Problems about bearings that even engineers can misunderstand

In mechanical processing, the use of bearings is very common, but there are always some people will misunderstand some problems in the use of bearings, such as the three misunderstandings introduced below.
Myth 1: Aren't bearings standard?
The person who puts forward this question has some understanding of bearings, but it is not easy to answer this question. It must be said that bearings are both standard parts and not standard parts.
The structure, size, drawing, marking and other aspects of the standard parts are completely standardized. It refers to the bearing of the same type, the same size structure, with interchangeability of installation.
For example, 608 bearings, their external dimensions are 8mmx inner diameter 22mmx width 7mm, that is to say, the 608 bearings bought at SKF and the 608 bearings bought at NSK are the same external dimensions, that is, a long appearance.
In this sense, when we say that the bearing is a standard part, it only refers to the same appearance and head.
The second meaning: bearings are not standard parts. The first layer means that, for 608 bearings, the external size is the same, the internal may not be the same! What really guarantees long-term use are internal structural parameters.

The same 608 bearing, the interior can vary greatly. For example, clearance can be MC1, MC2, MC3, MC4, and MC5, depending on fit tolerances; Cages can be made of iron or plastic; The precision can be P0, P6, P5, P4 and so on according to the purpose of selection; Grease can be selected from high to low temperature in hundreds of ways according to working conditions, and the amount of grease sealing is also different.
In this sense, we say that the bearing is not a standard part. According to the specific operating conditions, you can provide different performance of 608 bearings for your choice. To make it standardized, it is necessary to define the bearing parameters (size, sealing form, cage material, clearance, grease, sealing amount, etc.).
Conclusion: For bearings, you must not simply regard them as standard parts, we must understand the meaning of non-standard parts, in order to choose the right bearings.
Myth 2: Will your bearings last 10 years?
For example, when you buy a car, the 4S shop sells it and the manufacturer boasts about the warranty for 3 years or 100,000 kilometers. After using it for half a year, you find the tire is broken and seek the 4S shop for compensation. However, you are told that it is not covered by the warranty. It is clearly written in the warranty manual that the warranty of 3 years or 100,000 kilometers is conditional, and the warranty is for the core parts of the vehicle (engine, gearbox, etc.). Your tire is a wearing part and is not in the warranty scope.
I want to make it clear that the 3 years or 100,000 kilometers you asked for are conditional. So, you often ask "can bearings last 10 years?" There are also conditions.
The problem you are asking is the service life of bearings. For the service life of bearings, it must be the service life under certain service conditions. It is not feasible to talk about the service life of bearings without using conditions. Similarly, your 10 years should also be converted to hours (h) according to the specific use frequency of the product, because the calculation of bearing life can not calculate the year, only the number of hours (H).
So, what conditions are needed to calculate the service life of bearings? To calculate the service life of bearings, it is generally necessary to know the bearing force (axial force Fa and radial force Fr), speed (how fast to run, uniform or variable speed run), temperature (temperature at work). If it is an open bearing, you also need to know what lubricating oil to use, how clean and so on.
With these conditions, we need to calculate two lives.
Life 1: basic rated life of bearing L10 (assess how long bearing material fatigue spalling occurs)
It should be understood that the basic rated life of bearings is to examine the endurance of bearings, and the theoretical calculation life of 90% reliability is generally given. This formula alone may not be enough, for example, SKF or NSK may give you various correction coefficients.
Life two: the average life of grease L50 (how long the grease will dry up), the calculation formula of each bearing manufacturer is not the same.
Bearing average grease life L50 basically determines the final service life of bearing, no matter how good the quality is, no lubricating oil (grease dries up), how long can dry friction friction? Therefore, the average grease life L50 is basically considered as the final service life of the bearing (note: the average grease life L50 is the life calculated by the empirical formula with the reliability of 50%, which is only for reference and has a large discreteness in the actual test evaluation).
Conclusion: How long the bearing can be used depends on the actual conditions of bearing.
Myth 3: Your bearings are so brittle that they collapse under pressure
Bearing gently pressure is easy to have abnormal sound, indicating that the bearing internal scars, then, how is the bearing internal scars produced?
When the bearing is normally installed, if the inner ring is the mating surface, then the inner ring will be pressed, and the outer ring will not be stressed, and there will be no scars.
But what if, instead of doing that, the inner and outer rings were stressed relative to each other? This results in a Brinell indentation, as shown below.
Yes, you read right, is such a cruel reality, if the bearing inner and outer ring relative stress, just a gentle pressure, bearing is easy to produce damage indentation on the surface of the steel ball and raceway surface, and then produce abnormal sound. Therefore, any installation position that may make the bearing inner and outer ring bear relative force may cause damage inside the bearing.
Conclusion: At present, about 60% of bearing abnormal sound is caused by bearing damage caused by improper installation. Therefore, rather than trying to find the trouble of bearing manufacturers, it is better to use the technical strength of bearing manufacturers to test their installation posture, whether there are risks and hidden dangers.

Post time: Apr-12-2022