Introduction to Fault Diagnosis (Issue 4)

Introduction to Fault Diagnosis (Issue 4): Identification and Analysis of Power Frequency Faults - From Unbalance to Misalignment, Identify the True Cause of Equipment Jitter

In the previous episodes, we have already taken three steps together:

Episode 1: Clarified what vibration is and why faults can be identified;

Phase 2: Learned how vibration signals are collected by sensors;

Episode 3: Learned how to read the health information of devices from waveforms, spectra, envelopes, and time-frequency diagrams.

The most common type of question asked by many friends during the learning process is:

“What do 1X, 2X, and 3X on the spectrum represent? ”

“How should we view common faults such as imbalance and misalignment on the diagram？”

To put it simply, we have already understood what the diagram looks like, and now we want to further understand: what mechanical problem is the most common lump of power frequency and harmonics reflected in these diagrams?

So, starting from this issue, we enter a very practical sector:

Specifically discussing typical faults related to power frequency and harmonics.

There is only one goal for this period:

When you see the obvious 1X, 2X, 3X spectral lines on the spectrum, your mind immediately associates them with:

“This could be - imbalance/misalignment/looseness/coupling issue/pulley eccentricity/friction/shaft bending”

1、Let's review first: What are power frequency and harmonics?

In rotating machinery, the most fundamental frequency is the rotational frequency.

The number of revolutions per second of the rotor corresponds to the rotational frequency (in Hz);

Algorithm: RPM=rotational speed (r/min) ÷ 60.

In vibration analysis, we are accustomed to using the term 'doubling' to describe:

1X: frequency conversion;

2X: 2x frequency conversion;

3X: 3x frequency conversion;

... and so on.

Figure 1 Schematic diagram of frequency conversion and doubling on the spectrum

When you see the spectrogram:

The horizontal axis represents frequency (unit can be Hz or directly labeled as "multiplier X");

The appearance of obvious spectral lines or energy peaks at positions such as 1X, 2X, and 3X generally means that for every 1 revolution of the equipment, there will be 1, 2, or 3 repeated occurrences of some kind of 'abnormal force' or 'repeated impact'. (The original wording was "every revolution, every two revolutions, every three revolutions", which had a conceptual deviation and has now been revised)

Most common mechanical failures, especially imbalance, misalignment, looseness, eccentricity, friction, shaft bending, etc., are closely related to 1X, 2X, and low harmonics.

In this issue, we will focus on these "power frequency characteristics" and explain typical faults one by one.

2、Unbalanced rotor: "resident guest" at 1X frequency

1.What is imbalance?

Intuitive understanding:

When the rotor rotates, if the center of mass is not on the geometric center, centrifugal force will be generated. The faster it rotates, the greater the centrifugal force, and the more the equipment wants to shake.

Common reasons:

Dust, scale, and scaling on the rotor;

Blade wear and missing corners;

Processing and assembly eccentricity;

Changes in quality distribution caused by bearing replacement or component modifications.

2.The hazards of imbalance

Imbalance may seem like 'shaking up a bit', but ignoring it for a long time can be harmful

Centrifugal force increases with the square of the rotational speed → the higher the rotational speed, the more severe the vibration;

Vibration is transmitted through bearings, accelerating bearing wear, heating, and even early failure;

Easy to cause seal wear, oil film damage, and rotor deviation;

Vibration transmitted to the foundation may affect adjacent equipment and cause chain problems;

Long term neglect may lead to rotor fatigue cracking and even serious accidents.

So, imbalance is not a small problem that can be easily resolved, but a hidden danger that needs to be dealt with as soon as possible.

3.Vibration characteristics

On the spectrum, the most typical characteristic of imbalance is:

There is a clear main peak at 1X frequency, which is the "absolute C position";

High frequency harmonics are generally not prominent (unless other faults are superimposed);

In terms of vibration direction, radial (horizontal, vertical) vibration is significant, while axial vibration is relatively small;

The amplitude shows a significant "speed dependence" with increasing speed - as the speed increases, the amplitude of 1X increases significantly.

On the waveform:

Not a standard mathematical sine wave, but the overall shape is close to a sine wave;

The amplitude is relatively stable, with strong periodicity, and looks' quite regular '.

Figure 2 Unbalanced waveform and spectral characteristics

4.Practical diagnostic tips

When suspecting an imbalance, you can first ask yourself three things: Is 1X particularly prominent on the spectrum, while other harmonics are not obvious?

After cleaning the dirt, checking the weight block, and performing dynamic balancing, did 1X decrease significantly?

Is the vibration mainly concentrated in the radial direction, and is the axial vibration not particularly large?

If all three criteria are met, then imbalance is the primary suspect.

3、Coupling misalignment: 2X's' typical spokesperson '

Coupling misalignment is a very common and easily underestimated type of fault on site.

The so-called 'misalignment' means that two axes are not aligned in spatial position.

1.Main forms

There are three common types:

Parallel out of center: Two axes are parallel, but the axes are offset;

Misalignment of angles: there is an angle between the two axes;

Integrated misalignment: There are both parallel offsets and angular deviations - the most common on site.

