How to monitor and identify shaft misalignment

The cause of a misalignment condition is not always obvious. Vibration analysis may reveal a misalignment problem, but it does not necessarily identify the reason.

Capturing alignment data before the equipment is removed or disassembled, even when maintenance is performed for non-alignment reasons, can, over time, reveal hidden causes of misalignment.

Periodically checking and recording alignment conditions generates useful information about correctable conditions that, if addressed, will reduce failures, increase productivity, and save money.

Some experts also point to other factors (with actual reports) such as foundation problems (i.e., at the interface between the machine supports and the base or “flexible” foundation) and climatic problems.

VIBRATION AND TEMPERATURE FROM MISALIGNMENT

Generally, it is chosen to measure in the bearings close to the coupling and in radial and axial directions. Vibration caused by misalignment presents the following symptoms:

Continuous analysis: The RMS speed (0-1kHz) tends to rise a lot, which is the first indication of this type of failure.

Depending on the cause of the misalignment this increase may be more gradual.

Spectrum: It is expected to see a high 1X peak in the axial direction due to angular misalignment (gap) and high peaks at 1X, 2X, 3X and even 4X and 5X in the radial direction due to parallel misalignment (offset). In general, velocity is the best greatness to analyze the misalignment.

The peaks can be higher vertically at one end of the component (e.g. motor), but higher horizontally at the other end of the same component.

Waveform: The waveform will be a combination of 1X, 2X and possibly other sources, and may therefore include an “oscillation” or take the form of an “M” or “W”.

Phase: The components (e.g. motor and pump) will be out of phase axially, due to angular misalignment.

When comparing vertical and horizontal phase readings, they can be in phase or 180° out of phase. Vertical phase readings taken on opposite sides of the coupling (for example, motor end of motor and motor end of pump) will be out of phase.

Temperature: The increase in temperature near the coupling is noticeable, which depends on the level of misalignment and the type of coupling used.

In general, the temperature profile along the coupled shaft assumes a “W” shape, where temperature peaks occur at the coupling and at the bearings. This temperature distribution along the shaft is clearly visible in the figure below.

DIAGNOSTIC COMPLEXITY

Misalignment, even with flexible couplings, results in two forces, axial and radial, and consequently in increased vibration in the axial and radial directions.

Axial vibration is usually the best indicator of misalignment. In general, whenever the axial vibration amplitude is greater than one half of the highest radial vibration (horizontal or vertical), then misalignment should be suspected as the cause of the vibration.

However, the effect of misalignment on the vibration signature is complex and complicity can be summarized in the five general rules:

1. It is impossible to conclude that the cause of machinery malfunction in the real world is shaft misalignment, by only observing a single vibration spectrum in one operating condition.

2. The severity of the misalignment cannot be detected using vibration analysis. In other words, there is no exact relationship between the amount of misalignment and the vibration level/amplitude.

3. The vibration signature of misaligned rotating machines will be different with different flexible or rigid coupling designs.

4. The misaligned vibration characteristics of the rotors of machines supported in rolling bearings are typically different from the vibration characteristics of the machines rotors supported in plain bearings.

5. Simple testing is essential. The symptoms of misalignment are typically confused with unbalance problems, for example.

Tests with the engine running uncoupled and alignment measurements (with alignment history and monitoring, laser measurements, etc.) can help identify this condition.

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