Your plant is experiencing excessive vibration. How can you quickly identify the cause?

1 Basic Concepts

Before you start troubleshooting vibration, you need to understand some basic concepts and terms. Vibration is the oscillation of a system around its equilibrium position, measured by its amplitude, frequency, and phase. Amplitude is the magnitude of the displacement, force, or acceleration of the system, usually expressed in units of mm, N, or m/s^2. Frequency is the number of cycles per unit time, usually expressed in units of Hz or rpm. Phase is the angle of displacement relative to a reference point, usually expressed in degrees or radians. Vibration can be classified into two types: free and forced. Free vibration occurs when a system is disturbed and left to vibrate on its own, while forced vibration occurs when a system is subjected to an external force that varies with time.

2 Common Causes

Excessive vibration in a plant can have many causes, but some of the most common are unbalance, misalignment, looseness, and resonance. Unbalance is when the mass distribution of a rotating element is not symmetrical around its axis of rotation, creating a centrifugal force that causes vibration. This can be due to manufacturing defects, wear, corrosion, or dirt accumulation. Misalignment happens when the axes of two or more rotating elements are not parallel or perpendicular to each other, creating bending moments and axial forces that cause vibration. This can be caused by improper installation, thermal expansion, shaft deflection, or bearing failure. Looseness occurs when there is excessive clearance or play between two or more components, allowing relative motion that causes vibration. This can be due to wear, fatigue, corrosion, or improper tightening. Finally, resonance happens when the natural frequency of a system matches the frequency of an external force, resulting in large amplitude vibrations. This can be caused by changes in operating conditions, structural modifications, or environmental factors.

3 Measurement Tools

To identify the cause of vibration, you need to measure its amplitude, frequency, and phase at different locations and directions on the system. You can use various tools for this purpose, such as a vibration meter which is a handheld device that measures the vibration level in terms of acceleration, velocity, or displacement. This device can display the peak, RMS, or peak-to-peak values of the vibration signal, as well as the frequency spectrum. Additionally, a stroboscope emits a flashing light that synchronizes with the rotational speed of a rotating element and can help you visually inspect the condition and alignment of the element, as well as detect any cracks, defects, or unbalance. A dial indicator measures the linear displacement of a component relative to a fixed reference point and can be used to measure the runout, eccentricity, or misalignment of a rotating element. Finally, a phase meter measures the phase difference between two vibration signals and can help you determine the direction and magnitude of the unbalance, misalignment, or looseness of a system.

4 Analysis Techniques

Once you have collected the vibration data, you need to analyze it to identify the cause and severity of the vibration problem. Various techniques can be used for this purpose, such as spectrum analysis which converts the signal from the time domain to the frequency domain using a Fourier transform. This can help detect the dominant frequencies and sources of excitation, such as unbalance or misalignment. Time waveform analysis displays the signal in the time domain, showing amplitude and phase variations over time. This can help identify characteristics and patterns of vibration, such as periodicity or transient events. Envelope analysis extracts high-frequency components of the signal via demodulation, helping to detect and diagnose faults in rolling element bearings, like cracks or spalls.

5 Troubleshooting Tips

When analyzing the results of your vibration measurement, you may be able to troubleshoot and solve the vibration problem by looking for unbalance in the rotating element if the vibration level is high at one frequency and its harmonics. If the vibration level is high at two frequencies and their harmonics, you should check for misalignment between two or more rotating elements. Additionally, if the vibration level is high at multiple frequencies and their subharmonics, you should check for looseness in the components or connections. Lastly, if the vibration level is high at a frequency close to the natural frequency of the system, you should look for resonance, which can be avoided by changing the operating speed, adding damping, or modifying the stiffness or mass of the system.

6 Here’s what else to consider

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