2.Reasons and hazards of misalignment

Common reasons include:

Installation alignment is not strict, and the accuracy of cold alignment is insufficient;

Thermal expansion during operation leads to unit elongation and axis deviation;

Foundation settlement, improper or loose pre tightening of anchor bolts, and changes in the position of the machine base;

Coupling processing errors, poor key connection fit, etc.

The harm is often hidden but very "irritating":

The shaft is subjected to abnormal radial and axial additional loads, resulting in a significant reduction in its lifespan;

The wear and tear at the coupling, seal, and shaft seal increases, making it prone to leakage and heating;

Long term neglect can lead to more serious problems such as shaft bending and severe bearing wear.

3. Spectral characteristics

The typical spectral characteristics of misalignment can be summarized as follows:

Both 1X and 2X are relatively large, with 2X often being more prominent;

Under certain operating conditions, smaller harmonics such as 3X and 4X may also occur;

Axial vibration is usually more pronounced, especially when the angle is misaligned.

Simply remember a 'small formula':

Imbalance → 1X dominant;

Not centered → 1X+2X are not small, 2X is often very eye-catching, and the axial vibration is relatively active.

4. Waveform and on-site performance

On the waveform:

The overall time-domain waveform still has periodicity, but the amplitude will show significant periodic changes;

Sometimes you can see waveform "bulges" or local distortions.

Figure 3: Waveform and Spectral Characteristics of Coupling Misalignment

Common on-site phenomena include:

The temperature near the coupling is too high, with wear marks and rubber block shear deformation;

The wear and tear of the sealing area accelerates, making it prone to leakage;

The bearing temperature is sensitive to changes in load.

5. Diagnosis and treatment

Re alignment: Align the shaft system after cold/hot compensation according to the specifications;

Check whether the coupling and key connection are worn or offset;

Check whether the foundation and anchor bolts are loose or improperly pre tightened.

4、Loose: signals with increasing frequency doubling

Loose equipment is very common on site, such as:

Loose foundation bolts;

The machine base is not firmly connected to the foundation;

The bearing seat sways in the seat hole;

Some brackets, external pipelines, and accessories are not tightly connected.

1. Classification of looseness

It can be roughly divided into three categories:

Loose foundation/structure: loose feet, displacement of shims, foundation cracking, etc;

Loose connectors: loose threaded connections of flanges, bolts, brackets, etc;

Loose fittings: The outer ring of the bearing rotates in the seat hole, the key fit is loose, and the coupling sleeve is loose.

Different types of looseness have slightly different spectral and waveform performances, but they share a common characteristic: "multiple harmonics, unstable state, and chaotic waveform".

2. Spectral characteristics

Loose spectra often have these characteristics:

1X and its multiple multiples (2X, 3X, 4X...) may occur;

The amplitude of each harmonic is fluctuating and unstable;

Sometimes there may be an increase in broadband energy in a certain frequency band.

Loosening can be imagined as:

The place that should have been rigidly connected has become a "semi active joint", where the rotor "collides and shakes" every time it rotates, resulting in an increase in harmonic components and instability.

3. Waveform representation

On the time-domain waveform, it is often observed that:

The waveform envelope is irregular, with varying amplitudes;

There is a clear phenomenon of "beat vibration" (two frequencies superimposed, with periodic fluctuations in amplitude);

Occasionally experiencing short-term shocks and intermittent amplitude spikes.

Figure 4: Waveform and Spectral Characteristics of Structural Loosening

If you find that a certain device has not changed parts or adjusted working conditions recently, but the vibration suddenly increases and the frequency doubling significantly increases, you may want to first check if the bolts are loose.

5、Belt pulley eccentricity: a typical scene of shaking once after rotating one circle

In belt drive systems, eccentricity in the processing or installation of pulleys is a particularly common issue.

1.What is' eccentricity '?

Simply put:

The geometric center of the pulley is not on the rotation center.

The result is:

The tension of the belt changes once every revolution;

The periodic variation of tension force is directly manifested as the periodic fluctuation of vibration.

2. Spectral characteristics

Commonly seen in the frequency spectrum of belt pulley eccentricity:

At the belt pulley frequency (corresponding to 1X or a certain related frequency), there are clear spectral lines;

If the eccentricity is severe, there will be certain high-order harmonics;

The superposition of belt vibration itself may also result in low-frequency components related to tension and loose edge vibration.

The difference from a simple rotor imbalance is that:

Imbalance is more reflected in the rotor body;

Eccentricity is transmitted to another shaft system or equipment through belt tension, which may result in performance at both the motor end and the driven end measurement points;

On site, it is often observed that the belt bounces noticeably and wears unevenly.

6、Rotor Friction: Energy Dragging Tail in the Spectrum

Rotor friction refers to the contact between the rotor and the stator, sealing ring, baffle, etc. during operation. It may be a slight rubbing or intermittent or persistent friction.

