Main causes and dangers of misaligned shafts
Shaft misalignment is considered the second most prevalent source of vibration after unbalance, which occurs due to misalignment between corresponding parts, such as coupling halves, shafts, pulleys, and so on.
In a more technical way misalignment can be defined as the condition when the geometric centerline of two coupled shafts do not coincide along the axis of rotation.
These deviations can present themselves in three different ways:
- Parallel or radial misalignment;
- Angular or axial misalignment;
- Combined misalignment;
Parallel or radial misalignment, occurs when the centerlines of the shafts are apart in parallel.
On the other hand, in the case of angular or axial misalignment, this deviation is observed by the angle formed between the center lines in a plane in space.
And the combined misalignment is the most common situation, and occurs precisely when there are parallel and angular deviations in the same set of centerlines.
In the case of shaft alignment, in general, rigid and flexible couplings can be employed.
Although flexible couplings are preferable because they compensate for some of the misalignment, there are generally accepted standards for shaft misalignment with various types of couplings, so it is extremely important to have the shaft lines aligned as closely as possible.
SHAFT ALIGNMENT AND ITS IMPORTANCE
According to a survey of attendees (mostly maintenance and reliability professionals) at the International Maintenance Conference IMC-2012 on the most recurrent machine failures, misalignment stands out in first place or within the uncertainty margin of the survey is among the top 3.
Meanwhile, some studies point out that machine stoppages in Brazilian industries caused by problems related to inadequate shaft alignment reach more than 50%.
In addition, it is believed that 90% of machines run outside of the recommended alignment tolerances, which can lead to several machine performance, cost, and other component degradation problems.
PROBLEMS CAUSED BY MISALIGNMENT
- Regarding performance:
In the misaligned condition the increased temperature, noise and vibration dissipate some of the energy that should be converted into work, which leads to a direct reduction in the efficiency of the misaligned machine.
- Regarding cost:
There is a cost to produce such dissipated energy, which can directly impact the energy consumed by an electric motor, for example.
During its start, the electric motor consumes more energy (due to its inertia state) and the misalignment makes it difficult to enter the operating regime, increasing current consumption and generating problems in the sizing of the protection devices.
In addition, the motor consumes more energy to perform its work, generating a higher cost in the electricity bill.
Correct alignment can reduce energy consumption by up to 15%, perhaps even more.
Considering power consumption of a three-phase AC electric motor is given by:
Considering now a 25 HP motor under the conditions: volts= 380 V, efi=90% and PF=0.9, with current consumption before alignment of 36 A and after alignment of 32 A operating 350 days/year (which represents 8400 h), then 2.13 kW are consumed due to misalignment.
Assuming a kWh price of R$ 0.10, the annual savings generated by correcting this misalignment is R$ 1,790.00.
- Regarding degradation of other components:
Unfortunately, the costs are not only restricted to energy consumption; degradation in other components generated by misalignment can lead to premature component replacement:
– Bearings: a machine element that suffers the most from shaft misalignment, which receives a strain far above that for which it was designed.
In addition to the appearance of axial loads that damage, for example, ball bearings, which are normally not designed to receive axial loads.
– Sealing: the sealing elements do not achieve the ideal contact with the shaft, leading to leakage and contamination. This causes excessive wear to a certain part of the sealing element, which causes it to cease to perform its function.
It is noted that a misaligned shaft can cause up to 70% reduction in the life of a retainer, for example.
– Couplings: misalignment can cause overheating in the couplings, leading to drying of the rubber parts (commonly used in these elements).
In the representation shown in the figure below, there are the components with the most recurrent failures in machines.
TYPICAL SYMPTOMS OF MISALIGNMENT:
- Premature failure of bearing, seal, shaft or coupling;
- Excessive radial and axial vibration;
- High housing temperature near bearings or high discharge oil temperature;
- Excessive amount of oil leakage at bearing seals;
- Loose foundation screws (“false support”);
SOURCE OF 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 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 weather problems.
VIBRATION AND TEMPERATURE FROM MISALIGNMENT
Generally, it is chosen to measure in the bearing housing close to the coupling and in radial and axial directions. Vibration caused by misalignment presents the following symptoms:
- Continuous Analysis: The RMS velocity (0-1kHz) tends to rise significantly, being a first indication of this type of failure.
- Spectrum: Expect 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 quantity 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 (e.g. driving end of motor and driving end of pump) will be out of phase.
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 amplitude of the vibration 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 four general rules:
- It is impossible to conclude that the cause of machinery malfunction in the real world is shaft misalignment, by observing only a single vibration spectrum in one operating condition.
- The severity of the misalignment cannot be detected using vibration analysis. In other words, there is no relationship between the amount of misalignment and the vibration level/amplitude.
- The vibration signature of misaligned rotating machines will be different with different flexible or rigid coupling designs.
- The misaligned vibration characteristics of the rotors of machines supported in plain bearing housings, are typically different from the vibration characteristics of the rotors of machines supported in anti-friction bearing housings type.
- Temperature: The temperature rise 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.
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Vibration Engineer at Dynamox