Correcting complicated cases of misalignment takes more skill than simply measuring the vertical and horizontal deviation from zero under ambient temperature using a laser alignment system. In these cases, it is imperative to center the shaft(s) inside the journal bearing(s) during start-up to operational (“hot”) conditions. The cause of the misalignment may be that one of the bearings is unloaded, that the shaft is in the wrong quadrant, or both.
In this case study, we will explain the alignment decisions which were made during the diagnosis of a misaligned lemon-bore bearing.
The two traces measured represent the relative and absolute centerline (CL) shift. During start-up to nominal load, Cascade MVS acquired data using a multi-channel analyzer / recorder connected to two bearing orthogonal proximity probes. The analyzer/recorder externally attached a measuring system to the bearing case split lines. Both the absolute and the relative centerline split measurements are needed for proper calculation. These measurements must be recorded and considered for proper “cold” alignment move (i.e. preset position of the components).
The Cascade MVS Solution
Data is examined by imposing both centerline plots (shaft and bearing shift).
The red and blue arrows show the distances between the horizontal and vertical final positions of the shaft and bearing case CL positions that need to be evaluated.
- Both traces start from the same zero point. This establishes the end of both moves and relative relations to each other.
- Measuring the distances between two centerline final positions we find: the Horizontal (H) to Vertical (V) final position difference is about 90.0 and 82.0 microns (μ), respectively.
- After measuring the final position difference between the H and V centerlines, the vibration analyst must visualize which of the two centerline shifts, or both, are the cause of the “problem.” The analyst should be aware of the bearing type and what is the usual and expected final position of the shaft in such bearing with such clearances.
Cascade MVS Analysis
- Shaft centerline shifted almost vertically and settled in about the center of the bearing. It was too far right and high for the specific bearing type, meaning that it was at the wrong attitude angle/position, resulting in an “offloaded” bearing.
- The bear case data showed pedestal shifting up 20.0 and to the left about 98.0 μ.
- With the CW rotation, the shaft centerline (CL) should set somewhere at about 90 – 110.0 μ left and 15.0 – 20.0 μ up.
To achieve desired shaft running position inside bearing clearances, and get close enough to nominal bearing loading, the bearing “cold” position should be moved 90.0 μ to the right and 62 μ up (i.e. opposite to measured final “hot” centerline positions). Note that this move up takes in account the bearing “hot” position. Optimal 20.0 shaft lift from bearing surface is built in by deducting this value from shaft centerline “hot” position.
This new bearing centerline bore position will “wait” for BOTH centerlines “hot excursion” and their stop at desired optimal position, placing shaft in proper running quadrant attitude angle.
To learn more about our process, download our Lemon Bore Centerline Move Analysis PDF.