1. Spectral characteristics

Friction generally manifests in the frequency spectrum as:

A series of irregular sidebands appeared near 1X;

Generate broadband energy, especially with significant elevation in the mid to high frequency range;

Sometimes the main peak of 1X seems to be dragging a "tail", showing frequency band expansion.

It can be understood as follows:

The rotor was originally supposed to rotate smoothly, but now it has become "rubbing while rotating",

This nonlinear contact will excite a pile of chaotic frequency components

2. Waveform and on-site performance

On the time-domain waveform:

Often, irregular "burrs" and a feeling of waveform being gnawed on can be seen;

Sometimes there may be a form similar to "slipping" or "lagging".

Typical on-site phenomena:

There are obvious friction and screaming sounds;

Abnormal temperature increase in certain areas;

During inspection, it can be seen that the sealing ring, labyrinth, baffle, etc. have scratches, powder loss, and discoloration.

If there is suspicion of friction, do not just focus on the spectrum, but make a comprehensive judgment based on temperature, sound, and appearance inspection.

7、Rotor shaft bending: "mixed signal" of 1X and 2X

Axial bending can be caused by manufacturing errors, long-term operation, thermal deformation, or accidents.

1. Causes of Formation

The manufacturing or assembly of the rotor itself causes straightness deviation;

Long term high temperature and uneven heat distribution lead to thermal bending;

Plastic deformation caused by accidents such as jamming, overload, and overspeed operation.

2. Spectral characteristics

The spectral characteristics of axial bending have certain similarities with imbalance and misalignment:

The 1X component is often more prominent;

At the same time, 2X or even higher harmonics may occur;

Under certain operating conditions, the amplitude of 1X and 2X will also vary significantly with changes in load or temperature.

It can be roughly described as:

Unbalance: 1X is dominant, and the trend is relatively stable when the structure and operating conditions change;

Axial bending: 1X+2X is more pronounced and often affected by temperature and load, exhibiting the characteristic of "hot bending".

3. Key points for on-site judgment

Based on the following information, make a comprehensive judgment:

The difference in vibration between the cold and hot states of the rotor is significant;

It is difficult to align the rotor after shutdown, and there is still a significant 1X after alignment;

Persistent 1X/2X abnormalities that occur after accidents or blockages.

Axial bending problems are usually severe and may require shutdown, straightening, or rotor replacement.

8、Put these faults together: a simple 'power frequency characteristic table'

You can keep the following table in your mind as a "quick reference table" for preliminary on-site analysis:

Fault Type	Main frequency domain characteristics	Common directions/on-site characteristics
Unbalanced rotor	1X is the main factor, with no significant frequency doubling	Large radial vibration, significantly increasing with rotational speed
Coupling misalignment	Both 1X and 2X are relatively large, with 2X often protruding	Significant axial vibration, heating near the coupling, and easy sealing problems
Loose structure/foundation	Multiple harmonics (1X, 2X, 3X...) with unstable amplitude	The waveform envelope is chaotic, there is vibration, and the inspection of the foundation/machine base is prone to problems
Belt pulley eccentricity	The belt pulley has a noticeable frequency of 1X and may be accompanied by a small number of harmonics	The belt bounces and wears unevenly, and the tension changes significantly
Rotor friction	1X surrounding frequency+wideband energy+high-frequency lifting	Friction sound, temperature rise, and obvious wear marks on components
Bending of rotor shaft	1X+2X is more pronounced and sensitive to temperature/load changes	The difference between cold and hot states is large, and there is still significant residual vibration after multiple dynamic balances

However, in actual devices, it is often a combination of multiple problems, rather than simply "one frequency for one fault".

A more reasonable approach is to:

First, let's take a look: where is the main frequency? How is the doubling situation?

Looking again: measuring point location, vibration direction (horizontal/vertical/axial), operating conditions, historical trends;

When necessary, combine waveform, envelope spectrum, time-frequency diagram, and on-site inspection situation to make a judgment.

9、Summary: Learn to infer device status from "1X, 2X, 3X"

In this issue, we have taken a look at the most common types of faults on site, focusing on power frequency and low-frequency harmonics:

Imbalance → Large households with 1X;

Wrong →The combination fist of 1X+2X has relatively active axial vibration;

Loosening → increased frequency doubling and instability, waveform disorder, and oscillation;

Belt pulley eccentricity → belt pulley frequency 1X+tension fluctuation, significant belt jumping;

Friction → side frequency+wideband+high-frequency lifting, accompanied by friction sound and temperature rise;

Axial bending → 1X+2X, and often changes with temperature and load, sensitive to cold/hot states.

I hope that when you see those "eye-catching" spectral lines on the spectrum, your mind will no longer just be "there is a 1X here and a 2X over there", but will naturally associate them with:

“This 1X is so large, may it be unbalanced?

But 2X is not small and the axial vibration is large, is it not centered?

There are so many harmonics and they are still unstable. Is it possible that there is also a combination of looseness? ”

By observing multiple devices and conducting on-site interactions, you will quickly develop your own "spectrum intuition".

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