From the type of bearing design, we can see that this is a relatively rare two-row roller bearing, since it is two rows, then compared with the general single row (ball or roller) bearing, the bearing capacity is much higher, and this is a roller bearing, itself compared with the ball bearing, the bearing capacity will be higher.
Therefore, judging the use of bearings from the perspective of bearing design alone, this is a very suitable bearing for applications that require high load-bearing capacity.
Of course, judging from this point alone, it does not seem to be particularly convincing, and the general engineer also needs to calculate the life of the bearing to see if the bearing can achieve the required service life of the equipment or motor under the same size. After checking according to the general bearing life calculation formula, we found that, as theoretically imagined, the C value of this bearing is indeed higher than that of other single row bearings, and the life is far from meeting the needs of the application.
A series of judgments seem to have a given result, this bearing is a very ideal bearing for heavy-duty motors.
Especially in some vertical motors, not only is the load larger, but the bearing is required to bear the axial load. Due to the design features of spherical roller bearings, this bearing can withstand not only large radial loads, but also pure axial loads in theory.
However, in practical applications, we often find that the bearing has a high risk of failure when subjected to large axial loads, in other words, the life of this bearing is not as good as expected under this working condition.
Why? I can discuss it with you today.
About the equivalent dynamic load of the bearing
As the main load-bearing element in the shafting system, the bearing is often not subjected to a simple radial force, but is accompanied by a part of the axial force.
However, friends who have done bearing life calculations know that when calculating the fatigue life of the bearing, it is necessary to compare the actual force with the basic rated force of the bearing design, so there is a problem here, we need to bear the joint load of the bearing, equivalent to a load, and this load can be used as the actual load of the bearing for comparison, we call this load the equivalent dynamic load of the bearing.
As the name suggests, "equivalent" is the equivalent load of the bearing. Although it is not a "radial load" in the full sense of the word, here we can theoretically assume that it is an equivalent bearing load.
Fig. 1 shows the force situation that is usually easier to understand, the bearing bears a large radial load, and the axial load value is small;
Figure 2 illustrates a different situation, especially in vertical shaft applications, where the axial load on the bearing is much greater than the radial load (greater than a certain proportion, more on this in a later article).
In the catalogue of each bearing manufacturer, there will be a clear formula for the calculation of the equivalent dynamic load (P) of the bearing.
For the force situation in Figure 1:
For the force situation in Figure 2:
Let's take a spherical roller bearing of a certain brand with the model 22224 (the suffix of the bearing does not affect the calculation parameters) to see the values of these parameters:
(Note: Due to the different manufacturers of bearings, the data in the above table will be slightly different, please consult your bearing supplier for accurate data when actually using.) ）
From the formula and data, it can be seen that in fact, I have taken into account the role of the axial force in the equivalent load when the equivalent bearing is subjected to the combined load.
When the axial force on the bearing is larger, it can be seen from Equation 2 that the proportion of radial force in the entire equivalent dynamic load, or the influence of radial force on the equivalent load, is reduced by one-third, taking this 22224 as an example, the influence of axial force is much greater.
So, theoretically, when calculating fatigue life, we have fully considered the influence of axial force, so why is the actual bearing operation performance not as good as the theoretical conclusion?
This is because the actual result we see is the service life of the bearing, and this service life is affected by many more factors than the load. Similar to lubrication, installation and other factors that will also be affected by the factors, let's look at the spherical roller bearing, in the case of excessive axial load, what else will lead to the low service life of the bearing?
- single-column loading,
- Not subject to the slide of a column of rolling elements
- Unloaded column of rolling elements does not reach the minimum load
- Other effects of axial load on life
The above problems are the causes of a series of problems such as bearing failure and high noise when the axial load is too large, which will appear in spherical roller bearings, and how these problems arise and what are the harms, we will explain them in detail one by one in the follow-up articles.