Practical Machinery Vibration Analysis and Predictive Maintenance

Transcription

1 Practical Machinery Vibration Analysis and Predictive Maintenance By Steve Mackay Dean of Engineering Engineering Institute of Technology EIT Micro-Course Series Every two weeks we present a 35 to 45 minute interactive course Practical, useful with Q & A throughout PID loop Tuning / Arc Flash Protection, Functional Safety, Troubleshooting conveyors presented so far Upcoming: Electrical Troubleshooting and much much more.. Go to You get the recording and slides 1

2 Objectives Give some background to vibration measurement Give some suggested techniques Maintenance Philosophies 2

3 Vibration in everyday life Useful Vibration 3

4 Vibration Monitoring Vibration Monitoring 4

5 Vibration Monitoring Vibration Monitoring 5

6 Vibration Monitoring Vibration Monitoring 6

7 Vibration Monitoring Vibration Basics Any external force has to overcome structure s properties: - Mass - Stiffness - Damping A force cause vibration!!! 7

8 Vibration Basics Vibration can be described in terms of: - Acceleration - Velocity - Displacement Single degree of freedom system: - Forced vibration response - Free vibration response - Resonance is free vibration Frequency is number of vibration cycles / time! Vibration Basics 8

9 System Response 10N 2g Nature of Vibration - SHM X = Xo sin ω t X = Disp. at instant t Xo = Maximum disp. ω = 2.π. f (rad/s) f = frequency (Hz) t = time (seconds) 9

10 Wave Terminology Wave Terminology - Phase A time lag of T is a phase angle of 360º. A time lag of T/4 will be a phase angle of 90º. The two waves are out of phase by 90º! 10

11 A Fast Fourier Transform Also called the Frequency Domain or Vibration Spectrum Harmonics 11

12 Frequency and Time Domains FFT is the Frequency Domain. Time Waveform is the Time Domain. Frequency Analysis 12

13 Why do a Frequency Analysis? Overall Amplitude It is the total vibration amplitude over a wide range of frequencies. Acceleration, Velocity, or Displacement. 13

14 Which to choose? Real world vibration levels 14

15 Vibration Terminology Displacement [peak-peak] Velocity [peak] Velocity [rms] Velocity rms tends to provide the energy content in the vibration, whereas the Velocity peak depicts more of the intensity of vibration. Acceleration - peak Machinery Fault Diagnosis Vibration analysis is used to monitor the state of a machine. Detailed analyses can be made concerning the health of the machine and any faults, which may be arising or may have already arisen. The need for higher reliability and availability of critical machinery forces the use of this technique of PdM. 15

16 Common Machinery Faults Unbalance Bent shaft Eccentricity Misalignment Looseness Belt drive problems Gear defects Bearing defects Electrical faults Oil whip / whirl Cavitation Shaft cracks Rotor rubs Resonance Hydraulic + aerodynamic forces Amplitude due to unbalance will vary with the square of speed. The FFT will show 1 rpm frequency of vibration. It will be predominant. Phase difference is as shown Unbalance - Static 16

17 Unbalance - Couple Amplitude varies with square of speed. Predominant 1 peak. May cause high axial along with radial vibrations. Phase difference is 180º on shaft ends in both planes. Unbalance - Overhung Rotors Amplitude varies with square of speed. Predominant 1 peak. May cause high axial along with high radial vibrations. Axial plane phase difference is 0º. Radial direction phase is unsteady. 17

18 Bent Shaft Bend near centre: 1 is predominant. Bend at ends: 2 is predominant. No phase difference in radial direction at one location. 180º phase difference in axial plane. Misalignment After unbalance, misalignment is the major cause for high vibrations. Two kinds of misalignment: Angular - shaft ends meet an angle. Parallel - shaft ends are parallel but have an offset. 18

19 Angular Misalignment Angular Misalignment Predominant peak is 1. 1, 2, 3 may be present. High axial vibration with 1 and 2. Axial phase difference across the coupling is 180º. 19

20 Bent Shaft - Angular Misalignment Difference between angular misalignment and a bent shaft is differentiated only by phase difference. In misalignment, phase difference is 180º across the coupling. Parallel Misalignment 20

21 Parallel Misalignment The predominant peak is at 2. Vibrations in radial direction are higher than in the axial direction. Parallel Misalignment Phase difference in radial direction across the coupling is 180º. 21

22 Misaligned Bearing Cocked bearing show high axial vibrations Opposite ends have an axial plane phase difference of 180º. FFT may show peaks of 1, 2 and 3. Misalignment - Orbits 22

23 Mechanical Looseness Internal assembly looseness: Bearing liner in its cap. Sleeve or rolling element bearing. Impeller on a shaft. Looseness at machine to base plate interface: Loose bolts. Cracks in the frame structure or bearing pedestal. Structure looseness: Weakness of machine feet, baseplate or foundation. Loose hold-down bolts, distortion of frame or base. Internal Looseness Phase is unstable. Radial vibrations taken at 30º. Will see different spectrums for each (directional). FFT will show harmonics of or even. 23

24 Looseness at Machine to Base Plate interface Structure Looseness Measure each bolt, one at a time. 24

25 Resonance Every body has a resonance frequency. Frequency is dependant on mass, stiffness and damping. Forced and free vibrations (e.g. ringing of bell). Resonance is free vibrations. Bump test is simple technique to find resonant frequency of stationary mass or system. Bump Test 25

26 Thank You For Your Interest If you are interested in further training, please visit: IDC Technologies Short Courses: Two-day practical courses available to the public: IDC Technologies Conferences: The Engineering Institute of Technologies Online Certificate and Advanced Diploma programs: 26