Fault Diagnosis of Two Rotor System

Abstract

Misalignment and unbalance are the most probably common cause of machine vibration. Vibration monitoring is a useful technique for application to rotating machinery and provides valuable information regarding symptoms of machinery failure which may avoid costly breakdown. Understanding and practicing the fundamentals of rotating shaft parameter is the first step in reducing unwanted vibration. Experimental studies were performed on two rotor dynamic test apparatus to predict the vibration spectrum for different faults. The experimental test rig was designed and tested for misalignment and unbalance condition. The rotor shaft accelerations were measured at 1000 rpm speed using four channel vibration FFT (Fast Fourier Transform) analyzer. The experimental frequency spectra were obtained for misalignment and unbalanced condition leading to reliable diagnostic information.

Introduction

I. INTRODUCTION

Vibration analysis can identify improper maintenance or repair practices. These can include improper bearing installation and replacement, inaccurate shaft alignment or imprecise rotor balancing. Vibration analysis is used to determine the operating and mechanical condition of equipment. A major advantage is that vibration analysis can identify developing problems before they become too serious and cause unscheduled downtime. This with the equivalent theoretical forces due to faults. The fault condition and location of faults were can be achieved by conducting regular monitoring of machine vibrations either on continuous basis or at scheduled intervals. Regular vibration monitoring can detect defective bearings, mechanical looseness and worn or broken gears. Vibration analysis can also detect misalignment and               unbalance before these conditions result in bearing or shaft deterioration. All rotating machines produce vibrations that are a function of the machine dynamics, such as the alignment and balance of the rotating parts. Measuring the amplitude of vibration at certain frequencies can provide valuable information about the accuracy of shaft alignment and balance, the condition of bearings or gears, and the effect on the Machine due to resonance from the housings, piping and other structures.

Vibration measurement is an effective, non-intrusive method to monitor machine condition during start-ups, shutdowns and normal operation. Vibration analysis is used primarily on rotating equipment such as steam and gas turbines, pumps, motors, compressors, paper machines, rolling mills, machine tools and gear boxes. Interpreting the vibration signal obtained is a complex process that requires specialized training and experience. One commonly employed technique is to examine the individual frequencies present in the signal. These frequencies correspond to certain mechanical components such as the various pieces that make up a rolling-element bearing or certain functions such as shaft unbalance or misalignment. By examining these frequencies and their harmonics, the analyst can often identify the location and type of problem, and sometimes the root cause as well.

Abdullah M et al [1] investigated the relationship between Acoustic Emission r.m.s, amplitude and kurtosis for a range of different speed and load conditions for rolling element bearings.

A.W. Lees [2007] [3] observed the effects of rotor misalignments in rigidly coupled rotor and showed that excitation at twice synchronous speed was developed. Arun Kr .Jalan and A.R. Mohanty [2009] [7] used a model based technique for fault diagnosis of rotor–bearing system by generating residual vibrations for the rotor bearing system subject to misalignment and unbalance, and then the residual forces due to presence of faults were calculated. These residual forces were compared  successfully detected by this model based technique.

Tejas H. Patel, Ashish K. Darpe [2009] [5] Misalignment effect at coupling location of rotor FE model is simulated using nodal force vector. Effect of the types of misalignment (parallel and angular) on the vibration behavior of the coupled rotor is examined.

II. EXPERIMENTAL SET-UP

The experimental test rig used for this work is shown in fig 1.

Table No.1 List of Designed Parts of Systems

The test rig consists of a shaft with two rotor disc, which is supported on three ball bearings. A dc motor coupled by a flexible coupling drives the shaft. The parameters used in studies are given in table 1.

The actual rotor rig is also shown in fig 1 this rig consist of a shaft of 30 mm diameter and 600 mm length mounted in between the three SKF single row groove ball bearing. Two disc of 130 mm diameter and 18 mm thick is fixed at the mid span of the two bearing each. The disc consists of 4 drilled holes on the periphery at a radius of 45 mm to fix the balancing mass. The rotor rig is driven by 1 HP motor running at a speed of 300 RPM. A coupling connects the driver and driven shaft. The measuring instruments used in the experimental set-up. An accelerometer is mounted with a stud on the ball bearing housing. The output of the accelerometer is connected to the FFT.

A. Experiment Procedure

Experimental facility as shown in Figure 1 is used for unbalance and misalignment test. First the setup is run for few minutes to settle down all minor vibrations. Before creating the unbalanced, the shaft is checked for any misalignment and unbalance. Two dial gauge method is used to check the proper alignment and balancing. After this an unbalance has been created by placing a mass of 30 gram in the rotor at a radius of 45 mm. Accelerometer along with the FFT analyzer is used to acquire the vibration signals. Ch no. 1 of FFT is connected to vertical direction on drive end motor ch. no 2 of FFT is connected to horizontal direction on the same drive end as shown in figure 2. B&K 4384 charge-type accelerometers are used. Rotor speed is also measured with a B&K photoelectric tachometer probe. The Calibrated accelerometer is fitted over the motor and connected to the FFT. Vibration signals are measured at 1000 rpm.

