Less than 90 percent of all bearings ever achieve their design life. These tips can help you make sure they do.
During the development stage of a new product, design engineers and bearing engineers collaborate to select the best bearing to meet the operating conditions and desired product life. In this process, the life of the bearing can be predicted with a reasonably high degree of accuracy. Unfortunately, once the product leaves the drawing board and enters the real world, less than 90 percent of all bearings ever achieve their design life. The vast majority of these failures can be grouped into three easily corrected categories:
- Bearing handling and storage
- Bearing mounting/assembly
- Bearing lubrication
The first issue, handling and storage, is solvable through the application of a little common sense; i.e. do not use a bearing that has been dropped, do not store bearings in high vibration areas, do not unpackage bearings until they are to be used, etc. The other two issues deserve a more in depth discussion.
To understand why proper mounting techniques help to improve bearing life, it is first necessary to understand bearing radial internal clearance and how it changes from the unmounted condition to the mounted condition.
Simply stated, radial internal clearance is the distance that the inner ring of a bearing can be moved while the outer ring is held stationary. In a bearing, this clearance is quite small when compared to overall dimensions of the bearing. Table 1 shows both the external dimensions (and tolerances) and the C3 radial internal clearance for a 6210 deep groove ball bearing.
When this bearing is mounted into a device such as an electric motor, there should be an interference fit between the bearing’s inner ring and the shaft. As a result of this interference, the internal clearance of the bearing is decreased. For a 6210 ball bearing, the radial clearance will decrease from 0.018-mm ~ 0.036-mm to minus 0.002-mm ~ 0.034-mm (minus 0.0001-in ~ 0.0013-in) when it is mounted onto a shaft with the recommended k5 fit, which is 50.002-mm ~ 50.013-mm (1.9686-in ~ 1.9690-in) in diameter.
Now, how does any of this relate to bearing life? Figure 2 shows a graph of how radial internal clearance and bearing life are intimately tied together. As you can see, bearing life will exceed the predicted value (100 percent life) when the clearance of the bearing in operation is slightly negative, meaning that the bearing is radially preloaded.
Unfortunately, trying to design your fitting practices around achieving this added bearing life can be hazardous. Table 2 and Figure 3 show what might appear to be a minor machining mistake can have disastrous effects on the life of a 6210 ball bearing. Table 2 summarizes how slight changes in shaft diameter will drastically affect operating clearance.
To relate this back to bearing life, for the correctly-sized shaft, the bearing life when the operating clearance is at a minimum is approximately 102 percent of the calculated life. For the 0.025-mm oversized shaft this becomes 93.6 percent, for the 0.038-mm oversized shaft this decreases to 43 percent and for the 0.051-mm oversized shaft the life plummets to 17 percent of the predicted life.
To make matters worse, these numbers only look at the internal loading of the bearing. Other significant factors, such as excessive heat generation and lubricant breakdown, have not been considered. When these are factored in, the bearing could fail in a matter of days or even hours.
So far, we have focused on the results of a shaft that is too large. If a shaft were to be manufactured too small, then a similar failure could occur. First, the shaft would begin to creep (slip), then spin inside the bearing bore. When this occurs, heat gets generated due to the sliding friction between the two components. This would cause the bearing’s inner ring to expand, decreasing the operating clearance of the bearing. This reduction of clearance would eventually result in a radial preload, resulting in an increased amount of torque required to turn the bearing and causing the shaft to spin more easily inside the bearing bore.
These two events would continue to feed off of each other, along with the lubricant breakdown, until total bearing seizure occurs. In addition to the failed bearing, the shaft would have been galled and, in the worst case, the bearing would friction-weld to the shaft, which would seriously complicate any repair work on that machine.
Luckily, if these bearings are caught during regular maintenance before they catastrophically fail, then a good deal of evidence about the bearings can be learned by examining them. For instance, Figure 4 shows the bore of a bearing that exhibits a phenomenon called scuffing.