Premature bearing failure due to incorrect bearing alignment is very common. This article presents methods for inspecting, diagnosing, and resolving bearing misalignment.
Types of bearing misalignment
Misalignment typically refers to a situation where, after the bearing has been installed and is in motion, the axes of the bearing's inner and outer rings do not overlap and form a certain angle.
For a single bearing, if there is a certain angle between the axis of the bearing inner ring and the bearing housing, it is considered a misalignment of the individual bearing.
In a typical dual support axle system, the line connecting the two support points should be the axle, and the line connecting the centers of the two bearing housings should also be the axle. If these two axes form an angle, it is called axis misalignment.
In a shaft system, if one fulcrum (bearing) has good alignment between the axes of the inner and outer rings, but the other has an angle between the axes, the misaligned bearing will bear an additional misalignment load and will be subject to failures. Well-aligned bearing will also be affected indirectly.
In a shaft system, if both fulcrums (bearings) have good alignment, but there is misalignment in the line connecting the centers of the two fulcrums, then both bearings will be directly affected by the misalignment, resulting in additional load within of the bearings, which will reduce bearing life.
Many sources categorize bearing misalignment into shaft misalignment and bearing housing misalignment. Axis misalignment refers to the situation described above, where the line connecting two support points is misaligned with the axis; Bearing housing misalignment refers to the misalignment of a single bearing. This situation is often related to the precision of component processing.
The above introduces misalignment within a shaft system. When an axis is connected to another axis, there may be a situation where the two axes are not well aligned. If the two axes form a certain angle, it is an angular misalignment. If the two axes are parallel to each other but not in the same straight line, it is a parallel misalignment.
Checking bearing misalignment
For the alignment situation of two connected shafts, a dial indicator can be used directly to check the degree of alignment. This will not be repeated here.
For a single shaft, if there is misalignment after bearing installation, the basic inspection methods include: measurement checks, vibration checks, and bearing trace analysis.
Measurement check
After the bearing is installed, its alignment can be checked using the method illustrated below.
The example given employs a cylindrical roller bearing, but this technique is also applicable to other types of bearings. The procedure involves using a micrometer to rotate around the end face, measuring the difference between the maximum and minimum values, and then calculating the angle of misalignment.
Vibration inspection
During bearing operation, vibration analysis is typically conducted through frequency domain analysis using Fourier transformations of vibration signals. When the rotating axis presents misalignment, the amplitude of its double frequency will increase significantly.
If this amplitude exceeds half the amplitude of the single frequency (rotational frequency), it indicates that the misalignment must be corrected.
There are subtle differences in vibration characteristics for various misalignments. For more details, please refer to the appropriate vibration analysis literature or content related to 'Motor Bearing Failure Diagnosis and Analysis', without going into detail here.
Bearing trace analysis
Operating bearings often leave traces of contact inside the raceway. By identifying the characteristics of these contact traces, it can be determined whether there is any misalignment.
The image below shows misalignment marks resulting from an eccentric bearing housing.
Addressing measures for bearing misalignment
Misalignment in bearings imposes additional load, causing early bearing failure. Therefore, it is crucial to address and fix it. Specific measures include:
1. Inspect the positional accuracy of related components. If poor positional accuracy is due to component machining issues, remachining will be required with strict control over shape and positional accuracy. For specific control requirements, see “Motor Bearing Failure Diagnosis and Analysis”.
2. Damage due to poor alignment during the installation process. If the bearing is forcibly installed with poor alignment with the shaft during assembly, it may damage the bearing. At this point, it is advisable to design appropriate alignment accessories to ensure simple installation while maintaining alignment accuracy.
3. In multi-axis systems, if misalignment is found during inspection, it is necessary to use specialized instruments to adjust and correct the alignment between the axles.