B. Shaft Alignment

Shaft alignment is the process to align two or more shafts with each other to within a tolerated margin. Any misalignment between the two increases the stress on the shafts and will almost certainly result in excessive wear and premature breakdown of the equipment. This can be very costly. When the equipment is down, production might be down.

1. Parallel Misalignment: Parallel misalignment with, the center lines of both shafts are parallel but they are offset as shown in fig 4 The parallel misalignment can be further divided up in horizontal and vertical misalignment. Horizontal misalignment is misalignment of the shafts in the horizontal plane and vertical misalignment is misalignment of the shafts in the vertical plane. Parallel horizontal misalignment is where the motor shaft is moved horizontally away from the pump shaft, but both shafts are still in the same horizontal plane and parallel. Parallel vertical misalignment is where the motor shaft is moved vertically away from the pump shaft, but both shafts are still in the same vertical plane and parallel.

III. RESULTS AND DISCUSSION

A. Frequency spectrum of Baseline Condition (Healthy System)

The experimental frequency spectra were obtained to baseline condition. The perfect aligned and balance cannot be achieved in practice Thus, a baseline case is presented first to show the residual unbalance. The measured amplitude of a well balanced system at drive end (DE) with the self designed 3 pin type coupling at speed 1000 rpm shown in Figure 5 and 6 on CH. NO.1 and CH. NO.2 respectively. The base line spectrum is measured experimentally using four channel FFT analyzer.

The unbalance has been created by placing a mass of 30 gram in the rotor at a radius of 45 mm. Accelerometer along with the FFT analyzer is used to acquire the vibration signals. Ch no. 1 of FFT is connected to vertical direction on drive end motor Ch no 2 of FFT is connected to horizontal direction on the same drive end.

The Calibrated accelerometer is fitted over the motor and connected to the FFT. Fig 7 , shows frequency spectrum at 1000RPM in vertical direction (CH 1).Fig,8 shows the experiment vibration amplitude in mm/sec of DE at different speeds in horizontal direction (CH 2).

1. Frequency Spectrum of Unbalance of 30 Gram at a Radius of 45 mm in Rotor

2. Frequency Spectrum of Unbalance of 42 Gram at a Radius of 45 mm in Rotor

The unbalance has been created by placing a mass of 42 gram in the rotor at a radius of 45 mm. Accelerometer along with the FFT analyzer is used to acquire the vibration signals. Ch no. 1 of FFT is connected to vertical direction on drive end motor Ch no 2 of FFT is connected to horizontal direction on the same drive end.

In an unbalance fault the FFT spectrum displays a single 1Xrpm frequency peak. This defect may cause high horizontal direction than vertical direction. From that table if increase the unbalance mass the amplitude at 1X is also increases FFT gives more predictable results for unbalance fault.

Table No.3 Values of overall amplitude (mm/s) in vertical and horizontal directions of unbalanced mass 30 and 42 gram at a Radius of 45 mm in Rotor at 1000RPM

C. Frequency Spectrum for Misalignment condition

For generation of parallel misalignment in the healthy setup three holes for bolt on the output flange of the coupling are shifted on either side of the axis of coupling. Offset is 0.1 mm& 0.2 mm for parallel misalignment of 0.1mm and 0.2 mm respectively

1. Frequency Spectrum of Misalignment of 0.1 mm Condition

Table No.4 Values of overall amplitude (mm/s) in vertical and axial directions of different values of different parallel misalignments of rotor speed at 1000 RPM of Drive end

The maximum vibration amplitudes are obtained at second harmonics (2X) at speed (1000 rpm). This is indeed a good indication of the shaft misalignment. This defect may cause high axial vibration than vertical vibration. If we increase the misalignment the second harmonics (2X) has been increase. . If we increase the speed some external forces acts on rotor and it is somewhat unbalance condition. Experimental analysis can be used in predictive maintenance to monitor the parallel misalignment conditions. FFT gives more predictable results for parallel misalignment fault.

Conclusion

In case of unbalance, the amplitudes are measured at drive end in vertical and horizontal directions. The highest value of overall amplitude is produced in horizontal direction. It has been found that the amplitudes are higher in horizontal direction in the drive end and increased when unbalance increased In case of parallel misalignment the amplitudes are measured at drive end in vertical and Axial Directions. The highest value of overall amplitude is produced in vertical direction. It is observed that the amplitudes are higher in vertical direction in the drive end and increased when misalignments are increased

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Copyright

Copyright © 2022 Shantanu Metkar, Abhishek Ahirrao, Abhishek Avhad. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.