VIBRATION- SYNCHRONOUS ASYNCHRONOUS & SUB-SYNCHRONOUS SPECTRUM ANALYSIS

SYNCHRONOUS, ASYNCHRONOUS & SUB-SYNCHRONOUS SPECTRUM ANALYSIS

The raw vibration data consists of a wide range of frequencies & it’s analysis in raw form is very complex job. To analyse the data, we have to take the help of technology. FFT Analysis is a very good tool for analysing rotating assets. Raw vibration signal can be analysed in three ways-

·         Synchronous Vibration Analysis

·         Asynchronous Vibration Analysis

·         Sub-Synchronous Vibration Analysis  

Synchronous: The synchronous time averaging technique is used to detect the source of vibration at the 1X frequency. 1X frequency is the running speed of the machine. The vibration probe collects the data & the speed probe give the speed information mounted on the reference shaft. This running speed is tracked & the time averaged 1X data is monitored & analysed. This is very useful in the machine trains with variable speeds or if many other machines are running at close proximity to machine under measurement. Rotor related problems like imbalance, misalignment; rotor related looseness & rotor rub can be identified by using this technology. Other peaks are discarded.

Asynchronous Vibration Analysis: Asynchronous time averaging technique is used to detect the vibration components that are not related to rotating speed & are above 1X speed i.e. above the running speed. These faults are bearing fault, electrical noise, cavitation, etc. Resonance also produces asynchronous frequencies.

Sub-Synchronous Vibration Analysis: Sub-synchronous time averaging technique is used to detect problems like severe bearing looseness, rotor rub, vibration from nearby machines, belt vibrations & other low frequency components. These are below 1X running frequency.   

THERMOGRAPHY (THERMAL IMAGING) AN INTRODUCTION

In this section we will discuss about thermal imaging, another powerful tool in machine diagnostics & predictive maintenance arena. Thermography is an art & science to detect & measure radiation using electro-optical device & correlating to surface temperature of the subject. The history of thermal imaging can be understood by looking at the certain chronological developments in past. In 1672 Newton passed a beam of light though a prism & observed a coloured strip of light containing the colours purple, blue, green, yellow, orange and red. Later on Sir Frederick William Herschel in 1800 made an important discovery by moving Newton’s experiment one step ahead. He placed glass thermometers on different spectral colour patterns & observed a rising trend in temperature from blue to red colour spectrum. He also observed that the temperature beyond the visible colour spectrum was even higher. He concluded from this experiment that an invisible form of energy must be at work in that range, and that the sun emits invisible radiation beyond the visible light range. He called this radiation as ultra-red, now known as infrared radiation.

The laws of radiation that revolutionized the field of thermal imaging-

·      Kirchhoff’s Radiation Law: This states that every type of matter continuously radiates energy. The radiant energy is visible or invisible, depends on the temperature. Later on Kirchhoff coined the term Black Body which means that a body always emits precisely as much heat as it absorbs.

·       Hertz oscillator & Stefan-Boltzmann-law: In 1865 James C. Maxwell first predicted the assumption that light consists of electromagnetic waves which was later confirmed experimentally by Heinrich Rudolf Hertz. Josef Stefan and Ludwig Boltzmann refined Gustav Kirchhoff's theory in 1884 and established the findings of their research in the Stefan-Boltzmann law which states that the total thermal energy emitted by a black body depends on that body's intrinsic temperature.

·       Planck’s radiation law: In 1900 Max Planck did a breakthrough discovery which laid foundation of the present day understanding of electromagnetic radiation & gave birth to quantum physics, & still regarded as physical basis for thermography. It defines the intensity distribution of the electromagnetic energy emitted by a black body as a function of temperature, wavelength and frequency.

In principle thermograph is device to detect temperature pattern in the infrared wavelength of the subject. It started with the Herchel’s experiment & later on after discovery of seeback effect, which led to invention of the thermomultiplier, an early version of a thermocouple by Leopoldo Nobili. Samuel Langley used bolometer to detect body heat from a cow from a distance of 304 m. Sir William Herschel’s son Sir John Herschel, used a device called an evaporograph and produced the first infrared image in 1840. From photo-conducting detectors in early phase, the thermal imaging technology has advanced a lot till date. Unlike earlier cumbersome cooled detectors, now a days much sophisticated, uncooled simple detector technology is available at a reasonable price in the market. Till 1960’s the thermography technology was basically used in military applications but after that, non-military applications like medical, industrial & building maintenance stated taking prominance. Today thermography has emerged as a proven predictive maintenance technology in the industrial domain.     

VIBRATION ANALYSIS TECHNOLOGIES:

VIBRATION ANALYSIS TECHNOLOGIES:

Amplitude Monitoring: We all know that all machines vibrate. But the question is what the acceptable limit of this vibration is? Basically it all depends on the machine type & its design. OEM gives a different vibration limit of alarm & trip for different machines. The different vibration technologies that can be employed for root cause analysis depends on the vibration transducer involved in picking up the data.

With contact type transducers following analysis can be performed-

1.    Amplitude monitoring & analysis of asset deterioration over a period.

2.    Frequency (FFT technology) - Raw time domain signal is broken into frequency domain signal by FFT technique (Fast Fourier Transform). As different frequencies are associated with a peculiar machine characteristic, by comparing the amplitudes of a good machine spectra & test machine spectra, it is possible to pinpoint troubles very accurately. For example 1X (one times the running frequency) frequency is responsible for unbalance, 2X for misalignment etc.

With proximity (non-contact) type transducers following analysis can be performed-

1.    Amplitude monitoring & analysis.

2.    Frequency (FFT technology)

3.    Phase: Phase is the position of a rotating part at any instant with respect to a fixed point. Phase tells about the vibration direction. Phase becomes very useful when the source of the vibration is not clear. Various useful insights regarding problems like Machine Soft Foot, Cocked Bearings and Bent Shafts, Imbalance confirmation, Looseness, Bending or Twisting, Shaft misalignment etc. can be identified by phase analysis.

4.    Form Analysis (Orbit Analysis): X-Y plane time domain signal plot: For performing orbit analysis, the input signals from the two proximity sensor placed at 90⁰ with each other on the bearing & a key phase (tacho) probe is viewed by the software & the resulting lissajous pattern is analysed. Orbit plot gives visual graph of actual shaft central line movement in bearing housing. With accelerometers & velometers also orbits can be achieved and as the sensors are mounted outside the casing of the machine, these orbits are called case orbits and provide absolute shaft motion with respect to space. But the orbits taken from proximity sensors are more common & useful. The shape of the orbit tells about the nature of the machine fault.

5.    Position: Position monitoring is done with the help of non-contact type of probes & various parameters like shaft eccentricity, Axial shift, housing & rotor (shaft) expansion & valve position can be monitored (A part of TG TSI system). With the help of X-Y probes the average shaft centreline can be plotted & rotor lift can be seen. These parameters prove valuable information regarding present condition of the machine.

6.    Bode Plot: Bode plot contains two graphs- Amplitude vs machine speed & Phase vs machine speed. It gives valuable information regarding amount of run out associated with a proximity probe, balance condition & system damping.

7.    Polar Plots: In polar plots also same variables are used as in bode plot.  They also serve the same purpose, only the methid of representation is different.