under real working conditions, when the first fatigue fragmentation appears, the operational state of the bearing sometimes does not change significantly. For example, there may not be a noticeable increase in bearing vibration. Especially in the early stages of failure, it can be difficult to detect. At this point, the bearing temperature is generally normal. In the eyes of many users, this bearing is still “usable”.
Thirdly, if the above situation continues to evolve, bearing failure progresses, eventually leading to an increase in bearing vibration and temperature. At this time, you can determine whether the bearing is “unusable” according to the vibration or temperature standards of the related equipment. Therefore, the standard of “unusability” comes from relevant standards. Before reaching this standard, although the bearing may already be “sick”, it is still “usable”.
Fourth, in some cases where the shaft accuracy requirement is not high, even in the above situation, as long as the bearing can rotate, it is considered usable. It’s not considered “unusable” until the day it gets stuck and can’t turn.
Therefore, before analyzing the question of “how long can a bearing still be used”, it is necessary to determine what the standard for “still usable” is. Otherwise, it would be impossible to discuss such usability.
The factors that influence the usability of a bearing
We have previously discussed the different criteria for determining whether a bearing can still be used and introduced the concept of bearing failure. When discussing the usability of a bearing, we first need to establish a standard and then explore the factors that affect it within that standard.
Generally, basic rating life calculation is a common verification method for bearings. For our discussion, we can use the definition of this calculation, that is, the time until the first point of failure occurs within the bearing.
As we know, a bearing consists of components such as rolling elements, cages, races, seals and grease. As a whole, if any part of a bearing fails, the entire bearing is considered to have failed or “unusable”.
Considering the different components of a bearing, we have the following:
Rolling elements and tracks:
The service life of a bearing is generally defined by its fatigue life, which is related to the load and speed of the bearing. Thus, the service life of the rolling elements and races of a bearing is influenced by the load and speed of the bearing. Different loads and speeds result in different bearing lifetimes. Of course, mechanical engineers can look at these factors when calculating bearing life.
To illustrate this in simple terms: a bearing, when rotated manually (equivalent to no load, low speed), is almost impossible to wear out, implying a very long (almost infinite) useful life. However, the same bearing under a large load and high speed can quickly fail. Therefore, to approximately estimate the fatigue life of a bearing, load and speed must be taken into account. A simple question like “how long can a bearing last?” cannot provide an answer.
Cages:
Cages support alternating stresses within a bearing and under normal conditions would also experience fatigue. When operating conditions change, cages may also be subject to additional load.
However, under normal conditions, the load experienced by cages is significantly lower than that of rolling elements and tracks. Therefore, in most cases, the fatigue life of the cages exceeds that of the bearing. (Exceptions occur under abnormal operating conditions.) Additionally, cages made from different materials can be affected by ambient temperature, which we will not elaborate on here.
Stamps:
For non-contact seals such as dust covers, which normally do not experience significant loads, their fatigue life can be considered almost infinite (except under abnormal conditions). However, if we take into account chemical reactions between the dust cover and the external environment, such as pollutants (e.g. slow oxidation), further assessment is necessary.
For contact seals, generally made of rubber, their service life is influenced by edge wear and chemical reactions between the seal material itself and the external environment, including pollutants (such as slow oxidation). Wear on the sealing lip is related to the lip pressure, the relative speed of movement and the wear resistance of the sealing material. Seal manufacturers must have relevant test data.
Lubrication:
Bearings are commonly lubricated with grease, the service life of which can be estimated according to the general service life of lubricants. Interested readers can refer to books such as “Engine Bearing Application Technology” and “Engine Bearing Failure Diagnosis and Analysis” for calculations.
Furthermore, grease in non-operational bearings (in a non-working state) is subject to oxidation in air. This information can be obtained from the relevant manufacturers.
How long can bearings be used?
After discussing bearing usage patterns and the factors that influence a bearing's usage time, it's time to provide a clear answer to engineers who are interested in the results.
It's more accurate to say this is a requirement rather than an answer. Because when engine engineers design structures and select bearings, they need to calculate based on this service life. Otherwise, the selection process cannot be completed.
Generally, service life requirements for bearings in common small and medium-sized industrial engines can be found in mechanical design manuals and technical manuals provided by bearing suppliers.
Although there are some discrepancies in these recommended values, they are quite similar. Normally during calculations, a useful life of between 20,000 and 40,000 hours is considered.
Note that this track does not mention speed, load, bearing type, etc. Therefore, this is a design requirement and if the actual service life of a bearing cannot meet this requirement, discussions will need to be held with the relevant manufacturers.
Even if such discussions arise, it does not mean that the bearing is not compliant, as there are many relevant factors involved.
Finally, this range only applies to industrial engines in general. For specialized equipment, requirements may vary. For example, household appliances, wind energy equipment, etc. Engineers can consult the relevant standards for these cases.
