25 Sep

Everything on Vibration Analysis


What is Vibration Analysis?

Vibration analysis is the process of gathering vibration levels from the surface of machinery and then analyzing it to detect various faults or failures developing inside it.

The vibration levels are gathered with the help of vibration capturing sensors called accelerometers.

What defects can be captured with the Vibration Analysis Method?

Almost all types of faults that occur in rotating or reciprocating machines, like Unbalance, Misalignment, Looseness, Bearing defects, Belt defects, pump cavitation, etc can be determined by the Vibration Analysis technique.

How data captured is analyzed in Vibration Analysis?

The data captured with the help of sensors can be viewed in either of the two forms-

1. Time Domain 

2. Frequency Domain

Now let’s see how vibration data is analyzed in each measurement form.

1. Time Domain Form

The time-domain form shows data in an Amplitude(Y-axis) vs Time(X-axis) graph.

The vibration amplitude can be represented by various forms or parameters like Acceleration, Velocity, or Displacement. Although all these parameters are interrelated, each has its own importance in fault detection inside the machine which will be described later in this article.

The vibration data displayed in time domain form can also be called as Overall Vibration Measurement for a machine.

What are Overall Vibration Measurements?

The following are referred to as overall vibration measurements:

  • Displacement (Peak to Peak)

  • Velocity (Peak)

  • Acceleration (True Peak)

  • High-Frequency Accelerations (RMS)

RMS value is defined as the square root of means of squares of instantaneous values.

Peak (Derived Peak) = √2 x RMS . The Derived Peak is usually referred to as Peak.

True Peak is the maximum value attained by during one cycle of vibration waveform is called its True Peak value.

Peak to Peak is the difference between the maximum positive and the maximum negative amplitudes of a waveform.

Displacement - Is derived from the acceleration data or directly measured using an LVDT or laser probe. It is the total distance traveled by a vibrating body from one extreme to the other. Displacement is a measure of mechanical stress. It is measured in mills. Mills is 1/1000 of an inch.

Velocity - Is derived from the acceleration data and is measured in m/s or inch/sec. It is the rate of change of displacement. Velocity is a measure of mechanical fatigue. 

Acceleration - Is measured using an accelerometer and is measured in g’s (g=9.8 m/s2). It is the rate of change of velocity and represents the forces experienced by the equipment.

High-Frequency Accelerations or Energy - Is the acceleration RMS or the peak in the filtered high-frequency band ( Usually 5 kHz to 50 kHz band).

Why are they important?

These are called overall measurement as they provide an overall single value of measurement instead of a spectrum. Overall measurements can be used for identifying a developing fault in a piece of equipment. Overall measurements can be trended over time to make out the trend in machine health. Overall measurements have their own limitation and it is not possible to identify the specific fault as it requires identifying values at a specific frequency.

How to choose the right overall measurement metrics?

You must understand the use and limitations of each overall measurement before you start using them.

Overall Measurement

Application

Where to use?

Displacement Peak to Peak

Used for analyzing stress-related defects occurring in orders of rotating frequency.

 

Displacement is a good measure for low-frequency vibration and is not suited for high-frequency vibration. It can be used for analyzing frequencies of less than 20Hz.  It can be used on equipment running up to 1200 rpm.

 

Velocity Peak

 

Used for analyzing fatigue-related defects occurring in orders of rotating frequency.

 

Velocity is a good measure for medium frequency vibration. It can be used in the 10Hz (600 CPM)  to 1KHz (60,000 CPM) frequency range. It can be used on equipment running from 1200 to 3600rpm.

 

Acceleration True Peak or High-Frequency Accelerations RMS

 

Used for analyzing force related defects occurring in the high-frequency band.

 

Acceleration is a good measure of high-frequency vibration. It can be used for analyzing frequencies of more than 1KHz (60,000 CPM). It can be used for identifying bearing, cavitation, and lubrication issues.

 

  1. Frequency Domain Form To be continued...

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